# **Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe The MEET Project**

Edited by

Béatrice A. Ledésert, Ronan L. Hébert, Ghislain Trullenque, Albert Genter, Eléonore Dalmais and Jean Hérisson

Printed Edition of the Special Issue Published in *Geosciences*

www.mdpi.com/journal/geosciences

## **Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project**

## **Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project**

Editors

**B ´eatrice A. Led´esert Ronan L. H ´ebert Ghislain Trullenque Albert Genter El ´eonore Dalmais Jean H ´erisson**

MDPI ' Basel ' Beijing ' Wuhan ' Barcelona ' Belgrade ' Manchester ' Tokyo ' Cluj ' Tianjin


*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Geosciences* (ISSN 2076-3263) (available at: www.mdpi.com/journal/geosciences/special issues/ meet project).

For citation purposes, cite each article independently as indicated on the article page online and as indicated below:

LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. *Journal Name* **Year**, *Volume Number*, Page Range.

**ISBN 978-3-0365-6054-0 (Hbk) ISBN 978-3-0365-6053-3 (PDF)**

Cover image courtesy of Alexandre Nachbauer and Electricite´ de Strasbourg

© 2022 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications.

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## **Contents**


Reprinted from: *Geosciences* **2021**, *11*, 520, doi:10.3390/geosciences11120520 . . . . . . . . . . . . **207**


vi

Reprinted from: *Geosciences* **2021**, *11*, 349, doi:10.3390/geosciences11080349 . . . . . . . . . . . . **483**

## **Josipa Hrani´c, Sara Raos, Eric Leoutre and Ivan Rajˇsl**


## **About the Editors**

### **B ´eatrice A. Led ´esert**

Pr. Beatrice A. Led ´ esert is full professor at CY Cergy Paris Universit ´ e (France) where she teaches ´ applied geology, including geothermal exploration. Her research focuses on the exploration of deep geothermal reservoirs, based on mineralogy, petrography, and well data when available. The aim is to pinpoint the paths followed by the geothermal fluid within the reservoirs. She is also interested in processes that occur in surface installations (scaling and corrosion). Her research is supported by the French Agency for ecological transition, the European Research Council and the Eutopia European university alliance.

#### **Ronan L. H ´ebert**

Dr. Ronan L. Hebert is assistant-professor at CY Cergy Paris Universite (France). He is a ´ petrologist focusing on fluid–rock interactions in different contexts (geothermal, cultural heritage, building materials). His research focuses on hydrothermal alteration products, as well as clogging materials that may hinder the flow of fluid within geothermal reservoirs. These are crucial informations regarding reservoir stimulations. He has been working in the field of geothermal energy for more than 15 years.

#### **Ghislain Trullenque**

Dr. Ghislain Trullenque is a structural geologist with a solid fieldwork experience. His research is dedicated to rock petrophysical properties evolution during progressive deformation. He has expertise in the fields of experimental deformation, rock microfabric analysis and fluid–rock interaction processes. He is the scientific coordinator of the MEET project and leads several EU funded research and education projects.

#### **Albert Genter**

Dr. Albert Genter is a senior scientist and a worldwide expert in geothermal energy at Electricite´ de Strasbourg, France, a utility company providing energy on its territory in Eastern part of France. Specialized on structural geology and hydrothermal alteration of basement rocks, his research topics are related to fractured reservoirs in geothermal systems. By working for industry, he focused on environmental monitoring, non-technical barrier, and life cycle assessment of geothermal systems.

#### **El ´eonore Dalmais**

M.Sc. Eleonore Dalmais is a geosciences engineer with a solid experience in resources ´ exploration and estimation, 3D modelling and geophysical logging. She joined ES-Geothermie in 2013 ´ where she contributes to the exploration and drilling of deep wells in Alsace (France) for Enhanced Geothermal Systems and is also involved in geothermal plant exploitation. She participated to several national and international R&D projects dealing with geothermal energy.

#### **Jean H ´erisson**

Dr. Jean HERISSON studied chemistry, biology, environment, and material (culture heritage) building, making him a scientist capable of working in multidisciplinary environments. His experience with standardization committees, scientific networking through RILEM working groups and the different industrial projects he was responsible for, have oriented him toward consulting in the field of innovation. He is now a consultant for Benkei. He is currently managing 3 European projects and 3 French national ones.

## **Preface to "Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project"**

The contribution of geothermal energy to the energetic mix of European countries has been steadily increasing in the last two decades. This resource, being virtually infinite and permanently available, with a negligible environmental impact, is to be seen as a pillar of the energy transition from fossil and nuclear fuels towards renewable sources. In addition, geothermal brines might also be an important source for the extraction of raw materials such as lithium for battery production in the near future.

Depending on the existing surface infrastructures and needs, geothermal energy can be used directly, in the form of heat, or converted into electricity, and related applications, such as cooling and heat storage, are also feasible.

Gains in geothermal energy can be achieved using a variety of techniques, depending on the geological setting of the underground. Among the list of exploitation concepts, enhanced geothermal systems (EGS) are particularly interesting, as their application is much less independent of the underground setting, allowing, in turn, a large geographical deployment and market penetration in Europe. The challenges of EGS are multiple in terms of investment costs, the testing of novel reservoir exploitation approaches with an inherent risk of induced seismicity, and the presence of aggressive geothermal brines, damaging infrastructures.

The multidisciplinary and multi-context demonstration of enhanced geothermal systems exploration and exploitation techniques and potentials (MEET) project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 792037. A European consortium of academic and industrial partners aims to analyze these challenges and propose a series of tools dedicated to potential users and investors in terms of developing EGS and other deep geothermal applications throughout Europe

In order to reach its goal, the MEET project mainly addresses the need to capitalize on the exploitation of the widest range of fluid temperature in EGS plants, use co-produced hot brines in mature oil fields and apply EGS in different geological settings.

The approach is based on a combination of research and demonstration activities in order to make EGS safe and sustainable. This Special Issue summarizes the output of the MEET project based on laboratory experiments, geological field works on high-quality analogues, advanced reservoir modeling, the development of a decision-maker tool for investors and specific demonstration activities, such as chemical stimulation or the innovative monitoring of deep geothermal wells, and the production of electrical power via small-scale binary technology tested in various geological contexts in Europe.

## **B´eatrice A. Led´esert, Ronan L. H´ebert, Ghislain Trullenque, Albert Genter, El´eonore Dalmais, and Jean H´erisson**

*Editors*

#### *Editorial* **Editorial of Special Issue "Enhanced Geothermal Systems and Other Deep Geothermal Applications throughout Europe: The MEET Project" Other Deep Geothermal Applications throughout Europe: The MEET Project" Béatrice A. Ledésert 1,\*, Ronan L. Hébert 1, GhislainTrullenque 2, Albert Genter 3, Eléonore Dalmais 3 and Jean Herisson 4**

**Béatrice A. Ledésert 1,\* , Ronan L. Hébert <sup>1</sup> , Ghislain Trullenque <sup>2</sup> , Albert Genter <sup>3</sup> , Eléonore Dalmais <sup>3</sup> and Jean Herisson <sup>4</sup>** 1 Geosciences and Environment Cergy Laboratory, CY Cergy Paris Université, 1 Rue Descartes,

	- <sup>2</sup> UniLaSalle, 19 Rue Pierre Waguet BP30313, CEDEX, F-60026 Beauvais, France eleonore.dalmais@es.fr (E.D.)

2 UniLaSalle, 19 Rue Pierre Waguet BP30313, CEDEX, F-60026 Beauvais, France;

<sup>4</sup> Benkei, 97 Cours Gambetta, F-69003 Lyon, France **\*** Correspondence: beatrice.ledesert@cyu.fr; Tel.: +33-134-257-357

F-95000 Neuville-sur-Oise, France; ronan.hebert@cyu.fr

**\*** Correspondence: beatrice.ledesert@cyu.fr; Tel.: +33-134-257-357

#### **1. Introduction 1. Introduction**  The MEET project is a Multidisciplinary and multi-context demonstration of En-

The MEET project is a Multidisciplinary and multi-context demonstration of Enhanced Geothermal Systems exploration and Exploitation Techniques and potentials, which received funding from the European Commission in the framework of the Horizon 2020 program. During the four years of the project, two main types of exploitations were investigated: Enhanced Geothermal Systems (EGS) and oil-to-geothermal conversion or co-production. The following topics were addressed: the upscaling of thermal power production and optimized operation of EGS plants (papers [1–5]); variscan geothermal reservoirs in granitic and metamorphic rocks (papers [6–17]); and technical, economic and environmental assessment for oil-to-geothermal fields and EGS integration into energy systems (papers [18–20]). These 20 papers give an overview of some of the work performed in the MEET project, but they are not exhaustive of all the results obtained in this frame. Additional results are available at https://zenodo.org/communities/eu\_project\_meet/ (accessed on 1 September 2022). The MEET Project (Figure 1) received funding by the European Commission in the framework of the H2020 Program (Grant Agreement No. 792037) for a **M**ultidisciplinary and multi-context demonstration of **E**nhanced Geothermal Systems exploration and **E**xploitation **T**echniques and potentials. hanced Geothermal Systems exploration and Exploitation Techniques and potentials, which received funding from the European Commission in the framework of the Horizon 2020 program. During the four years of the project, two main types of exploitations were investigated: Enhanced Geothermal Systems (EGS) and oil-to-geothermal conversion or co-production. The following topics were addressed: the upscaling of thermal power production and optimized operation of EGS plants (papers [1–5]); variscan geothermal reservoirs in granitic and metamorphic rocks (papers [6–17]); and technical, economic and environmental assessment for oil-to-geothermal fields and EGS integration into energy systems (papers [18–20]). These 20 papers give an overview of some of the work performed in the MEET project, but they are not exhaustive of all the results obtained in this frame. Additional results are available at https://zenodo.org/communities/eu\_project\_meet/ (accessed on 1 September 2022). The MEET Project (Figure 1) received funding by the European Commission in the framework of the H2020 Program (Grant Agreement No. 792037) for a **M**ultidisciplinary and multi-context demonstration of **E**nhanced Geothermal Systems exploration and **E**xploitation **T**echniques and potentials.

**Figure 1.** Logo of the MEET project, URL (https://www.meet-h2020.com (accessed on 1 September 2022)) of its website and that of the platform (https://zenodo.org/communities/eu\_project\_meet/ (accessed on 1 September 2022)) on which all of the documents produced by the MEET project are stored. **Figure 1.** Logo of the MEET project, URL (https://www.meet-h2020.com (accessed on 1 September 2022)) of its website and that of the platform (https://zenodo.org/communities/eu\_project\_meet/ (accessed on 1 September 2022)) on which all of the documents produced by the MEET project are stored.

*Geosciences* **2022**, *12*, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/geosciences The MEET project dealt with the gains in geothermal energy that can be achieved using a variety of techniques, depending on the geological setting of the underground. Among the list of exploitation concepts, **Enhanced Geothermal Systems (EGS)** are particularly interesting, as their application is little dependent on the underground setting, allowing, in turn, for a large geographical deployment and market penetration in Europe. The

**Citation:** Ledésert, B.A.; Hébert, R.L.; Trullenque, G.; Genter, A.; Dalmais, E.; Herisson, J. Editorial of Special Issue "Enhanced Geothermal Systems and Other Deep Geothermal Applications throughout Europe: The MEET Project". *Geosciences* **2022**, *12*, 341. https://doi.org/10.3390/ geosciences12090341 Deep Geothermal Applications throughout Europe: The MEET Project". *Geosciences* **2022**, *12*, x. https://doi.org/10.3390/xxxxx Received: 5 September 2022 Accepted: 5 September 2022 Published: 13 September 2022 **Publisher's Note:** MDPI stays neu-

**Citation:** Ledésert, B.A.; Hébert, R.L.; Trullenque, G.; Genter, A.; Dalmais, E. Herisson, J. Editorial of

Special Issue "Enhanced Geothermal Systems and Other

Received: 5 September 2022 Accepted: 5 September 2022 Published: 13 September 2022 claims in published maps and institutional affiliations.

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This article is an open access article

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). tribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

challenges of EGS are multiple in terms of investment costs, the testing of novel reservoir exploitation approaches with an inherent risk of induced seismicity and the presence of aggressive geothermal brines that can damage infrastructures due to scaling and corrosion. The **use of co-produced hot brines in mature oil fields** is another target of the project. The challenges of EGS are multiple in terms of investment costs, the testing of novel reservoir exploitation approaches with an inherent risk of induced seismicity and the presence of aggressive geothermal brines that can damage infrastructures due to scaling and corrosion. The **use of co-produced hot brines in mature oil fields** is another target of the

The MEET project dealt with the gains in geothermal energy that can be achieved using a variety of techniques, depending on the geological setting of the underground. Among the list of exploitation concepts, **Enhanced Geothermal Systems (EGS)** are particularly interesting, as their application is little dependent on the underground setting, allowing, in turn, for a large geographical deployment and market penetration in Europe.

*Geosciences* **2022**, *12*, x FOR PEER REVIEW 2 of 9

MEET aimed at 1- gathering knowledge of EGS heat and power production in various geological settings; 2- helping increase heat production from existing plants and convert oil wells into geothermal wells; 3- enhancing heat-to-power conversion at low flow (<10 l/s) and/or low temperature (60–90 ◦C) by using smart mobile Organic Rankine Cycle (ORC) units; and 4- replicating the technology by promoting the penetration of EGS power and/or heat plants. project. MEET aimed at 1- gathering knowledge of EGS heat and power production in various geological settings; 2- helping increase heat production from existing plants and convert oil wells into geothermal wells; 3- enhancing heat-to-power conversion at low flow (<10 l/s) and/or low temperature (60–90 °C) by using smart mobile Organic Rankine Cycle (ORC) units; and 4- replicating the technology by promoting the penetration of EGS power

In order to reach those objectives, the MEET project mainly addressed the need to capitalize on the exploitation of the widest range of fluid temperature in oil and geothermal fields (from 90 ◦C to 160 ◦C) and apply EGS in different geological settings (sedimentary basins and basements affected or not by post extensional tectonics). The approach was based on a combination of research and demonstration activities. It relied on the study of demonstration sites (Figure 2), either at the **exploration stage** (United Downs Deep Geothermal Project (UDDGP)—UK; Göttingen University campus—-Germany; Havelange—Belgium) by studying **analogues** (e.g., Death Valley—CA, USA; Carnmenellis granite and Cornubian batholith—UK; Dinant synclinorium—Belgium; Rhenish massif and Harz mountains— Germany) or the **exploitation stage** (e.g., Soultz-sous-Forêts EGS plant—France; Chaunoy and Cazaux oil fields—France; Condorcet High School—France; Grásteinn farm and Krauma spa—Iceland). and/or heat plants. In order to reach those objectives, the MEET project mainly addressed the need to capitalize on the exploitation of the widest range of fluid temperature in oil and geothermal fields (from 90 °C to 160 °C) and apply EGS in different geological settings (sedimentary basins and basements affected or not by post extensional tectonics). The approach was based on a combination of research and demonstration activities. It relied on the study of demonstration sites (Figure 2), either at the **exploration stage** (United Downs Deep Geothermal Project (UDDGP)—UK; Göttingen University campus––Germany; Havelange—Belgium) by studying **analogues** (e.g., Death Valley—CA, USA; Carnmenellis granite and Cornubian batholith—UK; Dinant synclinorium—Belgium; Rhenish massif and Harz mountains—Germany) or the **exploitation stage** (e.g., Soultz-sous-Forêts EGS plant—France; Chaunoy and Cazaux oil fields—France; Condorcet High School— France; Grásteinn farm and Krauma spa—Iceland).

**Figure 2.** Location map of the sites and type of works performed in the MEET project. **Figure 2.** Location map of the sites and type of works performed in the MEET project.

Various geological settings are represented: volcanic areas (Krauma and Grásteinn), sedimentary basins (Chaunoy, Condorcet High School and Cazaux), granitic basements (Soultz-sous-Forêts; UDDGP; Death Valley analogue) and metamorphic basements Various geological settings are represented: volcanic areas (Krauma and Grásteinn), sedimentary basins (Chaunoy, Condorcet High School and Cazaux), granitic basements (Soultz-sous-Forêts; UDDGP; Death Valley analogue) and metamorphic basements (Havelange, Göttingen University campus, Harz Mountains and Rhenish Massif analogues).

After four years of transdisciplinary work, this Special Issue compiles some of the most recent geoscience results obtained by the 16 academic and industrial partners (ES-Géothermie (coordinator), UniLaSalle, GIM-Labs, CY Cergy Paris Université, Technische Universität Darmstadt, Universitätsenergie Göttingen GmbH, Georg-August-Universität Göttingen, Vermilion, Enogia, GFZ, Febus-Optics, University of Zagreb—Faculty of Electrical Engineering and Computing, ICETEC, Geological Survey of Belgium, GeoThermal Engineering and Benkei) coming from 5 countries (Belgium, Croatia, France, Germany and Iceland) that joined their efforts for the development of geothermal energy across Europe. The MEET project was organised in dedicated work-packages (WP) through the technical themes that were developed during the project: upscaling of thermal power production and optimized operation of EGS plants (WP3); enhancing and/or converting petroleum sedimentary basins for geothermal electricity and thermal power production (WP4); variscan geothermal reservoirs (granitic and metamorphic rocks; WP5); demonstration of electricity and thermal power generation (WP6); and economic and environmental assessment for EGS integration into energy systems (WP7). This *Geosciences* Special Issue is mostly presenting the geoscientific work performed within MEET. WP3 and WP5 have a dedicated section, whereas WP4, 6 and 7 are grouped in the section "Technical, economic and environmental assessment for oil to geothermal fields and EGS integration into energy systems". Other works performed within the MEET project can be found on the Zenodo platform: https://zenodo.org/communities/eu\_project\_meet/ (accessed on 1 September 2022).

The geoscience work performed in the MEET project was based on geological field work on high-quality analogues; laboratory experiments; advanced reservoir modelling; specific demonstration activities such as chemical stimulation, colder reinjection or innovative monitoring of deep geothermal wells; and production of electrical power via small-scale binary technology tested in the various geological contexts in Europe shown in Figure 2.

#### **2. Upscaling of Thermal Power Production and Optimized Operation of EGS Plants**

The Soultz-sous-Forêts demonstration site (called Soultz in the following) has been operated since 2016 for the production of electricity from a granitic basement. The temperature of the brine produced at the well head of GPK-2 drill hole is around 160 ◦C. It is currently reinjected in the ground at a temperature close to 70 ◦C.

In those operation conditions, minerals precipitate during the lowering of the temperature into the surface installations, producing deposits called scales. Scales have a negative impact on the power plant in lowering the electricity production and inducing specific waste management issues. Thus, scales have to be thoroughly characterized [1] and their deposition process modelled [2] to control scaling processes in surface installations. In order to increase energy output, a small-scale heat exchanger prototype, called SHEx in the following, has been tested for 3 months in order to lower the reinjection temperature down to 40 ◦C. The SHEx is of tubular type, made of an entrance, an exit, water boxes at each end, and tubes made of several alloys and metals in between. The lowering of the temperature of reinjection might have an impact on the geothermal reservoir ([3,4]) while the structure of the reservoir ([3,5]) is another key parameter to be considered for the sustainability of the EGS.

