Enhanced Geothermal System Reservoir Research Articles

Detecting fractures and monitoring hydraulic fracturing processes at the first EGS Collab testbed using borehole DAS ambient noise

Enhanced geothermal systems (EGS) require cost-effective monitoring of fracture networks. We validate the capability of using borehole distributed acoustic sensing (DAS) ambient noise for fracture monitoring using core photos and core logs. The EGS Collab project has conducted 10 m scale field experiments of hydraulic fracture stimulation using 50–60 m deep experimental wells at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. The first EGS Collab testbed is located at 1616.67 m (4850 ft) depth at SURF and consists of one injection well, one production well, and six monitoring wells. All wells are drilled subhorizontally from an access tunnel called a drift. The project uses a single continuous fiber-optic cable installed sequentially in the six monitoring wells to record DAS data for monitoring hydraulic fracturing during stimulation. We analyze 60 s time records of the borehole DAS ambient noise data and compute the noise root-mean-square (rms) amplitude on each channel (points along the fiber cable) to obtain DAS ambient noise rms amplitude depth profiles along the monitoring wellbore. Our noise rms amplitude profiles indicate amplitude peaks at distinct depths. We compare the DAS noise rms amplitude profiles with borehole core photos and core logs and find that the DAS noise rms amplitude peaks correspond to the locations of fractures or lithologic changes indicated in the core photos or core logs. We then compute the hourly DAS noise rms amplitude profiles in two monitoring wells during three stimulation cycles in 72 h and find that the DAS noise rms amplitude profiles vary with time, indicating the fracture opening/growth or closing during the hydraulic stimulation. Our results demonstrate that borehole DAS passive ambient noise can be used to detect fractures and monitor fracturing processes in EGS reservoirs.

Performance evaluation of enhanced geothermal systems with intermittent thermal extraction for sustainable energy production

Efficient extraction of thermal energy from deep geothermal reservoirs using Enhanced Geothermal Systems (EGS) is fundamental for sustainable clean energy generation. This study presents the Intermittent Thermal Extraction (ITE) method, addressing thermal breakthrough challenges encountered during continuous thermal extraction (CTE) within EGS reservoirs. Employing a thermal-hydraulic coupled approach and discrete fracture network modeling, the ITE process for EGS reservoirs is assessed, omitting considerations of stress-induced deformation. The research evaluates the ITE and CTE methods within a typical high-temperature geothermal reservoir in the Himalayan geothermal belt. Analysis includes variations in reservoir temperatures and pressures, alongside an examination of rock matrix permeability and injection fluid temperature impacts on EGS thermal extraction performance. Results demonstrate the ITE method's efficacy in prolonging the EGS reservoir lifespan by preventing thermal breakthroughs and maintaining pressure equilibrium. Under studied conditions, ITE extends the EGS reservoir lifespan by a minimum of 17.7 years, amplifying clean power generation by 13.1% compared to CTE. Additionally, ITE implementation substantially reduces greenhouse gas emissions by an estimated 1.46–5.03 million tons. The rock matrix permeability and injection fluid temperature significantly affect the EGS thermal extraction performance when the fracture aperture is constant. These findings highlight the ITE method's potential to optimize thermal extraction from EGS reservoirs, particularly in remote regions. This study contributes essential insights into advancing geothermal energy extraction techniques, emphasizing ITE as a promising approach for sustainable energy production.

Thermal‒hydraulic‒mechanical‒chemical coupling analysis of enhanced geothermal systems based on an embedded discrete fracture model

