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Thermally-induced changes in the fracture properties of geological reservoir rocks can influence their stability, transport characteristics, and performance related to various deep subsurface energy projects. The modified maximum tangential stress (MMTS) criterion is a classical theory for predicting the fracture instability of rocks. However, there is a lack of research on the accuracy of MMTS theory when rocks are subjected to different temperatures. In this study, mechanical theoretical analysis and failure and fracture mechanics experiments of granite under the influence of temperatures ranging from 20 °C to 600 °C are carried out. The results showed that the theoretical estimated value of MMTS differs significantly from the experimental data at 20 °C−600 °C. The Keff/KIC ratio is less than the experimental test value due to the critical crack growth radius (rc) estimated by the conventional method being larger than the critical crack growth radius (rce) derived from the experimental data. Varied temperatures affect the fracture process zone size of fine-grained, compact granite, and the MMTS theoretical estimation results. Therefore, it is essential to modify the critical crack growth radius for MMTS theory to accurately predict the fracture characteristics of thermally damaged rocks. In addition, the variation of the rock’s mechanical properties with temperature and its causes are obtained. Between 20 °C and 600 °C, the mode-I, mode-II, and mixed-mode (α = 30° and 45°) fracture toughness and Brazilian splitting strength of the granite decrease by 80% and 73%, respectively. When the rock is heated above 400 °C, its deterioration is mainly caused by a widening of its original cracks.

[Display omitted] • The low-velocity layer in the thickened crust provides the most important heat source in Tibetan Plateau. • Heat production of highly radiogenic rock is the main heat source of shallow intracrustal heat in southeastern China. • Heat upwelling along fractures is the main heat source of EGS in the north China sedimentary basin. A heat source is the basis for Enhanced Geothermal Systems (EGS) and is the most important factor for the site selection for EGS development. In this paper, we analyze regional variation patterns of mantle heat flow, study crustal thermal states, construct models of crustal steady-state temperature fields, and discuss the heat source mechanism of EGS in different zones, including the strongly tectonic activity zone of the Tibetan Plateau, North China sedimentary basin, and the highly radiogenic rock distribution zone of southeastern China. The results show that differences in the crustal thermal structure of different regions control their deep thermal conditions and EGS heat sources. The intense tectonic activity zone of the Tibetan Plateau, represented by the Gonghe Basin, shows an anomalously heated crustal thermal structure, and the low-velocity layer in its thickened crust provides the most important intracrustal heat source in the region. The local area in southeastern China is influenced by both highly radiogenic rocks and good sedimentary cover insulation, indicating a hot crust-cold mantle type of crustal thermal structure, thus reflecting the role of heat production from highly radiogenic rocks in controlling the shallow crustal temperature field in the area. The North China Plain has a typical cold crust-hot mantle type of crustal thermal structure, and the development of EGS in the area must consider the undulating bedrock pattern in the sedimentary basin and whether there is a deep fracture that connects the nearby mantle heat source. The results are of scientific significance for guiding future target site selection and EGS development in different regions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Hydraulic fracture (HF) propagation behavior is significant when building an enhanced geothermal system for hot dry rock (HDR) and evaluating the simulated reservoir volume (SRV) for shale gas reservoirs. The HF propagation behaviors are closely related to geologically structural discontinuities (SDs), which differ significantly between HDR and shale. Granite, one of the most common HDRs, mainly possesses natural fractures (NFs), quartz veins (QVs), and lithological interfaces (LIs). The SDs in the shale are mainly bedding planes (BPs) and NFs. According to the physical and mechanical property differences regarding the rock matrix, SDs can be divided into discontinuous planes and discontinuous rocks. The physical simulation experiment of hydraulic fracturing is an effective way to assess the geometry and propagation behaviors of HFs. However, the HF propagation behaviors are not generally well understood, especially the influence of multiple SDs on the HF geometry. To clarify this further, a comparative study of hydraulic fracturing on granite and shale was conducted to investigate the intersection mechanism between HFs and different SDs. The results show that HF propagation behaviors are characterized by six basic patterns: along the SD, crossing without dilation, crossing and dilation, captured by the SD, branching, and deflection. The intersection behaviors are closely associated with the cementing strength of the SD and differences in the fracture toughness of discontinuous rocks. Moreover, the fluctuation degree of the pressure-time curve and complexity of the HFs seem positively correlated. HF propagation in a rock matrix or cross interference with multiple SDs would induce a higher injection pressure and frequent fluctuations. The acoustic emission (AE) energy in granite was higher than that observed in shale. In addition, the generation of new HFs in the rock matrix can induce more AE events than those created simply by propagating along SDs. Experimental investigations can provide a theoretical basis to optimize the engineering parameters of field fracturing. [ABSTRACT FROM AUTHOR]