Ledésert et al. [1], in "Scaling in a Geothermal Heat Exchanger at Soultz-Sous-Forêts (Upper Rhine Graben, France): A XRD and SEM-EDS Characterization of Sulfide Precipitates", studied the sulfide scales that deposited in the SHEx by using X-Ray Diffraction (XRD) and a Scanning Electron Microscope (SEM) coupled with an Energy-Dispersive Spectrometer (EDS). The effect of the flow regime on the shape of sulfide crystals was questioned, as well as that of the lowering of temperature on the thickness of the deposit. The scales deposited in the SHEx were compared to scales deposited in normal industrial conditions.

In "Thermodynamic and Kinetic Modelling of Scales Formation at the Soultz-sous-Forêts Geothermal Power Plant", Kunan et al. [2] performed the thermodynamic and kinetic modelling of scale deposition by using Phreeqc and the Thermoddem database thanks to the data on the chemical elements, minerals, and gas it contains. The model generated a rough prediction of the scale formation when operating the plant with sulfate scales inhibitors at the Soultz geothermal plant and showed only a small deviation between simulated results and the actual case.

"Soultz-sous-Forêts Geothermal Reservoir: Structural Model Update and Thermo-Hydraulic Numerical Simulations Based on Three Years of Operation Data" by Baujard et al. [3] presents a thermo-hydraulic numerical simulation to better constrain the parameters that govern the functioning of the Soultz granitic exchanger at depth when colder reinjection is performed. In this article, a 3D hydrothermal study was performed in order to evaluate the spreading of the thermal front during colder reinjection and its impact on production temperature. The fault scale was investigated first, integrating pre-existing models from seismic profiles, seismic cloud structure and borehole image logs calibrated with well data. Secondly, this geometrical model was adapted to be able to run hydrothermal simulations. In a third step, a 3D hydrothermal model was built based on the structural model. After calibration, the effect of colder reinjection on the production temperature was calculated. Finally, the accuracy of the structural model on which the simulations are based is discussed and an update of the structural model is proposed in order to better reproduce the observations.

Mahmoodpour et al., [4] in "Hydro-Thermal Modeling for Geothermal Energy Extraction from Soultz-sous-Forêts, France", propose another hydro-thermal modelling for geothermal energy extraction from Soultz, based on structures identified in [3], at temperature lower than the current 70 ◦C fluid reinjection temperature. Two injectionproduction rate scenarios were modelled, and the drop in the production wellhead temperature for 100 years of operation was quantified. For each scenario, reinjection temperatures of 40°C, 50°C and 60°C were chosen and compared with the 70 ◦C current reinjection condition.

In "Sensitivity Analysis of FWI Applied to OVSP Synthetic Data for Fault Detection and Characterization in Crystalline Rocks", Abdelfettah and Barnes [5] used the Full Wave Inversion (FWI) method to detect, delineate and better characterize faults in the granitic geothermal reservoir, from Multi-Offset Vertical Seismic Profile (OVSP) data in order to further characterize the geothermal resource at Soultz. They made several sensitivity studies to show the dip and thickness of a fault that can be imaged by FWI, even in the presence of additive Gaussian noise. Their work was applied to the Soultz site to help a better characterization of the fracture network.

#### **3. Variscan Geothermal Reservoirs in Granitic and Metamorphic Rocks**

Given the depth of EGS reservoirs and the difficulty to obtain data to characterize them, surface analogues were studied in metamorphic and granitic environments. Analogues are surface sites that are easier to access and have similarities with the geothermal reservoirs in terms of rock nature and geological context. They allow researchers to better characterize the different kinds of EGS reservoirs.

#### *3.1. Granitic Rocks*

The granitic reservoir topic is developed through two sites: the Death Valley granitic surface analogue and the Cornish granites around and within the wells of the United Downs Deep Geothermal Project (UDDGP, Carnmenellis batholith). In the framework of MEET, reservoir improvements by using soft stimulation are planned in Cornwall at EDEN geothermal site where a 5 km deep well has been drilled in a fractured granite close to the UDDGP site.

#### 3.1.1. Death Valley Analogue

The Death Valley granitic analogue is located in the Noble Hills (Noble Hills Granite, NHG, CA, USA). Granitic rocks affected by present or past hydrothermal fluid circulation, typically undergo a variety of alteration processes called hydrothermal alteration. This is due to the instability of the primary mineralogical assemblages under the new physicochemical conditions, which leads to the formation of new mineral phases. Hydrothermal alteration was described for outcrops far from the faults [6] and close to them [7]. The

structural characterization of the fracture network that conducted the fluids responsible for the hydrothermal alterations is presented in [8,9].

Far from the faults, Klee et al. [6] present "Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, CA, USA). Part 1: Granite Pervasive Alteration Processes away from Fracture Zones", aimed at studying the impact of the regional geological context on the rock facies and alteration types. The NHG was first characterized from a geochemical and petrographical point of view. Once alteration parageneses were identified, an attempt was made to correlate the rock hydration rate given by the loss on ignition obtained during Inductively Coupled Plasma spectrometry to porosity, calcite content and chemical composition. Illite crystallinity was used to pinpoint a likely regional temperature gradient in the NHG.

In "Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, CA, USA). Part 2: The Influence of Fracturing on Granite Alteration Processes and Fluid Circulation at Low to Moderate Regional Strain", Klee et al. [7] decipher the role of fractures in the hydrothermal alteration of the rock they crosscut. Several generations of fluids have percolated through the granitic reservoir. The alteration degree, the porosity and the calcite content were evaluated approaching fracture zones. A correlation between the degree of alteration and the fracture density and the amount of strain is proposed.

Chabani et al. [8] proposed a geometrical description and a quantification of the multiscale network organization and its effect on connectivity using a wide-ranging scale analysis. The statistical analyses were performed from regional maps to thin sections. The aim of "Fracture Spacing Variability and the Distribution of Fracture Patterns in Granitic Geothermal Reservoir: A Case Study in the Noble Hills Range (Death Valley, CA, USA)" was to show which class of fractures (small, medium, large) and which orientations ruled the connectivity and hence the ability of fractures to conduct hydrothermal fluids.

In "Multiscale Characterization of Fracture Patterns: A Case Study of the Noble Hills Range (Death Valley, CA, USA), Application to Geothermal Reservoirs", Chabani et al. [9] further characterized the fracture network in the NHG by proposing geometric description and quantifying the multiscale network organization and its effect on connectivity, using a wide-ranging scale analysis and scale order classification. The statistical analyses were performed on real (measured in the field) and virtual (established from photographs) scanlines intersecting fracture networks.

#### 3.1.2. United Downs Deep Geothermal Project (UDDGP) Demonstration Site

In the United Downs Deep Geothermal Project (UDDGP) demonstration site settled in another granite body in Cornwall (UK), both field and borehole samples were collected and analyzed, and numerical simulations were performed. A hydrothermal doublet system was drilled in a fault-related granitic reservoir. It targets the Porthtowan Fault Zone (PTF), which transects the Carnmenellis granite, one of the onshore plutons of the Cornubian Batholith in SW England. At 5058 m depth (TVD; 5275 m MD) up to 190 ◦C were reached in the dedicated production well. The injection well, UD-2, is aligned vertically above the production well, UD-1, and reaches a depth of 2393 m MD.

Schulz et al. [10] propose a "Lab-Scale Permeability Enhancement by Chemical Treatment in Fractured Granite (Cornubian Batholith) for the United Downs Deep Geothermal Power Project, Cornwall (UK)". Lab-scale acidification experiments were performed on outcrop analogue samples from the Cornubian Batholith, which include mineralized veins. The experimental setup is based on autoclave experiments on sample powder and plugs and core flooding tests on sample plugs. These tests were designed to investigate to what degree the permeability of natural and artificial (saw-cut) fractures can be enhanced. Petrological and petrophysical analysis of the samples was performed before and after the acidification experiments to track changes resulting from the acidification for the prediction of likely chemical stimulation.

In "Hydrothermal Numerical Simulation of Injection Operations at United Downs, Cornwall, UK", Mahmoodpour et al. [11] present numerical simulations to analyze the

hydraulic stimulation results and evaluate the increase in permeability of the reservoir. Experimental and field data were used to characterize the initial reservoir static model. Based on experimental and field data, stochastic discrete fracture networks (DFN) were developed to mimic the reservoir permeability behavior. Equivalent permeability fields were calculated to create a computationally feasible model. Hydraulic testing and stimulation data from the UD-1 borehole were used together with hydraulic testing and stimulation data from the UD-2 borehole used for validation.

#### *3.2. Metamorphic Rocks*

The geothermal potential of metamorphic basement was also investigated through the study of the Harz Mountains and Rhenish Massif as analogues for the EGS projects in Göttingen and Havelange. The inputs and difficulties of studying surface analogues were investigated [12]. Fluid flow laboratory experiments were conducted within slate samples from the Harz Mountains [13] and water samples collected from springs around a very deep borehole in Belgium were analyzed [14]. The fluid flow pathways were investigated through the use of the Mohr diagram [15] and the experimental determination of hydraulic properties of fractures [16]. The placement of wells for geothermal projects in rock basements was also investigated [17].

In "Use of Analogue Exposures of Fractured Rock for Enhanced Geothermal Systems", Peacock et al. [12] present an overview of the input and difficulties of the study of field exposures as analogues for EGS sub-surface reservoirs. This contribution discusses general lessons learnt about the use of deformed Devonian and Carboniferous meta-sedimentary rocks in the Harz Mountains (Germany), as analogues for a proposed EGS at Göttingen University campus (Germany). It indicates that the objectives of analogue studies must be clarified in order to explain to people from other disciplines the information that can and cannot be obtained from surface exposures. The parameters that have to govern the choice of an analogue are also highlighted.

Cheng et al. [13], in "Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)", present an experimental study for the evaluation of long-term evolution of fracture permeability in saw-cut slate samples from the Harz Mountains (Germany). The purpose was to investigate fracture permeability evolution at temperatures up to 90 ◦C using both deionized water (DI) and a NaCl solution as the pore fluid.

Cabidoche et al. [14] characterized water samples collected in 50 springs to evaluate the geothermal potential around the Havelange (Belgium) deep borehole, within the Rhenohercynian fold and thrust belt. They based their work on a heat map as well as on hydrogeochemistry and geothermometry analyses to define the main water types, and produced the paper entitled "Spring Water Geochemistry: A Geothermal Exploration Tool in the Rhenohercynian Fold-and-Thrust Belt in Belgium".

As regards fluid flow pathways, Peacock et al. [15] propose a "Use of Mohr Diagrams to Predict Fracturing in a Potential Geothermal Reservoir". Inferences have to be made about likely structures and their effects on fluid flow in a geothermal reservoir at the predrilling stage. Simple mechanical modelling was used here to predict the range of possible structures that are likely to exist in the sub-surface and that may be generated during the stimulation of a potential geothermal reservoir. In particular, Mohr diagrams are used to show under what fluid pressure and stress different types and orientations of fractures are likely to be reactivated or generated. The approach enables the effects of parameters to be modelled individually and defines the type and orientation of fractures to be considered. This modelling is useful for helping geoscientists to consider, model and predict the ranges of mechanical properties of rock, stresses, fluid pressures and the resultant fractures that are likely to occur in the sub-surface. Here, the modelling was applied to folded and thrusted greywackes and slates, which are planned to be developed as an Enhanced Geothermal System beneath Göttingen (Germany).

In "Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS)", Herrmann et al. [16] experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For slate samples, the hydraulic transmissivity of the fractures was measured at confining pressures up to 50 MPa, temperatures between 25 and 100 ◦C and differential stress perpendicular to the fracture surface of up to 45 MPa.

In their paper "Impact of Well Placement in the Fractured Geothermal Reservoirs Based on Available Discrete Fractured System", Mahmoodpour et al. [17] show how necessary well placement in a given geological setting for a fractured geothermal reservoir is for enhanced geothermal operations. Fully coupled thermo-hydromechanical (THM) processes are simulated in 2D in the fractured reservoir to estimate maximum geothermal energy extraction potential by optimizing well placement. To enhance the knowledge of well placement for different working fluids, the authors examine different injection–production well doublet positions in a given fracture network using coupled THM numerical simulations. Thermal breakthrough time, mass flux, and the energy extraction potential are examined to assess the impact of well position in a two-dimensional reservoir framework.

#### **4. Technical, Economic and Environmental Assessment for Oil to Geothermal Fields and EGS Integration into Energy Systems**

Numerous oil fields are approaching the end of their lifetime and have great geothermal potential considering temperature and water cut. EGS is also a promising source of energy. However, electricity and thermal power generation is threatened by technical issues [18], and a proper economic evaluation of different scenarios is crucial for further implementation of these solution at larger scale [19,20].

In "Study of Corrosion Resistance Properties of Heat Exchanger Metals in Two Different Geothermal Environments", Davíðsdóttir et al. [18] investigated the corrosion resistance of different alloy candidates for heat exchangers. They exposed in situ corrosion-resistant alloy coupon samples 316L, 254SMO, Inconel 625 and titanium grade 2 at two locations and geological settings (Triassic clastic sediments, Paris Basin, France; volcanic setting, Iceland). Coupons were exposed for four months at the Chaunoy oil field in France and one month at the Reykjanes powerplant in Iceland. After exposition, the tested alloys were analysed regarding corrosion with macro- and microscopic techniques using optical and electron microscopes.

Romanov and Leiss [19] focused their study entitled "Analysis of Enhanced Geothermal System Development Scenarios for District Heating and Cooling of the Göttingen University Campus" on potential scenarios of EGS development in the poorly known Variscan basement below Göttingen, for district heating and cooling of the University campus. On average, they demonstrated that a single EGS doublet could cover about 20% of the heat demand and 6% of the cooling demand of the campus. The levelized cost of heat (LCOH), net present value (NPV) and CO<sup>2</sup> abatement cost were evaluated. Based on a sensitivity analysis, the EGS heat output was estimated for potential profitability. The most influential parameters on the outcome were identified and are presented in this paper. Key prerequisites for launching EGS project in Göttingen are also given.

Hrani´c et al. [20] worked on oil fields that are approaching the end of their lifetime and have great geothermal potential considering temperature and water cut, for which oil companies consider switching from oil business to investments into geothermal projects on existing oil wells. The used methodology presents the evaluation of the existing geothermal potential for several oil fields in terms of water temperature, flow rate and spatial distribution of existing oil wells. This paper entitled "Two-Stage Geothermal Well Clustering for Oil-to-Water Conversion on Mature Oil Fields" proposes a two-stage clustering approach for grouping similar wells in terms of temperature and then spatial arrangement to optimize the location of production facilities. The outputs regarding the production quantities

and economic and environmental aspects provide insight into the optimal scenario for oil-to-water conversion. A case study has also been developed.

This Special Issue hence compiles 20 scientific contributions resulting from some of the work performed during the H2020 MEET project. It shows that a multidisciplinary approach including geology, material science, petrophysics, geophysics, reservoir modeling and the collaboration between academics and the industry is essential. This Special Issue brings some tangible scientific content to convince the readers about the opportunity to generalize EGS in Europe in different geological contexts and cogenerate hot water and oil in order to tackle the challenge of energy transition.

**Funding:** This work has received funding from the European Union's Horizon 2020 research and innovation program (Grant agreement No. 792037—MEET Project).

**Acknowledgments:** The partners of the MEET project wish to thank GEIE Exploitation Minière de la Chaleur, UDDGP, EDEN, Vermilion, Krauma spa and Grásteinn farm for site access and data sharing. The Fonds Régional d'Aide aux Porteurs de Projets Européens (FRAPPE) of Hauts-de-France is aknowledged for its support.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Article* **Scaling in a Geothermal Heat Exchanger at Soultz-Sous-Forêts (Upper Rhine Graben, France): A XRD and SEM-EDS Characterization of Sulfide Precipitates**

**Béatrice A. Ledésert 1,\* , Ronan L. Hébert <sup>1</sup> , Justine Mouchot <sup>2</sup> , Clio Bosia <sup>2</sup> , Guillaume Ravier <sup>2</sup> , Olivier Seibel <sup>2</sup> , Éléonore Dalmais <sup>2</sup> , Mariannick Ledésert 3,†, Ghislain Trullenque <sup>4</sup> , Xavier Sengelen <sup>1</sup> and Albert Genter <sup>2</sup>**


**Citation:** Ledésert, B.A.; Hébert, R.L.; Mouchot, J.; Bosia, C.; Ravier, G.; Seibel, O.; Dalmais, É.; Ledésert, M.; Trullenque, G.; Sengelen, X.; et al. Scaling in a Geothermal Heat Exchanger at Soultz-Sous-Forêts (Upper Rhine Graben, France): A XRD and SEM-EDS Characterization of Sulfide Precipitates. *Geosciences* **2021**, *11*, 271. https://doi.org/ 10.3390/geosciences11070271

Academic Editors: Jesus Martinez-Frias and Matteo Alvaro

Received: 5 May 2021 Accepted: 23 June 2021 Published: 28 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Abstract:** The Soultz-Sous-Forêts geothermal site (France) operates three deep wells for electricity production. During operation, scales precipitate within the surface installation as (Ba, Sr) sulfate and (Pb, As, Sb) sulfide types. Scales have an impact on lowering energy production and inducing specific waste management issues. Thus scaling needs to be reduced for which a thorough characterization of the scales has to be performed. The geothermal brine is produced at 160 ◦C and reinjected at 70 ◦C during normal operation. In the frame of the H2020 MEET project, a small heat exchanger was tested in order to allow higher energy production, by reinjecting the geothermal fluid at 40 ◦C. Samples of scales were analyzed by XRD and SEM-EDS, highlighting that mostly galena precipitates and shows various crystal shapes. These shapes can be related to the turbulence of the flow and the speed of crystal growth. Where the flow is turbulent (entrance, water box, exit), crystals grow quickly and mainly show dendritic shape. In the tubes, where the flow is laminar, crystals grow more slowly and some of them are characterized by well-developed faces leading to cubes and derived shapes. The major consequence of the temperature decrease is the increased scaling phenomenon.