Enhanced geothermal system (EGS) is subject to the comprehensive effects of multiple physical fields during the long-term heat extraction process, including hydraulic (H), thermal (T), mechanical (M) and chemical (C) fields. The embedded discrete fracture model (EDFM) can effectively simulate the variations of flow, temperature, mechanical and concentration fields in fractured reservoirs. At present, however, the thermo-hydro-mechanical-chemical (THMC) coupling model based on EDFM is less researched. In this paper, the THMC coupling model of fractured reservoir is established based on EDFM by considering the changes in reservoir heterogeneity and physical properties as well as water–rock reactions. Then, the spatiotemporal evolution of flow, temperature, displacement and concentration fields in the operation process of EGS is simulated and analyzed. And the following research results are obtained. First, when the permeability of the basement rock is low, the production temperature decrease during exploitation is gradual, allowing EGS to maintain a high exploitation temperature for an extended period. However, lower permeability may result in a decrease in the quality flow rate from production wells, thereby affecting net heat extraction power. Second, when fracture permeability or fracture opening changes, EGS can output higher temperature stably for a certain period and then the temperature decreases at different amplitudes. When the fracture permeability increases to a certain value or the fracture opening decreases to a certain value, the influence of the change in fracture parameters on production temperature gets weak. Third, After 40 years of EGS operation, considering variable property fluids results in a 22 °C lower exploitation temperature compared to using constant property fluids, and considering water–rock reactions results in a 15 °C lower exploitation temperature, with a 12.5 % increase in reservoir average porosity. In conclusion, when researching a long-term operating EGS, it is necessary to comprehensively consider the influences of reservoir rock parameters, physical properties of injected fluid, water–rock reaction and other factors. And in the future, attention shall be paid to the two-way coupling of chemical reaction and mechanical deformation of other mineral compositions in the reservoir to the hydro-thermo-chemical field influence, so as to provide more accurate and reliable prediction for the engineering development and utilization of EGS reservoirs.

Reservoir and Fracture Characterization for Enhanced Geothermal Systems: A Case Study Using Multifractured Wells at the Utah Frontier Observatory for Research in Geothermal Energy Site

Summary For enhanced geothermal systems (EGS), multistage hydraulic fracturing along a deviated or horizontal well is a key technology used to create a high-conductivity fracture network between injection and production wells in deep, low-permeability geothermal reservoirs. The purpose of the created fracture network is to allow for the efficient transfer of fluid, heated by the geothermal reservoir, from the injection to the production well; therefore, well spacing (between injection and production wells) and hydraulic fracturing must be designed not only to promote connectivity between well pairs but also to mitigate thermal short-circuiting and thermal breakthrough. Analysis of post-fracture pressure decay (PFPD) data measured after each stage of a hydraulic fracturing treatment can be used to provide critical reservoir and fracture parameters required for well and hydraulic fracturing design optimization; this method provides a low-cost alternative and complementary approach to in-situ observation techniques, such as core-through experiments, fiber optics, or image logs in offset wells. Until now, PFPD has primarily been applied to multifractured horizontal wells (MFHWs) completed in low-permeability hydrocarbon reservoirs. The goal of this study is therefore to develop a methodology to estimate fracture and reservoir parameters using stage-by-stage PFPD data associated with EGS projects. An analytical model is proposed herein to estimate fracturing fluid efficiency, fracture length, average fracture aperture, average fracture conductivity, and reservoir permeability for different possible fracture geometries in EGS reservoirs. PFPD data collected for three hydraulic fracture stages in the injection well at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) site were analyzed to demonstrate the practical application of the proposed method. The results of this study indicate that, due to the presence of natural fractures in the target (granitic) reservoir, the hydraulic fracturing treatment (using slickwater) in the openhole section resulted in low fracturing fluid efficiency and small hydraulic fractures. In contrast, hydraulic fracturing treatments conducted in the perforated casedhole wellbore resulted in higher fracturing fluid efficiency and created larger hydraulic fractures even with smaller injected volumes. The results of the PFPD analysis were confirmed using a Formation MicroScanner image log and microseismic data collected during each stage of hydraulic fracturing.

