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Article

Increasing the Thermal Resistance of Water-Based Mud for Drilling Geothermal Wells

by
Sławomir Błaż
*,
Grzegorz Zima
,
Bartłomiej Jasiński
and
Marcin Kremieniewski
*
Oil and Gas Institute—National Research Institute, 31-503 Krakow, Poland
*
Authors to whom correspondence should be addressed.
Energies 2024, 17(18), 4537; https://doi.org/10.3390/en17184537
Submission received: 16 July 2024 / Revised: 23 August 2024 / Accepted: 4 September 2024 / Published: 10 September 2024
(This article belongs to the Section H: Geo-Energy)
Browse Figures
Versions Notes

Abstract

:
Energy demand and growing environmental concerns have fueled increased interest in geothermal drilling in recent decades. The high temperature and pressure in the boreholes present significant challenges to drilling, particularly in terms of the selection of suitable drilling mud, cement slurry, and drilling equipment. Drilling mud is regarded as one of the primary factors that affect the cost and success of geothermal drilling. This paper presents experimental studies aimed at assessing the thermal stability of drilling muds for geothermal drilling. Research on the antidegradation of polymers contained in drilling muds is presented. The thermal stability of drilling fluids was evaluated on the basis of changes in rheological and filtration parameters under the influence of a temperature of 160 °C. Attempts were made to increase the thermal resistance of drilling fluids by using antioxidants and glycol compounds. The effectiveness of increasing the thermal resistance of muds by adding synthetic polymers, nanomaterials, and graphite was tested. A new way of increasing the thermal resistance of drilling muds by using fatty amine compounds in combination with the amine agent ‘TEA’ was proposed. Tests showed that the addition of polyglycol and the antioxidant agent sodium ascorbate to the mud did not protect the polymers from decomposition at 160 °C. There was no effect of increasing the thermal conductivity on improving the thermal resistance of the scrubber. Based on the analysis of results from laboratory tests, a composition of a water-based drilling mud without bentonite was developed for drilling geothermal wells. The developed drilling mud is characterized by thermal resistance up to 160 °C, stable rheological parameters, low filtration, and appropriate thermal conductivity characteristics.

