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Review

Geothermal Potential of Hot Dry Rock in South-East Baltic Basin Countries—A Review

by
Rafał Moska
1,*,
Krzysztof Labus
2,*,
Piotr Kasza
1 and
Agnieszka Moska
1
1
Oil and Gas Institute—National Research Institute, 25A Lubicz Str., 31-503 Krakow, Poland
2
Faculty of Mining, Safety Engineering and Industrial Automation, Silesian University of Technology, 2 Akademicka Str., 44-100 Gliwice, Poland
*
Authors to whom correspondence should be addressed.
Energies 2023, 16(4), 1662; https://doi.org/10.3390/en16041662
Submission received: 11 December 2022 / Revised: 29 January 2023 / Accepted: 3 February 2023 / Published: 7 February 2023
Browse Figures
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Abstract

:
The beginning of 2022 was a time of major changes in the perception of energy availability and security in European countries. The aggression of Russia against Ukraine destabilizing the European energy economy, combined with the withdrawal from fossil fuels which has been going on for a dozen years, has strengthened activities to introduce new energy technologies based on renewable energy sources. One of the most promising and stable sources of renewable energy is geothermal energy, in particular enhanced geothermal systems (EGS) in hot dry rocks (HDR). These deposits occur at a great depth in almost every place on Earth, but due to their low permeability, they require hydraulic fracturing, which results in high investment costs. This technology has been developed for several decades. The current situation in Europe seems to confirm that its rapid development to a level that guarantees stable and profitable operation is crucial. This is of particular importance in the case of former member states of the economic zone of the Council for Mutual Economic Assistance, which until recently were heavily dependent on Russian energy. This review, based on the latest available data, covers potential HDR prospective areas in the countries of the south-eastern Baltic basin, including Lithuania, Latvia, Estonia and Poland. It is specific to this region that the original heat flux density is lower as a result of the paleoclimatic effect associated with the youngest ice age; however, thermal conditions do not deviate too much compared to western Europe, especially Rhine Graben, and significantly exceed the conditions of Finland, where an EGS project is currently being operated. In Lithuania, the most prospective area is the ZNI intrusion (south of Klaipeda), characterized by a geothermal gradient of up to 40 °C/1000 m. In addition, the Precambrian batholith south of Liepāja (Latvia) and the Rapakivi granites in the north and center of Estonia are promising EGS sites. Poland has relatively the most explored EGS potential, in both volcanic, crystalline and sedimentary rocks, especially in the area of the Szczecin Trough, Gorzów Block, Moglino-Łódź Trough and Karkonosze Mountains. Unfortunately, local tectonic conditions, in particular the development of faults and natural fracture zones that affect the directions of fracture propagation during hydraulic fracturing, have not been sufficiently recognized, which is one of the main barriers to the expansion of EGS pilot projects in these countries. These issues present challenges for the researchers, especially in terms of petrophysical analyses of rocks in target zones and local stress conditions, which have a key impact on fracturing operations and profitability of the systems. Despite high investment costs on the one hand and a significant slowdown in the global economy in 2022 on the other, it remains hopeful that the authorities of individual countries will decide to accelerate research work, leading to the implementation of pilot projects of EGS installations, and that this technology will be further improved to ensure a stable clean energy supply.

1. Introduction

The beginning of the 2022 year was a time when the geopolitical and energy supply situation in Europe changed significantly. The aggression of Russia against Ukraine made the European countries aware that the directions of energy supply need to be immediately diversified, especially regarding natural gas, oil and steam coal. These mineral resources have become a strong economic weapon in the hands of Russia and are used to effectively blackmail energy-dependent European countries, expecting them to accept military activities near their borders. It seems clear that now is the last moment to intensify the trend away from fossil fuels towards renewable energy in the European energy market. The leaders of the energy transformation process are highly developed, “old European Union” countries such as Germany, the Benelux Union and France, which are gradually abandoning not only the use of fossil fuels, but also nuclear energy. This trend is also observed in Central and Eastern Europe; however, the lower level of energy industry development, resulting from the Soviet influence until the 1990s, resulted in a much less developed renewable energy sector in these countries. In this light, it seems natural to quickly search for and develop new energy solutions that, on the one hand, will be emission-free and stable over time and, on the other, will increase the energy independence of countries.
One of the most promising clean energy technologies is enhanced or engineered geothermal systems (EGS), which use the petrothermal energy of hot dry rocks (HDR), accessible at great depths, almost everywhere on Earth [1,2,3]. These technologies have been developed for years in many countries around the world and are still being improved, breaking technical barriers and increasing profitability [4]. This review presents the current state and prospects for the development of EGS in countries of the south-east Baltic Basin, including Lithuania, Latvia, Estonia and Poland. The focus was primarily on the analysis of areas where geothermal collectors could potentially be developed in HDR rock, and the conditions that should be met, similar to the installations operating in western Europe. This study was based on a wide set of data from the latest, currently available literature, which have not been compiled in this way so far. New general maps and summary tables of data were developed, in which potential prospective areas for EGS-type installations are indicated.