Monitoring seismicity triggered by geothermal site shutdown with a surface DAS array at Brady Hot Springs

SUMMARY Seismicity induced by fluid injection including wastewater injection, hydrofracking and enhanced geothermal system (EGS) site production draws public attention. Dense arrays have been deployed to improve monitoring capability. In 2016 March, the PoroTomo experiment deployed an 8.6-km-long fibre-optic cable for distributed acoustic sensing (DAS) in the geothermal field at Brady Hot Springs, Nevada, covering an area of 1.5 by 0.5 km. The goal was to assess an integrated technology for characterizing and monitoring temporal changes in the rock mechanical properties of an EGS reservoir in three dimensions. We applied a neural network designed for earthquake detection called ADE-Net2 to the DAS data set to detect seismic events in continuous records. We were able to detect a total of 90 earthquakes, which included 21 events that had not been reported by a previous template-matching study. Additionally, we were able to successfully detect almost all of the active source signals, with only seven events being missed. We used the STA/LTA (short-/long-term average) method to pick arrivals and a clustering method to remove outliers. We initially tried a standard event location algorithm, but the low signal-to-noise ratio resulted in significant picking uncertainty that is up to ∼0.5 s, leading to large location uncertainty. Therefore, we developed a new location method based on the similarity between the theoretical traveltime curve and picked moveout. A grid search scheme was adopted to find the optimal point at which the traveltime curve is most similar to the picked one. Most newly detected earthquakes locate southwest of the DAS array, where five earthquakes were reported by a local seismic network. The plant began shutting down at 19:15 UTC on the March 13, and most earthquakes occurred on the March 14, indicating a relationship between the seismicity and the pressure changes caused by the shutdown of the plant. The pressure changes at epicentres obtained from a simplified model range from 71 to 157 kPa, exceeding a typical earthquake trigger threshold of 10 kPa.

Effect of temperature-dependent rock thermal conductivity and specific heat capacity on heat recovery in an enhanced geothermal system

The modeling of heat recovery from an enhanced geothermal system (EGS) requires rock thermal parameters as inputs such as thermal conductivity and specific heat capacity. These parameters may encounter significant variations due to the reduction of rock temperature during heat recovery. In the present study, we investigate the effect of temperature-dependent thermal conductivity and specific heat capacity on the thermal performance of EGS reservoirs. Equations describing the relationships between thermal conductivity/specific heat capacity and temperature from previous experimental studies were incorporated in a field-scale single-fracture EGS model. The modeling results indicate that the increase of thermal conductivity caused by temperature reduction accelerates thermal conduction from rock formations to fracture fluid, and thus improves thermal performance. The decrease of specific heat capacity due to temperature reduction, on the contrary, impairs the thermal performance but the impact is smaller than that of the increase of thermal conductivity. Due to the opposite effects of thermal conductivity increase and specific heat capacity decrease, the overall effect of temperature-dependent thermal parameters is relatively small. Assuming constant thermal parameters measured at room temperature appears to be able to provide acceptable predictions of EGS thermal performance.

A three-dimensional coupled thermo-hydro model for geothermal development in discrete fracture networks of hot dry rock reservoirs

Three-dimensional (3D) fluid flow and thermal transport modeling in discrete fracture networks (DFNs) is critical for geothermal energy recovery from enhanced geothermal systems (EGSs). Considering nonlinear fluid flow models, a 3D coupled thermo-hydro (T-H) model based on the Galerkin finite element method was developed for DFNs. Forchheimer flow was considered in this fluid flow model, and a simplified thermal transport model was adopted based on the local thermal non-equilibrium theory. Finite element spatial discretization of fluid flow and thermal transport models in DFNs was formulated. A simple upwind scheme was adopted as the numerical strategy of the thermal transport model, avoiding its solution oscillation. In addition, the proposed numerical method for the fluid flow and thermal transport was verified by comparing the model results with experimental, analytical, and numerical solutions. This approach was further applied to the modeling of heat extraction in a Habanero EGS reservoir for a period of 40 years under different injection pressures. The results demonstrated that the proposed approach was effective for simulating coupled T-H processes in 3D DFNs. With increasing the injection pressure, the power generation of reservoir increased, but its life span decreased. Injection temperature had positive effect on life span for power generation.