1. Introduction

The drilling of boreholes to make thermal water available places increasing demands on the properties of the drilling muds used. The temperature in the borehole and the formations being drilled have significant impacts on the stability of the hole and the density and rheology of the drilling mud. The drilling fluids used play an important role, so optimizing their properties is crucial in the process of drilling a hole. The rheological and structural properties of the drilling fluid determine, among other things, the effectiveness of cleaning the hole during drilling, transporting drill cuttings to the surface, and keeping drill cuttings in a suspended state during lack of circulation [1,2,3,4,5].
During the drilling process of a geothermal well, circulation of drilling fluid serves to cool the bit, transport drill cuttings to the surface, and control subsurface pressures. This circulation process causes an undesirable cooling effect on the temperature of the formation surrounding the borehole. Thus, drilling mud becomes an important tool for predicting circulating and surrounding formation temperatures during the geothermal well drilling process [6,7,8,9].
The high pressures and temperatures in geothermal boreholes require special drilling muds with high thermal stability. Oil-based muds have significant thermal resistance, but they are used less and less due to costs and environmental issues. Therefore, relatively cheaper and more environmentally friendly water-based drilling muds (WBM) are preferred. Water-based drilling muds, in turn, often do not meet the requirements for drilling in difficult geological conditions, particularly those connected with high pressure and temperature (HPHT). Such conditions affect the efficiency of water-based drilling muds, causing adverse changes in their properties, which may increase the risk of drilling failures [10,11,12,13,14].
Thermal stability is an important factor to consider when selecting the composition of the mud for geothermal drilling. Water-based muds usually start to break down at temperatures over 120 °C, which reduces their usefulness in HTHP conditions, except in situations that involve environmental concerns [15].
At high temperatures, the properties of drilling muds can change rapidly, leading to reduced rheological parameters and increased mud filtration. Consequently, deterioration of the drilling fluid’s rheological properties can result in poor hole cleaning performance, formation damage, and other drilling problems that may occur under various conditions. Thus, drilling fluid stability is crucial to the success of drilling operations. Therefore, it is important to develop and use additives for muds that are resistant to high temperatures [8,11,12,13].
Drilling muds for geothermal drilling usually contain natural polymers, such as xanthan gum (XCD). This water-based biopolymer is produced by fermentation using the Xanthomonas campestris bacterial strain. Water-based XCD solutions have a high yield point and structural strength combined with a fairly low plastic viscosity. They also have pseudoplastic properties, i.e., small viscosity at high shear rates and high viscosities at small shear rates [2,14,15,16,17].
However, such polymers degrade more quickly at high temperatures, which reduces the rheological parameters of the muds [18,19].
Polymer degradation is an inevitable process that includes mechanical and chemical, mechanical, oxidative, biological, and thermal degradation, which causes the rupture of the main and side chains of the polymer molecular chain and the formation of small molecules that affect the physical and chemical properties of the polymer. Polymers used as additives in drilling fluids undergo mechanical and chemical degradation during the drilling process. Mechanical degradation is primarily caused by the mechanical energy generated by the activity of the drilling tool in the well. Mechanical cutting disrupts the structure of the polymer molecular chains, which is an irreversible change process [19,20]. Natural polymers have ether–oxygen bonds with low bond energy and ester bonds sensitive to water and heat. Exposure to temperature causes a significant decrease in the molecular weight of the biopolymer, which reduces the viscosity of the solution. The decomposition of the XCD polymer matrix is accompanied by the removal of pyruvate and acetate groups, which are most susceptible to hydrolysis. Oxidation and reduction reactions by free radicals are probable causes of biopolymer degradation in drilling fluids and the associated loss of viscosity. In the absence of dissolved oxygen and depending on pH, acid-catalyzed hydrolysis reactions and base-catalyzed reactions have a significant impact on biopolymer degradation at high temperatures.
The degradation mechanism of synthetic polymers includes the elimination of side groups, random cleavage, and depolymerization, which leads to the breakdown of the polymer backbone and the formation of small molecules [21,22,23].
Knowledge of biopolymer degradation mechanisms at high temperatures is, therefore, required for the stabilization of biopolymers in drilling muds. These mechanisms can be prevented, neutralized, or delayed in order to achieve thermal stability in the muds. They can be prevented using mud additives designed to protect the biopolymers. Extensive research has been carried out to prevent the breakdown of biopolymers in drilling muds at high temperatures, e.g., using formic salts, antioxidants, and polyglycol [13,19,20,24,25]. Formic salts are used during high-temperature borehole drilling to stabilize biopolymers because they function as antioxidants. They include sodium formate, potassium formate, and cesium formate. These agents have physical and chemical properties that qualify them for use in difficult borehole conditions for borehole drilling, deposit opening, and well workover. An essential aspect of drilling fluids is their high water solubility, which is required to produce heavy alkaline solutions with densities that cannot be achieved by some inorganic salts, as well as compatibility with the formation waters and protective colloids commonly used in the mud, particularly due to their positive contribution to thermal resistance and compliance with environmental requirements [1,9,22,26,27].
Formic salts stabilize biopolymers at high temperatures, increasing their breakdown (transition) temperature, and ensure free-radical properties that delay oxidation processes. At the breakdown transition temperature, the biopolymer undergoes a conformational change that results in the loss of viscosity and an increase in the rate of hydrolytic degradation by two orders of magnitude [6,28]. The second method of biopolymer stabilization in drilling muds is the use of polyglycols. Biopolymers and polyglycols in solutions combine by intramolecular hydrogen bonds and hydrophobic interactions. These associations and interactions stabilize biopolymers at high temperatures [8,18,20,24].
There have been attempts to increase the thermal resistance of biopolymers using buffering and antioxidant agents. This method is not very effective at high temperatures in case of changes in molecular conformation in the biopolymer [21].
Synthetic polymers show much better thermal stability and resistance to salt contamination in comparison with natural polymers. Some synthetic polymers are better than modified natural polymers in terms of their performance in HPHT conditions and their salt resistance [20,28]. Synthetic polymers have a macromolecular structure. Depending on the shape of the molecule, polymer compounds can be divided into linear, branched, and cross-linked. Some synthetic polymers are useful additives to water-based muds, improving the rheological parameters, filtration, and salt resistance. Mixtures of multiple polymers (copolymers) can have a bigger impact on the properties of water-based muds. Such copolymers depend on specific monomer combinations. Although monomers are added directly to muds, the connection between the structure and properties of monomers defines the specific properties of the copolymer. Synthetic polymers are most effective in water-based muds (WBM) with the addition of bentonite (clay, montmorillonite). The interaction of bentonite particles with synthetic polymers contributes to the formation of a stable colloidal dispersion, necessary to ensure the desired filtration and rheological properties at high temperatures [18,19,29,30].
One of the polymers that improve the stability of drilling mud in adverse high-temperature and high-salinity conditions is a polymer synthesized based on a monomer of the 2-acrylamido-2-methylpropane-sulphate (AMPS). It has been proven that AMPS shows high resistance to electrolyte contamination, and it can also stabilize viscosity while reducing filtration. However, the disadvantages of synthetic polymers are their high price, limited quantity, and negative impact on the permeability of productive horizons [1,24].
Current ongoing research focuses on the development of new water-based muds (WBM) for drilling deep geothermal wells using nanomaterials.
Depending on the number of nano-additives, the drilling muds can be divided into simple nanofluids and advanced nanofluids. Muds that contain a single type of nanomolecule are referred to as simple nanofluids, and muds with more than one type of nanomolecule are defined as advanced nanofluids [31,32,33,34]. Additives that are now finding increasing use in drilling operations include carbon nanoparticles such as nanotubes and graphene. The literature indicates that the main advantages of fluids with nanomaterials include reduced damage to the deposit, uninterrupted drilling, reduced filtration of drilling mud, improved thermal resistance, and effective elimination of lost circulation [35,36,37,38,39,40,41,42]. Increasing thermal resistance of drilling muds by adding nanomaterials is based on the use of unique properties of nanoparticles, which can significantly improve thermal and mechanical parameters of the fluid. Adding nanomaterials to drilling fluid significantly increases its thermal resistance by improving thermal conductivity and structural stability, reducing thermal losses, and modifying molecular interactions. Thanks to these properties, nanomaterials are becoming a key element in the development of more effective and thermally resistant fluids used for geothermal drilling [32,33,34,35,36,37,43].
To date, there has been extensive research into ways of improving the thermal resistance of drilling mud. There have been attempts to improve the thermal resistance of drilling mud using antioxidants such as sodium ascorbate, potassium formate, and glycol compounds. There have been many attempts to increase the thermal resistance of drilling mud by adding synthetic polymers or nanomaterials. In light of the above, the authors of this paper verified the impact of the above-mentioned agents and presented a new way of improving the thermal resistance of drilling muds. The innovation of this method is obtaining appropriate thermal resistance of the water-based mud by using fatty amine compounds in combination with the amine compound “TEA”, which, based on available data, has not been tested in this context so far. The research presented in the article allowed for the development of the composition of water-dispersed drilling mud with resistance up to 160 °C without the addition of a clay phase and significant amounts of formate salts and a limited amount of specialized synthetic polymers. The new method of increasing the thermal resistance of water-based muds presented in the article sets a new direction for research that will be continued to further increase the scope of applicability of water-dispersion mud to temperatures up to 200 °C.