Under the significant pressure to diversify energy supply and reduce the dependence on fossil fuels, harnessing the vast geothermal resources has been made a priority in China's renewable energy development plans. This review provided an overview on the development of geothermal power generation and direct use in China, identified the key barriers for geothermal energy utilization, particularly in power generation, and proposed corresponding measures for boosting the growth of geothermal industry. Little progress was made in geothermal power generation over the past decades, while geothermal direct use, particularly geothermal heat pumps (GHPs) and district heating, has been growing quickly. Development of geothermal power generation was primarily limited by the lack of high temperature hydrothermal resources near the major demand centers. A series of technological challenges associated with drilling under extremely high pressure and temperature conditions, transportation of heavy equipment, long-distance electricity transmission, and hot dry rock (HDR) exploitation, also hindered large-scale development of geothermal resources for power generation and district heating. There were no social barriers for geothermal exploitation except in hot spring tourism areas, while the high cost could limit the consumer acceptance of geothermal heating/cooling solutions. Significant investment in research and development of enhanced geothermal system (EGS) technologies for exploiting HDR resources and establishment of favorable policy instruments are recommended as key technological and policy measures to accelerate the growth of geothermal energy market, while the potential social barriers for the development EGS projects in the densely populated areas should also be recognized and mitigated in the future. [ABSTRACT FROM AUTHOR]

A new analytical solution for the thermal-hydraulic coupling process is derived with a 1-D steady state conductive heat flow in the body of hot rock with perpendicular water flow in the single fracture and transient heat transfer from rock to water. The heat produced from the hot rock via water flow in the idealized single fracture is demonstrated by arithmetic equations. The applicability of the analytical solution is verified by numerical calculations and is limited to conditions with fast water flow rates or high water flux and long fluid pathways. 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 × 106 m2 was determined. The total mass flow rate of water injected into the large fracture was 57 kg/s. The discussion shows the ability of the model to estimate heat production and reservoir scale.