**Keywords:** Soultz-Sous-Forêts; geothermal site; heat exchanger; scales; sulfates; sulfides; As and Sb-bearing galena; crystal growth; crystal shapes

## **1. Introduction**

Geothermal power production is a very attractive resource with characteristics such as low cost, little environmental pollution and worldwide distribution [1–3]. In addition, it is available all the time, whatever climatic conditions and day/night alternation, as opposed to wind- or solar-derived energy. Geothermal power plants take their energy from deep underground water that is pumped to the surface. During its residence time in the ground at sometimes high temperature (150–300 ◦C), the water acquires specific properties by interaction with the rock reservoir, generally resulting in high salinity and acidity. These characteristics are responsible for corrosion and scaling (deposition) issues in wells and in surface installations. Those phenomena are known from the very beginning of industrial high-temperature geothermal operations and are identified to be responsible for economic issues [4,5]. Scaling is encountered in both low enthalpy [6] and high enthalpy [5,7] geothermal systems. According to [8], among the most abundant scales are silica, carbonates, sulfates, sulfides, and native metals such as antimony (Sb). It is also

known that metal sulfide scaling frequently occurs in volcanic geological context or high Cl environments [9]. *Geosciences* **2021**, *11*, x FOR PEER REVIEW 3 of 24

Among all the geothermal geological contexts, grabens present specific characteristics. In the Upper Rhine Graben (URG; at the border between France and Germany) hydrothermal fluids percolate within fault zones and are brought relatively close to the surface, favoring the development of high-energy geothermal plants. These are dedicated to the production of either electricity (e.g., Soultz-Sous-Forêts, called Soultz in the following) or heat (e.g., Rittershoffen). The URG deep ground water system is characterized by a brine with high salinity (99–107 g/L at Soultz) and moderate low pH (around 5) [10] responsible for strong corrosion of geothermal surface installations (pipes, heat exchangers) and also deposition of minerals within these installations [11,12]. close to the surface, favoring the development of high-energy geothermal plants. These are dedicated to the production of either electricity (e.g., Soultz-sous-Forêts, called Soultz in the following) or heat (e.g., Rittershoffen). The URG deep ground water system is characterized by a brine with high salinity (99–107 g/L at Soultz) and moderate low pH (around 5) [10] responsible for strong corrosion of geothermal surface installations (pipes, heat exchangers) and also deposition of minerals within these installations [11,12].

The Soultz geothermal production plant is based on three 5000 m-deep wells, GPK-2, GPK-3 and GPK-4 (Figure 1) penetrating the granitic basement. GPK-2 is the production well while the total reinjection of the geothermal fluid is performed through GPK-3 and GPK-4. In industrial operation conditions, the brine is produced at 160 ◦C and is reinjected at 70 ◦C [13], showing no difference in its chemical composition when the temperature decreases. In the framework of the H2020 MEET European program [14,15], an additional small heat exchanger (SHEx) has been installed temporarily in order to assess the optimization of energy production by lowering the temperature of the reinjected fluid from 70 ◦C to 40 ◦C [16]. This SHEx received 10% of the total flow and was tested over 3 months. It was designed with six alloys in order to test their reaction to corrosion and scaling. Only scaling phenomena are described here and several points are addressed: (1) determination of the chemical and mineralogical composition of the scales, (2) impact of temperature lowering on the scaling processes (composition/morphology/thickening of the deposits), (3) influence of alloys on scaling development. An X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS) survey was performed to answer those questions. The Soultz geothermal production plant is based on three 5000 m-deep wells, GPK-2, GPK-3 and GPK-4 (Figure 1) penetrating the granitic basement. GPK-2 is the production well while the total reinjection of the geothermal fluid is performed through GPK-3 and GPK-4. In industrial operation conditions, the brine is produced at 160°C and is reinjected at 70 °C [13], showing no difference in its chemical composition when the temperature decreases. In the framework of the H2020 MEET European program [14,15], an additional small heat exchanger (SHEx) has been installed temporarily in order to assess the optimization of energy production by lowering the temperature of the reinjected fluid from 70 °C to 40 °C [16]. This SHEx received 10% of the total flow and was tested over 3 months. It was designed with six alloys in order to test their reaction to corrosion and scaling. Only scaling phenomena are described here and several points are addressed: (1) determination of the chemical and mineralogical composition of the scales, (2) impact of temperature lowering on the scaling processes (composition/morphology/thickening of the deposits), (3) influence of alloys on scaling development. An X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS) survey was performed to answer those questions.

**Figure 1.** The Soultz Enhanced Geothermal System with its 5 wells among which the 3 deepest ones (GPK-2, GPK-3 and GPK-4) are used for electricity production. The brine is produced at 160 °C in GPK-2 well and reinjected at 70 °C in GPK-3 and GPK-4 in normal operation. The temperature gradient recorded at Soultz (red dashed line) and key figures are indicated on the left. Figure modified after [17,18]; size of the deep reservoir from [19]. **Figure 1.** The Soultz Enhanced Geothermal System with its 5 wells among which the 3 deepest ones (GPK-2, GPK-3 and GPK-4) are used for electricity production. The brine is produced at 160 ◦C in GPK-2 well and reinjected at 70 ◦C in GPK-3 and GPK-4 in normal operation. The temperature gradient recorded at Soultz (red dashed line) and key figures are indicated on the left. Figure modified after [17,18]; size of the deep reservoir from [19].

site analyses on scaling issues occurring in various geothermal plant types are reported in

**2. Technical Context** 

#### **2. Technical Context**

#### *2.1. Scaling Phenomenon in Geothermal Power Plants Worldwide*

Scaling is a common phenomenon in geothermal power plants worldwide. Specific site analyses on scaling issues occurring in various geothermal plant types are reported in abundant literature [12,20–30], showing the importance of this topic for the plant operation. Scales act as insulators and thus lower the thermal exchanges. They also reduce the diameter of the pipes and inside volume of the exchangers which lowers the overall productivity of the geothermal power plants. Scales may also trigger the accumulation of toxic chemical elements, inducing additional risk and cost issues during the operational phase. In addition, scale formation is generally linked to degassing process, which has a major impact in terms of corrosion on the surface installation of geothermal power plants. Reducing the formation of scales is a challenge for operators who thus use inhibitors in their industrial process in order to prevent their formation [2,21,31].

#### *2.2. Scaling Phenomeon at Soultz Geothermal Power Plant*

At Soultz, surface installations are composed of several parts among which pipes and heat exchangers in which the natural geothermal fluid provides its thermal energy to an industrial fluid. An Organic Rankine Cycle (ORC) allows the production of electricity. The temperature of the brine lowers within the exchangers and the chemical equilibrium changes, resulting in the precipitation of minerals. The geochemistry of the brine provided by [10,13] shows a high salinity due to a great abundance of Na, K and Ca as major cations and Cl and SO<sup>4</sup> as major anions. It is summarized in Table 1, for the elements found in the scales. Li, Zn, Ba and other minor elements are also present in significant abundance. In addition, the geothermal fluid is characterized by a high CO<sup>2</sup> content, and natural anoxic conditions [10,13].

**Table 1.** Chemical composition of the brine after [10,13] from 22 brine samples collected in GPK-1, and the three deep wells at Soultz. Sb is not given in [13] (ng: not given), the only data about Sb comes from [10]. MRCC: most representative chemical composition of the native geothermal brine, in [13].


From this brine, sulfates of barite type ((Sr, Ba)SO4) and minor sulfides of galena type ((Pb, As, Sb)S) precipitate during the lowering of the temperature in surface installations when no antiscalants are used [27,30]. The same phenomenon is also encountered in German geothermal plants located in the URG [12]. In the URG, and at Soultz in particular where the geothermal brine circulates within a granitic basement, those scales are known to accumulate radionuclides, <sup>226</sup>Ra for sulfates and <sup>210</sup>Pb for sulfides [30], and are thus to be disposed of as Naturally Occurring Radioactive Material waste (NORM classification, [32]. In such conditions when no inhibitors were used, the Soultz power plant needed to be stopped and cleaned three times a year, inducing high maintenance cost, loss of energy production and waste management issues [33].

For safety reasons and power plant healthy operation, the formation of barite needs to be inhibited continuously [31,33]. Antiscalants are well known from the oil and gas industry and mainly consist of phosphonates and polycarboxylates when preventing barite formation [34]. The scaling phenomenon being closely linked with corrosion phenomena described by [35–37], both an antiscalant and a corrosion inhibitor are currently used at Soultz. The corrosion inhibitor agent is based on amines. Each type of mineral scale has an antiscalant which is more suitable for lowering its deposited amount. Antiscalants are

either inhibitors of crystallization known to be very powerful to control barite deposition or dispersants that better control metal sulfide scaling [33], or even a mixture of both. When sulfate production is made impossible thanks to antiscalants, sulfides precipitate as observed by [12] in geothermal power plants of the URG. In the following, chemicals used to prevent the deposition of scales will be simply called inhibitors. *2.3. The Tested Small Heat Exchanger (SHEx; Soultz)*  In a geothermal exchanger, the natural hot brine provides its thermal energy to a working fluid and then is reinjected. Both flows are totally independent of one another

sulfides precipitate as observed by [12] in geothermal power plants of the URG. In the following, chemicals used to prevent the deposition of scales will be simply called

*Geosciences* **2021**, *11*, x FOR PEER REVIEW 5 of 24

#### *2.3. The Tested Small Heat Exchanger (SHEx; Soultz)* and never mix. The SHEx was installed as bypass on the reinjection line [17]. It was tested

inhibitors.

In a geothermal exchanger, the natural hot brine provides its thermal energy to a working fluid and then is reinjected. Both flows are totally independent of one another and never mix. The SHEx was installed as bypass on the reinjection line [17]. It was tested over three months (late January to April 2019) in the presence of inhibitors, after which it was dismantled to allow scaling and corrosion studies. The SHEx consists of a tubular heat exchanger made with tubes of six different alloys (Figure 2), an entrance, an exit, and one water box at each end with different designs (Figure 2A) made of a seventh alloy. The west water box is separated into three compartments, while the east one is made of only two parts (Figure 2B). The cross-section of the shirt and the included tubes with their alloy is shown in Figure 2C. The tubes are organized in three parallel layers. The hot fluid comes into the SHEx through the entrance and flows through the three layers of tube with a constant decrease of the temperature: around 65–70 ◦C at the entrance, ~60 ◦C in the upper layer of tubes, 50 ◦C in the intermediate layer, then 40 ◦C in the lower layer and the exit (Figure 2B). Each water box is closed by a flange (Figure 2D). over three months (late January to April 2019) in the presence of inhibitors, after which it was dismantled to allow scaling and corrosion studies. The SHEx consists of a tubular heat exchanger made with tubes of six different alloys (Figure 2), an entrance, an exit, and one water box at each end with different designs (Figure 2A) made of a seventh alloy. The west water box is separated into three compartments, while the east one is made of only two parts (Figure 2B). The cross-section of the shirt and the included tubes with their alloy is shown in Figure 2C. The tubes are organized in three parallel layers. The hot fluid comes into the SHEx through the entrance and flows through the three layers of tube with a constant decrease of the temperature: around 65–70 °C at the entrance, ~60 °C in the upper layer of tubes, 50 °C in the intermediate layer, then 40 °C in the lower layer and the exit (Figure 2B). Each water box is closed by a flange (Figure 2D).

**Figure 2.** (**A**) overview of the SHEx; (**B**) schematic section of the cooling-down loop with 4 passes; (**C**): schematic front view with the tested alloys after [17]; (**D**): location of samples on flange closing the east water box. Note that only one tube is represented in (B) for each temperature (see Figure 2C for exact front representation of the location of tubes, after [17]). The intermediate layer of tubes is separated vertically in two parts (Figure 2B,C and Figure 3) as can be seen in the east water box. **Figure 2.** (**A**) overview of the SHEx; (**B**) schematic section of the cooling-down loop with 4 passes; (**C**): schematic front view with the tested alloys after [17]; (**D**): location of samples on flange closing the east water box. Note that only one tube is represented in (**B**) for each temperature (see Figure 2C for exact front representation of the location of tubes, after [17]). The intermediate layer of tubes is separated vertically in two parts (Figures 2B,C and 3) as can be seen in the east water box. Only the circulation of the geothermal brine is schematized.

shown in Figure 3: turbulent in the entrance and in water boxes, laminar into the tubes, and perpendicular to the flanges, hence allowing to examine the likely influence of the flow regime on the scales. The three layers of tubes allow the examination of the likely impact of temperature on the scaling phenomenon. The six tested alloys are 1.4539 (904 L), 1.4547 (254 SMO), 1.4462 (DX 2205), 1.4410 (SDX 2507), 2.4858 (Alloy 825) and 3.7035 (TiGr2) [17] as visible in Figure 2C. The potential influence of the alloys on the scales will also be discussed. Mundhenk (2012) [26] proposed a ranking of metals as regards

Only the circulation of the geothermal brine is schematized.

operated at Soultz and Rittershoffen are made of 1.4410 (SDX 2507).

corrosion in geothermal brine conditions of the URG. The industrial exchangers currently

**Figure 3.** Temperature and flow inside the exchanger. Note that only one tube is represented for each temperature (see Figure 3C for exact cross section representation of the location and alloy of tubes). The intermediate layer of tubes is separated in two parts by a vertical panel. Thus, the flow occurs in both directions but in separate tubes. The west water box is not represented. **Figure 3.** Temperature and flow inside the exchanger. Note that only one tube is represented for each temperature (see Figure 2C for exact cross section representation of the location and alloy of tubes). The intermediate layer of tubes is separated in two parts by a vertical panel. Thus, the flow occurs in both directions but in separate tubes. The west water box is not represented.

**3. Material and Methods**  *3.1. Scales*  In this fluid circulation test performed with the use of inhibitors, scales occur as black deposits, either as a powder (for example in water boxes, Table 2), or as a continuous plating forming a thin solid layer (like in tubes, Figure 4). The path followed by the geothermal fluid within the SHEx and the flow regime is shown in Figure 3: turbulent in the entrance and in water boxes, laminar into the tubes, and perpendicular to the flanges, hence allowing to examine the likely influence of the flow regime on the scales. The three layers of tubes allow the examination of the likely impact of temperature on the scaling phenomenon. The six tested alloys are 1.4539 (904 L), 1.4547 (254 SMO), 1.4462 (DX 2205), 1.4410 (SDX 2507), 2.4858 (Alloy 825) and 3.7035 (TiGr2) [17] as visible in Figure 2C. The potential influence of the alloys on the scales will also be discussed. Mundhenk (2012) [26] proposed a ranking of metals as regards corrosion in geothermal brine conditions of the URG. The industrial exchangers currently operated at Soultz and Rittershoffen are made of 1.4410 (SDX 2507). **Figure 3.** Temperature and flow inside the exchanger. Note that only one tube is represented for each temperature (see Figure 3C for exact cross section representation of the location and alloy of tubes). The intermediate layer of tubes is separated in two parts by a vertical panel. Thus, the flow occurs in both directions but in separate tubes. The west water box is not represented.

#### **3. Material and Methods 3. Material and Methods**

contact with a 1.4410 steel.

contact with a 1.4410 steel.

*3.2. Preparation of Samples* 

#### *3.1. Scales 3.1. Scales*

In this fluid circulation test performed with the use of inhibitors, scales occur as black deposits, either as a powder (for example in water boxes, Table 2), or as a continuous plating forming a thin solid layer (like in tubes, Figure 4). In this fluid circulation test performed with the use of inhibitors, scales occur as black deposits, either as a powder (for example in water boxes, Table 2), or as a continuous plating forming a thin solid layer (like in tubes, Figure 4).

conditions for each alloy. They were neither rinsed with clear water nor ground. They were simply dried at ambient temperature. The industrial sample was collected in 2017 in the operated power plant at the exit of the industrial heat exchanger, just before the **Figure 4.** Macroscopic view of continuous scales while still in contact with a metal tube during dismantling of the SHEx in April 2019. The tube has been cut all along for the recovery of scales. **Figure 4.** Macroscopic view of continuous scales while still in contact with a metal tube during dismantling of the SHEx in April 2019. The tube has been cut all along for the recovery of scales.

reinjection line, where the geothermal fluid is circulating at a temperature of 75–65 °C in

dispersive spectrometry (EDS) for local chemical analyses. For each sample, the side in

Millimeter-size fragments of continuous solid scales were collected and glued on metal stubs with carbon lacquer (Figure 5) for observation by reflection optical microscopy (ROM) and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS) for local chemical analyses. For each sample, the side in

were simply dried at ambient temperature. The industrial sample was collected in 2017 in the operated power plant at the exit of the industrial heat exchanger, just before the reinjection line, where the geothermal fluid is circulating at a temperature of 75–65 °C in

Millimeter-size fragments of continuous solid scales were collected and glued on

The SHEx was only drained and not rinsed before dismantling. Thirty-five samples were collected in the SHEx and one in the operated industrial plant for SEM-EDS analyses

#### *3.2. Preparation of Samples*

The SHEx was only drained and not rinsed before dismantling. Thirty-five samples were collected in the SHEx and one in the operated industrial plant for SEM-EDS analyses (Table 2). They were collected in order to be representative of hydrodynamic and thermal conditions for each alloy. They were neither rinsed with clear water nor ground. They were simply dried at ambient temperature. The industrial sample was collected in 2017 in the operated power plant at the exit of the industrial heat exchanger, just before the reinjection line, where the geothermal fluid is circulating at a temperature of 75–65 ◦C in contact with a 1.4410 steel. *Geosciences* **2021**, *11*, x FOR PEER REVIEW 7 of 24

> Millimeter-size fragments of continuous solid scales were collected and glued on metal stubs with carbon lacquer (Figure 5) for observation by reflection optical microscopy (ROM) and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS) for local chemical analyses. For each sample, the side in contact with the metal (called metal side, MS in the following), the side in contact with the fluid (called fluid side, FS in the following) and the cross-section were prepared systematically (Figure 5). The MS surface looks bright and smooth while FS surface is velvety and rough (Figures 4 and 5). The cross-section was prepared in order to study the thickness of the deposits, but the preparation frequently failed. It is to be noted that on alloy 1.4462, the scales separated systematically into two layers (MS and FS) during sampling, which is the reason why three samples were collected for each temperature (MS, FS and total). Scales from industrial sample, entrance and water boxes of the SHEx occur as a powder (Figure 6) and they were just spread over carbon lacquer (Figure 6). contact with the metal (called metal side, MS in the following), the side in contact with the fluid (called fluid side, FS in the following) and the cross-section were prepared systematically (Figure 5). The MS surface looks bright and smooth while FS surface is velvety and rough (Figures 4 and 5). The cross-section was prepared in order to study the thickness of the deposits, but the preparation frequently failed. It is to be noted that on alloy 1.4462, the scales separated systematically into two layers (MS and FS) during sampling, which is the reason why three samples were collected for each temperature (MS, FS and total). Scales from industrial sample, entrance and water boxes of the SHEx occur as a powder (Figure 6) and they were just spread over carbon lacquer (Figure 6).

**Figure 5.** ROM view of tiny samples of continuous scales with smooth MS (tube side), velvety rough FS and section of samples collected in a tube made of 1.4410 super duplex steel, at 60 °C, glued with carbon lacquer on a metal stub for SEM-EDS analysis. **Figure 5.** ROM view of tiny samples of continuous scales with smooth MS (tube side), velvety rough FS and section of samples collected in a tube made of 1.4410 super duplex steel, at 60 ◦C, glued with carbon lacquer on a metal stub for SEM-EDS analysis.

**Figure 6.** Powder collected in the entrance of the SHEX made of 1.4307, (**A**) as viewed by ROM and (**B**) by SEM.

More than 1 g of scales being necessary for X-ray diffraction (XRD), the amount of

scales was insufficient in any tube of the SHEx. Only entrance, exit and water boxes 1 and 3, all of them made of 1.4307, provided 4 samples. XRD was performed by ORANO company with a Panalytical diffractometer using Co Kα radiation (λ = 1791 Å) in order to avoid potential Fe fluorescence. Neither internal nor external standards were used. The

*3.3. X-ray Diffraction* 

carbon lacquer on a metal stub for SEM-EDS analysis.