2. Materials and Methods

2.1. Materials

Laboratory tests are performed using natural and synthetic polymers used in drilling muds and agents designed to improve the thermal resistance of the polymers as well as agents used to increase thermal conductivity:
  • Biopolymer (xanthan), obtained in the process of fermentation of polysaccharides with microorganisms (Xanthomonas campestris bacteria). It is used to increase the viscosity of drilling muds of various degrees of salinity and to hold the weighting agents in suspension.
  • Polyanionic cellulose, referred to as highly purified carboxymethylcellulose, is used mainly to regulate the filtration and viscosity of drilling muds, whereas it can be used for the chemical treatment of all types of water-based muds. The tests were performed using two versions of this agent: low-viscosity (LV) and regular (R).
  • Starch HHT—starch with improved thermal resistance.
  • Carbonate blocker (CaCO3), a weighting agent obtained as a result of the mechanical processing of marble. Due to its solubility in acids, it is used primarily for weighting drilling muds and workover fluids used in opening the deposit and in workover works in boreholes
  • Carbon nanotubes—an agent used to increase the thermal resistance of the polymers and improve thermal conductivity.
  • PSP109—sulphonated amide polymer.
  • CR480—binding retarder and an agent used to reduce the filtration of cement slurry.
  • SR5—ethoxylated fatty amine.
  • TEA—amine compound.
  • Cocamide DEA.
  • Graphite—an agent used to improve thermal conductivity.
  • Ferohem—weighting material.
  • PEG8000—polyethylene glycol with a molecular weight of 8000.
  • HCOOK, KCl.

2.2. Preparation of Invert Emulsion Drilling Mud

The water-based mud (Table 1) for the laboratory tests was prepared as follows. The XCD biopolymer, rheological regulators PAC R and PAC L, and filtration-reducing starch HHT were added successively to 1 dm3 of mains water. Then, after the polymers dissolved, the KCl ionic inhibitor and M25 carbonate blocker were added to the solution. Finally, after determining the properties of the prepared mud, further agents were added to modify its thermal resistance.

2.3. Experimental Procedures

The following procedure is conducted to determine the thermal resistance of the polymer in drilling muds to regulate rheological parameters and assess the thermal resistance of the mud (Scheme 1). Attempts to increase the thermal resistance of the prepared mud are conducted by selecting suitable chemicals. The first attempts used glycol compounds with different molecular weights specifically PEG 8000, R 2100, and P 600, and agents designed to improve thermal conductivity, such as carbon nanotubes and graphite. The authors tested the impact of antioxidants such as sodium ascorbate or potassium formate. There were also attempts using ethoxylated fatty amine in combination with the “TEA” amine compound.

2.3.1. Testing Procedures

Laboratory tests of drilling muds such as density, rheological parameters, and filtration were determined in accordance with the standard PN-EN ISO 10414-1 [44,45]. Petroleum and natural gas industries—Field testing of drilling fluids—Part 1: Part 1: Water-based fluids (ISO 10414-1:2012)

2.3.2. Density Measurement

The density of the drilling mud was measured using a mud-type balance Baroid at 20 °C and atmospheric pressure. This scale consists of an arm with a mud container on one side and a calibration container on the other. The balance arm is equipped with a weight moved along the scale and a level, enabling precise weighing [44,45].