• Two-Well EGS Approach : Demonstrated a promising method for extracting heat from hot dry rock (HDR) systems using a two-well injection-production system. • Comparative Analysis : A comparative study between the Utah FORGE project and Fervo Energy's efforts showcases an interplay of advancements and methodologies in the field of Enhanced Geothermal Systems (EGS). • Laboratory Core Analysis : Utilized comprehensive core sample analysis to underpin field data interpretations, confirming key geothermal parameters like permeability and porosity. • Advanced Analytical Techniques : Utilized analytical techniques, routinely used in unconventional oil and gas reservoirs, to precisely measure effective formation permeability from pressure falloff analysis. • Numerical Modeling Insights : Provided critical insights through numerical modeling, particularly on the permeability enhancement via hydraulic fracturing of EGS systems for heat extraction purposes. In this paper, we present analyses of laboratory and field data indicating that the current two-well, injection-production system, connected with multiple hydraulic fractures, is a very promising method for extracting heat from hot dry rock (HDR) systems to generate electricity. The current two-well system could be expanded to a three-well system consisting of one injection well and two symmetric producing wells connected via the central hydraulic fracture emanating from the injection well. To improve a uniform distribution of the injected fluid among all hydraulic fracture stages in the injection well, we advocate for field implementation of a newly designed well stimulation technique, the GeoThermOPTIMAL. We first present an analysis of the post-fracturing flow data obtained from an HDR geothermal injection well at the Utah FORGE Enhanced Geothermal System (EGS) research field site. The site is adjacent to the Roosevelt hydrothermal (HT) field. The objective of the study is to assess the effectiveness of well stimulation in extracting heat from the low-permeability, hot dry granitoid rock in the Utah FORGE research site. The study includes interpreting pressure falloff data obtained during the well stimulation process and employing laboratory-measured core data as a major input in the interpretation of the field falloff data. As a confirmation of the robustness of our analysis in Utah FORGE, we reviewed and analyzed the flow test results published for an injection-production doublet at the Blue Mountain EGS (Project Red) commercial site in Nevada. From the analyses of these two field tests, we have concluded that the interpretation and findings of the Blue Mountain EGS pilot test are consistent with the interpretation and findings from the Utah FORGE field research project test results. In summary, our engineering assessments began with laboratory experiments conducted on various core samples, including those from a granite outcrop and the Utah FORGE geothermal reservoir. These experiments aimed to measure key parameters such as matrix and fracture permeabilities, and porosities (k m ≈ 10 − 18 m 2 , k f , e f f ≈ 10 − 15 m 2 , ϕ m ≈ 10 − 1 , and ϕ f ≈ 10 − 4 ). These data served as guides and inputs for analytical and numerical solutions used to match the field pressure response of the geothermal wells. While we did not have core samples from the Blue Mountain EGS wells, we successfully applied the Utah FORGE analysis approach to the Blue Mountain site with cautious optimism. [ABSTRACT FROM AUTHOR]

• The study delineates the heat contribution of the deep and shallow magma chambers beneath Mt. Changbai. • The study investigated the thermal energy transfer induced by shallow hydrothermal circulation in the Mt. Changbai area. • The study evaluates hydrothermal and hot dry rock geothermal resources in the Mt. Changbai area. Mount Changbai is an active volcano in Northeast China that has erupted several times since the Miocene. Investigating the potential high–temperature geothermal reservoirs associated with magma chambers under the volcano has a significant meaning for clean energy development and utilization. Through a geological field survey combined with drilling data and collected geophysical data, including seismic and magnetotelluric data, of the Mt. Changbai area, we constructed a geological model extending from the upper mantle to the surface. Based on the geological model, we performed thermal simulations to understand the evolution of temperature profiles under Mt. Changbai since the Miocene (∼25 Ma). The results were compared with temperature measurements from geothermal drilling and hot springs for validation. We found that: (1) The total temperature increase can reach ∼263 °C at –6 km (absolute depth) under the Mt. Changbai volcano. While the temperature increase has reached ∼224 °C since the shallow magma chamber appeared at ∼1.5 Ma, which accounts for >80 % of the total temperature increase. (2) Surface cold–water infiltration caused the decrease of subsurface temperature. However, the water can result in local accumulation of heat with a temperature increase up to 44 °C. (3) Both hydrothermal and hot dry rock geothermal resources are abundant in the Mt. Changbai area. Sedimentary strata of the Meso–Neoproterozoic and above that have good porosity and permeability, forming hydrothermal reservoirs. The temperature can reach 150 °C at a depth of 3 km under the Tianchi crater and are transferred to the surface by water through faults. While, metamorphic rock of the Archean–Paleoproterozoic and intrusive rock of the Mesozoic can form hot dry rock geothermal reservoirs with a temperature of 200–400 °C at a depth of 4–7 km under the Tianchi crater and surrounding areas. [ABSTRACT FROM AUTHOR]

Hot dry rock (HDR) is considered a new clean and renewable energy source. China is rich in HDR resources, but viable and reliable resource evaluation techniques and exploration methods have not been available yet. This research method, consisting of deep exploration combined with shallow drilling, screens the geothermal anomaly regions using heavy magnetic, electric measurement, seismic and thermal joint interpretation technology, and verifies the geophysical multiplicity with massive oilfield drilling data to locate the favorable HDR target region. The research results show a regionally geothermal abnormal area. We use relative impedance combined with seismic profile to recognize the HDR target region. The geophysical exploration and geological joint interpretation technology are faster and more accurate than the single geophysical exploration technology. The comprehensive analysis believes that the high resistance granite body in the southern Paleo-central uplift zone, with its area 500 km2, its thickness 150–330 m, its static formation temperature reaching 152° in the depth of 3360 m, its high rock thermal conductivity, its good deep fracture, and base fracture development, is a fragmented granite and weathered shell, easy for artificial reservoir re-constructing. There is high preciseness in locating the favorable target region for HDR development.