**Figure 6.** Powder collected in the entrance of the SHEX made of 1.4307, (**A**) as viewed by ROM and (**B**) by SEM. **Figure 6.** Powder collected in the entrance of the SHEX made of 1.4307, (**A**) as viewed by ROM and (**B**) by SEM.

#### *3.3. X-ray Diffraction 3.3. X-ray Diffraction*

More than 1 g of scales being necessary for X-ray diffraction (XRD), the amount of scales was insufficient in any tube of the SHEx. Only entrance, exit and water boxes 1 and 3, all of them made of 1.4307, provided 4 samples. XRD was performed by ORANO company with a Panalytical diffractometer using Co Kα radiation (λ = 1791 Å) in order to avoid potential Fe fluorescence. Neither internal nor external standards were used. The More than 1 g of scales being necessary for X-ray diffraction (XRD), the amount of scales was insufficient in any tube of the SHEx. Only entrance, exit and water boxes 1 and 3, all of them made of 1.4307, provided 4 samples. XRD was performed by ORANO company with a Panalytical diffractometer using Co Kα radiation (λ = 1791 Å) in order to avoid potential Fe fluorescence. Neither internal nor external standards were used. The results were compared to JCPDS files for the determination of the mineral phases present in the samples.

**Figure 5.** ROM view of tiny samples of continuous scales with smooth MS (tube side), velvety rough FS and section of samples collected in a tube made of 1.4410 super duplex steel, at 60 °C, glued with

contact with the metal (called metal side, MS in the following), the side in contact with the fluid (called fluid side, FS in the following) and the cross-section were prepared systematically (Figure 5). The MS surface looks bright and smooth while FS surface is velvety and rough (Figures 4 and 5). The cross-section was prepared in order to study the thickness of the deposits, but the preparation frequently failed. It is to be noted that on alloy 1.4462, the scales separated systematically into two layers (MS and FS) during sampling, which is the reason why three samples were collected for each temperature (MS, FS and total). Scales from industrial sample, entrance and water boxes of the SHEx occur as a powder (Figure 6) and they were just spread over carbon lacquer (Figure 6).

#### *3.4. Scanning Electron Microscopy Coupled with Energy Dispersive Spectrometry (SEM-EDS)*

The SEM used in this study, a Zeiss GeminiSEM 300 coupled with a Bruker EDS, belongs to the IMAT analysis facility of CY Cergy Paris Université. No metal coating was necessary prior to observation. Observation was performed with a secondary electron detector by using a chosen acceleration voltage (between 10 and 15 kV) at a working distance between 6 and 8 mm that allowed EDS analyses in high vacuum mode. A low acceleration voltage was deliberately chosen in order to lower the beam/sample interaction volume size and to ascertain that the X-ray pulses came exclusively from the scale particles. This modest acceleration voltage presents a second advantage as it reduces significantly charging phenomena. However, this induces a poor quality of analyses enhanced by the marked topography of samples which is not ideal in terms of quantification as the beam/sample interaction volume might be truncated or shadowed. Thus, the analyses can only be used for relative abundance of the elements within and between the different scale samples.

#### **4. Results**

Results obtained by XRD and SEM-EDS are presented below and discussed in Section 5.

#### *4.1. X-ray Diffraction (XRD)*

XRD patterns that were obtained for the four samples are provided in Figure 7. Entrance and exit of the SHEx show the same diffractograms, indicating the presence of galena (PbS), and likely minor dufrénoysite (Pb2As2S5) as regards the weak intensity of the peaks, over the whole range of temperatures (65 ◦C and 40 ◦C). The water boxes also show the presence of galena and likely dufrénoysite, together with that of halite (NaCl) for the two temperatures under concern (65 ◦C and 40 ◦C). The samples not being reduced into powder it was possible to determine hkl diffraction planes for galena, namely 111, 200, 220, 311 and 222 (Figure 7).

*Geosciences* **2021**, *11*, x FOR PEER REVIEW 9 of 24

*Geosciences* **2021**, *11*, x FOR PEER REVIEW 9 of 24

**Figure 7.** X-ray diffraction diagrams obtained for scales collected in the entrance, the exit and water boxes 1 and 3 of the SHEx. They show galena (Gal; As and Sb bearing PbS), additional halite (Hal; NaCl) in the water boxes, and likely traces of dufrénoysite (Duf; Pb2As2S5), all of these sites being made of 1.4307 alloy. The numbers indicated vertically represent the hkl diffraction planes of galena crystals. *4.2. Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM-EDS)*  The observation by SEM allows to distinguish several features. **Figure 7.** X-ray diffraction diagrams obtained for scales collected in the entrance, the exit and water boxes 1 and 3 of the SHEx. They show galena (Gal; As and Sb bearing PbS), additional halite (Hal; NaCl) in the water boxes, and likely traces of dufrénoysite (Duf; Pb2As2S5), all of these sites being made of 1.4307 alloy. The numbers indicated vertically represent the hkl diffraction planes of galena crystals. of dufrénoysite (Duf; Pb2As2S5), all of these sites being made of 1.4307 alloy. The numbers indicated vertically represent the hkl diffraction planes of galena crystals. *4.2. Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM-EDS)*  The observation by SEM allows to distinguish several features.

#### 4.2.1. Structure of the Scales As indicated before, scales occur as a continuous solid deposit (Figure 5) or as a *4.2. Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM-EDS)* The observation by SEM allows to distinguish several features. 4.2.1. Structure of the Scales

#### the geothermal brine (Figure 8A,B) and a smooth MS in contact with the metal (Figure 8C,D). 4.2.1. Structure of the Scales As indicated before, scales occur as a continuous solid deposit (Figure 5) or as a

As indicated before, scales occur as a continuous solid deposit (Figure 5) or as a powder (Figure 6, Table 2). In that first case, the deposit shows a rough FS in contact with the geothermal brine (Figure 8A,B) and a smooth MS in contact with the metal (Figure 8C,D). powder (Figure 6, Table 2). In that first case, the deposit shows a rough FS in contact with the geothermal brine (Figure 8A,B) and a smooth MS in contact with the metal (Figure 8C,D).

powder (Figure 6, Table 2). In that first case, the deposit shows a rough FS in contact with

**Figure 8.** Sample 1.4539, 50°C, surface of the scales: rough in contact with the brine (**A** and zoom in **B**), smooth in contact with the metal (**C** and zoom in **D**), showing As-Sb-galena and halite on MS and only galena on FS in this example. **Figure 8.** Sample 1.4539, 50 ◦C, surface of the scales: rough in contact with the brine (**A** and zoom in **B**), smooth in contact with the metal (**C** and zoom in **D**), showing As-Sb-galena and halite on MS and only galena on FS in this example.

*Geosciences* **2021**, *11*, 271



Tube 3.7035 40 Continuous SEM

9B).

Tube (FS) 1.4462 40 Continuous SEM Tube (MS) 1.4462 40 Continuous SEM

**Sampling** 

Wwb 2

Wwb 3

Flange A

Flange B

Flange C

**Points Alloy** 

**Temperatu re (°C)** 

Exit 1.4307 40 Powder XRD

**Structure** 

(East) 1.4307 50 Continuous SEM 110 15

**of scales Analyses** 

Industrial 1.4410 65–75 Powder SEM x Entrance 1.4307 65 Powder XRD, SEM x

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**Table 2.** Summary of sampling points, performed analyses and observation by SEM. Industrial: industrial heat exchanger. All other sampling points are within the SHEx. FS: fluid side, MS: metal side, Total: FS+MS sampled at the same time when they separate. The alloys are from [17]. The analyses performed on each sample are indicated. Wwb: West water box. (co): cuboctahedron. (oct): octahedron. Underlined figures are for 1.4410 alloy for comparison between three temperatures.

> **Whole Thickness of Scales (µm)**

Wwb 1 (top) 1.4307 65 Powder XRD, SEM x x x x

(Bottom) 1.4307 40 Powder XRD, SEM x x

(East) 1.4307 60 Continuous SEM - 15 x x x

Tube 1.4539 50 Continuous SEM x x x Tube 1.4547 50 Continuous SEM 270 - x x

Tube 1.4410 50 Continuous SEM 80 10 x x Tube 2.4858 50 Continuous SEM 50 30 x x x

Tube (Total) 1.4462 40 Continuous SEM x x x (oct) x

Tube 2.4858 40 Continuous SEM 60 30 x x

(East) 1.4307 50 Continuous SEM 30 12 x x x

Tube 1.4539 60 Continuous SEM x x x (co)

Tube 1.4547 60 Continuous SEM x Tube (FS) 1.4462 60 Continuous SEM - Tube (MS) 1.4462 60 Continuous SEM - Tube (Total) 1.4462 60 Continuous SEM 22 5 x? Tube 1.4410 60 Continuous SEM 50 10 x Tube 2.4858 60 Continuous SEM x Tube 3.7035 60 Continuous SEM 15 6 -

Tube (FS) 1.4462 50 Continuous SEM - Tube (MS) 1.4462 50 Continuous SEM - Tube (Total) 1.4462 50 Continuous SEM x?

Tube 3.7035 50 Continuous SEM x Tube 1.4539 40 Continuous SEM 200 10 x Tube 1.4547 40 Continuous SEM 270 25 x

Tube 1.4410 40 Continuous SEM 220 10 x

(Middle) 1.4307 55 Powder SEM x

**Thickness of Smooth Zone (µm)** 

**Dendrite Needle Coral Cube Fibro-**

**Radiated** 

The scales are composed of several superimposed layers (Figure 9A) and the smooth layer in contact with the metal is generally divided in several parallel sub-layers (Figure 9B). layer in contact with the metal is generally divided in several parallel sub-layers (Figure

The scales are composed of several superimposed layers (Figure 9A) and the smooth

**Figure 9.** Sample of scales collected in a tube made of 2.4858 at 40 ◦C. (**A**) Cross-section of the scales divided into several layers, (**B**) the very first one (in contact with the metal) being itself composed of several micro-porous sub-layers. B is a focus on the white rectangle in A and shows five successive sub-layers that deposited on the top of one another. Sub-layer 1 is the oldest, in direct contact with the metal.

#### 4.2.2. Thickness of the Scales

The thickness of scales was measured whenever it was possible, which was not very frequent. It could be measured systematically only for scales deposited in tubes made of 1.4410 (Figure 10) at decreasing temperatures. One has to note that measuring the thickness by SEM provides values with a non-negligible uncertainty as the measurement was sometimes not exactly normal to the deposit. However, the magnitude of the measurement remains true. The thickness observed in tubes of 1.4410 after the 3-month test was around 50 µm at 60 ◦C, 80 µm at 50 ◦C and locally up to 220 µm at 40 ◦C (Table 2, Figure 10), thus representing a deposition rate of about 17 µm/month at 60 ◦C to 73 µm/month at 40 ◦C, considering a constant deposition rate. The same trend of increasing thickness with decreasing temperatures tends to be seen on other alloys (Table 2): small at 60 ◦C (22, 50, and 15 µm, in tubes made of 1.4462, 1.4410, 3.7035 respectively), generally greater at 50 ◦C (270, 80, and 50µm, in tubes made of 1.4547, 1.4410, 2.4858 respectively) and in general the biggest at 40 ◦C (200, 270, 220, 60 µm, in tubes made of 1.4539, 1.4547, 1.4410, 2.4858 respectively).

Wherever it could be measured, the smooth zone always shows a thickness smaller than or equal to 30 µm, while the whole thickness of the deposit reaches 270 µm. Nothing much can be said about the thickness of scales deposited on the east flange as it could not be measured as a whole at 60 ◦C and varies from 30 to 110 µm at 50 ◦C. No sample is available at 40 ◦C as the flange is separated in only two zones: 60 ◦C and 50 ◦C.

#### 4.2.3. SEM-EDS Chemistry of the Samples

Whatever the location (water box, entrance, exit and tubes), either on MS or FS of the scales when they are continuous, or in powder, and whatever the alloy on which they deposited, SEM-EDS spectra and maps show the presence of Pb, S ± As and Sb compounds of galena type (Figures 11 and 12). Halite (NaCl) is also frequently observed. Both of these two phases were encountered on the XRD patterns (Figure 7). Because of the analytical limitations exposed in the Methods section and their likely small size, crystals of sulfosalts such as dufrénoysite were not identified by SEM-EDS.

contact with the metal.

4.2.2. Thickness of the Scales

1.4410, 2.4858 respectively).

**Figure 9.** Sample of scales collected in a tube made of 2.4858 at 40 °C. (**A**) Cross-section of the scales divided into several layers, (**B**) the very first one (in contact with the metal) being itself composed of several micro-porous sub-layers. B is a focus on the white rectangle in A and shows five successive sub-layers that deposited on the top of one another. Sub-layer 1 is the oldest, in direct

The thickness of scales was measured whenever it was possible, which was not very

Wherever it could be measured, the smooth zone always shows a thickness smaller

than or equal to 30 µm, while the whole thickness of the deposit reaches 270 µm. Nothing much can be said about the thickness of scales deposited on the east flange as it could not

available at 40 °C as the flange is separated in only two zones: 60 °C and 50 °C.

frequent. It could be measured systematically only for scales deposited in tubes made of 1.4410 (Figure 10) at decreasing temperatures. One has to note that measuring the thickness by SEM provides values with a non-negligible uncertainty as the measurement was sometimes not exactly normal to the deposit. However, the magnitude of the measurement remains true. The thickness observed in tubes of 1.4410 after the 3-month test was around 50 µm at 60 °C, 80 µm at 50 °C and locally up to 220 µm at 40 °C (Table 2, Figure 10), thus representing a deposition rate of about 17 µm/month at 60 °C to 73 µm/month at 40 °C, considering a constant deposition rate. The same trend of increasing thickness with decreasing temperatures tends to be seen on other alloys (Table 2): small at 60 °C (22, 50, and 15 µm, in tubes made of 1.4462, 1.4410, 3.7035 respectively), generally greater at 50°C (270, 80, and 50µm, in tubes made of 1.4547, 1.4410, 2.4858 respectively) and in general the biggest at 40 °C (200, 270, 220, 60 µm, in tubes made of 1.4539, 1.4547,

**Figure 10.** Tubes made of 1.4410, thickness as a function of temperature. The deposit in contact with the metal is smooth while it is rough when in contact with the brine. The thickness of the scales tends to increase when the temperature decreases within the SHEx, from 50 µm at 60 °C to 220 µm **Figure 10.** Tubes made of 1.4410, thickness as a function of temperature. The deposit in contact with the metal is smooth while it is rough when in contact with the brine. The thickness of the scales tends to increase when the temperature decreases within the SHEx, from 50 µm at 60 ◦C to 220 µm at 40 ◦C. (**A**) 60 ◦C, (**B**) 50 ◦C, (**C**,**D**) 40 ◦C. two phases were encountered on the XRD patterns (Figure 7). Because of the analytical limitations exposed in the Methods section and their likely small size, crystals of sulfosalts such as dufrénoysite were not identified by SEM-EDS.

**Figure 11.** EDS map of FS of scales collected in a tube made of 2.4858, at 40°C (**A**–**F**) showing (**A**) the SEM image and the black rectangle in which the elementary maps were performed, (**B**,**C**) maps of elementary concentration for Na and Cl characteristic of halite (NaCl) with its common cubic shape, (**D**) resulting map with all elements, (**E**,**F**) maps of elementary concentration for Pb and S characteristic of galena (PbS). As and Sb were also encountered together with Pb and S but in such **Figure 11.** EDS map of FS of scales collected in a tube made of 2.4858, at 40 ◦C (**A**–**F**) showing (**A**) the SEM image and the black rectangle in which the elementary maps were performed, (**B**,**C**) maps of elementary concentration for Na and Cl characteristic of halite (NaCl) with its common cubic shape, (**D**) resulting map with all elements, (**E**,**F**) maps of elementary concentration for Pb and S characteristic of galena (PbS). As and Sb were also encountered together with Pb and S but in such a small amount that the images are not contrasted enough to be included.

a small amount that the images are not contrasted enough to be included.

**Figure 12.** EDS spectra obtained on FS of scales collected (**A**) on a flange at 50 °C and (**B**) in a tube made of 1.4539, at 60

also locally detected in the samples.

°C.

Table 3 shows examples of semi-quantitative analyses. The same elements (Pb, As, Sb, S) are present in each of the studied samples (analyses 1, 2 and 3), but in varying relative abundance. Sb is sometimes quite abundant (analyses 2 and 3) but no Sb-bearing sulfosalts were discovered either on XRD diagrams (Figure 7) or by SEM. Na and Cl are °C.

a small amount that the images are not contrasted enough to be included.

**Figure 12.** EDS spectra obtained on FS of scales collected (**A**) on a flange at 50 ◦C and (**B**) in a tube made of 1.4539, at 60 ◦C.

**Figure 12.** EDS spectra obtained on FS of scales collected (**A**) on a flange at 50 °C and (**B**) in a tube made of 1.4539, at 60 Table 3 shows examples of semi-quantitative analyses. The same elements (Pb, As, Sb, S) are present in each of the studied samples (analyses 1, 2 and 3), but in varying relative abundance. Sb is sometimes quite abundant (analyses 2 and 3) but no Sb-bearing Table 3 shows examples of semi-quantitative analyses. The same elements (Pb, As, Sb, S) are present in each of the studied samples (analyses 1, 2 and 3), but in varying relative abundance. Sb is sometimes quite abundant (analyses 2 and 3) but no Sb-bearing sulfosalts were discovered either on XRD diagrams (Figure 7) or by SEM. Na and Cl are also locally detected in the samples.

two phases were encountered on the XRD patterns (Figure 7). Because of the analytical limitations exposed in the Methods section and their likely small size, crystals of sulfosalts

**Figure 11.** EDS map of FS of scales collected in a tube made of 2.4858, at 40°C (**A**–**F**) showing (**A**) the SEM image and the black rectangle in which the elementary maps were performed, (**B**,**C**) maps of elementary concentration for Na and Cl characteristic of halite (NaCl) with its common cubic shape, (**D**) resulting map with all elements, (**E**,**F**) maps of elementary concentration for Pb and S characteristic of galena (PbS). As and Sb were also encountered together with Pb and S but in such

such as dufrénoysite were not identified by SEM-EDS.

sulfosalts were discovered either on XRD diagrams (Figure 7) or by SEM. Na and Cl are also locally detected in the samples. **Table 3.** Three normalized semi-quantitative analyses obtained by SEM-EDS. Note the limitations of accuracy due to the low acceleration voltage, the topography of samples and their low thickness. The analyses can only be used for relative abundance of the elements within and between the different scale samples and mostly indicate the elements present in the samples.


From these semi-quantitative results, no difference appears in the chemistry of the scales, whatever the alloy on which they deposited or the temperature of the brine from which they precipitated.

#### 4.2.4. Shapes of Galena Crystals

Various galena crystal shapes were observed thanks to SEM on the tiny fragments described in Section 3 (Materials and Methods). The crystals are of micrometer size in millimeter-sized samples. Thus, these observations might not be exhaustive but give an overview of the crystal shapes of galena.