2.3.3. Measurements of Rheological Parameters

The rheological parameter of drilling mud is measured at 20 °C with the OFITE 900 viscometer. The viscometer is used to directly determine the relationship between the shear rate of the fluid and the existing shear stress to calculate the plastic viscosity, apparent viscosity, yield point, and gel strength [29,30].
Plastic viscosity (PV) = reading at 600 rpm − reading at 300 rpm [mPas]
Yield point (YP) = reading at 300 rpm − plastic viscosity (PV) [lb/100 ft2].
The measurement of rheological parameters at higher temperatures was performed using the Ofite 77 high-temperature high-pressure viscometer. The muds were tested both in a heating and cooling cycle in a temperature range from 20 °C to 160 °C at shear rates from 5.1 s−1 to 1020 s−1.

2.3.4. Filtration Measurement

Filtration tests of drilling muds at a temperature of 20 °C were determined using a low-pressure filter press at a pressure of 0.7 MPa for a period of 30 min. However, at temperatures of 120 °C, 140 °C, and 160 °C, filtration was measured using a static HTHP filter press with a pressure of 3.4 MPa [45].

2.3.5. Measurements of the Thermal Conductivity of Drilling Muds

The thermal conductivity of drilling mud was determined using an analyzer of thermal properties, ISOMET 2114 (APPLIED PRECISION, Bratislava, Slovakia), used to directly measure thermal conductivity using the so-called “hot wire” method. It is one of the simplest methods, and it can be used both in a laboratory and in field conditions during borehole drilling.
The device can determine the thermal conductivity, thermal diffusivity, and volumetric heat capacity. ISOMET 2114 has two types of measurement probes: a needle probe and surface probe. The device uses the dynamic measurement method to shorten the time of measurement in comparison with steady-state measurement methods. The measurement is based on an analysis of the temperature response of the analyzed material to heat flow impulses. Heat flow is excited by the electrical heating of a resistor heater placed in the probe, which is in direct thermal contact with the tested specimen. The thermal conductivity and volumetric heat capacity are assessed based on periodically sampled temperature records as a function of time, provided that heat propagation occurs in an unlimited medium.

3. Results and Discussion

The water-based mud used for geothermal drilling contains polymers in its composition that are sensitive to high temperatures. Most natural polymers can withstand temperatures up to 120 °C. Above this temperature, processes occur that lead to structural changes in the polymers, including the breaking of chemical bonds, which in turn can lead to the loss of polymer-specific properties. High temperature can initiate various types of reactions, such as thermal degradation and oxidation. In addition, it can also increase the rate of chemical reactions occurring within the polymer, which can lead to the breakdown of the polymer chain into lower-molecular-weight components.
Therefore, drilling geothermal wells requires muds with high thermal resistance. Increasing the thermal resistance of muds for geothermal drilling is only possible by using specially modified synthetic polymers in their composition or by using a mineral phase (bentonite muds). In other cases, it is possible to retard polymer decomposition by using antioxidant compounds, nanomaterials, and inhibitors that retard polymer decomposition.
The aim of the presented laboratory tests is to develop the composition of the water-based mud with stable rheological and structural properties and filtration at temperatures up to 160 °C. Overcoming this challenge is crucial for the development of effective clay-free water-based muds. Consequently, the main objective of this study is to determine the decomposition temperature of the polymers and the use of effective agents that increase the decomposition temperature of the polymer.