Hot dry rock (HDR) geothermal energy, as a clean and renewable energy, has potential value in meeting the rapid demand of the social economy. Predicting the temperature distribution of a subsurface target zone is a fundamental issue for the exploration and evaluation of hot dry rock. Numerical finite–element simulation is currently the mainstream method used to study the variation in underground temperature fields. However, it has difficulty in dealing with multiple geological elements of deep and complex hot dry rock models. A Unity networking for hot dry rock temperature (HDRT–UNet) is proposed in this study that incorporates the matrix rock temperature field equation for relating the three parameters of density, specific heat capacity and thermal conductivity. According to the numerical geological structures and rock parameters of cap rocks, faults and magma intrusions, a new dataset simulated by the finite element method was created for training the HDRT–UNet. The temperature simulation results in the Gonghe basin show that the predicted temperatures within faults and granites were higher than their surrounding rocks, while a lower thermal conductivity of the cap rocks caused the temperature of overlying strata to be smaller than their surrounding temperature field. The simulation results also prove that our proposed HDRT–UNet can provide a certain evolutionary knowledge for the prediction and development of geothermal reserves. [ABSTRACT FROM AUTHOR]

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.

In the Enhanced Geothermal System (EGS), the flow state of the fracture of the thermal reservoir (granite) is important for heat transfer. In this paper, the effects of reservoir temperature, flow rate and fracture shape on the thermal convection that occurs in the fracture of rough granite are studied based on the fractal dimension theory. The rough fracture is manufactured regularly by carving technology. The shear flow experiments were carried out at confining pressures of 10, 15 and 20 MPa to analyze the flow state. The convective heat transfer experiments were conducted at 60 and 70 °C to simulate different heat reservoir conditions. The results indicated that the outlet temperature and overall heat transfer coefficient (OHTC) increased with the increase in the rock temperature. A higher flow rate can enhance the thermal performance. The complexity of the fracture surface is also an important factor that affects the convective heat transfer process. The larger fractal dimension of the fracture surface can more easily excite turbulence, affecting the thickness of the boundary layer. The OHTC will increase as the fractal dimension increases.

Hot dry rock (HDR) geothermal energy, as a clean and renewable energy, has potential value in meeting the rapid demand of the social economy. Predicting the temperature distribution of a subsurface target zone is a fundamental issue for the exploration and evaluation of hot dry rock. Numerical finite–element simulation is currently the mainstream method used to study the variation in underground temperature fields. However, it has difficulty in dealing with multiple geological elements of deep and complex hot dry rock models. A Unity networking for hot dry rock temperature (HDRT–UNet) is proposed in this study that incorporates the matrix rock temperature field equation for relating the three parameters of density, specific heat capacity and thermal conductivity. According to the numerical geological structures and rock parameters of cap rocks, faults and magma intrusions, a new dataset simulated by the finite element method was created for training the HDRT–UNet. The temperature simulation results in the Gonghe basin show that the predicted temperatures within faults and granites were higher than their surrounding rocks, while a lower thermal conductivity of the cap rocks caused the temperature of overlying strata to be smaller than their surrounding temperature field. The simulation results also prove that our proposed HDRT–UNet can provide a certain evolutionary knowledge for the prediction and development of geothermal reserves.

ABSTRACT: Geothermal energy is a precious resource, which is widely distributed, varied, and abundant. China has entered a period of rapid development of geothermal energy since 2010. As shallow geothermal energy promoting, the depth of hydrothermal geothermal exploration is increasing. The quality of Hot Dry Rock (HDR) and related exploratory technologies are better developed and utilized. On the basis of geothermal development, this paper reviews the geothermal progress during the “12th Five-Year Plan”, and summarizes the achievements of hydrothermal geothermal and hot dry rocks from geothermal survey and evaluation aspects. Finally, the authors predict the development trend of the future geothermal research to benefit geothermal and hot dry rock research.