A three-dimensional investigation of the thermoelastic effect in an enhanced geothermal system reservoir

During heat extraction in enhanced geothermal system (EGS) reservoirs, thermal contraction is induced by cold fluid injection, resulting in thermoelastic deformation. The induced thermoelasticity can alter rock properties, including their fracture aperture and permeability; therefore, the thermoelastic effect is crucial in understanding EGS reservoir behaviour. Based on coupled thermo-hydro-mechanical (THM) processes extended in COMSOL Multiphysics to include the thermoelastic effect, this paper presents a three-dimensional (3D) numerical model of an EGS reservoir with a multiple planar fracture system to investigate the influence of thermoelasticity on reservoir thermal performance. The model is used to perform an in-depth analysis to determine the rate at which the thermoelastic effect develops during heat extraction in relation to a baseline THM model without thermoelasticity. After demonstrating that thermoelasticity influences the thermal performance of EGS reservoirs, the study is further extended to investigate the effect of injection temperature and Young's modulus on fracture aperture opening, reservoir impedance, thermal front propagation, and flow rate. The results show that thermoelasticity affects the long-term thermal performance of EGS reservoirs by reducing the energy extraction rate due to increased flow pathways. Due to high reservoir impedance, the thermoelastic effect appears to cause thermal short circuits (growth of rapidly cooling paths with high flow rates). The results suggest that thermoelasticity has a significant impact on system thermal performance for deep EGS reservoirs.

Study on heat extraction characteristics in a rock fracture for the application of enhanced geothermal systems

Enhanced geothermal systems (EGS) technologies aim to extract geothermal energy from hot dry rock (HDR). Heat extraction performance is of great importance for thermal efficiency and life cycle of EGS. It is intuitively considered that EGS reservoir modeling should be established based on accurate measured data of thermal and hydraulic properties of rock matrix and fluid, such as thermal conductivity, heat capacity and fracture permeability, however, which are time-consuming to obtain in some projects. This paper aims to clarify the influence of these parameters on heat extraction performance, which can provide basis for reservoir model simplification during preparation process of engineering scheme. Numerical simulations were carried out based on a rough fracture of granite rock at core scale, which has been validated by amount of experiments. First, the differences between simulation results and the previous experiment results were presented and analyzed, for variations of heat extraction rate per flow rate. Compared with the method with constant flow rate, the compensating method with various flow rate greatly increased the heat extraction rate and postpone thermal breakthrough time. Therefore, a simplified method was proposed in this paper to optimize the flow rate or predict the heat extraction rate when EGS operation with various flow rates. The method are applicable for geothermal reservoir management for efficient and stable heat extraction. Finally, the effects of properties of rock mass and fracture were studied. The production temperature and heat extraction rate were larger with larger thermal conductivity and heat capacity. It was found that the maximum influence on heat extraction rate were less than 20% and 10% within the common value ranges of thermal conductivity and heat capacity, respectively. There were almost no differences in the heat transfer process in fractures with different fracture permeabilities. It can be concluded that the prediction error of heat extraction performance is tolerable for engineering reservoir simulations, even without precise experimental test for thermal and hydraulic properties variations with temperature or pressure.

Modeling flow and heat transfer of fractured reservoir: Implications for a multi-fracture enhanced geothermal system

The EGS (enhanced geothermal system), particularly the multi-fracture EGS, is expected to be a key method to effectively develop the deep geothermal resource. The accurate modeling of flow and heat transfer in an EGS reservoir, particularly the fracture, is challenging. Therefore, an improved model considering the LTNE (local thermal non-equilibrium) and non-Darcy flow in a rough fracture is proposed. Then, the flow and temperature fields in the matrix are comprehensively investigated. The relationship between matrix and fracture is analyzed. The LTNE and LTE (local thermal equilibrium) are compared to determine the specific thermal process in the fracture. The difference between non-Darcy and Darcy flow in the fracture is studied to evaluate the actual flow behavior. The influences of the rough and smooth fractures are discussed. The results show that the direct contribution of the fracture to production is close to 100%, while the matrix mainly plays an indirect role as a heat source. There is a noticeable local low-pressure region induced by fluid density change in the matrix, while the heat conduction, especially rock conduction, dominates the heat transfer of the matrix. The intense heat convection in the fracture leads to a heat transfer rate difference of up to 5000 times compared with the heat conduction of the matrix, causing a marked thermal breakthrough. The non-Darcy flow represents the actual flow in the fracture, and the velocity is significantly lower than the Darcy flow velocity when a high injection flow rate is employed in the EGS production. The total extracted heat difference between rough and smooth fractures reaches 7.51%. • A model considering LTNE and non-Darcy flow in a rough fracture is established. • The heat conduction of the rock dominates the heat transfer process in the matrix. • The direct contribution of the EGS fracture to production is close to 100%. • The heat transfer rate in EGS fracture can be thousands of times that in the matrix. • The non-Darcy flow may well represent the actual flow behavior in EGS fracture.