Tests of the Improvement of the Thermal Resistance of Water-Based Drilling Mud

Successful drilling of a geothermal well depends largely on the rheological properties of the mud, i.e., plastic viscosity, apparent viscosity, and yield point. Therefore, the appropriate selection of drilling materials ensuring the maintenance of appropriate rheological parameters at high temperatures is very important. Laboratory tests began with determining the initial composition of the water-based mud, determining its basic properties and identifying the temperature threshold at which polymers contained in the drilling mud are damaged. The results of testing the impact of a temperature of 160 °C on the properties of the water-based mud are graphically presented in Figure 1 and Figure 2. Based on the tests performed, it was established that the thermal resistance of the mud was 130 °C. Above this temperature, processes occur in the mud leading to a decrease in rheological and structural parameters and, in the final stage, to the complete decomposition of polymers. This was proven by the measured rheological parameters. As a result of the temperature of 160 °C, the plastic viscosity of the mud was reduced by 92.6% and the yield strength by 97.1% Pa. Thermal degradation of polymers also increased the filtration of the mud from 8 to 54 cm3/30 min (Table 2, Figure 1 and Figure 2).
This thermal degradation makes a mud with these rheological parameters unsuitable for high-temperature geothermal applications. Therefore, the article presents a series of laboratory tests aimed at modifying its composition to improve its thermal resistance.
Various attempts are made to increase the thermal resistance of water-based muds. One way to increase the scope of applicability of water-based mud is to modify its composition by adding polyglycol compounds. Polyglycols in drilling mud connect through intermolecular hydrogen bonds and hydrophobic interactions with biopolymers. These associations and interactions stabilize biopolymers at high temperatures. Our research did not confirm this thesis. The presented test results show that polyglycols have a small impact on improving the temperature resistance of the drilling mud. As a result of heating the mud with the addition of PEG 8000 at a temperature of 160 °C, the plastic viscosity decreased by 86.9% and the limit by 93.2% (Figure 1 and Figure 2). The polyglycol compound PEG8000 did not protect the water-dispersion mud against thermal decomposition.
Another way to protect the water-based mud against the destructive effects of temperature is the possibility of introducing antioxidant agents into the mud. Oxidation and reduction reactions involving free radicals are the probable cause of the degradation of biopolymers in drilling muds and the associated loss of viscosity. Sodium ascorbate was used as an antioxidant in the studies. The addition of 1% sodium ascorbate to the mud did not protect the polymers against thermal decomposition. The mud with the addition of 1% sodium ascorbate retained only 8.7% of the plastic viscosity and 4.8% of the yield point of the base mud (Figure 1 and Figure 2).
Formate salts are also used to protect biopolymers from high temperatures. One of the most important characteristics of formate salts used successfully in drilling mud technology is their high water solubility and low environmental impact. Formate salts stabilize biopolymers at high temperatures by increasing their decomposition (transition) temperature and providing radical properties that retard oxidation processes. At the decomposition transition temperature, the biopolymer undergoes a conformational change that causes a loss of viscosity and an increase in the rate of hydrolytic degradation by two orders of magnitude. The addition of formate salts to drilling fluids increases their thermal resistance by stabilizing polymers, preventing salt precipitation, and stabilizing the pH value. In the present study, (Figure 1 and Figure 2), the antioxidant effect of potassium formate HCOOK was checked. Potassium formate was introduced into the water-based mud in an amount of 5%. Potassium formate, depending on its concentration in the solution, can change the conformation of the polymer and thus in-crease its temperature resistance. The use of 5% potassium formate in the mud did not in-crease its temperature resistance. Heating the water-based mud with 5% HCOOK at 160 °C resulted in an 87.5% decrease in the plastic viscosity of the mud and a 97% decrease in yield stress (Figure 1 and Figure 2). An increase in the thermal resistance of the water-based mud was achieved only by adding potassium formate by about 46%, i.e., at a solution density of 1.2 g/cm3. At a lower addition of potassium formate, no effect of increasing the thermal resistance of the mud was observed. On the other hand, at higher concentrations of HCOOK, flocculation processes can occur in the water-based mud at 160 °C.
Synthetic polymers are characterized by much greater thermal resistance than natural or modified polymers. They are also more resistant to salt contamination. Due to the properties of synthetic polymers at high temperatures, sulfonated amide polymers PSP109 and CR480 were introduced into the composition of the initial mud.
The use of these two agents increased the plastic viscosity and yield point and increased the thermal resistance of the water-based mud. The increase in plastic viscosity affects thermal resistance mainly by affecting the flow dynamics. In the case of drilling fluids with high plastic viscosity, the flow rate can be significantly reduced, which can lead to less efficient heat transfer. This reduced flow can cause thermal resistance because the heat may not be evenly distributed, leading to temperature gradients.
Applying “TEA” to the base mud in an amount of 3% vol. increased its pH from 9.4 to 10.5 and slightly increased plastic viscosity from 23 to 24 mPa·s (Table 2) at room temperature. Heating the mud at 160 °C showed that the addition of 3% TEA increased the thermal resistance of the mud. After being exposed to 160 °C, the mud retained 66.7% of the plastic viscosity and 54.7% of the yield point (Table 2, Figure 3 and Figure 4).
Analyzing the current research results, it was concluded that an appropriate direction for further research would be to check the effect of the amine agent “TEA” in combination with ethoxylated fatty amine SR5 (Table 2). The tests carried out showed that the use of “TEA” and SR5 further improved the thermal resistance of the drilling mud. The synergistic effect of these agents resulted in an increase in the rheological parameters of the mud compared to the base mud: the plastic viscosity increased to 34 mPa·s and the yield point to 29.2 Pa. The filtration value was also reduced to 3.6 cm3/30 min. As a result of heating it at a temperature of 160 °C, the mud retained 88.2% of its plastic viscosity and 45.9% of the yield point of the mud before heating (Table 2, Figure 3 and Figure 4).
Most likely, the amine compound “TEA” added to the mud acts as an antioxidant, which increases the polymer decomposition temperature. Compared to other antioxidants, its effect in synergy with fatty amine is much more stable.