The paper describes application of the cross-impact method in the process of selecting locations and technologies used in a geothermal system based on energy accumulated in a dry rock formation, where CO2 is used as the working medium. The survey is based on the opinion of a group of 20 experts representing different fields of earth and technical sciences. They represent Norway and Poland, where the location of such a system is considered. Based on experts’ experience and opinions, all factors that seem to be significant were classified into the following groups: targets, key factors, results, determiners, motor and brakes, regulating factors, external factors, auxiliary factors, and autonomous factors. Direct influences between variables were indicated. Due to major differences in geological conditions in Poland and Norway, the factor of on- or offshore technology was pointed out as the primary determiner. Among key factors, the system operation’s long-term safety and level of technological readiness were indicated. As a target factor, an interest of local authority was pointed out. Among the variables that are important when selecting locations for this type of system, nine are essential: (1) Formal constraints related to local nature protection areas—this variable is essential in the case of an onshore system; (2) Availability of CO2 sources; (3) Level of geological recognition; (4) The distance of the CO2-EGS from a thermal energy user and electricity grid; (5) Existing wells and other infrastructure; (6) Depth of the EGS system; (7) Water depth if offshore, this variable is only important when offshore systems are involved; (8) Physical parameters of reservoir rocks; (9) Reservoir temperature.

Enhanced geothermal system (EGS) provides a feasible way for extracting geothermal energy from hot dry rock (HDR) reservoirs, whereas seismic risk it might induce and enormous cost impair its prospects. For sustainability, circulating fluid in a closed loop to extract thermal energy from a deep geothermal reservoir seems more reasonable and receives increasing attention. With this aim, using a super long gravity heat pipe (SLGHP) to extract geothermal energy from HDR reservoirs was tried by several researchers due to its extremely extraordinary heat transport ability. Jiang et al. designed a SLGHP, by which a thermal power as high as 200 kW was achieved from a 3000 m depth HDR reservoir successfully. The pioneering work proves the feasibility of a safer way for harvesting geothermal energy other than EGS, more importantly, it does not consume power and avoids the toughest issue of pipe scaling in a conventional geothermal production system. The article reviews the latest advances in SLGHPs for geothermal energy extraction. Laboratory experiments and representative in-situ tests are thoroughly analyzed with respect to technologies employing natural circulation inclusive of gravity-assisted heat pipe (GHP) to harvest geothermal energy. Theoretical and numerical work is also reviewed as significant supplements to experimental investigation. Key factors involved are discussed to clarify the challenges of GHPs in engineering for geothermal energy extraction. Low thermal extraction power and economic benefits as well as lack of highly efficient technology to convert heat to electricity are the main barriers of the application of SLGHPs in geothermal energy extraction and utilization. Nonetheless, previous investigation on SLGHPs offers new insights into geothermal energy exploitation from HDR reservoirs. • Advances of geothermal energy extraction using of SLGHP are summarized. • Prospects of SLGHP in harvesting geothermal energy are discussed. • Measures to improve the performance of a SLGHP are suggested. • A combined heat and power cogeneration system is proposed for in-situ geothermal energy utilization. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is often conducted to create artificial fractures as flow paths in Hot Dry Rock (HDR) geothermal reservoirs. However, the fractures could be closed due to the high geo-stress conditions, resulting in low efficiency of geothermal productivity. Hence, maintaining a high hydraulic permeability of the created fractures is a key point for the success of Enhanced Geothermal Systems (EGS). In this study, chemical stimulation was carried out to improve the hydraulic properties of fractured granite. The effects on the hydraulic properties of two sets of samples were examined by laboratory flowing tests. Results show that chemical stimulation solution with mixture of 12%HCl+5%HF improved efficiently permeability. The permeability was increased by four orders of magnitudes with an optimized reaction time. The mud acid was found to react fast with minerals such as feldspars and biotite, but rather inactive with quartz as revealed by the microscopic structure and ionic detection of the fluid and rocks. Thereby, the quartz particles can be used as fracturing proppant to support opening of aperture. The findings obtained from this study indicated that a hydraulic fractured granite based HDR geothermal reservoir can be effectively improved by chemical stimulation. [ABSTRACT FROM AUTHOR]