2. Current State of Energy Production and Consumption in the Baltic States

Until recently, the energy systems of the Baltic states were very different. The Estonian system was purely thermal; the Latvian system was based on water and thermal technologies; and the Lithuanian system was dominated by nuclear energy [5]. The changes and modernizations carried out in the energy industry in the last decade, as well as the abandonment of nuclear energy, have led to major changes in the production structure. According to Eurostat data for 2020, primary energy consumption in the Baltic states is not very high compared to other countries in the Euro area. Renewable sources have a higher share in energy production than the Euro area average, with the highest value in Latvia, which also has the highest participation of renewables in electricity production. The share of renewable energy for cooling and heating in the Baltic countries is more than twice the EU average, and the Baltic countries have a lower dependence on fossil fuels. Estonia, due to the extensive use of oil shale (phasing out by 2040 at the latest) and increasingly renewable fuels, has the lowest dependence on energy imports, with a share of only 10.5% (Figure 1).

3. Development of Enhanced Geothermal Systems in the World

The first HDR/EGS pilot project was launched in Fenton Hill, New Mexico, USA in 1979. It served as a testing ground for many years when significant discoveries were made, and the principles of engineered geothermal systems, including hydraulic stimulation, and operation methods were described, both from the point of view of geology, tectonics, field engineering and surface thermal energy recovery installations [4]. Although the EGS technology was very underdeveloped at this time, Fenton Hill became the first site to confirm that energy production from HDR on a commercial scale is possible and could be profitable in the future. This project resulted in increased interest in this technology and the launch of research projects around the world. In the 1970s and 1980s, research work on hydraulic fracturing technology in HDR continued in Rosemanowes, UK [7,8]. In the underground geothermal collector target zone, an unfavorable phenomenon of thermal short circuiting was observed, consisting of creating a preferred flow path in the rock, which resulted in cooling the formation and reducing the temperature of the liquid produced. The unfavorable effect of the proppant used to support fractures formed in the hydroshearing mechanism was also observed [4,9]. In the 2000s, a large commercial project was implemented in Habanero (Cooper Basin, Australia), where specific, favorable geological and tectonic conditions prevailed in the HDR target zone: temperature reaching 250 °C at a depth of less than 4000 m, a compressive stress regime causing the formation of horizontal fractures, and the presence of horizontal natural fractures facilitating fluid flow between vertical wells [10,11,12]. The operation of a 1 MW power plant was demonstrated there, but despite wide-ranging plans to launch a 40 MW power plant, the work was finally abandoned in 2016.
Most of the EGS installations function in intrusive igneous rock, i.e., granite, granodiorite; however, projects were also carried out in low-porosity, tight sedimentary and mixed rocks, e.g., Genesys, Groß Schönebeck in Germany [13,14] (Table 1). The working fluid that transports thermal energy to the surface was usually fresh water; however, liquids based on linear and cross-linked polymers were also used in sedimentary formations e.g., Groß Schönebeck [14]. They also experimented with the use of supercritical CO2 for these purposes [15]. In single cases, work was carried out on the functioning of EGS technology in rocks made available through a single well [16]. Currently, work on the development of EGS technology is being carried out in China and the United States, among others [17,18,19]. In Europe, research work continues on projects such as Soultz-sous-Forêts (France), Landau, Groß Schönebeck and ST1 Deep Heat (Finland) (Table 1) [14,20,21]. The opening of pre-existing or newly formed fractures as a result of pumping the treatment liquid into HDR is associated with the risk of seismic events caused by the activation of fault surfaces subjected to stresses in the formation. These low-magnitude local events were commonly reported in all ongoing projects: however, in two cases (Basel, Switzerland and Phoang, Republic of Korea) relatively high-magnitude events that caused damage to the surface were a reason to stop the projects [22,23,24,25,26]. The best example how the EGS can provide energy for local needs for a long period of time are projects carried out on the Rhine Graben (France, Germany). Small-scale power plants or geothermal heating plants, operating in Soultz-sous-Forêts, Landau and Rittershoffen, provide several MWe and several dozen MWth power [2].