The thermal conductivity of a drilling mud refers to its ability to conduct heat. This is an important parameter in the context of drilling operations because it can contribute to lower circulating temperatures at the bottom of the borehole, allowing for faster temperature equalization and thus faster stabilization when the borehole remains static. On the other hand, the ability of the drilling fluid to resist heat flow is called thermal resistance and is the inverse of thermal conductivity. Drilling muds with high thermal conductivity generally have lower thermal resistance. On the other hand, drilling muds with low thermal conductivity heat up more slowly in the borehole and cool more slowly at the surface, resulting in higher thermal resistance [38,39,40]. High thermal conductivity fluids are recommended for geothermal drilling, where temperatures are high and heat must be removed from the borehole quickly. This characteristic is not beneficial, however, in the case of drilling failures and technological downtimes, because under statistical conditions, the fluid can quickly undergo thermal decomposition at high temperatures. Drilling fluids with low thermal conductivity can often be used as an insulator, reducing heat transfer and thus protecting the borehole structures from excessive heating. Determining the appropriate thermal conductivity of the drilling fluid is therefore essential for managing borehole temperatures and protecting the drilling equipment and the stability of the borehole. One way to increase the thermal conductivity of drilling muds is to modify its composition and properties by adding nanomaterials.
Nanomaterials are agents with sizes in the range of 1 nm to 100 nm. Nanomaterials exhibit completely different physicochemical properties compared to their macroscopic properties [41,42,43]. Nanomaterials such as carbon nanotubes are highly thermally conductive. Adding them to a drilling fluid can increase its ability to conduct heat, allowing for more efficient heat removal from the drilling area. This, in turn, prevents the fluid from overheating, which could lead to thermal degradation. Nanomaterials can create a barrier that restricts heat flow in undesirable directions, which can help maintain optimal operating temperatures. Nanomaterials neutralize free radicals and other reactive oxygen species, which further increases the thermal resistance of the fluid [41,42,43].
We increase the thermal conductivity of the mud using carbon nanotubes and graphite, selected on the basis of preliminary tests.
During further research, the composition of the mud was modified to improve thermal conductivity. The properties of mud weighted to 1.2 g/cm3 were modified by adding 1.0% carbon nanotubes (Table 3) and 3% graphite (Table 3).
The tests carried out on muds with the addition of nanocomposites showed that the increase in thermal conductivity depends on the type of nanoadditives used, their properties, and their concentration in the drilling mud.
Among the tested nanoadditives, the best results were achieved when using carbon nanotubes. The addition of 1% carbon nanotubes to the mud increased the thermal conductivity of the mud by 12.2% (Table 3, Figure 5). Another material that, due to its properties, can increase the thermal conductivity of drilling fluids is graphite.
Graphite is a type of carbon with a black-gray color and a metallic luster. The basic properties of graphite are good thermal and electrical conductivity, high thermal resistance, and excellent lubricating properties. Moreover, graphite does not dissolve in water, has low chemical activity, and has no negative impact on the environment. For laboratory tests, flake graphite was used and added to the drilling fluid in an amount of 3%. The test results are presented in Table 3 and Figure 5. Graphite, as a material with good thermal conductivity, also improves the thermophysical properties of the mud. The mud containing 3% graphite is characterized by thermal conductivity higher than that of the base mud by approximately 27.6% (Table 3, Figure 5).
The impact of the increased thermal conductivity on the rheological and structural parameters of the prepared muds after heating to 160 °C indicates that the highest parameters were recorded for the base mud, which has 72.5% of the plastic viscosity and 64.1% of the yield point of the mud before heating, with a filtration of 2.6 cm3/30 min (Table 3, Figure 6 and Figure 7), whereas the lowest rheological parameters were recorded for the mud with 1% carbon nanotubes, whose plastic viscosity was reduced by 34.1% and whose yield point decreased by 67.9% (Table 3, Figure 6 and Figure 7). For comparison, in the potassium-polymer mud, exposure to a temperature of 160 °C reduces the plastic viscosity by 86.2% and lowers the yield point by 93.8% (Table 3, Figure 6 and Figure 7).
A very important parameter that determines the possibility of using a given mud for drilling thermal wells is their filtration under high temperature and pressure conditions. Mud filtration measurements were tested on an HPHT filter press at temperatures of 120, 140, and 160 °C and a differential pressure of 0.7 MPa. The compositions and properties of muds selected for filtration tests are presented in Table 3.
The analyzed muds have the same density (Table 3) but differ in the type of the weighting material and thermal conductivity. The filtration value of the developed WBM SR5 mud with a thermal conductivity of 0.5637 W/m·K (Figure 5) was the lowest and amounted to 11.8 cm3/30 min at a temperature of 160 °C (Figure 8). Modifying the composition of the WBM SR5 mud to improve its thermal properties by adding 3% graphite and changing the type of loading material did not reduce the filtration value. The filtration of the mud with increased thermal conductivity of 0.7129 W/m·K (Figure 5) at a temperature of 160 °C was 16.4 cm3/30 min. For comparison, the filtration value of potassium-polymer mud at a temperature of 160 °C is 148 cm3/30 min (Figure 8). Modification of the composition of the developed mud to improve its thermal conductivity did not reduce the filtration of the drilling mud.
Measurements of rheological and structural parameters were also carried out for the developed mud under high temperature and pressure conditions. The tests were carried out in semi-dynamic conditions using an Ofite 77 viscometer in a heating cycle at temperatures from 20 to 160 °C and then cooling them back to ambient temperature. The measurement results are presented in Figure 9 and Figure 10 in the form of a graph of changes in plastic viscosity and flow limit with temperature.
The tests were conducted for the developed mud WBM SR5 with increased thermal conductivity of 0.7129 W/m·K with added 3% graphite and weighted to a density of 1.2 g/cm3 with hematite (Table 3).
The simulated mud circulation in the borehole, which accounts for the impact of temperature (20 °C to 160 °C), indicates a systematic and regular reduction of the viscosity and yield point of the mud during heating to 160 °C and a high capability to recover its structure during cooling to 20 °C. The gradual increase in mud temperature to 160 °C systematically reduces the plastic viscosity of the mud. At 90 °C, the mud reaches a plastic viscosity of 25 mPa·s and at 160 °C—10 mPa·s (Figure 9). As a result of the gradual cooling of the mud to the ambient temperature, the plastic viscosity of the mud was restored to 49 mPa·s (Figure 9).
The chart of changes in the yield point of the drilling mud depending on temperature shows that the reduction of the yield point occurred until the temperature of 160 °C. At 90 °C, the yield point was reduced to 9.1 Pa, and at 160 °C, the yield point reached the lowest value—1.4 Pa (Figure 10). When the mud cooled back down to the ambient temperature, the yield point was restored to 21 Pa (Figure 10).
The conducted laboratory tests enable the selection and use of thermal inhibitors in the mud to increase the polymer breakdown temperature in the mud. Thanks to the synergism of TEA and SR5 and added sulphonated polymer PSP109 and the CR480 agent, the produced drilling mud had a high resistance to temperatures up to 160 °C, stable rheological parameters, and low filtration.