The convective heat transfer coefficient (HTC) is a useful indicator that characterizes the convective heat transfer properties between flowing fluid and hot dry rock. An analytical method is developed to explore a more realistic formula for the HTC. First, a heat transfer model is described that can be used to determine the general expression of the HTC. As one of the novel elements, the new model can consider an arbitrary function of temperature distribution on the fracture wall along the direction of the rock radius. The resulting Dirichlet problem of the Laplace equation on a semi-disk is successfully solved with the Green's function method. Four specific formulas for the HTC are derived and compared by assuming the temperature distributions along the radius of the fracture wall to be zeroth-, first-, second-, and third-order polynomials. Comparative verification of the four specific formulas based on the test data shows that the formula A corresponding to the zeroth-order polynomial always predicts stable HTC values. At low flow rates, the four formulas predict similar values of HTC, but at higher flow rates, formulas B and D, respectively, corresponding to the first- and third-order polynomials, predict either too large or too small values of the HTC, while formula C, corresponding to the second-order polynomial, predicts relatively acceptable HTC values. However, we cannot tell which one is the more rational formula between formulas A and C due to the limited information measured. One of the clear advantages of formula C is that it can avoid the drawbacks of the discontinuity of temperature and the singular integral of HTC at the points (± R, 0). Further experimental work to measure the actual temperature distribution of water in the fracture will be of great value. It is also found that the absorbed heat of the fluid, Q, has a significant impact on the prediction results of the HTC. The temperatures at the inlet and the outlet used for Q should be consistent with the assumptions adopted in the derivation of its corresponding HTC formula. A mismatched value of Q might be the reason that some existing HTC formulas predict negative or extremely large HTCs at high flow rates. [ABSTRACT FROM AUTHOR]

The local heat transfer coefficient (LHTC) can be an effective indicator to describe the local heat transfer characteristics of fractures but has rarely been employed to investigate the local heat exchange properties of a hot dry rock fracture compared to the overall heat transfer coefficient (OHTC). The aim of this paper is to investigate the LHTC of water flowing through a single fracture within a cylindrical granite specimen under confining pressure by combining the numerical modeling approach and the experimental results. A numerical model was successfully developed and verified by the test data. It is found that the local heat transfer coefficient is obviously closely related to the fluctuation of the fracture morphology. Within a certain range of aperture, narrower fracture has higher local heat transfer ability. Increasing the flow rate did not significantly improve the local heat transfer ability in the flat area of the fracture for a fixed fracture aperture. However, at the rougher areas, the flow rate has a higher effect, and the sunken positions at the fracture surface have much larger LHTC. A correlated model was developed to describe the relationship between waviness and LHTC. In addition, a comparison between the LHTCs and the OHTCs shows that the definition-based method presents the smallest values of the OHTCs, which are the closest to the arithmetic mean values of the corresponding LHTCs. [ABSTRACT FROM AUTHOR]