Thermo-hydro-mechanical optimization of the enhanced geothermal system for commercial utilization

Enhanced geothermal system (EGS) has been commercialized worldwide, yet the complex thermal-hydraulic-mechanical coupling processes were not well illuminated. With the reservoir regarded as the fractured porous media of rock matrix and discrete fractures, this work establishes a three-dimensional EGS model based on the thermal non-equilibrium theory to study the heat extraction in the EGS. After model validations, the evolution of three fields (i.e., temperature field, seepage field, and stress field) and the effects of several crucial operating parameters (e.g., injection flow rate, depth, and length) on heat extraction performance are discussed. The results show that the change of pressure and temperature in the reservoir alters the fracture displacement and the flow preferential paths and rock matrix permeability. For the given EGS reservoir, the total heat extraction rate reaches 60% at 50 years and approaches 86% at 100 years. Increasing the elastic modulus of the rock matrix promotes the overall recovery and then decreases the average temperature of the rock matrix. Decreasing injection flow rate, increasing length, increasing depth, and decreasing the elastic modulus of rock matrix give rise to (i) increasing the period for maintaining stable average outlet temperature; (ii) elevating the average outlet temperature after specific years. Running for 100 years, the final injection pressure of the EGS is 130 MPa, 48 MPa, and 45 MPa for the three fracture widths, 26%, 14%, and 10% lower than the initial injection pressure. Changing the fracture width from 0.005 m to 0.05 m is beneficial for the EGS operation on economic efficiency. Based on the commercial standards, the optimal operating conditions are proposed for the EGS.

Optimization Design of Multi-Factor Combination for Power Generation from an Enhanced Geothermal System by Sensitivity Analysis and Orthogonal Test at Qiabuqia Geothermal Area

In order to explore the optimal mining strategy of a fractured Enhanced Geothermal System (EGS) reservoir, we numerically investigated the influence of seven factors on heat production and conducted an optimization analysis of a multi-factor and multi-level combination by an orthogonal test based on the geological data at the Qiabuqia geothermal field. Seven factors were considered, including five reservoir factors (fracture spacing, fracture permeability, fracture permeability anisotropy, matrix permeability, and heat conductivity) and two operation factors (injected section length and injection rate). The results show that injection rate and fracture permeability have the greatest influence on production performance. Different factor combinations have a great influence on the productivity. The multi-factor and multi-level combination optimization is needed, and the optimization scheme of the EGS can be achieved through the orthogonal test and range analysis. The order of influence degree on the power generation is injection rate > fracture permeability > fracture permeability anisotropy > injected section length > matrix permeability > fracture spacing > heat conductivity. The order of influence degree on the coefficient of performance of the EGS is fracture permeability > injection rate > injected section length > fracture permeability anisotropy > matrix permeability > fracture spacing > heat conductivity. For reservoir stimulation, the stratum with dense natural fractures should be selected as the target EGS reservoir. It is not advisable to acidify the EGS reservoir too much to widen the apertures of the natural fractures. Fracture permeability anisotropy will increase pump energy consumption, but this adverse effect can be greatly reduced if the other parameters are well matched. Matrix permeability and heat conductivity may not be used as indicators in selecting a target reservoir. For project operation, the injected section length should be as long as possible. The injection rate plays a major role in all factors. Special attention should be paid to the value of the injection rate, which should not be too large. The appropriate injection temperature should be determined in accordance with the water source condition and the engineering requirement. If a commercial rate (100 kg/s) is to be obtained, the permeability of the reservoir fracture network needs to be stimulated to be higher. Meanwhile, in order to ensure that the production temperature is both high and stable, it is necessary to further increase the volume of the EGS reservoir.