4. Technical Aspects of Geothermal Collectors in Hot Dry Rocks

Extracting a large amount of thermal energy, which can be used not only in heating but also in the production of electricity, requires access to rocks deposited at great depths, where high temperature prevails. The high crustal stresses in these zones cause rocks to have different petrophysical properties, compared to the generally highly porous and permeable rocks in shallower conventional geothermal reservoirs. Due to the fact that hot dry rocks have very low porosity and permeability, it is necessary to create an artificially hydraulic connection between drilled wells, to allow the working fluid to circulate in a loop between two or more wells and transfer heat energy into surface. For this purpose, a hydraulic fracturing technology, which originated in the oil industry, is mainly used. It involves injecting a fracturing fluid at high pressure through the well into the low permeability rocks. Recalling principles of fracking in unconventional gas deposits, the pressure of the fluid creates new fractures in the rock or opens pre-existing ones, thus creating highly permeable arteries through which gas can enter into the well. The composition of the fracking fluid depends on the type of rock. In ductile formations, characterized by a low Young’s modulus and a high Poisson’s ratio, crosslinked or linear gel fluids are usually used, while for brittle ones (high Young’s modulus, and low Poisson’s ratio), slickwater or two-phase fluids (foams) are applied. Fracturing fluid usually contains a small amount of environmentally safe chemical additives depending on the specific needs. Proppant material is often added to the fluid, to keep the fractures open after pumping pressure ceases [34]. More viscous fluids are capable of transporting higher-weight proppants, while slickwater and foams are characterized by significantly lower transport properties.
Since the 1970s, hydraulic fracturing technology has also been used to create underground heat exchangers in EGS. The design and development of both HDR and gas wells are very similar, except for the higher depths of the EGS and therefore the stresses in the reservoir, the higher temperature and the presence of crystalline or igneous rocks at the target depth with high density and mechanical strength. The objective of fracturing, in HDR, is to hydraulically connect two or more wells in such a way that the flow of working fluid guarantees the economically optimal operation of the system, i.e., the temperature and volume of the fluid obtained from the production well are stabilized at an appropriate level. Excess flow may cause unfavorable short circuiting, which causes cooling of the formation, and consequently, lowering the temperature of the obtained fluid. The main types of underground heat exchanger systems are presented in Figure 2. The most commonly used are type A and B—EGS doublet or triplet—where cool water is pumped into the injection well and the heated water is received from production wells. On the other hand, the less expensive single-well circulation concepts (C, D) are rarely used because of lower efficiency.
In HDR-type formations, the dominant role in hydraulic conductivity of rocks is played by natural fractures, activated by shear forces acting in their planes (hydroshearing stimulation mechanism), whereas inducted fractures, opening in a direction perpendicular to the minimum horizontal stress (pure opening mechanism), are in the minority. Open fractures in hydroshearing are formed at relatively low pressures because their strength is lower than that of the surrounding rock. Pumping pressure that is too high can “overstimulate” HDR formation, causing irreversible rock movements and leading to a short circuit (e.g., Rosemanowes, Fjalbacka) [4]. The walls of the shear-activated fractures move relative to each other during treatment and shift slightly when pumping stops. This results in desired natural fracture conductivity and consequently no need for expensive proppants and more viscous fluids with higher transport capacities during operations (Figure 3). In most cases, when there are no water-sensitive clay minerals in the formation, fresh water without any additives is used. If clay minerals are presented, i.e., in sedimentary formations, clay control additives are recommended. If proppants are used (especially in tight sedimentary HDR formations), there is generally a small risk of the embedment phenomena (pressing the grains of proppant into the fracture surface) due to the high strength of the rocks [3]. Fracking operations in HDR formations are usually massive type, using large volumes of fresh water (more than 1000 m3 per stage or more than 10,000 m3 per entire operation). At the design of the fracturing procedure in HDR formations, it is important to know the preferred orientation of natural fractures, because even if new fractures develop, they will only reach a few meters in length and then intersect the natural ones. Henceforth, the conductivity will occur through the natural fractures. Therefore, the correct sequence of work during the development of an EGS reservoir is to drill a well, then perform hydraulic fracturing and record microseismic events that inform about the direction and intensity of energy propagation in the reservoir, and finally drill a second well passing through the seismic cloud in a suitable location [3].

5. Potential of Hot Dry Rock in Countries of the South-East Baltic Basin

5.1. Lithuania

Lithuania is located in the central-eastern part of the Baltic sedimentary basin in central-eastern Europe. Although the country does not have volcanic activity or hot springs, it does have several of geothermal reservoirs that have been explored for years [35]. The only geothermal heating plant in Lithuania, Klaipeda Geothermal Demonstration Plant, was built in 2000 with capacity 13.6 MW [36,37]. The plant uses a Devonian aquifer in the western part of the country, which supplies geothermal water from 1100 m at 38 °C. The plant operated intermittently over the years 2001–2017, due to financial and technical problems, and has been stopped now. There are also small-scale shallow geothermal pump systems in the country used not only for heating but also balneology and fish and shrimp farming [38,39]. The geothermal gradient in Lithuania varies from 12 °C/km in the east to 42 °C/km in the west [40]. In Lithuania, there are several prospective areas to obtain petrothermal energy from hot dry rocks. The crystalline basement of the country is a part of the East European Craton of the Early Precambrian consolidation. The basement, overlain by sedimentary cover with a thickness of several hundred meters to more than 2 km, is composed of two lithotectonic zones. The eastern part of the country is attributed to the East Lithuanian Domain composed mainly of mafic metavolcanic and felsic metasedimentary rocks, metamorphosed in amphibolite to granulite facies conditions and strongly migmatised [41]. A characteristic feature of this area is the occurrence of lenticular belts, interpreted as fold and thrust sheet structures. The western part of the country comprises the West Lithuanian Granulite, with felsic, and metapelitic gneisses, originating from psammites, pelites, felsic and intermediate volcanics. Intrusive Saxony-type charnockites, as well as garnet- and cordierite-bearing granitoids, form a major part of this zone. In the Mesoproterozoic, the Riga pluton intruded on the northern part of the West Lithuanian Granulite domain, forming a batholith up to 250 km in diameter, and creating the most prospective zone in terms of geothermal potential of Lithuania. This area is generally characterized by high heat flow and high heat production by the granitoids, due to the presence of radiogenic heat-producing elements: Th, U, and K. The most prospective geological formation is the Zemaiciu Naumiestis intrusion (ZNI) in southwest of the country (Figure 4). This pluton consists mainly of massive, biotite, medium- to coarse-grained monzogranites, along with a minority of biotite syenogranites and porphyric quartz monzodiorites, with fracture populations at a density of several to several dozen per 10 m. Exploration boreholes and modeled profiles of this structure revealed an intrusion thickness of about 4 km and its bottom at a depth of 6 km. The ZNI heat flow anomaly ranges from 83 to 100 mW/m2, while the geothermal gradient measured in deep wells averages between 35–40 °C depending on the area, which gives a modelling temperature of 150 °C at the target depth (4.5–5.0 km). According to preliminary assumptions [41], the geothermal potential of ZNI would probably be sufficient for a power plant of up to 5 MW, assuming single injection and two producing wells at a 500 m offset, an injection rate of approximately 150 L/s and a production rate of 75 L/s per well.