4. Conclusions

  • The conducted laboratory tests indicate that adding glycol compounds with a molecular weight of 8000 and sodium ascorbate to the mud does not protect the polymers from breakdown at 160 °C.
  • The conducted research showed that the thermal resistance of the potassium-polymer mud was increased by the use of fatty amine compounds and the “TEA” amine compound. After being exposed to 160 °C, the mud with 3% “TEA” retained 66.7% of the plastic viscosity and 54.7% of the yield point.
  • Modifying the composition of the water-based drilling mud using 3.0% graphite and changing the type of the weighting material based on iron compounds (hematite) improved thermal conductivity by 21.6% relative to the base mud. The mud with carbon nanotubes, in turn, had a thermal conductivity of 0.6265 W/m·K.
  • The increase in thermal conductivity of drilling mud after adding 1% of carbon nanotubes resulted in a decrease in its thermal resistance. The plastic viscosity of the drilling mud with thermal conductivity of 0.6265 W/m·K, as a result of its heating, decreased by 34.1%, and the yield point by 67.9%. The drilling mud with lower thermal conductivity of 0.5637 W/m·K was characterized by higher thermal resistance, which retained 72.5% of the plastic viscosity and 64.1% of the yield point of the mud before heating. The improvement in the thermal properties of the mud to 0.7129 W/m·K did not reduce the filtration after exposure to temperature. The filtration of the mud with 3% graphite at 160 °C was 16.4 cm3/30 min. For comparison, the filtration of the developed mud WBM SR5 at 160 °C was 11.8 cm3/30 min. The filtration of the potassium-polymer mud, in turn, was 148 cm3/30 min.
  • The extent of the conducted laboratory tests is sufficient to develop the composition of a water-based drilling mud for geothermal drilling. The developed drilling mud had a high resistance to temperatures up to 160 °C, stable rheological parameters, and low filtration (Table 3, item 3).
  • Obtaining positive results from the tests conducted allows us to conclude that further work on increasing the thermal resistance of the water-based mud will be continued. In the longer term, research is planned to increase the thermal resistance of water-based drilling mud without the clay phase up to 200 °C.

Author Contributions

Conceptualization, S.B., G.Z., B.J. and M.K.; methodology, S.B.; validation, S.B., G.Z., B.J. and M.K.; formal analysis, S.B. and G.Z.; investigation, S.B.; resources, G.Z., M.K. and B.J.; data curation, S.B.; writing—original draft preparation, S.B.; writing—review and editing, G.Z., B.J. and M.K.; visualization, S.B.; supervision, S.B.; project administration, S.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Science and Higher Education Warsaw (Internal order Oil and Gas Institute—National Research Institute Project No. 0057/KW/23 and No. 0051KW23).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Nomenclature

HTHPHigh temperature high pressure
λThermal conductivity
PVPlastic viscosity
AVApparent viscosity
YPYield point
WBMWater-based mud