### 1. Dendrites

Dendritic crystal shape was observed in the scales collected in the industrial installation (reference sample), as well as in the entrance of the SHEx, and on the upper part of the eastern flange (Figure 13). The industrial sample and that collected at the entrance of the SHEx are made of only dendrites (Figure 13A,B). The only dendrite observed on the flange (Figure 13C) seems to have been deposited by the flow as it is free and not embedded in the matrix. No dendrites were found in any of the tubes, whatever the temperature.

**Table 3.** Three normalized semi-quantitative analyses obtained by SEM-EDS. Note the limitations of accuracy due to the low acceleration voltage, the topography of samples and their low thickness. The analyses can only be used for relative abundance of the elements within and between the

 **1 2 3** 

S 11.75 16.28 15.00 As 3.89 8.57 3.70 Sb 11.46 25.08 17.82 Pb 72.90 50.07 63.48 Total 100 100 100

From these semi-quantitative results, no difference appears in the chemistry of the scales, whatever the alloy on which they deposited or the temperature of the brine from

Various galena crystal shapes were observed thanks to SEM on the tiny fragments described in Section 3 (Materials and Methods). The crystals are of micrometer size in millimeter-sized samples. Thus, these observations might not be exhaustive but give an

Dendritic crystal shape was observed in the scales collected in the industrial installation (reference sample), as well as in the entrance of the SHEx, and on the upper part of the eastern flange (Figure 13). The industrial sample and that collected at the entrance of the SHEx are made of only dendrites (Figure 13A,B). The only dendrite

Tube 2.4855 60 °C

Content (wt.%)

Tube 2.4855 40 °C

Content (wt.%)

different scale samples and mostly indicate the elements present in the samples.

Tube 3.7035 60 °C

(wt.%)

Elements Content

which they precipitated.

1. Dendrites

the temperature.

2. Needles

4.2.4. Shapes of Galena Crystals

overview of the crystal shapes of galena.

**Figure 13.** Dendritic shapes of galena crystals (**A**) in the industrial sample (1.4410 alloy), (**B**) in the entrance of the SHEx and (**C**) on the upper part of the eastern flange (both made of 1.4307). No **Figure 13.** Dendritic shapes of galena crystals (**A**) in the industrial sample (1.4410 alloy), (**B**) in the entrance of the SHEx and (**C**) on the upper part of the eastern flange (both made of 1.4307). No dendrites were found in any of the tubes.

dendrites were found in any of the tubes. 2. Needles

Needle shape is found in tubes of 1.4462 (50 °C and 40 °C), 1.4539 (60 °C and 50 °C), and 2.4858 (50 °C), in water boxes (1.4307 alloy, at 65 °C, 55 °C and 40 °C), and on a flange (1.4307 alloy, 50 °C), thus at various temperatures (from 65 °C to 40 °C, Figure 14) and alloys. Needles were not observed on other samples. Needles can be parallel to each other (A) or perpendicular (B to H) and sometimes in three orthogonal directions (C). Needles Needle shape is found in tubes of 1.4462 (50 ◦C and 40 ◦C), 1.4539 (60 ◦C and 50 ◦C), and 2.4858 (50 ◦C), in water boxes (1.4307 alloy, at 65 ◦C, 55 ◦C and 40 ◦C), and on a flange (1.4307 alloy, 50 ◦C), thus at various temperatures (from 65 ◦C to 40 ◦C, Figure 14) and alloys. Needles were not observed on other samples. Needles can be parallel to each other (A) or perpendicular (B to H) and sometimes in three orthogonal directions (C). Needles have a square section as visible mostly in B and F. They were observed in zones where the flow is rather laminar (tubes) or turbulent (water boxes and flanges). *Geosciences* **2021**, *11*, x FOR PEER REVIEW 14 of 24 have a square section as visible mostly in B and F. They were observed in zones where the flow is rather laminar (tubes) or turbulent (water boxes and flanges).

**Figure 14.** Needle shapes of galena crystals (**A**,**B**) on a flange and (**C**) in water box and (**D**–**F**) in tubes, at different temperatures and on three different alloys (1.4307, 1.4462, 1.4539). (**A**) needles parallel to each other, (**B**–**F**): needles grew in perpendicular directions. (**B**,**F**): Note that the needles are monocrystals with square section. **Figure 14.** Needle shapes of galena crystals (**A**,**B**) on a flange and (**C**) in water box and (**D**–**F**) in tubes, at different temperatures and on three different alloys (1.4307, 1.4462, 1.4539). (**A**) needles parallel to each other, (**B**–**F**): needles grew in perpendicular directions. (**B**,**F**): Note that the needles are monocrystals with square section.

3. Coral Shapes 3. Coral Shapes

Coral-like shapes are of various types (Figure 15) that all show an important internal porosity. They are the most common shapes encountered in the exchanger (Table 4). They were observed in different zones of the SHEx (entrance, 1.4307 alloy; water box, 1.4307 Coral-like shapes are of various types (Figure 15) that all show an important internal porosity. They are the most common shapes encountered in the exchanger (Table 4). They were observed in different zones of the SHEx (entrance, 1.4307 alloy; water box, 1.4307

alloy; flange, 1.4307 alloy) and in tubes of all alloys, and at temperatures varying from 65

**Figure 15.** PbS coral shapes of different kinds. All of them show an important internal porosity. The zones to be observed are highlighted by red ellipses. Hal: halite. (**A**) halite crystal embedded in coral shape galena, water box, 1.4307, 65°C, (**B**) 3D view of coral shape galena, water box, 1.4307, 65°C, (**C**) coral shape made of botryoids with abundant porosity, flange A, 1.4307, 65°C, (**D**) coral shape

porosity. They are the most common shapes encountered in the exchanger (Table 4). They were observed in different zones of the SHEx (entrance, 1.4307 alloy; water box, 1.4307

alloy; flange, 1.4307 alloy) and in tubes of all alloys, and at temperatures varying from 65 ◦C (entrance and water box) to 40 ◦C (water box and tubes). alloy; flange, 1.4307 alloy) and in tubes of all alloys, and at temperatures varying from 65 °C (entrance and water box) to 40 °C (water box and tubes).

**Figure 15.** PbS coral shapes of different kinds. All of them show an important internal porosity. The zones to be observed are highlighted by red ellipses. Hal: halite. (**A**) halite crystal embedded in coral shape galena, water box, 1.4307, 65°C, (**B**) 3D view of coral shape galena, water box, 1.4307, 65°C, (**C**) coral shape made of botryoids with abundant porosity, flange A, 1.4307, 65°C, (**D**) coral shape **Figure 15.** PbS coral shapes of different kinds. All of them show an important internal porosity. The zones to be observed are highlighted by red ellipses. Hal: halite. (**A**) halite crystal embedded in coral shape galena, water box, 1.4307, 65 ◦C, (**B**) 3D view of coral shape galena, water box, 1.4307, 65 ◦C, (**C**) coral shape made of botryoids with abundant porosity, flange A, 1.4307, 65 ◦C, (**D**) coral shape made of botryoids with abundant porosity, water box, 1.4307, 55 ◦C, (**E**) coral shape made of numerous contiguous needles, water box, 1.4307, 55 ◦C, (**F**) cross section of coral shape galena. made of botryoids with abundant porosity, water box, 1.4307, 55°C, (**E**) coral shape made of numerous contiguous needles, water box, 1.4307, 55°C, (**F**) cross section of coral shape galena.

4. Cube and Cubic-Derived Shapes 4. Cube and Cubic-Derived Shapes

*Geosciences* **2021**, *11*, x FOR PEER REVIEW 14 of 24

**Figure 14.** Needle shapes of galena crystals (**A**,**B**) on a flange and (**C**) in water box and (**D**–**F**) in tubes, at different temperatures and on three different alloys (1.4307, 1.4462, 1.4539). (**A**) needles parallel to each other, (**B**–**F**): needles grew

in perpendicular directions. (**B**,**F**): Note that the needles are monocrystals with square section.

3. Coral Shapes

flow is rather laminar (tubes) or turbulent (water boxes and flanges).

have a square section as visible mostly in B and F. They were observed in zones where the

Several cubic or cubic-derived shapes were observed in the samples. Cubes were found in a tube at 60 ◦C (1.4539) where it shows exactly the same hollow shape as on a flange (1.4307, 40 ◦C; Figure 16A). Cubes were also observed in a water box (1.4307, 40 ◦C; Figure 16B,C) where they occur either as massive structures (Figure 16B) or as a kind of skeleton made of needles oriented in the three directions of space (Figure 16C), those two features being in close contact in the same sample. Cubes are thus found on at least two different alloys (1.4307 and 1.4539) and at temperatures from 60 ◦C to 40 ◦C. Several cubic or cubic-derived shapes were observed in the samples. Cubes were found in a tube at 60 °C (1.4539) where it shows exactly the same hollow shape as on a flange (1.4307, 40 °C; Figure 16A). Cubes were also observed in a water box (1.4307, 40 °C; Figure 16B,C) where they occur either as massive structures (Figure 16B) or as a kind of skeleton made of needles oriented in the three directions of space (figure 16C), those two features being in close contact in the same sample. Cubes are thus found on at least two different alloys (1.4307 and 1.4539) and at temperatures from 60 °C to 40 °C.

**Figure 16.** Cubic-derived shape of PbS crystals, occurring as (**A**) hollow cubes, (**B**) massive cubes, or (**C**) skeleton made of needles in three orthogonal directions inside the orange ellipse. The massive cube (**B**) is seen in the lower left-hand corner of (C). **Figure 16.** Cubic-derived shape of PbS crystals, occurring as (**A**) hollow cubes, (**B**) massive cubes, or (**C**) skeleton made of needles in three orthogonal directions inside the orange ellipse. The massive cube (**B**) is seen in the lower left-hand corner of (**C**).

> Other PbS shapes derived from the cube were locally observed in tubes, as shown in Figure 17, such as a cuboctahedron (Figure 17A) and an octahedron (Figure 17B), on two different alloys and at 60 °C and 40 °C respectively. The octahedron (Figure 17B) in found Other PbS shapes derived from the cube were locally observed in tubes, as shown in Figure 17, such as a cuboctahedron (Figure 17A) and an octahedron (Figure 17B), on two different alloys and at 60 ◦C and 40 ◦C respectively. The octahedron (Figure 17B) in found in the vicinity of orthogonal needles not visible on the photograph.

> **Figure 17.** PbS cuboctahedron (**A**) and PbS octahedron (**B**) with their indexed faces, observed in two

Thus, cubes and cubic-derived shapes were observed on three different alloys and at

tubes made of 1.4539, at 60 °C (**A**) and 1.4462, at 40 °C (**B**).

temperatures from 65 to 40 °C (Table 4). 5. Fibro-Radiated Botryoidal Shape

in the vicinity of orthogonal needles not visible on the photograph.

50°C.

**5. Discussion** 

shapes [52], nanocoral [53], and many others.

*5.1. Structure and Chemistry of Scales* 

of (C).

in the vicinity of orthogonal needles not visible on the photograph.

**Figure 16.** Cubic-derived shape of PbS crystals, occurring as (**A**) hollow cubes, (**B**) massive cubes, or (**C**) skeleton made of needles in three orthogonal directions inside the orange ellipse. The massive cube (**B**) is seen in the lower left-hand corner

**Figure 17.** PbS cuboctahedron (**A**) and PbS octahedron (**B**) with their indexed faces, observed in two tubes made of 1.4539, at 60 °C (**A**) and 1.4462, at 40 °C (**B**). **Figure 17.** PbS cuboctahedron (**A**) and PbS octahedron (**B**) with their indexed faces, observed in twotubes made of 1.4539, at 60 ◦C (**A**) and 1.4462, at 40 ◦C (**B**).

made of botryoids with abundant porosity, water box, 1.4307, 55°C, (**E**) coral shape made of numerous contiguous needles, water box, 1.4307, 55°C, (**F**) cross section of coral shape galena.

different alloys (1.4307 and 1.4539) and at temperatures from 60 °C to 40 °C.

Several cubic or cubic-derived shapes were observed in the samples. Cubes were found in a tube at 60 °C (1.4539) where it shows exactly the same hollow shape as on a flange (1.4307, 40 °C; Figure 16A). Cubes were also observed in a water box (1.4307, 40 °C; Figure 16B,C) where they occur either as massive structures (Figure 16B) or as a kind of skeleton made of needles oriented in the three directions of space (figure 16C), those two features being in close contact in the same sample. Cubes are thus found on at least two

Other PbS shapes derived from the cube were locally observed in tubes, as shown in Figure 17, such as a cuboctahedron (Figure 17A) and an octahedron (Figure 17B), on two different alloys and at 60 °C and 40 °C respectively. The octahedron (Figure 17B) in found

4. Cube and Cubic-Derived Shapes

Thus, cubes and cubic-derived shapes were observed on three different alloys and at temperatures from 65 to 40 °C (Table 4). Thus, cubes and cubic-derived shapes were observed on three different alloys and at temperatures from 65 to 40 ◦C (Table 4). *Geosciences* **2021**, *11*, x FOR PEER REVIEW 16 of 24

#### 5. Fibro-Radiated Botryoidal Shape 5. Fibro-Radiated Botryoidal Shape

The fibro-radiated botryoidal type (Figure 18A) is made of needles organized in 3D fan shape (Figure 18B,C) with several superimposed layers (Figure 18B,C). No to minor porosity is observed as opposed to coral shape. These three examples were observed occurred at a 50 ◦C temperature, on three different alloys. Table 2 shows all the locations where botryoids were observed, from 65 ◦C to 40 ◦C. The fibro-radiated botryoidal type (Figure 18A) is made of needles organized in 3D fan shape (Figure 18B,C) with several superimposed layers (Figure 18B,C). No to minor porosity is observed as opposed to coral shape. These three examples were observed occurred at a 50 °C temperature, on three different alloys. Table 2 shows all the locations where botryoids were observed, from 65 °C to 40 °C.

**Figure 18.** Fibro-radiated botryoidal shape from three different samples collected on a flange, and in two tubes of different alloys. All of the examples presented here were observed for a 50 °C temperature, but they also occurred at 65 °C and 40 °C (see Table 2). (**A**) general view, on a tube made of 2507 at 50°C, (**B**,**C**) close view of a cross-section of botryoid on a flange and on a tube, at **Figure 18.** Fibro-radiated botryoidal shape from three different samples collected on a flange, and in two tubes of different alloys. All of the examples presented here were observed for a 50 ◦C temperature, but they also occurred at 65 ◦C and 40 ◦C (see Table 2). (**A**) general view, on a tube made of 2507 at 50 ◦C, (**B**,**C**) close view of a cross-section of botryoid on a flange and on a tube, at 50 ◦C.

Table 2 recapitulates all of the shapes that were observed by SEM, as a function of the location inside the SHEx, the alloy type and the temperature. Some samples do not show any characteristic crystal shapes, because of their poor quality (tubes of 1.4462 and 3.7035 alloys, Table 4). In the other samples, the coral shape is the most widely observed, whatever the alloy and the temperature. Other crystal shapes are frequently found in association with it in samples collected in tubes. Cubes are associated with coral shape in the west water box and on a flange, and in association with needles in both water boxes and in some tubes. Dendrites were observed in great abundance and not in association with other shapes in the industrial installation and in the entrance. The only dendrite found on a flange appears to be free on the surface of the scale and not embedded in the Table 2 recapitulates all of the shapes that were observed by SEM, as a function of the location inside the SHEx, the alloy type and the temperature. Some samples do not show any characteristic crystal shapes, because of their poor quality (tubes of 1.4462 and 3.7035 alloys, Table 4). In the other samples, the coral shape is the most widely observed, whatever the alloy and the temperature. Other crystal shapes are frequently found in association with it in samples collected in tubes. Cubes are associated with coral shape in the west water box and on a flange, and in association with needles in both water boxes and in some tubes. Dendrites were observed in great abundance and not in association with other shapes in the industrial installation and in the entrance. The only dendrite found on a flange appears to be free on the surface of the scale and not embedded in the deposit. The various shapes were observed whatever the temperature and the alloy, except for dendrites which were observed only at the highest temperature.

In the SHEx, scales occur either as a powder or as layered deposits (Table 4).

Galena is studied in ore deposits for scientific and economical purposes [38–40], and because of its toxicity in mining environments [41–43]. Galena is also well known in the industry, in particular for its semi-conductor properties and is thus thoroughly studied [44–46]. Natural galena is rarely a pure PbS component and frequently contains arsenic [47] and antimony [40,48]. Various galena shapes and chemical compositions are reported in natural environments [49]. Laboratory growth experiments show that the shape and chemistry of galena crystals can be controlled by several factors among which time, temperature and concentration of elements in the solvent [44–46,50]. All these previous studies might be useful for understanding the growth process in the SHEx at Soultz, even though the chemistry of the solution and other parameters are different. As regards shapes of PbS crystals, the literature reports laboratory growth of hopper (skeletal) crystals [51], dendrites, nanocubes, and truncated nanocubes [50], dendrites with different

except for dendrites which were observed only at the highest temperature.

deposit. The various shapes were observed whatever the temperature and the alloy,

### **5. Discussion**

Galena is studied in ore deposits for scientific and economical purposes [38–40], and because of its toxicity in mining environments [41–43]. Galena is also well known in the industry, in particular for its semi-conductor properties and is thus thoroughly studied [44–46]. Natural galena is rarely a pure PbS component and frequently contains arsenic [47] and antimony [40,48]. Various galena shapes and chemical compositions are reported in natural environments [49]. Laboratory growth experiments show that the shape and chemistry of galena crystals can be controlled by several factors among which time, temperature and concentration of elements in the solvent [44–46,50]. All these previous studies might be useful for understanding the growth process in the SHEx at Soultz, even though the chemistry of the solution and other parameters are different. As regards shapes of PbS crystals, the literature reports laboratory growth of hopper (skeletal) crystals [51], dendrites, nanocubes, and truncated nanocubes [50], dendrites with different shapes [52], nanocoral [53], and many others.

### *5.1. Structure and Chemistry of Scales*

In the SHEx, scales occur either as a powder or as layered deposits (Table 4).


**Table 4.** Structure of the scales sampled in the SHEx.

Where the flow is turbulent, the scales deposit as a powder. Layered deposits are structured into sub-layers likely related to the operation of the power plant. In tubes made of 1.4462, the scales occurred as two major layers which were difficult to extract together and fell into small pieces during sampling, inducing a poor quality of samples. Scales deposited in 3.7035 tubes were strongly attached to the metal and were difficult to collect, resulting also in a bad quality of samples. This explains the lack of information about crystal shapes for those two alloys (Table 2). The micro-porosity observed in the superimposed thin layers, as well as between and inside the crystals (coral-shape for example) might be due to local turbulence of the flow.

The deposits that formed in the SHEx indeed contain galena, whatever the occurrence (entrance, water boxes or exit), as indicated by XRD diagrams (Figure 7) when compared to [50] who also used Cobalt anticathod. Their characteristics are summarized in Table 5.

**Table 5.** Characteristics of galena crystals determined from XRD.


XRD analyses of the scales indeed show the presence of galena but give no information about its chemistry which was surveyed by SEM-EDS. It is homogeneous with the systematic presence of As and Sb in varying abundance, in addition to Pb and S, which is a well-known phenomenon in natural ore systems [40,47,48]. The chemistry of scale surveyed by SEM-EDS is summarized in Table 6.