5.2. Latvia

Latvia, similar to Lithuania, is located in the south-eastern part of the Baltic sedimentary basin in central-eastern Europe. It borders Lithuania to the south, and geologically, lies in the Precambrian East European Craton. The Cambrian strata covering the crystalline bedrock are characterized by an anomalously high average temperature, ranging from 38 °C in the northwest to more than 120 °C in the southwest, which has been explained in terms of mantle processes and high heat generation of crustal lithologies [43]. The average geothermal gradient varies within the interval of 8 °C/1000 m–19 °C/1000 m in the northern and eastern part of the country, but the average geothermal gradient reaches 35 °C/1000 m in the central and southwestern parts. In the geological cross section of Latvia, three lithological structures of different geothermal gradients can be distinguished: 1. Terrigeneous Devonian and Carboniferous rocks, 2. Silurian and Ordovician Carboniferous clay deposits, and 3. Cambrian and Venda terrigeneous rocks [44]. In the Cambrian and Venda area, the gradients vary from 6 °C to 31 °C/1000 m, whereas the maximum is reached in the southwest of Latvia. The temperature reaches 38–62 °C at the depth of 1281–1714 m in the southwest of Liepaja city, and 33–55 °C at the depth of 1100–1436 m, near the Lithuanian border (Figure 5). The bedrock of the Cambrian and Venda area is Precambrian crystalline basement, similar to the Lithuanian area. Despite the lack of literature data on temperature at depths over 2000 m in this area, taking into account the thermal gradients, it can be assumed that the potential drilled borehole should be at least 4000 m deep, to reach temperature of not less than 140 °C at the target HDR zone. This temperature can meet the expected values for small-scale enhanced geothermal systems (about one to several MW power), such as projects like Groß Schönebeck (Germany) and Phoang (South Korea) [25,29]. However, EGS projects are also known where the target zone was characterized by lower temperature conditions, which in this case would allow the drilling of a shallower well [21,33].

5.3. Estonia

Estonia is the northernmost among the countries of the south-east Baltic basin. The Precambrian crystalline basement here is a continuation of the north-eastern Latvian basement, and a part of the Baltic-Belorussian granulite belt. The crystalline bedrock is overlain by sedimentary rocks (sandstones, dolomites, limestones, silts and clays) of the Ediacaran to Paleozoic age. The sedimentary cover is about 100 m thick in the north and more than 780 m thick in southern Estonia [46]. Granulites are deposited in most of the country area and are represented by intermediate and mafic metavolcanic rocks with minor felsic bodies. High-grade is usually well preserved, but particularly in western Estonia strong retrograde and metamorphism have occurred [47,48]. In the western part of Saaremaa island and in the Naissaar and Mariamaa zones (Figure 6) the gneissic rocks are intersected by several generations of intrusive rocks, of which the most important ones are the rapakivi plutons: Naissaar, Neeme, Märjamaa, Ereda and Riga [49,50]. This formation can be correlated with those in southern Finland [49,51]. Approximate temperatures in the bedrock at a depth of 500 m range between 11 °C and 16 °C, with the highest values in the north and north-east of the country [52], and are similar to the temperatures in southern Finland [53]. According to Jõeleht [54], the heat flow density, adjusted for paleoclimatic effects, varies between 28–68 mWm−2. In the upper basement of northern Estonia, the temperatures are somewhat higher, due to the thermal shielding effect of Cambrian clays, with low thermal conductivity. The highest values of heat flow density (68 mWm−2) were calculated for the Meriküla well, in north-east Estonia, and the Saviranna well located about 25 km NEE from Tallinn (48 mWm−2). This is attributed to the high heat production of the Neeme rapakivi granite, compared to gneiss, which is generally dominant in the area [55]. Granulites are characterized by a heat production of 0.94 µW·m−3, which is slightly higher compared to regions analyzed in Finland [56].
The ST1 Deep Heat geothermal project in Espoo, southern Finland, could serve as a reference point for potential EGS locations. The typical geothermal gradient in this area is relatively low (15–17 °C/km), which corresponds to a heat flow of 52 mW/m2 [58], therefore, the goal was to achieve 40 MW in a combined heat and power plant, fed with 115 °C water from hot dry rock at a depth of 6000 m, making this the deepest EGS in the world. By 2020, two wells were drilled, the main (OTN-3) and the observation well (OTN-2), of depths 6400 and 3300 m, respectively. The target zone is a Precambrian crystalline basement consisting of granite, pegmatite, gneiss as well as amphibolite-rocks of similar origin to the potential sites in Estonia. Due to a very low porosity of 0.5%, total flow is realized there through structures subjected to brittle deformation, that is, induced and natural fractures [21], similar to typical natural fractures in HDR granitic rocks in the world. Drill bit seismics and vertical seismic profiling in OTN-2 revealed a fissured structure with a dip of 44° ENE. The final trajectory of OTN-3 was continued into this structure at a distance of 1 km. In June and July 2018, hydraulic stimulation was performed in the interval of 5800–6100 m. During a 49-day period, fresh water was pumped with a capacity of 400–800 m3/min, allowing wellhead pressure to fluctuate in the range of 60–90 MPa [59]. In 2020 another stimulation was performed, pumping about 7000 m3 of fresh water through the 1300 m open hole. The stimulation zone is characterized by a reverse faulting regime (unlike the regional strike-slip regime in this area) [33], resulting in a preferred vertical direction of the propagation of the new induced fractures. Information about the mechanism of stimulation in OTN-2 well is not available in the literature; however, the world experiences in granitoids stimulation suggest that the mechanism could be hydroshearing-slip and dilatation in the plane of natural fractures. The discrepancy between the regional state of stress and the local actual state in the area of the OTN boreholes may be an important hint for the designers of the potential HDR stimulation technology in Estonia, drawing attention to the fracture propagation directions in the rock mass and thus the quality of connections between injector and producer wells. At the same time, it appears that the local reverse faulting regime could impede the use of a single well EGS arrangement as in the GeneSys Hannower project [13,16]. The horizontal propagation of the inducted fractures could restrict the flow between the stimulated rock horizons in the vertical well. It is worth mentioning that in the ST1 project, a system of so-called “Road Lights” was implemented, consisting in special procedures for responding to the revealed seismic events. Reaching an event magnitude above 2.1 means that the operation is aborted. More than 50,000 microevents below magnitude 1.9 have been recorded so far. Lessons learned from the failure of “cyclic soft stimulation” in Phoang [24,60] and events in the BS-1 well in Basel [22], as well as carrying out works at a great depth, could significantly reduce the risk of relatively strong seismic events felt on the surface of the earth, as a result of hydraulic stimulation operations. The ST1 project is currently being continued.