The Effect of Silicate Ions on the Separation of Lithium From Geothermal Fluid

In an enhanced geothermal system (EGS), geothermal energy in rocks with insufficient permeability or fluid saturation can be used by creating artificial geothermal reservoirs. Generally, EGS geothermal fluid contains high concentrations of total dissolved solids that originated from various geochemical reactions between the fluid in the reservoir and the minerals in the rock. For example, the concentration of lithium ions are measured approximately 150 mg/L, and several researchers have focused on the recovery of lithium in the geothermal fluid using various methods, one of which is liquid extraction. Solvent extraction has been used to recover lithium from various sources, and successful recovery efficiency have been attained. However, the geothermal fluid in EGS reservoirs contains high concentrations of SiO2, which might inhibit the selective recovery of lithium. Thus, in this study, two consecutive stages of solvent extraction were used to separate the lithium from the geothermal fluid that contained different concentrations of SiO2 ions. The divalent ions were removed in the first stage, and the lithium ions were extracted effectively in the second stage. The SiO2 inhibits the selective recovery of lithium in the first stage to a greater extent than it does in the second stage. The spectroscopy data shows a decrease of the organic solvents main functional group (P=O & P-O-H) absorbance that reacts with the metal ions of the geothermal water after extraction however the intensity difference was reduced as the SiO2 concentrations increases. Silicate ions can be problematic due to the formation of scaling in EGSs, so controlling its concentration in the geothermal reservoir would be beneficial for the long-term operation of EGSs and for the successful recovery of valuable metal resources from EGS reservoirs.

Subsalt Rotliegend Sediments—A New Challenge for Geothermal Systems in Poland

New seismic data and the completion of the K-1 petroleum exploratory well, located close to the axial zone of the Mogilno-Łódź Trough (Polish Lowlands) delivered new insight into local structural, tectonic, facial and thermal variability of this geological unit. In this paper, the two variants of 3D models (SMV1 and SMV2) of Permian-Mesozoic strata are presented for the salt pillow related Kłecko Anticline, while resources assessment was confined to the Rotliegend Enhanced Geothermal System (EGS) type reservoir, that is divided into Playa, Eolian and Fluvial facies-based complexes. Using very conservative assumptions on the methods of the EGS reservoir development, authors assessed that heat in place and technical potential for eolian sandstones are about 386 PJ and ca. 2814 kW, respectively, and for Fluvial 367 PJ and ca. 2850 kW in relation to the volume of 1 km3 at depths of about 5000 m b.s.l. The authors recommend for the further development of the Eolian complex because of its low shale content, influencing the high susceptibility to fracking. The presented research is the first Polish local resources assessment for an EGS reservoir in sedimentary Rotliegend, within thermal anomaly below the salt pillow, which is one of over 100 salt structures mapped in Poland.