**Table 6.** Summary of the chemistry of scales as surveyed by SEM-EDS.

Other sulfosalts might also occur given the high relative amount of As and Sb given by EDS analyses, but their small abundance did not allow us to see them on the diffractograms. In addition, the semi-quantification performed thanks to SEM-EDS did not allow to determine either their presence or their amount in the samples.

Halite was detected by XRD neither in the SHEx entrance, nor in its exit, probably because of its too small abundance, but it was seen by SEM-EDS. Indeed, the intensity of the peaks related to halite in the water boxes is weak on the XRD patterns (Figure 7).

To summarize, the composition of scales is homogeneous whatever the metal on which they formed and whatever the temperature of deposition (from 65 ◦C to 40 ◦C). Thus, it appears that these two parameters (alloy and temperature) do not influence the chemistry of scales.

### *5.2. Thickness of Scales*

It is to be noted that the measurement of the deposit thickness (Table 2) might be considered as only semi-quantitative. Indeed, it could not always be performed strictly perpendicular to the deposit, which induced an uncertainty in the obtained value. However, a general tendency is observed: the thickness increases (e.g., from 50 to 220 µm for 1.4410 tubes) when the temperature decreases (from 60 ◦C to 40 ◦C). The deposition of scales in an exchanger has several effects, some of them positive, such as protection against corrosion, others negative, such as insulation reducing the heat exchange and hence energy production, or the decrease of the diameter of tubes which reduces the fluid flow. The thicker the deposit, the better the protection against corrosion but the lower the energy production. The thickness of scales has thus to be carefully monitored and controlled by addition of inhibitors to the process and maintenance when necessary, in order to allow optimal energy production.

#### *5.3. Conditions of Scale Formation*

It is likely that halite crystals developed when the SHEx was dismantled, during its draining and drying, as they are not embedded in the scales and sometimes grew on the MS of the scales (Figure 8A).

The conditions for scale formation during the geothermal process are summarized in Table 7 which shows the changes undergone by the deposit through time, with a decreasing influence of metal.


**Table 7.** Summary of conditions for scale formation.

The influence of alloy nature on the shape of galena crystals being eliminated, the likely influencing parameters that remain to explain the crystal shapes are the chemical composition of the brine, its temperature and the flow regime.

Dendrites were observed exclusively at the exit of the industrial installation and in the entrance of the SHEx. Indeed, dendrites are known to crystallize quickly [31], which is permitted by the highest temperature (75–65 ◦C) and the turbulence in these locations.

Abundant recent literature presents the conditions of galena synthesis in laboratory and the shapes of crystals obtained [44,45,51–56]. Shapes identical to those found in the SHEx are encountered in conditions described as hydrothermal, meaning with water as a solvent and maintained at a temperature of, for example, 80 ◦C [50] to 200 ◦C [45], with durations of 2 h [45] to 48 h [50]. Song et al. (2012) [44] report that the concentration of the solution and reaction time (24 h at 170 ◦C) are key parameters for obtaining controlled PbS crystal shapes. In those conditions, they report cubes, dendrites, stars, and wires. These various shapes are required for specific industrial uses where the optical, magnetic and electronic properties of semi-conductors are of high importance [45]. Other authors [52] conducted solvothermal syntheses meaning with organic solvents and imposed temperature conditions leading to PbS dendrites. Nanocoral shape was obtained by [53], by vaporsolid deposition at high temperature (1050 ◦C) and thus at conditions drastically different from ours. [51] provide examples of various shapes obtained at a constant temperature (120 ◦C) but for various synthesis durations (3 to 24 h). [57] report various shapes of PbS nanoparticles (cubic, needle-like, spherical) due to the use of a number of capping agents. Wang et al. (2003) [52] who conducted their syntheses at the constant 120 ◦C temperature with various starting agents and several solvents, including water, obtained various types of PbS dendrites and other shapes. Hence, all these experiments show that there is no clear relationship between the parameters of the synthesis and the shapes obtained.

At Soultz, the solvent of the brine is water, but the inhibitors that are injected in the process are composed of organic molecules which play a role in the crystallization of galena. In fact, when no such agent is used, mostly sulfates (barite group (Ba,Sr,Ca)SO<sup>4</sup> solid-solution) are produced in the URG [12] and at Soultz in particular [27,31,33].

In the case of galena crystallization (use of inhibitors), when it occurs as cubes or derived shapes it results from a preferential growth along <111> direction inducing {100} faces to develop, which is not consistent with the major growth in <100> direction deduced from XRD. However, this is not abnormal since cubes and cubic-derived shapes are rarely found. Indeed, [54] proposed relations between crystal structure and crystal morphology on an energy basis. According to [54] the morphology of a crystal is governed by chains of strong bonds running through the structure, called periodic bond chain (P.B.C.) vectors. Crystal faces are divided into three classes. Flat faces (F) contain two or more coplanar P.B.C. vectors and are the most important faces. Stepped faces (S) are parallel to only one P.B.C vector and are of medium importance. Kinked (K) faces are parallel to no P.B.C. vector and are very rare or do not occur at all. For PbS crystals that belong to the fcc structure, F faces are {100}, S faces are {110} and K faces are {111}. According to Hartmann's and Perdok's theory [54] only F faces should appear at equilibrium, giving cubes as in Figure 16. In fact, during crystal growth, impurities such as the inhibitors used in the geothermal industrial process at Soultz, or As and Sb ions present in the geothermal brine, are adsorbed on K faces ({111} in this case) which promotes their development, together with that of cube (faces {100}), inducing the formation of cuboctahedrons (Figure 17A). In other cases (Figure 17B), only faces {111} develop, leading to octahedral crystals. Crystals with such planar faces (cubes, cuboctahedron, octahedron) appear in conditions of small growth rate, here in laminar flow, as opposed to dendrites which are obtained by a high growth rate in a single direction, here in the hottest and most turbulent flow. As seen in Figure 19, when a crystal grows, the faces which are kept at the end are those where the setting up of atoms is the slowest (faces {100} in the case of galena cubes). Indeed [55] indicates that the faster the growth in a given direction, the smaller the area of the face developed perpendicular to that direction (Figure 19, face {111}).

**Crystal Shape** 

**Figure 19.** Schematic growth of a crystal face {xxx} as a function of the atom setting up rate in the direction perpendicular to that face <xxx>. **Figure 19.** Schematic growth of a crystal face {xxx} as a function of the atom setting up rate in the direction perpendicular to that face <xxx>.

Hence, F faces are visible in the final crystal. Changes in morphology are related to F faces showing a different (higher) growth rate. As a consequence, dendrites and needles that develop in a preferential direction grow very quickly (turbulence and/or high temperature) while cubes and derived shapes grow slowly, in laminar flow and at temperatures which can be low (down to 40 °C). In Figure 14C, needles developed in three perpendicular directions grow on a preexisting 30µm wide galena cube. This succession of shapes might be controlled by very local changes in the parameters of the surrounding Hence, F faces are visible in the final crystal. Changes in morphology are related to F faces showing a different (higher) growth rate. As a consequence, dendrites and needles that develop in a preferential direction grow very quickly (turbulence and/or high temperature) while cubes and derived shapes grow slowly, in laminar flow and at temperatures which can be low (down to 40 ◦C). In Figure 14C, needles developed in three perpendicular directions grow on a preexisting 30µm wide galena cube. This succession of shapes might be controlled by very local changes in the parameters of the surrounding medium.

medium. Table 8 summarizes the occurrence of galena crystal shapes.

Coral +++ Water box, flange, tubes 65–40 Turbulent, laminar

Table 8 summarizes the occurrence of galena crystal shapes. **Table 8.** Occurrence of galena crystal shapes as a function of their location, the temperature and the flow.


Cube + Water box, flange, tubes 65–40 Turbulent, laminar Fibro-radiated ++ Water box, flange, tubes 65–40 Turbulent, laminar Thus, except for dendrites, the location and hence turbulence degree of the place, nature of alloy and temperature are not controlling the shape of PbS crystals that formed in the SHEx, which can be found mixed at given places (Table 2) as opposed to what is Thus, except for dendrites, the location and hence turbulence degree of the place, nature of alloy and temperature are not controlling the shape of PbS crystals that formed in the SHEx, which can be found mixed at given places (Table 2) as opposed to what is described in the literature for syntheses in the laboratory. This might be due to the fact that in the SHEx, the parameters are not controlled as in the laboratory, which allows various shapes to crystallize at the same place. In addition, the temperature range is low, from 65 ◦C to 40 ◦C and is probably not discriminating for promoting specific crystal shapes.

described in the literature for syntheses in the laboratory. This might be due to the fact that in the SHEx, the parameters are not controlled as in the laboratory, which allows various shapes to crystallize at the same place. In addition, the temperature range is low, from 65 °C to 40 °C and is probably not discriminating for promoting specific crystal The presence of As sulfosalts such as dufrénoysite, and maybe others containing Sb as indicated by the EDS semi-quantitative analyses (Table 5), might also be responsible for some of the shapes that were encountered during this study. However, it was not possible to identify them during SEM-EDS survey.

#### shapes. **6. Conclusions and Outlook**

The presence of As sulfosalts such as dufrénoysite, and maybe others containing Sb as indicated by the EDS semi-quantitative analyses (Table 5), might also be responsible for some of the shapes that were encountered during this study. However, it was not possible to identify them during SEM-EDS survey. **6. Conclusion and Outlook**  One way to improve the energy production of geothermal power plants in the URG is to decrease the reinjection temperature. This might induce several problems in the One way to improve the energy production of geothermal power plants in the URG is to decrease the reinjection temperature. This might induce several problems in the power plants including cooling of the rock reservoir, promotion of a chemical disequilibrium into it, and increase of scaling phenomenon as observed for the samples collected in the SHEx at Soultz. When inhibitors are used, those scales are mostly composed of lead sulfide (galena, PbS) together with minor sulfosalts The galena crystals collected at the interface between the metals from which the SHEx was made and the geothermal fluid, after three months of operation, show a homogeneous chemical composition including As and Sb, whatever the alloy on which the scales deposited and whatever the temperature (65 ◦C to

power plants including cooling of the rock reservoir, promotion of a chemical disequilibrium into it, and increase of scaling phenomenon as observed for the samples

composed of lead sulfide (galena, PbS) together with minor sulfosalts The galena crystals collected at the interface between the metals from which the SHEx was made and the

40 ◦C). Thus, there is no influence of the alloy on the scaling phenomenon, as opposed to what is observed for corrosion. Those galena crystals show several shapes that cannot be evidently connected to the alloy, the temperature or the large-scale flow regime, except for dendrites. Indeed, the alloy is insulated from the fluid by the very first layers of deposit, the temperature does not vary drastically from the entrance to the exit of the SHEx (25 ◦C gradient only, at rather low temperatures), the chemical composition of the brine is constant during the industrial process (Ravier, personal communication), and after three months of operation, the scales are rough at the contact with the fluid and the flow is certainly very slow because of this rugosity, allowing the slow growth of crystals with various shapes including cubes and derived shapes. Dendrites are the only shape to be found exclusively at the highest temperature (65–75 ◦C) and in a turbulent environment. To go further, investigation could be performed with Raman to characterize the sulfosalts likely present in the samples, and with XANES to assess the oxidation state of As, Pb and Sb. In addition, statistics of the various shapes encountered at the different locations ought to be performed to pinpoint likely influencing parameters at the micro-scale. Furthermore, one could also conduct laboratory experiments with the brine produced from the geothermal reservoir, and with varying parameters such as temperature, alloy, speed of flow rate, type and amounts of inhibitors, etc. Finally, at present, scales produced at Soultz have to be disposed of as waste due to their toxicity. One can rather imagine an industrial valorization, especially for those deposited at the entrance and exit of the exchangers where mostly dendrites are formed, which is a sought-after shape for the industry [52].

**Author Contributions:** Conceptualization, B.A.L. and M.L.; methodology, G.R., O.S., B.A.L., É.D., A.G., X.S.; validation, J.M., C.B., É.D., A.G., Ayming and H2020 MEET partners; formal analysis, B.A.L.; investigation, B.A.L.; resources, G.R., O.S., J.M., C.B., É.D.; data curation, B.A.L. and M.L.; writing—original draft preparation, B.A.L. and M.L.; writing—review and editing, R.L.H., J.M., C.B., G.R., O.S., É.D., G.T., X.S., A.G.; supervision, B.A.L., A.G., G.T; project administration, A.G., É.D., G.T.; funding acquisition, all the co-authors and H2020 MEET consortium. All authors have read and agreed to the published version of the manuscript. Authorship is limited to those who have contributed substantially to the work reported.

**Funding:** This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 792037 (MEET project).

**Acknowledgments:** The authors thank the Soultz-Sous-Forêts site owner, GEIE Exploitation Minière de la Chaleur, for giving access to their geothermal installation. Valérie Granger (Orano) is acknowledged for XRD analysis and its interpretation. We gratefully thank Jean Hérisson (Ayming) for reviewing this paper as well as relationship with European Commission and overview of the H2020 MEET project. We gratefully acknowledge the two reviewers for their helpful review which greatly helped us to improve the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


## *Article* **Thermodynamic and Kinetic Modelling of Scales Formation at the Soultz-sous-Forêts Geothermal Power Plant**

**Pierce Kunan <sup>1</sup> , Guillaume Ravier 1,\* , Eléonore Dalmais <sup>1</sup> , Marion Ducousso <sup>2</sup> and Pierre Cezac <sup>2</sup>**


**Abstract:** Geothermal energy has been a subject of great interest since the 1990s in the Upper Rhine Graben (URG), where the first European Enhanced Geothermal System (EGS) pilot site has been developed, in Soultz-sous-Forêts (SsF), France. Several studies have already been conducted on scales occurring at the reinjection side at the geothermal plants located in the URG. It has been observed that the composition of the scales changes as chemical treatment is applied to inhibit metal sulfate. The purpose of this study was to model the scaling phenomenon occurring in the surface pipes and the heat exchangers at the SsF geothermal plant. PhreeqC, a geochemical modelling software, was used to reproduce the scaling observations in the geothermal plant during exploitation. A suitable database was chosen based on the availability of chemical elements, minerals, and gas. A thermodynamic model and a kinetic model were proposed for modelling the scaling phenomenon. The thermodynamic model gave insight on possible minerals precipitated while the kinetic model, after modifying the initial rates equation, produced results that were close to the expected scale composition at the SsF geothermal plant. Additional laboratory studies on the kinetics of the scales are proposed to complement the current model.

**Keywords:** Upper Rhine Graben; Soultz-sous-Forêts; geothermal brine; scaling; metal sulfides; thermodynamic; kinetics

## **1. Introduction**

*1.1. Geothermal Energy in the Upper Rhine Graben*

The Upper Rhine Graben (URG) is a rifting formation, oriented NNE, part of the European Cenozoic rift system. It extends for 300 km of length, from Basel (Switzerland) in the south to Mainz (Germany) in the north. Important thermal anomalies have been identified in the URG thanks to a rich geological exploration (Figure 1, [1]). These anomalies delineate thermal gradient locally over 100 ◦C/km in the first km of sediments and controlled with normal faults parallel to the graben direction. The first European Geothermal research project of Soultz-sous-Forêts (SsF) was conducted initially in the early 1990s. This project was based on the Hot Dry Rock (HDR) concept, where the goal was to create an artificial heat exchanger in the basement rocks by hydraulic fracturing [2]. However, the results obtained after the drilling of the first well at SsF showed the presence of natural fluid circulation through the existing fracture network of the reservoir [3]. Since then, the Enhanced Geothermal System (EGS) technology was incorporated into future development of the URG geothermal project. This approach consists of exploiting the natural thermal brine circulation by improving, if necessary, the connection between the geothermal wells and the reservoir with various chemical, hydraulic, and thermal treatments [4].

**Citation:** Kunan, P.; Ravier, G.; Dalmais, E.; Ducousso, M.; Cezac, P. Thermodynamic and Kinetic Modelling of Scales Formation at the Soultz-sous-Forêts Geothermal Power Plant. *Geosciences* **2021**, *11*, 483. https://doi.org/10.3390/ geosciences11120483

Academic Editors: Antonio Paonita and Jesus Martinez-Frias

Received: 22 September 2021 Accepted: 18 November 2021 Published: 23 November 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

There are several geothermal projects that have been developed in the French, German, and Swiss URG region over the past years. In France, two notable geothermal plants are in operation at SsF and Rittershoffen, respectively, for power and heat production while in Germany, three geothermal plants are in operation for power generation.

### *1.2. SsF Geothermal Power Plant*

The Soultz-sous-Forêts geothermal project started in 1987 and is the cradle of the geothermal energy European research in granitic and fractured systems. Over 30 years of research, the geothermal site at SsF continues to exploit commercially the fractured basement for the EEIG Heat Mining. The actual geothermal system consists of three wells: one production well named GPK-2 and two injection wells named GPK-3 and GPK-4 which are drilled 5 km into the granitic basement. The geothermal brine is produced at a temperature of 150 ◦C, reaching the wellhead with a nominal flow rate of 30 kg/s provided by a downhole production Line Shaft Pump [5]. The installed gross capacity of the binary plant is around 1.7 MWe (Figure 2).

**Figure 2.** The SsF geothermal power plant (Source: EEIG Heat Mining).

The geothermal brine is flowed through a system that consists of three consecutive double pass tubular heat exchangers which supply heat to an Organic Rankine Cycle (ORC) to produce electricity. The geothermal brine is then fully reinjected into the granitic basement at around 65–70 ◦C. The volume of reinjected brine is split between the two injection wells without the need of reinjection pumps. The well-head overpressure in the surface infrastructure is regulated by using production pump which reaches about 23 bars to keep the gas dissolved in the brine. The reinjection temperature is linked to the conversion process. The geothermal plant has been successfully producing electricity commercially since September 2016, with an availability rate of about 90% for the past four years [6]. The granite reservoir is made of a porphyritic monzogranite rich in K-feldspar megacrysts. Primary silicate minerals are quartz, plagioclase, biotite, and hornblende. A chemical analysis on the composition of the brine was taken in February 2020 (Table 1, [7]), while an analysis on the gas dissolved in the brine was taken in April 2019 (Table 2, [7]).


**Table 1.** Composition of brine at the production well of the SsF geothermal plant [7].

**Table 2.** Composition of gas in brine at the production well of the SsF geothermal plant [7].


#### *1.3. Geochemical Characterization of the Scale during Operation*

In the Upper Rhine Graben region, scaling commonly occurs at the cold side of the SsF geothermal plant [8]. Therefore, in the Upper Rhine Graben, scale formation before the application of sulfate scale inhibitors was dominated by (Ba, Sr, Ca)SO<sup>4</sup> solid–solution scaling containing minor amounts of galena, pyrite, or poly-metallic sulfides phases [8–10]. The main scales observed related to deep geothermal activity have been studied not only because when represented at a significant amount of secondary precipitations they could plug the geothermal infrastructures (pipe, heat exchanger, well-head), but also because

the scales have the properties to trap radiogenic elements such as <sup>226</sup>Ra and <sup>210</sup>Pb in their crystalline lattices [8,9].