This article presents a parametric study of closed loop geothermal systems (CLGSs) design with multiple lateral horizontal wells, drilled in Hot Dry Rocks (HDRs), by developing a coupled finite element method (FEM) model for the problem of transient heat production in the reservoir and an Organic Rankine Cycle (ORC) model for electricity generation evaluation at the surface. The design parameters are well spacing and geometry, the number of wells, injection flow rate, energy losses along the well, and ORC working and cooling fluid mass flow rates. The study evaluates how these parameters affect the evolution of the spatial distribution of the temperature of the rock mass, the circulation fluid temperature leaving the reservoir, and the rate of electrical power generation with seasonal variations in surface air temperature. Analysis shows that the temperature evolution of the reservoir over a wide range of realistic properties is not influenced significantly by the rock and fluid properties, injection flow rate, nor well length and diameter and is a function of time only, and the initial rock mass and injection fluid temperatures. Therefore, in a CLGS with a hexagonal array of horizontal wells, the distance between the wells, beyond which the performance of a hexagonal array of wells is practically indistinguishable from that of a single isolated well, is 200 m over 30 years; i.e., the produced fluid temperature is practically unaffected by heat extraction from neighboring wells, regardless of reservoir properties. For smaller well spacing (100 m), the system shows practically negligible interaction in the first 15 years and only a small impact (∼ 1%) during the next 15 years of heat production. The performance of a CLGS mainly depends on the fluid temperature leaving the reservoir (falling rapidly in the first few months) and ambient surface temperature (varying over a year). Using data from the FORGE Utah site, analysis suggests that a CLGS with four 1 km horizontal wells operated at 2 m 3 /min/well can produce ∼ 5 MW of electrical power during cold months and none during hot months. • A parametric study of Closed Loop Geothermal Systems in hot dry rock. • Evaluated the effects of well spacing, diameter and flow rates on electrical power. • Determined an optimal inter-well spacing of 100m in a hexagonal array • Demonstrated the impact of seasonal air temperature on power generation. • Determined the optimum ORC mass flow rates for 5 MW CLGS. [ABSTRACT FROM AUTHOR]

• Reconstruction of reservoir rock samples by three-dimensional carving technology. • The effect of fracture morphology on fracture evolution was analyzed. • Distilled water and simulated geothermal fluid were used in seepage experiments. • The error between the actual aperture and the inversion aperture was verified. Long-term seepage under high temperature and high pressure affects fracture morphology, ultimately impacting the development efficiency of Enhanced Geothermal Systems (EGS). In this study, rock samples with different morphologies were produced and their morphologies were described quantitatively using fractal dimension theory. Experimentally, distilled water and simulated geothermal fluid were injected into fractures at a constant flow rate for 360 h. The behaviors of different fluid components during the fracture evolution process under high temperature and high pressure were observed. Results show that apertures and permeability decreased by 7.1% and 19.5% when distilled water was used, while they decreased by 0.6% and 1.8% respectively when simulated geothermal fluid was injected. The fracture apertures first decreased and then gradually increased. The more complex the fracture shape, the faster the mineral dissolution rate. With fractal dimensions of 1.007, 1.027 and 1.056, the dissolution rates of rock-like minerals in deionized water were 9.43×10−2, 1.33×10−1 and 1.51×10−1 mg/L per 12 hours, respectively. This study lays a foundation for further research on fracture network evolution and provides a basis for the long-term efficient and stable operation of EGS. [ABSTRACT FROM AUTHOR]

• Thick-walled hollow cube specimens with pre-crack were employed to measure fracture toughness during hydraulic fracturing of high temperature rock. • Hydraulic fracture tests were carried out for each temperature group of specimens using different injection flow rates to measure the fracture toughness under different test conditions. • The reduction in injection flow rate degraded the specimen's fracture toughness both independently and in a mutually reinforcing and controlled manner in conjunction with the temperature factor. • Comparison of the test results with the fracture toughness measured with NSCB specimens demonstrated the feasibility of the test method. The fracture toughness of hot dry rock (HDR) is a test parameter affected by the injection flow rate and temperature in the hydraulic fracturing of enhanced geothermal systems (EGS), and it is crucial to the determination of fracture expansion criteria in artificial thermal reservoirs. To investigate the fracture toughness of hot dry rock during hydraulic fracturing as influenced by the injection flow rate and temperature, hydraulic fracturing tests were conducted using thick-walled hollow cubic granite specimens with pre-cracks at different specimen temperatures and injection flow rates on a self-developed real-time high-temperature true triaxial load hydraulic fracturing apparatus. The specimen's fracture toughness was calculated by plugging the breakdown pressure into the corresponding fracture mechanics equation. Additionally, the acoustic emission events of the specimens during fracturing were recorded. The test results show a 14.11%, 21.11%, 24.66%, 33.54%, 43.84%, and 57.89% decrease in fracture toughness for six temperature groups of specimens from 25 to 500 °C when the injection flow rate was varied from 10 ml/min to 1 ml/min, and the fracture toughness decreased by 45.45%, 56.99%, and 73.26% at the three injection flow rates of 10 ml/min, 5 ml/min, and 1 ml/min, respectively, when the specimen temperature was from 25 °C to 500 °C. This indicates that a reduction in the injection flow rate could lower the specimen's fracture toughness both independently and in a mutually reinforcing and controlled way together with the temperature factor. The independent effect of the injection flow rate was attributed to the mean fracture power, and the thermal shock excited by the rapid cooling of high-temperature specimens during hydraulic fracturing was believed to be the essential cause of its interaction with temperature to affect the fracture toughness. The test results were closer to the actual fracture toughness of high temperature rocks during hydraulic fracturing than the NSCB results, demonstrating the feasibility of this test method. To date, this work is the most reliable method for calculating the fracture toughness of hot dry rock during hydraulic fracturing. [ABSTRACT FROM AUTHOR]