5.4. Poland

Poland is not included among the Baltic states in the political sense of the word; however, in terms of considerations on EGS, it seems to be a good comparison area, first due to its geographical location, second as the former member of Eastern Block belonged to the economic zone of the Council for Mutual Economic Assistance, which determined the slower energy development compared to the countries of western Europe, and finally due to the current rapid economic development, enabling the allocation of significant funds to renewable energy sources. Despite significant changes in the Polish energy sector in recent years, fossil fuels still occupy the largest percentage share in the energy balance, covering over 85% of the total demand (Figure 1). Due to the restrictive regulations of the European Union in the field of greenhouse gas emissions, the desire to reduce the consumption of fossil fuels in the overall energy balance and thus to reduce high emission fees, a significant increase in interest in renewable energy sources, including geothermal and especially unconventional EGS-type systems, is currently observed in Poland. The first steps in this direction were made in the first decade of the 21st century, when a preliminary scientific project was carried out to assess the thermal potential of geological structures for the purposes of EGS in Poland [31]. Three types of structures were considered: crystalline bedrock, volcanic rocks under the sedimentary cover and sedimentary basins. This project was based on analyses of the Earth’s crust temperature and heat flux density maps. Additionally, a series of field geophysical studies, laboratory tests and numerical simulations were carried out, which allowed several prospective areas to be identified [3,31].
The highest heat flux density of earth in Poland is observed in the western and central parts of the country [61]. The values correlate well with the crustal temperature at a depth of 2 km [62]. Geothermal gradients and heat flow anomalies are higher than the corresponding values in the most promising areas of Lithuania, Latvia and Estonia. In this area, the Permian trachyandesites of the Gorzów Block were deposited (Figure 7, Table 2). This formation is located at a depth below 4300 m, is characterized by large thickness, and contains gas bubbles, which ensures good prospects for fracturing. The location described shows a meaningful analogy to the Groß Schönebeck reservoir near Berlin, where the geothermal gradient is 3.5–4.0 °C/100 m, and the temperature at a depth of 4.3 km reaches approximately 150 °C [31,63].
Two potential EGS areas were identified in the Polish Lowlands: the Szczecin and the Mogilno-Łódź Troughs. The first one forms a fold element elongated along the NW–SE axis, covering the Zechstein–Mesozoic complex. This structure was influenced by the block tectonics of the basement as well as by halokinetic movements, which mobilized Zechstein salts [71]. In the Szczecin Trough, the EGS prospects concern the Permian or Carboniferous strata, located more than 5000 m below sea level, with temperatures exceeding 150 °C [3]. Potential petrogeothermal reservoirs in the Mogilno-Łódź Trough area comprise Lower Triassic, such as the Buntsandstein formation of claystone and siltstones with limestone and sandstone beds located about 5700 m. The compact sandstones of Lower Buntsandstein can be considered as having the EGS potential, due to their thicknesses locally exceeding 1500 m. The Middle Triassic limestones and marls (Muschelkalk), are significantly less thick, up to 300 m. Upper Triassic clastic sediments and evaporates (Keuper) and clayey sandstones (Rhaetian) form a complex of 2400 m thickness [63]. In addition, extensive massifs of igneous rocks situated in southern Poland are considered suitable for EGS technology. The most promising is the Karkonosze granitoid pluton in the Sudeten Mountains, with a geothermal gradient of approximately 40 °C/1000 m, and temperatures reaching 160 °C at 4.0 km below ground level [31]. An intense heat flux is typical for the Variscan fold belt and in the northern zone of the Lower Silesian internides. The Upper Paleozoic, Devonian and Carboniferous sediments form a continuous cover on the Precambrian and Lower Paleozoic complexes. In the Upper Silesian Coal Basin, the deeper part of the Carboniferous profile consists of carbonates and clastic, flysch, and molasse sediments. Carbonates are 200 to 1500 m thick, while Carboniferous clastic sediments, which form a potential petrogeothermal reservoir, reach more than 5000 m in thickness [72].