By using sulfate inhibitors in the Upper Rhine Graben region, barite precipitation was strongly reduced [8,11]. However, brittle grey–dark scales are still precipitating on the pipe walls consisting of PbS, and elemental Pb, As, Sb are precipitating in the geothermal infrastructures [11]. Traces of halite are present on some samples, but it corresponds to a drying residue from the geothermal brine [11]. Based on Raman spectrum of the sulfide phase, a hydrothermal Pb-Sb-Cu-sulfide (Pb13CuSb7S24) has been characterized as well as an amorphous phase [11].

Several studies at SsF geothermal plant [6,12] report on the effects of the chemical treatment used to inhibit the formation of sulfate scales at SsF geothermal plant. Complementary studies have been carried out in the framework of the MEET research project at temperature below 65 ◦C with a test heat exchanger [13]. A typical black scale deposit at the wall of a tube pipe of this heat exchanger is shown in Figure 3. CY Cergy Paris Université conducted a study on different scales found in the test heat exchanger with a Zeiss GeminiSEM 300 Scanning Electron Microscopy, coupled with a Bruker Energy Dispersive Spectrometry. Figure 4 details this typical scale, a (Pb,As,Sb)S fibro-radiated hilly scale found at 50 ◦C on 1.4410 stainless steel tube [14].

**Figure 3.** PbS scales deposited in tubes from the test heat exchanger.

**Figure 4.** Microscopic photo of (Pb,As,Sb)S scale found at SsF plant [14].

Scales in the range between 150 ◦C and 65 ◦C have been sampled in June 2018 before cleaning operation in the ORC evaporator and preheaters after nearly one year of operation. Figure 5 presents a schematic drawing of the geothermal loop at SsF and the temperature gradient in the heat exchangers between the production well GPK-2 and injection wells GPK-3 and GPK4. Chemical composition of these scales has been determined using ICP MS method which is a type of mass spectrometry that uses an inductively couple plasma to ionize the sample. Scales in the range between 60 ◦C and 40 ◦C have been sampled in April 2019 in a test heat exchanger (HEX) designed with different metallurgy and

installed at the SsF geothermal plant during three months in the framework of the MEET research project [13]. The latest chemical composition of scales observed at SsF geothermal plant within a range of temperature between 150 ◦C to 40 ◦C are presented in Table 3. Table 3 considers only scaling samples from tubes with 1.4410 metallurgy like the ORC heat exchanges to have a good comparison. A detail description of these scales is given by Ledésert et al. (2021) [14], and chemical composition was also determined using ICP MS method. Chemical treatment of the brine was almost the same for the two sets of scales. These scales consist of S, Pb, Sr, Ba, Sb, As, Fe, Si, and Cu elements.

**Figure 5.** Schematic drawing of the geothermal loop at SsF.

**Table 3.** The mass composition of scales formed in the heat exchangers at the geothermal plant and in the test heat exchangers in percentage.


The presentation of the mass percentage of scales is based on the total elements found in the scales. Certain compounds, mainly carbonates, were omitted from Table 3 because they are not the main focus of this study which is dedicated to low temperature scale formation. There are also lesser amounts of the scales deposited in the higher temperature heat exchangers (ORC heat exchangers), while more scales are deposited in the lower temperature heat exchangers (Test HEX).

Lead is found primarily at lower temperatures notably at temperatures below 120 ◦C. Sulfur, arsenic, silicon, and antimony are also deposited at large quantities after lead. The rest of the elements are found in smaller traces (less than 5%). The test heat exchanger has a different concentration of scales compared to the ORC heat exchangers at the geothermal plant due to the difference in temperature. In the test heat exchanger, lead has a higher concentration than those in the main exchangers. The chemical treatment on the sulfate scales proved to be effective as the quantity of barium sulfate (barite) and strontium sulfate (celestite) are found in very small quantities which are less than 4% for any point of temperature, while before the application of such treatment (Ba, Sr, Ca)SO<sup>4</sup> solid–solution was dominating [8].

The main objective of this study was to model the scaling phenomenon occurring in the surface pipes and heat exchangers at the SsF geothermal plant. Scaling formation was firstly modelled according to thermodynamic perspective and the results are compared to the geochemical analyses presented in Table 3 and used as references. A previous investigation was conducted on available thermodynamical databases to find the most suitable one regarding geochemical elements and possible scaling minerals. Thermodynamical modeling was then completed with kinetic modeling to better represent real operational conditions in heat exchangers. The results of both modeling are later discussed.

#### **2. Methods**

The modelling of the geochemical fluids is done through the software, PhreeqC 3.6.4 which is a computer program that is written in C++ programming language. It is designed to perform numerous aqueous geochemical calculations. PhreeqC implements several types of aqueous models depending on the database used. This program was created by the U.S. Geological Survey (USGS). PhreeqC is freely distributed by the USGS and is currently an open source software.

PhreeqC uses a pre-established thermodynamic database to perform the calculations during modelling of a fluid. Each database has different sets of elements and aqueous species as well as different thermodynamic data which are taken from different references sources. There are several databases found within the installation of the PhreeqC program. Supplementary databases were also found in the PhreeqC Users forum. There are databases taken from studies such as e THERMOCHIMIE [15] and THEREDA [16]. The PhreeqC manual [17] was referred to when performing the modelling of formation of scales with PhreeqC. Table 4 shows the list of databases gathered which are listed from D1 to D19:

**Table 4.** PhreeqC databases and allocated nomenclature.


#### *2.1. Verification: Elements*

In order to verify the validity of the databases to be used in the modelling process, the sets of elements available within the databases were compared to the elements found in the geothermal fluid at the SsF plant. The latest chemical analysis (taken in February 2020) on the composition of the brine at the SsF plant was used to cross-reference with the sets of elements found in the databases to narrow down the list of valid databases. This analysis showed that there was high concentration of Na and Cl ions in the brine. The recent study by Bosia et al. (2021) [7] provides further details on the geochemical dataset used. Databases with more supplementary elements were taken more into consideration due to the likelihood of simulating the actual fluid. Thus, the presence of the elements in the databases are compared to the elements found in the geothermal fluid at the SsF plant (Table 5)


**Table 5.** Geochemical elements in the databases.

\* = limited.

The geochemical elements from the Table 5 are represented in their aqueous state. From this study, the Thermoddem (D14 and D15) [18] and LLNL (D7) [19] databases, having respectively 57 and 55 elements of the 57 SsF brine chemical composition, are observed to be suitable for the purpose of this study as they possess the most amount elements found in the brine at the SsF plant. Further reference to the Thermoddem database will be the Thermoddem (D15) database instead of the Thermoddem (D14) database, because D15 is the latest version for the Thermoddem database.

#### *2.2. Verification: Minerals*

Another criterion set for the validation of the databases is the formation of probable minerals in the geothermal fluid at the SsF plant. A list of known minerals precipitated was made to compare to the minerals found in the databases. Furthermore, a list of probable minerals precipitated was made for minerals that have not been identified before in previous studies. These minerals that are susceptible to precipitation are identified by listing out minerals from the databases that consist of at least two of nine elements that are the majority in the analysis of scales conducted at the site. The nine principal elements are sulfur, lead, strontium, barium, antimony, arsenic, iron, silicon, and copper. A similar approach to the verification of elements was used in the verification of minerals in which a table with the list of minerals was cross-referenced with the database. The occurrences of known minerals and minerals susceptible to precipitation in the databases are tabulated (Table 6).


**Table 6.** Minerals in the databases.


**Table 6.** *Cont.*

The similar conclusion as before can be drawn from this verification in which the two databases, Thermoddem (D15) and LLNL (D7) are suitable for the modelling of the geothermal fluids at the SsF plant due to possessing an extensive amount of thermodynamic data on known mineral found as deposits in the plant as well as possible minerals precipitated. The LLNL database has 33 mineral datasets out of the 42 possible minerals deposited, while the Thermoddem database has 35 out of the 42 possible minerals deposited.

Another step was carried out to verify the domain of validity for the minerals in the LLNL and Thermoddem databases. The range of temperature valid for each mineral was verified to ensure that it corresponds with the maximum modelling temperature of 200 ◦C. For the Thermoddem database, the thermodynamic data of all the minerals are valid within 0 ◦C to 300 ◦C. On the other hand, the LLNL database has different limits for each mineral. Fortunately, the minerals that were identified in Table 6 are well within the limits proposed in the LLNL database, as the lowest maximum temperature for the minerals found is at 200 ◦C.

#### *2.3. Verification: B-Dot Model Database*

The two databases of interest, the Thermoddem database and the LNLL database, utilize the B-Dot equation for the calculation of activity of the elements. The B-dot model is also known as the Truesdell–Jones model (TJ model). The ionic strength of the fluid was calculated from the major elements mentioned in the most recent published geochemical datasets in Bosia et al. (2021) [7] and found to be at 1.79 mol/kg for GPK-2 and at 1.8 mol/kg for GPK-3 (Table 7). The unit for the ionic strength can be represented as mol/L or mol/kg since the fluid is primarily composed of water while the effects of the ions in the conversion can be ignored due to their miniscule presence in the fluid. The validity of the B-dot model is verified in Figure 6 [20] as the ionic strength is well within the limit of the TJ model for both wells. The higher the ionic strength, the less accurate the results produced. When the ionic strength of the brine exceeds the limits of the TJ model (2.2 mol/kg), the results obtained from using the B-dot databases will no longer be valid.

**Figure 6.** Schematic plot showing the general applicability of different activity coefficient models as a function of ionic strength for a divalent cation. The dashed tangent to the curve at its origin is a plot of the Debye–Hückel limiting law for the ion [18].


**Table 7.** Ionic strength calculations of the geothermal fluid sampled at GPK-2 and GPK-3.

Since the ionic strength of the fluids at the SsF geothermal plant are well within the limits of the zone of validity, the two databases are thus used for the modelling of the fluids. Alsemgeest et al. (2021) [21] suggest being cautious when applying B-dot equation to SsF high saline geothermal brine. Nevertheless, they are also the most documented in terms of the geochemical elements and minerals.

44

#### *2.4. Verification: Gas*

The data available on the gases in the databases are compared to those required for modelling the geothermal fluid. The databases are then analyzed by initiating a preliminary modelling of the fluids to compare the results of the modelling with the results at the plant. For this preliminary modelling, the mixture of the gas dissolved in the brine (Table 2) was used. The conditions of the preliminary modelling are done at pH 5.2 and at two different temperatures, 80 ◦C and 150 ◦C. The saturation pressure of each database is compared and analyzed. For this analysis, the Thermoddem database, the LLNL database, and the Pitzer database were used. For the Thermoddem database and the LLNL database, as they were deemed suitable for the modelling of scales through the verification of elements and minerals, they are thus analyzed for the verification of gases. Even though the Pitzer database lacks several data on the elements and minerals, it is still considered for modelling of dissolved gases in the geothermal fluid because this database uses a different model for the calculation of activity of the elements. This may then give a more accurate result in the modelling of dissolved gases in the geothermal fluid. The results of the preliminary modelling at two different temperatures steps in terms of saturation pressure with the three databases are recorded in Table 8.

**Table 8.** Results of the saturation pressure of SsF gas for each database at two temperature steps.


The LLNL and Thermoddem databases give out a similar result at both tested temperature while the Pitzer database shows a higher pressure compared to the two previous databases (Table 8). The saturation pressure obtained from modelling at 150 ◦C with the Pitzer database (18 atm = 18.2 bar) is closer to the actual case observed at the SsF plant [22] at the same temperature which ranges between 18.0 and 18.5 bar at relative pressure. The Thermoddem and LLNL databases provided results outside the range of saturation pressure observed at the SsF plant. Thus, the Pitzer database is found to be more suitable than the Thermoddem and LLNL databases for the gas modelling of the SsF plant.

Overall, the Thermoddem database was selected for the modelling of the formation of scales in the geothermal fluids as this database has more data than the LLNL database on the geochemical elements and possible minerals precipitated. Furthermore, the Thermoddem database has been compiled by a French geological survey company, BRGM which is specifically designed for waste derived from natural fluid precipitation [18]. As for modelling of the dissolved gas in the fluid, the Pitzer database was observed to have given a more satisfactory result as mentioned in the previous paragraph. Thus, the Pitzer database should be used for the modelling of the solubility of gas in the geothermal fluid.

#### *2.5. Scale Modelling*

When modelling the formation of scales with PhreeqC, the physical properties of the fluids such as the temperature, pressure and pH of the fluid are inputted into the software. The initial temperature, pressure and pH of the fluid are 25 ◦C, 1 bar, and pH 5.2 respectively representative of the laboratory conditions for brine analysis. The temperature and pressure were later changed to the production conditions of the brine at the SsF geothermal plant which are at 150 ◦C and 20 bars respectively. The pH of the fluid is also adjusted by the software to reflect the temperature and the composition of the fluid, thus there was no need to modify it. The unit for the concentration of each component in the fluids is also user-defined. In the case of this study, the unit used is in mg/kgw where kgw stands for a kilogram of water. Thus, the unit mg/kgw is the mass in milligrams of the element for each kilogram of water.

The formation of scales at the SsF plant is initially modeled by using thermodynamic modelling. This method uses the thermodynamic database researched in the previous

section. The saturation index of each mineral is studied in this modelling process. For any minerals with a saturation index equal or higher than zero for the conditions of the fluid at the geothermal plant, that mineral can potentially precipitate. The amount of minerals precipitated was then calculated. This method provided insight on the potential minerals that could precipitate aside from the minerals already observed in previous studies such as those mentioned in Scheiber et al. (2012) [8], Sanjuan et al. (2011) [9], and Nitschke (2012) [10]. However, this method is limited to cases where thermodynamic equilibrium is reached.

Kinetic modelling was also considered to represent accurately the situation of the formation of scales at the geothermal plant. For this method, the amount of time that the fluids pass through the plant's exchangers is needed. It takes around 3 min for the fluid to circulate from the entrance of the first ORC heat exchanger to the exit of the final ORC heat exchanger. In these conditions, the kinetics of the reaction is also a crucial factor for the kinetic modelling. The kinetic data for chalcopyrite, galena, orpiment, and pyrite was taken from the database made by Zhang et al. (2019) [23]. The kinetic constant for stibnite was taken from Biver et al. (2011) [24] and adjusted into a modified kinetic equation for galena. For other minerals without any kinetic data, a modified kinetic equation of a similar mineral was used. The amount of minerals precipitated is calculated using its kinetic equation. This method refers to the saturation index of the mineral before calculating with the kinetic information available. As stated before, when the saturation index of the mineral is below zero, the kinetic calculation is skipped as the mineral does not precipitate. The duration for the kinetic modelling at each temperature was set to one minute because the velocity of the brine is estimated to be slightly less than 1 m/s and the length of the tubes of heat exchanger (30 m). This gives a duration of about 30 s to pass through a heat exchanger. Another 30 s was added to take into account the head cover and the pipes between each heat exchanger.

#### **3. Results**

As mentioned in the previous section, the modelling of scales in the geothermal fluids was done in Phreeqc with the Thermoddem database. For this modelling sequence, the range of temperature and pressure were set. The temperature starts from 150 ◦C which is the highest observable temperature at the SsF plant. The temperature then reduces until the lowest temperature found in the test heat exchanger which is at 40 ◦C. Additionally, two fictional temperatures were added which are at 175 ◦C and 200 ◦C in order to simulate the influence of such high temperatures on the formation of scales. These two temperatures are representative of temperatures found in the geothermal reservoir that is four to five kilometers deep under. The pressure was then fixed at 20 bars to simulate the exact conditions at the SsF geothermal plant.

#### *3.1. Thermodynamic Modelling*

The precipitation of the minerals was first studied through the observation made on the saturation index of each mineral. For the minerals with a saturation index equal or higher than zero, they are minerals that could possibly be present in the scales at thermodynamic equilibrium (Appendix A, Table A1). A list of potential minerals present within the set range of temperature was constructed from the observation of the saturation index of each mineral (Table 9).


**Table 9.** Presence of potential minerals at the set range of temperature according to saturation index.

The next step for the modelling of scales formation at the SsF geothermal plant is to calculate the quantity of minerals precipitating in the given temperature range. An initial modelling based on the present minerals (Table 9) was done and the results showed that not all minerals with a positive saturation index precipitated (Table 10, left side). This is explained by the higher saturation index of several minerals which have higher priority to precipitate. The results of the thermodynamic modelling (Table 11) from using the minerals of the left side of Table 10 showed that majority of the minerals consist of silicates because of the high concentration of O and Si. At the range of temperature between 40 ◦C to 150 ◦C, silicate scales are not usually found at high amounts at the SsF geothermal plant.

**Table 10.** Mineral precipitated for thermodynamic modelling (For 40–200 ◦C).



**Table 10.** *Cont.*

**Table 11.** Results of first thermodynamic modelling in weight percentage.


To have a better focus on the modelling of scales at the SsF geothermal plant, the minerals considered for the thermodynamic model were then identified (Table 10, right side). Barite and celestite were excluded from future modelling sequence, because inhibitors are used by the operator to prevent the formation of these scales. For silicates, it is suspected that kinetic reaction prevents their deposition. That is why they were excluded to focus on the primary elements found in the scales found at the SsF geothermal plant as mentioned before. The results of the calculation are done at the different temperatures (Table 12).

**Table 12.** Results of refined thermodynamic modelling in weight percentage.


For each step of temperature, the modelling results show that sulfur and iron are the major elements with concentrations of 45% and 53% respectively (Table 12). On the other hand, the total amount of the other elements represents less than 3% of the total. Copper is only found at 40 ◦C and in extremely small quantities. Antimony and lead are also found in small quantities (less than 1.5%) at any given step of temperature.

#### *3.2. Kinetic Modelling*

The results given out by the calculation of the thermodynamic model gives insight on the precipitation of the minerals at thermodynamic equilibrium which may not necessarily be respected in the conditions studied. Modelling done from a kinetics aspect was proposed and the results from the thermodynamic model were compared and complimented with literature review and field knowledge to select the proper minerals which could precipitate. The kinetic information was mainly obtained from Zhang et al. (2019) [23] as mentioned in the Section 2. Initially, the model had little modification to the kinetic information used from the source with exception for minerals lacking their kinetic information. Rates equations for metal sulfides including the concentration of oxygen into the calculation are removed, because they serve no purpose due to the little to no oxygen content in the brine at the SsF geothermal plant.