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Scheme 1. Tests of the thermal resistance of drilling muds.
Scheme 1. Tests of the thermal resistance of drilling muds.
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Figure 1. Comparison of the plastic viscosity of muds with modifying additives before heating and after heating at 160 °C.
Figure 1. Comparison of the plastic viscosity of muds with modifying additives before heating and after heating at 160 °C.
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Figure 2. Comparison of the yield point of muds with modifying additives before heating and after heating at 160 °C.
Figure 2. Comparison of the yield point of muds with modifying additives before heating and after heating at 160 °C.
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Figure 3. Comparison of the plastic viscosity of muds with modifying additives before heating and after heating at 160 °C.
Figure 3. Comparison of the plastic viscosity of muds with modifying additives before heating and after heating at 160 °C.
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Figure 4. Comparison of the yield point of muds with modifying additives before heating and after heating at 160 °C.
Figure 4. Comparison of the yield point of muds with modifying additives before heating and after heating at 160 °C.
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Figure 5. Average thermal conductivity of the muds weighted to a density of 1200 kg/m3.
Figure 5. Average thermal conductivity of the muds weighted to a density of 1200 kg/m3.
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Figure 6. Comparison of the plastic viscosity of muds with additives increasing the thermal conductivity before heating and after heating at 160 °C.
Figure 6. Comparison of the plastic viscosity of muds with additives increasing the thermal conductivity before heating and after heating at 160 °C.
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Figure 7. Comparison of the yield point of muds with additives increasing the thermal conductivity before heating and after heating at 160 °C.
Figure 7. Comparison of the yield point of muds with additives increasing the thermal conductivity before heating and after heating at 160 °C.
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Figure 8. Filtration loss drilling mud at temperatures of 120 °C, 140 °C, and 160 °C.
Figure 8. Filtration loss drilling mud at temperatures of 120 °C, 140 °C, and 160 °C.
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Figure 9. Dependence of the plastic viscosity of potassium-polymer mud weighted with hematite to a density of 1.2 g/cm3 and containing 3% graphite on temperature variations.
Figure 9. Dependence of the plastic viscosity of potassium-polymer mud weighted with hematite to a density of 1.2 g/cm3 and containing 3% graphite on temperature variations.
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Figure 10. Dependence of the yield point of potassium-polymer mud weighted with hematite to a density of 1.2 g/cm3 and containing 3% graphite on temperature variations.
Figure 10. Dependence of the yield point of potassium-polymer mud weighted with hematite to a density of 1.2 g/cm3 and containing 3% graphite on temperature variations.
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Table 1. Composition of applied drilling muds.
Table 1. Composition of applied drilling muds.
CompositionWater-Based Mud
H2O1000 cm3
Biocyde0.1%
XCD0.4%
PAC R0.3%
PAC L0.5%
Starch HHT0.5%
KCl5%
Carbonate blocker7%
Table 2. Impact of modifying additives on the properties of water-based mud before heating and after heating at 160 °C.
Table 2. Impact of modifying additives on the properties of water-based mud before heating and after heating at 160 °C.
No.Water
Based Mud
WBM		Density,
g/cm3		Plastic Viscosity,
mPa.s		Apparent Viscosity, mPa.sYield Point,
Pa		Gel Strength,
I/II, Pa		Filtration
API
[cm3]		pH
±0.05±1±1±0.75±0.25±0.1±1
1WBM
before heating		1.082344.520.54.8/7.28.09.4
2after heating at 160 °C 1.081.72.350.60.05/0.0556.06.9
3WBM No.1
+1% PSP109
+1% CR480		1.093258.525.34.8/7.26.49.4
4after heating at 160 °C 1.0916269.60.96/1.46.88.9
5WBM No.1
+3% TEA
before heating		1.08244419.23.8/6.77.810.6
6after heating at 160 °C1.08162710.51.4/1.91810.4
7WBM No.3
+3% TEA
+2% KAD
before heating		1.09336025.85.3/7.25.210.2
8after heating at 160 °C1.09273810.51.4/2.46.69.7
9WBM No.3
+3% TEA
+2% SR5
before heating		1.093464.529.24.8/7.63.610.5
10after heating at 160 °C1.09304413.41.4/2.94.810.3
Table 3. Impact of additives improving thermal conductivity on the properties of water-based mud before heating and after heating at 160 °C.
Table 3. Impact of additives improving thermal conductivity on the properties of water-based mud before heating and after heating at 160 °C.
No.Water
Based Mud
WBM		Density,
g/cm3		Plastic Viscosity,
mPa.s		Apparent Viscosity, mPa.sYield Point,
Pa		Thermal Conductivity,
W/m·K		Filtration
API
cm3		pH
±0.05±1±1±0.75±0.0001±0.1±1
1WBM
+ barite
before heating		1.19296130.60.55855.29.5
2after heating at 160 °C1.19461.9-1207.7
3WBM No.1
+1% PSP109
+1% CR480
+3% TEA
+2% SR5		1.21407230.60.56372.010.5
4after heating at 160 °C 1.212949.519.6-2.610.3
5WBM
+1% PSP109
+1% CR480
+3% TEA
+2% SR5
+3% graphite
+ hematite
before heating		1.24070.529.20.71293.210.5
6after heating at 160 °C1.2284819.1-6.410.3
7WBM No.3
+3% carbon nanotubes
before heating		1.214481.535.80.62652.810.5
8after heating at 160 °C1.21294111.5-5.210.3
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Błaż, S.; Zima, G.; Jasiński, B.; Kremieniewski, M. Increasing the Thermal Resistance of Water-Based Mud for Drilling Geothermal Wells. Energies 2024, 17, 4537. https://doi.org/10.3390/en17184537

AMA Style

Błaż S, Zima G, Jasiński B, Kremieniewski M. Increasing the Thermal Resistance of Water-Based Mud for Drilling Geothermal Wells. Energies. 2024; 17(18):4537. https://doi.org/10.3390/en17184537

Chicago/Turabian Style

Błaż, Sławomir, Grzegorz Zima, Bartłomiej Jasiński, and Marcin Kremieniewski. 2024. "Increasing the Thermal Resistance of Water-Based Mud for Drilling Geothermal Wells" Energies 17, no. 18: 4537. https://doi.org/10.3390/en17184537

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