6. Discussion

For the analyzed region of Europe, the lowered primary heat flux density is specific as a result of the paleoclimatic effect associated with the youngest ice age, when the ice cover cooled the surficial layers of the earth’s crust for tens of thousands of years. This effect is observed at least to a depth of 1000 m [53]. However, the distribution of the geothermal gradient and the heat flow density in this region do not differ significantly from the values measured in Western Europe, especially in the Rhine Graben, and significantly exceed the thermal conditions parameters for Finland, where the EGS project is currently being developed (Table 1 and Table 2). HDR-type rocks, characterized by low porosity and permeability, are found in each of the countries analyzed. Prospective areas in Poland, i.e., the Gorzów Block, the Szczecin and Mogilno-Łódź Troughs and the Upper Silesian Block, are located in the eastern margin of the Western European Paleozoic Platform, where prospective EGS rocks are tight sedimentary rocks, volcanites and granitoids in the Karkonosze region. In the Baltic countries (Lithuania, Latvia and Estonia), the potential of HDR lies in the crystalline basement. The thickness of the overlying sedimentary formations decreases eastward, from just over 2000 m in the western part of Lithuania to less than 500 m in the Estonian Laeva (Figure 8).
Prospective hot dry granites and volcanites in Poland are deposited at a depth of about 4000 to 4300 m, which is a similar depth range as EGS installations in Groß Schönebeck and shallower than in Soultz-sous-Forêts. The temperature of the target zone, calculated from the geothermal gradient (35–40 °C/km), is comparable to the eastern Germany EGS sites (about 150 °C), and significantly exceeds the temperature in Espoo (Finland) (Table 1 and Table 2). The tight sedimentary formations of Poland in Mogilno-Łódź and Szczecin Troughs are located deeper (5000 m to over 6500 m) comparable to the crystalline basement target zone in Espoo and porphyric granites in Rhine Graben; however, they are warmer than the rock of Finland (much above 150 °C), and at the deepest parts reach predicted temperatures similar to those in Soultz-sous-Forêts (up to 195 °C).
The top of the intrusive igneous rocks, which are prospective formations in Lithuania, Latvia and Estonia, are deposited relatively shallow (from about 2500 m in Latvia, to about 500 m in Estonia, Figure 8); however, looking at the geothermal gradient and thermal heat flow data at these depths, temperature there is too low for the implementation of profitable collectors. Higher temperatures are reached at a depth of 4000–5000 m, where predicted values are comparable to temperatures in western Poland and eastern Germany (150 °C). This does not apply to Estonia, where a geothermal gradient in the range of less than 26–28 °C/km, determines a temperature in the range of 58–73 °C at a depth of 4000 m, which means conditions similar to those in Finland.
The predicted regional tectonic regime in prospective Polish locations is normal with a strike-slip component [3,76,77], which in general corresponds to the regime in western Europe. A separate issue is the local tectonic conditions, in particular the development of faults and natural fracture zones, affecting the directions of fracture propagation during the fracking operation. These conditions in the discussed countries, especially in Lithuania, Latvia and Estonia, have not been sufficiently recognized, creating one of the main barriers to the development of EGS pilot projects in these countries. Detailed recognition of local geological and tectonic conditions is possible after drilling the well to the appropriate expected target depth and performing well logging, as well as a set of laboratory measurements including rock mechanics tests on the core samples. So far, such deep exploratory wells have not been drilled. The analyses can be and usually are based on data from hydrocarbon or geothermal wells; however, they usually do not reach the target depths for EGS technology, and the extrapolations may be fraught with significant errors.
For Poland, a recent tectonic stress regime was designated, based on analyses of hydraulic fracturing of borehole walls [76,77]. However, these studies concern only selected areas that do not correspond well with the potential areas for EGS in this country. Furthermore, the methodology of this research was undermined by other authors [78], who pointed out that the results obtained do not justify drawing conclusions for the entire country, or even on a regional scale.
In order to deepen the knowledge about the geothermal conditions of south-east Baltic basin countries, closely related to regional and local geological/ thermal conditions, laboratory tests should be performed including measurements of core samples from the target zones. In particular, thermal and petrophysical parameters should be assessed, including thermal conductivity, rock fracture conductivity, rock strength (UCS, Young’s modulus, Poisson’s ratio in the reservoir conditions), and field testing should be conducted, i.e., minifracs to determine instantaneous shut-in pressure, corresponding to minimum principal stress. and analysis of tensile fractures to designate the rock anisotropy.
In the context of the development of EGS, the legal aspects of hydraulic fracturing operations in geothermal wells are also of key importance. In the individual analyzed countries, the legislation is very diverse: while in Poland and Estonia HF operations are allowed by law, in Lithuania there are heavy restrictions, and in Latvia the situation is not clearly standardized. Despite the uncertainties mentioned above, including technical difficulties, lack of research data and cost intensity, the development of EGS technology seems inevitable. Currently, Eastern European countries cannot afford to increase their dependence on energy from any other country, hence the development of unconventional technologies, including EGS, can greatly help in the pursuit of full energy independence.