Effects of vertical anisotropy on optimization of multilateral well geometry

Multilateral wells have been increasingly used in recent years by different industries such as the oil- and gas industry (incl. coal-bed methane (CBM)) and geothermal energy production. The common purpose of these wells in the oil industry and partially in the geothermal industry is to achieve a higher production rate per well without increasing the hydraulic gradient significantly. In geothermal systems the ultimate objective is to improve heat extraction performance by enhancing productivity but also by increasing the heat transfer area. Optimal design of multilateral wells has been the focus of numerous publications in CBM and unconventional oil- and gas production, but as a result of cross-fertilization and technology-transfer among different sectors in the energy industry in the last few years, the focus of research on multilateral wells has turned to enhanced geothermal systems (EGS). Recently, several papers have been published on the use of multilateral wells in deep geothermal reservoirs. Coaxial closed-loop geothermal systems with multilateral wells utilize only the increased heat transfer area due to the laterals while EGS's with multilateral injection and production wells also benefit from the hydrodynamic advantages of such well configurations. However, substantial emphasis is – understandably – placed on the effect of artificial and natural fracture networks of EGS reservoirs and less focus is devoted to the impact of vertical permeability anisotropy which can seriously influence the flow pattern of a given configuration. Similarly, in CBM-related studies, the effect of horizontal permeability anisotropy along the face cleats to the butt cleats on multilateral well-design was investigated and vertical permeability anisotropy was not considered. In the current work we present a generalized approach for evaluating the effects of vertical permeability anisotropy on multilateral well geometry by numerical hydrodynamic modelling in an idealized, homogeneous hydrogeological setting. Current paper proposes an alternative modelling approach for representing the non-horizontal branches of multilateral wells by introducing a thin layer for the laterals with the same inclination as the well branches. This approach is then is used to investigate the effect of anisotropy on the flow pattern and near-wellbore drawdown and hydraulic gradient in the case of different branch deviations. Results suggest that the benefits of highly deviated or horizontal laterals emerge when vertical anisotropy is high. Evaluation of branch deviation vs. anisotropy indicates that above approx. 60° there is no significant benefit in increasing deviation which implies that very highly deviated or horizontal laterals might not necessarily pay off the associated technical challenges and extra costs.

Comparative Study of Acid and Alkaline Stimulants with Granite in an Enhanced Geothermal System

AbstractThe Enhanced Geothermal System (EGS) is an artificial geothermal system that aims to economically extract heat from hot dry rock (HDR) through the creation of an artificial geothermal reservoir. Chemical stimulation is thought to be an effective method to create fracture networks and open existing fractures in hot dry rocks by injecting chemical agents into the reservoir to dissolve the minerals. Granite is a common type of hot dry rock. In this paper, a series of chemical stimulation experiments were implemented using acid and alkaline agents under high temperature and pressure conditions that mimic the environment of formation. Granite rock samples used in the experiments are collected from the potential EGS reservoir in the Matouying area, Hebei, China. Laboratory experimental results show that the corrosion ratio per unit area of rock is 3.2% in static acid chemical experiments and 0.51% in static alkaline chemical experiments. The permeability of the core is increased by 1.62 times in dynamic acid chemical experiments and 2.45 times in dynamic alkaline chemical experiments. A scanning electron microscope analysis of the core illustrates that secondary minerals, such as chlorite, spherical silica, and montmorillonite, were formed, due to acid‐rock interaction with plagioclase being precipitated by alkaline‐rock interactions. Masking agents in alkaline chemical agents can slightly reduce the degree of plagioclase formation. A chemical simulation model was built using TOUGHREACT, the mineral dissolution and associated ion concentration variation being reproduced by this reactive transport model.

Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)

The long-term sustainability of fractures within rocks determines whether it is reasonable to utilize such formations as potential EGS reservoirs. Representative for reservoirs in Variscan metamorphic rocks, three long-term (one month each) fracture permeability experiments on saw-cut slate core samples from the Hahnenklee well (Harz Mountains, Germany) were performed. The purpose was to investigate fracture permeability evolution at temperatures up to 90 °C using both deionized water (DI) and a 0.5 M NaCl solution as the pore fluid. Flow with DI resulted in a fracture permeability decline that is more pronounced at 90 °C, but permeability slightly increased with the NaCl fluid. Microstructural observations and analyses of the effluent composition suggest that fracture permeability evolution is governed by an interplay of free-face dissolution and pressure solution. It is concluded that newly introduced fractures may be subject to a certain permeability reduction due to pressure solution that is unlikely to be mitigated. However, long-term fracture permeability may be sustainable or even increase by free-face dissolution when the injection fluid possesses a certain (NaCl) salinity.