Although Australia is not usually associated with geothermal energy, it possesses significant amounts of both conventional (wet) geothermal and hot dry rock (HDR) geothermal resources. The country’s conventional geothermal resources are extensive, but are low temperature and are located in areas of low population density with the result that they are not used extensively for either electricity generation or for direct heating. Australia’s HDR resources, on the other hand, represent a world‐class resource but have not been used to date because the technology for converting HDR resources into electricity is not yet fully commercial. The effort being directed towards exploring economically useful HDR resources and in the development of technology to convert this energy resource into electricity could make HDR resources an important part of Australia’s energy mix in the near future, but there are major barriers. [ABSTRACT FROM AUTHOR]

Abstract: Utilizing information from plate tectonics characteristics, volcanic activities, and geothermal anomaly, this paper identifies areas where hot dry rock (HDR) may exist as potential geothermal resource in China. Further investigations are also carried out in the paper based on results from regional tectonics, volcanic geology and lithology, as well as data from geothermal displays, geochemistry, geophysics, and shallow borehole temperature measurements. The study reveals several promising areas of HDR geothermal resource in China, including Tengchong of Yunnan province, Qiongbei of Hainan province, Changbaishan of Jilin province, Wudalianchi of Heilongjiang province, and the Southern Tibet area. A 3D static heat conduction model was developed to study the underground temperature gradient characteristics of the Rehai geothermal field in Tengchong and the Yangbajing geothermal field in Tibet. The model adopted is a geological block 10km deep from the ground surface and 50km wide in each of the horizontal directions (2500km2 area). The numerical simulation results in evaluations on the quantities of the HDR geothermal resource in Rehai and Yangbajing geothermal fields. The paper shows that there is abundant HDR geothermal resource with large exploitation value in China. If developed with a power capacity of 1×108 kW, the Rehai and Yangbajing fields along would be able to generate electricity for 1560 years. [Copyright &y& Elsevier]

Enhanced geothermal systems (EGS) is currently the most efficient method to mine heat from hot dry rock (HDR) reservoirs. The coupled thermo-hydro-mechanical (THM) effect therein is of critical importance to predict the long-term performance of EGS. A novel 3D THM coupling model integrating displacement discontinuity method (DDM), finite volume method (FVM) and finite element method (FEM) is developed in this study to aid such prediction. The present model is first validated against analytical solutions, and then applied to a penny shape EGS to simulate a 30-year prolonged injection. The simulation results reveal that EGS maintains high productivity during the early injection stage but once thermal drawdown takes place, the EGS productivity degrades rapidly. The fracture aperture model employed in the present study is capable of simulating hydraulic fractures in which the fluid pressure is large enough to overcome the normal stress exerted to the fracture faces and thus jack the fracture open. In contrast, most early studies adopted a compressive aperture model, where a substantial part of the normal stress is borne by fracture gouge/roughness. The strength of DDM + FVM + FEM scheme lies in the efficiency and versatility of simulating fractures. Our study indicates that the DDM + FVM + FEM scheme has high potential to serve as a competent alternative to performing the THM coupling simulation in EGS. [ABSTRACT FROM AUTHOR]