7. Conclusions

The south-east Baltic Basin countries (Lithuania, Latvia, Estonia and Poland), as former members of the Eastern Bloc, are not as well developed in terms of power engineering as the countries of western Europe. In 2022, the conflict in Ukraine made it necessary to significantly intensify the diversification of energy sources and increase the use of renewable energy, including EGS in these countries. In this paper, a wide set of the latest available literature data on potential areas for EGS technology in these countries were compiled and compared with the respective data from western European EGS sites.
As a result of the paleoclimatic effect associated with the youngest ice age, the south-east Baltic basin area is characterized by a lowered heat flux density, the highest values of which were recorded in Poland and Lithuania (65–105 mW/m2). Heat flux density decreases towards the north-east, to 40–50 mW/m2 in Estonia. The distribution of the geothermal gradient shows a similar trend, from 35–45 °C/km in Poland and the western part of Lithuania (which is comparable to the values in the western European EGS sites-Groß Schönebeck and Soultz-sous-Forêts), to below 30 °C/km in Estonia. In Poland, the prospective HDR-type formations of the Gorzów and Silesia Blocks, Karkonosze Mountains, Mogilno-Łódź and Szczecin Troughs are associated to volcanic, intrusive and tight sedimentary rocks, petrophysically similar to HDR in western Europe.
In Lithuania, Latvia and Estonia, HDR potential is shown by the crystalline basement, located in the west and south of each of these countries. Local tectonic conditions, in particular the development of fault and natural fracture zones in relation to the necessary hydraulic fracturing operations have not been sufficiently recognized, creating significant barriers in the development of EGS pilot projects in these countries. In addition, deep wells reaching potential target zones have not yet been drilled, especially in Lithuania, Latvia and Estonia, hence the detailed analyses require extrapolation of data from shallower zones. These issues present challenges for the researchers, especially in terms of detailed thermal and petrophysical analyses of target zone rocks and local stress state conditions, having a key impact on fracturing operations and the profitability of the system.
Fracking operations in the Baltic states are not completely banned as in western Europe. It seems that the main factor holding back the pilot projects is the relatively high costs of this type of investment, which must be covered by the investor, which in most cases is the State Treasury. We remain hopeful that, in light of the political and economic situation of Europe in 2022, the authorities of individual countries will decide to start pilot work in this type of installation and EGS will be able to be further refined to ensure stable supplies of clean energy for future years.

Author Contributions

Conceptualization, literature review, investigation, manuscript writing, visualization, overall supervision, R.M.; investigation, manuscript writing, manuscript review, K.L.; supervision, manuscript review, P.K.; manuscript writing, visualization, A.M. All authors have read and agreed to the published version of the manuscript.