Coupled thermal–hydraulic–mechanical model for an enhanced geothermal system and numerical analysis of its heat mining performance

The operation of an enhanced geothermal system (EGS) involves a complex thermal–hydraulic–mechanical (THM) coupling process, which is important for exploiting the geothermal energy contained in hot dry rocks. In this study, the THM coupling mechanism in an EGS reservoir is explained, and a fully coupled THM mathematical model with local thermal nonequilibrium of a dual media is established, considering the dynamic changes in the properties of the rock matrix, fractures, and fluid during EGS operation. The model is verified through an example of the thermoelastic consolidation of a saturated soil. The heat transfer, fluid flow, and mechanical characteristics of a geothermal reservoir over 40 years are studied via a numerical simulation of a 2D geometric model. The effects of fracture occurrence, coupling conditions, and model parameters on the mining performance are analyzed. The results show that the dual-media model can reflect the anisotropy in the temperature, pressure, and stress due to the presence of fractures. An area with a more complex fracture network and smaller spacing is found to be more conducive for heat transfer. The inlet–outlet pressure differences, thermal expansion coefficient of the rock matrix, and initial fracture permeability have significant effects. When the inlet–outlet pressure difference is increased from 4 to 10 MPa, the heat extraction ratio increases from 56% to 89% at 40 years, and the initial output thermal power is increased from 9.2 to 24.9 MW. When the coefficient of thermal expansion is increased from 1 × 10−6 to 7 × 10−6 K−1, the heat extraction ratio is increased from 54% to 84%, and the initial output thermal power is increased from 8.5 to 19.7 MW. When the initial permeability is increased from 1 × 10−11 to 2.5 × 10−11 m2, the heat extraction ratio increases from 74% to 92%. These results provide a reference for optimizing the mining effect of the EGS.

Modeling heat transport processes in enhanced geothermal systems: A validation study from EGS Collab Experiment 1

Heat recovery from an enhanced geothermal system (EGS) is a complex process involving heat transport in both fracture networks and rock formations. A comprehensive understanding of and the ability to model the underlying heat transport mechanisms is important for the success of EGS commercialization but remains challenging in practice due to the generally insufficient characterization of EGS reservoirs. In the present study, we analyze an extensively monitored intermediate-scale EGS field experiment performed in a well-characterized testbed. The high-resolution, high-quality measurements from the field experiment enable the development of a high-fidelity model incorporating a well-constrained fracture network. Based on the field experiment, we investigate the complex heat transport processes in an EGS-relevant environment and validate the capability of a numerical approach in simulating these inherently coupled heat transport processes. A series of numerical simulations were performed to study the effects of different heat transport mechanisms, including thermal convection with fracture flow, thermal conduction in rock formations, and the Joule-Thomson effect. The agreement of thermal responses between field measurements and simulation results indicates that our numerical approach can appropriately model the heat transport processes pertaining to heat recovery from EGS reservoirs.

Evaluation of Heat Production from Single Fracture Hot Dry Rock with Applications for EGS Reservoir Design

Enhanced Geothermal Systems (EGS) are the most effective methods for extracting geothermal energy from deep hot dry rock (HDR). The EGS reservoir framework is envisioned as independent-multiple fractures connecting injection and production wells. The reservoir is further conceptually represented by a single fracture with homogeneous features, and a unit fracture is defined as having extendibility to a larger surface area for heat extraction. An analytical solution for the thermal-hydraulic coupling process is derived with a 1-D steady state conductive heat flow in the HDR with perpendicular water flow in the single fracture and transient heat transfer from rock to water. The heat produced from the HDR via water flow in the idealized single fracture is demonstrated by arithmetic equations. The lifetime of an EGS reservoir in these reference conditions is 23.2 years and is confined by the produced water temperature of 150°C for commercial utilization. The heat recovery factor is 12.4%. With a power plant capacity of 5 Mw installed, the total area for extracting recoverable heat within the projected lifetime of a fracture surface of 1.58×106m2was determined. The discussion shows the ability of the model to estimate heat production and reservoir scale.