For the initial model, two different sets of minerals were considered. The first set of minerals are galena (PbS), orpiment (As2S3), pyrite (FeS2), amorphous silica (SiO2), quartz (alpha) (SiO2), and stibnite (Sb2S3). Galena and stibnite are known minerals already observed at the SsF plant [14]. Pyrite was considered over arsenopyrite (AsFeS) and chalcopyrite (CuFeS), because pyrite has a higher saturation index than arsenopyrite (Appendix A, Table A1); thus pyrite is more susceptible to precipitate than arsenopyrite. Chalcopyrite was dismissed as the principal provider of Fe precipitation because there is only a small amount of copper found in the analysis done at the SsF plant (Table 3) which is negligible compared to the quantity of Fe found. As for orpiment, this mineral is the only representative for presence of the element As. For amorphous silica and quartz (alpha), they were considered as they had a major influence in the thermodynamic modelling. Unfortunately, the desired modelling conditions do not fall within the domain of validity for the initial kinetic model created. For the formation of galena, this model is only valid for a temperature between 25 ◦C to 70 ◦C and a pH between one and three. For the formation of pyrite, this model is only valid for a temperature between 20 ◦C to 40 ◦C and a pH between one and four. For both cases, the range of pH is too acidic compared to the actual case. The model for the formation of orpiment is only valid for a temperature between 25 ◦C to 40 ◦C and a pH between 7.3 and 9.4 which is too alkaline. For the formation of amorphous silica, the model is only valid for a pH around 5.7, which is a bit too alkaline compared to the pH of the fluid at the SsF geothermal plant. For the formation of quartz (alpha), the model is within the proper zone of validity. Regardless, this model was used as an initial approach to modelling the minerals precipitated. For stibnite, no source for its kinetic information aside from its kinetic constant is found [24]. Thus, the kinetic equation of galena was taken and modified to suit the kinetic rate of stibnite. Minerals such as barite and celestite were not added, because their exclusion serves as a proxy to their inhibition by chemical treatment.

The second set of minerals consists of the same minerals from the first set, but excluding amorphous silica and quartz (alpha). These two minerals were excluded to better focus on the main minerals identified in the scales at the SsF geothermal plant. The modelling with both set of minerals was only done from 200 ◦C to 65 ◦C as it is complicated to model the circulation of fluids in the pipes between the ORC heat exchangers and the test heat exchangers. Furthermore, the residence time and the surface area of the heat exchangers in contact with the brine are different in both cases which will thus further complexify the model. To simplify the model, the ORC heat exchangers were chosen as the standard for the temperature to be modelled.

The first results showed that for the temperatures between 65 ◦C and 150 ◦C, S and Fe are the major elements in the simulated scales (Table 13). From 175 ◦C onwards, Si and O are the major elements while Pb, As, and Sb are found in negligible amounts.


**Table 13.** Results for initial kinetics model with first set of minerals in weight percentage.

The results show that sulfur is the majority for every step of temperature taking up to 53.4% of the composition of scales (Table 14). Iron is shown to be in second largest mass quantity with a weight percentage of around 46% except at 65 ◦C which is at 40.9%. Lead is shown to be in smaller quantity such as 8.9% at 65 ◦C and 2.2% at 90 ◦C, respectively. Between 200 ◦C and 120 ◦C, the quantity of lead is less than 1%. As for antimony and arsenic, both are found in extremely small quantities where antimony is at 1.1% and arsenic is at 0.14% for the temperature of 65 ◦C. Antimony and arsenic are not found at higher temperatures (above 150 ◦C).

**Table 14.** Results for initial kinetics model with second set of minerals in weight percentage.


#### **4. Discussion**

#### *4.1. Introduction*

In this discussion, an analysis is done on the thermodynamic modelling and the kinetic modelling to identify the utility and shortcomings of each method. The factors that affect the results of each method are also discussed. Modifications were done on the kinetic model to better fit with the chemistry of scale observed at the SsF plant. Finally, new perspectives are proposed and discussed to improve further the proposed predictive kinetic model.

#### *4.2. Thermodynamic Modelling Analysis*

The thermodynamic modelling provides insight on possible precipitation of minerals at each temperature step. It can be observed that minerals containing strontium such as celestite were not listed as minerals precipitated by the modelling software (Table 9). In the analysis made on the scales at the SsF plant, traces of strontium were found and were identified to be celestite [8,9]. This discrepancy can be explained by the fact that the supposed mineral found at the plant, celestite, dissolves in favor of the precipitation of barite [25]. Since the results are calculated at thermodynamic equilibrium, the total consumption of celestite was already considered during the calculations made by PhreeqC. Another explanation is that the PhreeqC software does not consider the existence of solid solutions like barium/strontium sulfates. Hence, the software considers barite over celestite for their precipitation. Thus, strontium was excluded from the comparison of the weight percentage of the elements between the Ssf plant analyses, the thermodynamic models, and the kinetic models. Barite is shown to potentially precipitate at the given range of temperature (Table 9). However, as the temperature decreases, the saturation index of barite increases thus increasing its potential to precipitate (Appendix A, Table A1). A similar situation is observed in the formation of galena, albeit with a higher saturation

index. For pyrite, it can also potentially precipitate at the given range of temperature. Its saturation index increases from 200 ◦C to 90 ◦C in which it starts to decrease thereafter. Precipitation of native metals could not be observed in neither thermodynamics modelling nor kinetics modelling, because the modelling software cannot take into account their formation.

When silicates were considered for the thermodynamic model, the results (Table 11) showed that Si and O take up the majority of the elements until it rendered the rest of the elements negligible in the simulated scales. This is not the case at the SsF geothermal plant as there were tiny amounts of silicate in the actual analyses. A second model was constructed by excluding the silicates to have a better focus on the known minerals found at the geothermal plant.

The amount of galena formed in the thermodynamic models is greatly inferior to the actual scaling at the SsF geothermal plant (Table 15). There is an unusually high amount of iron and sulfur in the thermodynamic modelling. Furthermore, the quantity of lead is still in the minority. Another problem is that the thermodynamic modelling simulates the precipitation of the minerals over a great amount of time which is until the fluid reaches thermodynamic equilibrium. At the SsF geothermal plant, the precipitation of the minerals is not necessarily at thermodynamic equilibrium since the residence time of the brine in the exchanger is only around three minutes. Furthermore, the initial amount of lead (Pb) (Table 2) is smaller than the rest of elements in the brine. This could explain the low amount of lead found in simulated scales compared to the other elements in this modelling method. Thus, the thermodynamic model proved to be not sufficient for the prediction of formation of scales at the SsF geothermal plant and kinetic effect must be considered.


**Table 15.** Comparison between Soultz-sous-Forêts, thermodynamic model, and kinetic model results in relative percentage by weight.

#### *4.3. Kinetic Modelling Analysis*

The kinetics model with the first set of minerals (Table 13) showed improvements in the results when compared to the first thermodynamic model (Table 11). The kinetic model with the first set of minerals (Table 13) has significantly reduced the Si and O contents for the temperatures between 65 ◦C and 150 ◦C. This confirms that the kinetic effect controls the absence of silicates in the SsF scales.

However, for this range of temperature, sulfur (S) and iron (Fe) have the highest concentration with the highest percentage being 52.2% and 45.1% respectively (Table 15). Regardless, the concentration of each element for the kinetic model 1 does not reflect the actual concentration found in the SsF plant analyses.

As for the kinetic model 2, it showed similar improvements in the results to the results of kinetic model 1. At 65 ◦C, the quantity of lead has increased from 0.25% (thermodynamic model 2) to 8.9% (kinetic model 2) in the composition of elements found in the modelled scales (Table 14). However, iron and sulfur are still the major elements in the modelled scales. The lack of kinetic information on the formation of stibnite could also lead to inaccuracies in the results such as the low amount of antimony. In addition, the amount of sulfur present at each temperature is larger than the actual case. The discrepancies can be explained by the conditions of the modelled scales being outside the domain of validity for temperature and pH of the kinetic information used.

Therefore, to better simulate the scale formation at the SsF geothermal plant, a modified version of the initial model was created. In this second model, the kinetic information of the minerals was modified to reflect closely to the analyses done at the geothermal plant. The kinetic information was purposely modified until the model produces a result similar to the ones obtained at SsF geothermal plant at one temperature step. The modification was done iteratively until the results were in an approximate range of the actual case. Thus, the modified kinetic information is not indicative of any actual kinetic values. The two minerals (arsenopyrite and chalcopyrite) were added to compensate for the low amount of arsenic and the high amount of sulfur and iron. The kinetic information of chalcopyrite is taken from Zhang et al. (2019), whereas no kinetic data was found on arsenopyrite. Thus, the kinetic data of chalcopyrite was taken and modified for arsenopyrite. Next, the kinetic rate of pyrite was slowed down as this mineral has the greatest influence on the increases of percentage of iron and sulfur (Table 16). Overall, the kinetic information of all the minerals except galena and chalcopyrite was modified to obtain a general model for the formation of scales.


**Table 16.** Modification of the first kinetic model. nx: representing the index used in the rates equation (Appendix B).

The modified model presented a result that is closer to the analyses of scales at the geothermal plant (Tables 15 and 17). The percentage of sulfur is still higher than the actual case, but the increase in quantity of sulfur scales better than the unmodified kinetic information models. The quantity of iron is higher than the actual case for the temperature between 90 ◦C and 150 ◦C. In addition, there are no other minerals that contain antimony and arsenic that has a positive saturation index for temperatures above 120 ◦C. This leads to having small and negligible quantities of both elements at the mentioned temperature. All things considered, this model allows a rough prediction on the scale formation when operating the plant with sulfate scales inhibitors at the SsF geothermal plant as there is only a small deviation between simulated results and the actual case. The model becomes

less accurate at higher temperatures such as at 150 ◦C, because of the lack of antimony and arsenic at this temperature (Table 15).


**Table 17.** Mass of elements in percentage for modified kinetics model.

#### *4.4. New Perspectives*

For the modelling of scales for the SsF geothermal plant, a lot of information was lacking such as the kinetic information that is suited for the operating conditions of the plant. Future studies and analyses on the precipitation of the minerals are to be arranged to obtain the missing kinetic information and challenge the modified kinetic model. A laboratory study is necessary to investigate the precipitation of minerals at conditions of the SsF geothermal plant which is at around pH 5.2 and the temperature range of the ORC heat exchangers. The kinetic model for pyrite might also not be suitable for modelling the scales at the pH, pressure, and temperature of SsF geothermal plant which led to inaccuracies in the results pertaining to the amount of Fe and S. Therefore, the kinetic information of the precipitation of pyrite as well as galena, arsenopyrite, chalcopyrite, arsenides, sulfosalts, selenides, and other base metal sulfides are needed to be determined through this laboratory study so that a proper kinetic model can be constructed.

Furthermore, the inhibition of sulfates such as barium and celestite was just excluded from the calculation due to lack of information on their kinetics. Therefore, the inhibition process should also be analyzed and studied to obtain its kinetic information that can be integrated into the kinetic model. With a proper kinetic model, a more precise result can be obtained through the simulation on the formation of scales in the pipes and exchanger at the geothermal plant. Besides that, other reactions aside from precipitation should also be studied and integrated into the model such as the possibility of heavy metal corrosion in the pipes and heat exchanger, as mentioned in Lichti and Brown (2013) [26] and Lichti et al. (2016) [27]. This phenomenon should be studied at the SsF geothermal plant and be verified whether it affects the amount of scales formed at the plant. A study should also be conducted on the possibility of a chemical interaction between FeS and PbS. The results from the laboratory studies on this chemical interaction at the SsF operational condition could be integrated into the current prediction model for a more accurate result.

#### **5. Conclusions**

From the geochemical analyses done on the SsF geothermal plant, lead is found to be the major element in the composition of scales formed when operating the plant with sulfate anti-scales. The principal mineral formed was identified to be galena. This could change when additional chemical treatment is added to the process. To have an accurate prediction on the mineral and elements formed during the scaling phenomenon, a prediction model needs to be created.

The main goal of this study was to better characterize the scales formed at the SsF geothermal plant by establishing a geochemical model that allows the prediction of the formation of scales. Intensive bibliographic research was done to obtain the necessary thermodynamic and kinetic information used in the modelling of the formation of scales at the SsF geothermal plant. The two methods of modelling present their own set of challenges to reflect accurately the actual case.

For the thermodynamic modelling, this method is done over a great amount of time which is impractical for predicting the formation of scales in an actual case. The saturation index obtained from thermodynamic modelling however is a good indication on which mineral can precipitate in function of the temperature. Minerals such as silicate scales could potentially precipitate at the right conditions.

For the kinetic modelling, specific kinetic information such as the rates equation and the kinetic constant for the precipitation of the mineral are lacking for the desired range of temperature. Nevertheless, the modelling shows that silicate precipitation is strongly controlled by kinetic. Additionally, this method allows a more accurate prediction for the formation of scales with the caveat of having the proper kinetic information.

The results obtained in this study open up to new perspectives on the issue of lack of kinetic information. The proposed steps from the new perspectives can improve the current prediction model for future uses.

**Author Contributions:** Conceptualization, P.K., G.R. and M.D.; methodology, P.K., G.R. and M.D.; software, P.K.; validation, G.R., E.D., M.D. and P.C.; formal analysis, P.K., G.R. and M.D.; investigation, P.K. and G.R.; resources, P.K. and G.R.; data curation, G.R.; writing—original draft preparation, P.K. and G.R.; writing—review and editing, P.K., G.R., E.D., M.D. and P.C.; visualization, P.K. and G.R.; supervision, G.R. and M.D.; project administration, G.R. and E.D.; funding acquisition, G.R. and E.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the European Union's Horizon 2020 research and innovation program under grant agreement No 792037 (MEET project), as well as the Agence Nationale de la Recherche under grant agreement No 10-IEED-0811-05 (THERMA'LI project).

**Data Availability Statement:** All data supporting this paper can be found in the references.

**Acknowledgments:** The authors thank the Soultz-sous-Forêts site owner, GEIE Exploitation Minière de la Chaleur, for giving access to their geothermal installation. The authors would also thank Laurent André (BRGM) for the technical support, guidance, and insight on the modelling process. Finally, the author would like to thank Beatrice Ledésert (CY Cergy Paris Université) for providing the microscopic image of the (Pb,As,Sb)S scale found at SsF plant.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

## **Appendix A**


**Table A1.** Saturation Index of minerals with potential to precipitate.

**Pressure (bar) 20** Fe10S<sup>11</sup> Fe10S<sup>11</sup> −19.75 −15.47 −11.34 −9.49 −2.77 0.56 1.65 1.98 2.14 Fe11S<sup>12</sup> Fe11S<sup>12</sup> −21.56 −16.83 −12.28 −10.23 −2.81 0.92 2.19 2.61 2.84 Fe7.016S<sup>8</sup> Fe7.016S<sup>8</sup> −11.61 −8.67 −5.83 −4.55 0.01 2.12 2.67 2.74 2.72 Fe9S<sup>10</sup> Fe9S<sup>10</sup> −16.97 −13.14 −9.45 −7.79 −1.8 1.11 2 2.23 2.31 PbS Galena 2.57 2.54 2.51 2.49 2.18 1.6 1 0.53 0.05 FeS<sup>2</sup> Marcassite 4.15 4.39 4.63 4.72 4.87 4.42 3.73 3.13 2.56 As2S<sup>3</sup> Orpiment 0.92 0.97 1.04 1.04 0.58 −0.82 −2.6 −4.1 −5.52 FeS<sup>2</sup> Pyrite 4.84 5.06 5.28 5.36 5.45 4.96 4.23 3.6 3 SiO<sup>2</sup> Quartz (alpha) 1.43 1.31 1.21 1.16 0.95 0.73 0.54 0.4 0.28 SiO<sup>2</sup> Quartz (beta) 1.21 1.11 1.02 0.97 0.78 0.59 0.42 0.29 0.18 Na2(Fe3Fe2)Si8O22(OH)<sup>2</sup> Riebeckite −7.54 −6.95 −6.34 −6.05 −4.66 −3.17 −1.68 −0.44 0.8 Sb2S<sup>3</sup> Stibnite 3.25 2.76 2.29 2.08 1.25 0.7 0.02 −1.4 −3.01

**Table A1.** *Cont.*

#### **Appendix B**

The PhreeqC program code can be divided into several parts which signify different simulation iterations. Every part is ended with the line "End" to carry on to the next simulation. Each part is divided into several sections that carry out the different calculations for the modelling of fluids. Certain sections are not mandatory for the simulation as each of them serves different purposes. The first section is the "Database" in which we define the database to be use as a reference for the calculations. The next section is "Solution" in which the properties of the fluid are defined. Examples of the properties of the fluids which are added in this section are the temperature, pressure, and pH of the fluid. Furthermore, the composition of the fluid is also added in this section. The unit for the concentration of each component in the fluids is defined by the user. In the case of this study, the unit that was used is in mg/kgw (milligrams per kilogram of water).

The next section is the "Gas\_Phase". For this section, it functions similarly as the "Solution" section in which the properties of the gas are defined and the composition in percentage of the gas is declared. The properties of the gas can be modified for the different simulation iterations by using the line "Gas Phase Modify". This enables the modification of volume, pressure, and the concentration of each component of the gas. In the case of this study, this line is only used to modify the pressure of the gas.

The line "Reaction\_Temperature" is used to modify the temperature of the solution after the first simulation iteration. This section allows the modification of the initial temperature of the fluid to another designated temperature or to a range of temperature. The line "Equilibrium\_Phases" is used to model and simulate the precipitation of minerals in the brine. This line allows the user to obtain the number of moles of the minerals precipitated or dissolved at thermodynamic equilibrium. The user is required to provide the saturation index of each the corresponding minerals at the desired temperature.

The fluid can also be simulated from a kinetic aspect by using the lines "Rates" and "Kinetics". In the "Rates" section, the user is required to provide the rate equation for the given mineral as well as the kinetics constant of the rate equation. The "Kinetics" section uses the information from the "Rates" section to properly calculate the number of moles of minerals precipitated for a given duration. In this section, the user is required to provide information on the number of moles of minerals present initially in the fluid, the desired duration of the precipitation of the minerals, the number of intervals between the given duration and the type of Runge Kutta equation used. The Runge Kutta method is a family of implicit and explicit iterative methods that includes the Euler method. This method is used in temporal discretization for the approximate solution of differential equations.

The final command line used is the "Selected\_Output" command line. This section allows the user to output the certain parts of the results of the simulation into a text file or a csv file.

DATABASE C:\phreeqc\database\PHREEQC\_ThermoddemV1.10\_15Dec2020.dat SOLUTION 1

Units mg/L

Temperature 25.0 Pressure 1.0 pH 5.2 Cl 55942 Na 26412 Ca 7018 K 3357 S 64.4 Pb 0.113 Sr 422.415 Ba 25.55 Sb 0.0645 As 9.676 Fe 26.3 Si 179 Cu 0.001 GAS\_PHASE 1 -Pressure 1.0 -Fixed\_Pressure -Temperature 25 -Volume 1.03 CO2(g) 0.882 N2(g) 0.0908 CH4(g) 0.0239 END USE SOLUTION 1 USE GAS\_PHASE 1 GAS\_PHASE\_MODIFY 1 Pressure 19.7385 RATES ################ #arsenopyrite ################ -start 1 rem assuming Fe(III)>1e-4M is the switch point for Fe-promoted mechanism 10 R=8.31451 20 if TOT("Fe(3)")<=1e-4 then J=(10ˆ-1.52)\*EXP(-28200/(R\*TK))\*ACT("H+")ˆ0.8 30 if (parm(1)>0) then SA0=parm(1) else SA0=1 40 if (M0<=0) then SA=SA0 else SA=SA0\* (M/M0)ˆ0.67 70 SR\_mineral=SR("Arsenopyrite") 80 if (M<0) then goto 150

90 if (M=0 and SR\_mineral<1) then goto 150

100 rate=J\*SA\*(1-SR\_mineral) \*parm(2)

120 moles=rate\*Time

150 Save moles


################

56