Funding

This paper is based on research carried out at the Oil and Gas Institute-National Research Institute (Poland), and the Silesian University of Technology, as part of the implementation doctorate program RJO/SDW/005-33, founded by Ministry of Education and Science of Poland.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Dependence on energy imports and share of energy from renewable sources and fossil fuels in Baltic countries, Poland and the Euro area in 2020 (after: [6]). (The presented data also include Poland, which, like the Baltic countries, joined the EU in 2004, but previously belonged to the Eastern Bloc and thus to the economic zone of the Council for Mutual Economic Assistance).
Figure 1. Dependence on energy imports and share of energy from renewable sources and fossil fuels in Baltic countries, Poland and the Euro area in 2020 (after: [6]). (The presented data also include Poland, which, like the Baltic countries, joined the EU in 2004, but previously belonged to the Eastern Bloc and thus to the economic zone of the Council for Mutual Economic Assistance).
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Figure 2. The most common types of underground heat exchanger systems. SV—principal vertical stress; SH, Sh—maximum and minimum horizontal stress, respectively. (A)—doublet in extensional stress regime, (B)—doublet in lateral compressive regime, (C)—single directional well for injection and production cycling in extensional stress regime, (D)—single directional well for continuous injection and production in extensional stress regime.
Figure 2. The most common types of underground heat exchanger systems. SV—principal vertical stress; SH, Sh—maximum and minimum horizontal stress, respectively. (A)—doublet in extensional stress regime, (B)—doublet in lateral compressive regime, (C)—single directional well for injection and production cycling in extensional stress regime, (D)—single directional well for continuous injection and production in extensional stress regime.
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Figure 3. Basic principles of hydroshearing. Blue arrows depict fluid flow, black—principal stress directions, red—fracture opening forces.
Figure 3. Basic principles of hydroshearing. Blue arrows depict fluid flow, black—principal stress directions, red—fracture opening forces.
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Figure 4. Generalized map of the Lithuanian crystalline basement in the background of geothermal gradient distribution (based on [40,42]).
Figure 4. Generalized map of the Lithuanian crystalline basement in the background of geothermal gradient distribution (based on [40,42]).
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Energies 16 01662 g005 550
Figure 5. Average temperature in Cambrian underground water horizon, on the background of the geological setting of Latvia (based on [44,45]).
Figure 5. Average temperature in Cambrian underground water horizon, on the background of the geological setting of Latvia (based on [44,45]).
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Energies 16 01662 g006 550
Figure 6. Elements of the Estonian crystalline basement, heat flow density and estimated temperature (based on [49,57]).
Figure 6. Elements of the Estonian crystalline basement, heat flow density and estimated temperature (based on [49,57]).
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Energies 16 01662 g007 550
Figure 7. Prospective areas for conventional and unconventional geothermal energy utilization and Earth’s heat flow density in Poland against the background of simplified geologic setting (based on [31,61,64]).
Figure 7. Prospective areas for conventional and unconventional geothermal energy utilization and Earth’s heat flow density in Poland against the background of simplified geologic setting (based on [31,61,64]).
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Energies 16 01662 g008 550
Figure 8. Simplified lithostratigraphic profiles of selected areas in Baltic countries. Based on [31,63,73,74,75].
Figure 8. Simplified lithostratigraphic profiles of selected areas in Baltic countries. Based on [31,63,73,74,75].
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Table
Table 1. Selected geological and technological data on European EGS sites.
Table 1. Selected geological and technological data on European EGS sites.
Country
(EGS Site), Activities, Duration		Depth, mTemperature, °C
(Geothermal Gradient, °C/km)		Geological ConditionsTarget Zone Stimulation MechanismReferences
United Kingdom
(Rosemanowes)
1977–1985		about 2000below 100Granite Carnmenellis batolith, Permian. Strike-slip tectonic regimehydraulic, hydroshearing[4]
France (Soultz-sous-Forêts)
1990-present		5200about 200
(about 38)		Porphyric granites
MFK, two-mica
granites, Carboniferous,
Normal with strike-slip component tectonic regime		hydraulic, hydroshearing[20,27,28]
Germany (Groß
Schönebeck)
2006-present		about 4100149
(about 36)		Clastic and volcanic rocks (rhyolite, andeside) Rotliegend, Permian.
Normal-strike-slip tectonic regime		hydraulic[29,30,31]
Germany
(Landau)
2005-present		2200about 160
(about 72)		Porphyric granites
MFK, two-mica
granites, Carboniferous,
Normal with strike-slip component tectonic regime		hydraulic, hydroshearing[27,28,32]
Finland (ST1 Espoo)
2014-present		6400100–110
(15–17)		Low porosity, natural fractured granite, pegmatite, gneiss and amphibolite. Precambrian
Reverse faulting regime.		hydraulic[21,33]
Table
Table 2. Selected parameters of the potential EGS sites in the countries of the south-east Baltic basin.
Table 2. Selected parameters of the potential EGS sites in the countries of the south-east Baltic basin.
Potential EGS SiteGeothermal Gradient,
°C/1000 m		Heat Flow Anomaly, mW/m2Target Depth,
m, Temperature,
°C		Lithology, StratigraphyReferences
LithuaniaZemaiciu Naumiestis intrusion (ZNI), south of Klaipeda35–4083–1004500–5000,
150		syenogranite, porphyric quartz monzodiorite[41,65]
LatviaPrecambrian batholith, south of Liepaliaup to 35No databelow 1740,
140 at 4000 *		intrusive igneous rocks[44,66]
Precambrian batolith, center and south of the countryup to 35No databelow 1500,
140 at 4000 *		intrusive igneous rocks[44]
EstoniaNothern and center of the country,
Precambrian basement		26–28 in sedimentary cover;
14 in the Precambrian		40–50
42		4000,
58–73 ** depending on the site		rapakivi granites[49,67,68]
PolandKarkonosze Mountains,
SW of the country		44654000,
165		intrusive igneous rocks,
Carboniferous		[31,61]
Gorzów Block
NW of the country		35–401054300,
160		trachyandesites,
Permian		[31,61]
Mogilno-Łódź Trough
country center		up to 34 in the best target area75–905000–6500,
165–195		sandstones,
buntsandstein, lower Triassic		[31,61,63]
Szczecin Trough
NW of the country		up to 2785–100>5000,
above 150		sediments, Carboniferous,
Permian		[61,63,69]
Upper Silesian Block
S of the country		up to 4580–95>5000,
170		clastic deposits of Carboniferous[61,63,70]
* Calculated taking into account the given thermal gradient, ** Calculated based on data from [49].
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Moska, R.; Labus, K.; Kasza, P.; Moska, A. Geothermal Potential of Hot Dry Rock in South-East Baltic Basin Countries—A Review. Energies 2023, 16, 1662. https://doi.org/10.3390/en16041662

AMA Style

Moska R, Labus K, Kasza P, Moska A. Geothermal Potential of Hot Dry Rock in South-East Baltic Basin Countries—A Review. Energies. 2023; 16(4):1662. https://doi.org/10.3390/en16041662

Chicago/Turabian Style

Moska, Rafał, Krzysztof Labus, Piotr Kasza, and Agnieszka Moska. 2023. "Geothermal Potential of Hot Dry Rock in South-East Baltic Basin Countries—A Review" Energies 16, no. 4: 1662. https://doi.org/10.3390/en16041662

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