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• 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]

• 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]

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]

• The continuous variation of radiogenic heat production with depth was initially obtained from a 4 km geothermal well within the Gonghe basin. • An unusually high concentrations of uranium were detected, and a high level of radiogenic heat production was found in the granitic basement within the geothermal well. • High radiogenic heat production values are very closely related to high uranium concentrations. • The existence of granitic rocks with high radiogenic heat production may not be the sole origin of the geothermal anomaly, but it gives rise to an important effect on the formation of HDR resources within the Gonghe basin. With the advancement of Hot Dry Rock (HDR) geothermal resources exploration in deep buried geological formations, the high radiogenic characteristics have garnered increasing attention and are considered to be the primary heat source for HDR development. The deep wells in the northeastern Gonghe basin geothermal area revealed a geothermal gradient of up to 45.2 ℃/km within the basal granitic basement, indicating its suitability for the exploration and development of HDR geothermal resources. Nonetheless, the research on HDR formation mechanisms is still under debate so far. Understanding the distribution of radiogenic heat production (RHP) with depth is crucial for comprehending the origin of the heat source mechanisms. Previous RHP calculations were mainly focused on samples extracted from outcroppings. Due to the limited available RHP datasets from deep wells, the variations of RHP with depth remain unclear. In the present research, 134 new samples were continuously extracted from a 4 km-deep geothermal well to decipher the in-situ RHP characteristics with depth within the Gonghe basin. These samples contain, on average, 2.40 % potassium, 12.93 ppm thorium and 2.87 ppm uranium for sedimentary cover, and on average 3.61 % potassium, 24.11 ppm thorium and 14.01 ppm uranium for granitic basement. On average, the radioactive isotopes in sediments and granitoids generate 1.65 ± 0.81 and 5.54 ± 0.61 µW/m3 of heat, respectively. Additionally, a 1D thermal simulation model was established to assess the impact of RHP on the formation of HDR. Modeling results indicate that RHP has a significant impact on the origin of the geothermal anomaly. For the proposed geothermal models, the presence of RHP within granitoids contributed 39–70 °C to the formation of HDR at the depth of 4 km. While RHP may not be the sole origin of the geothermal anomaly in the Gonghe basin, it does have a substantial impact on the thermal structure. Our findings in this study enhance the understanding of the heat source of the HDR resource within the Gonghe basin. [ABSTRACT FROM AUTHOR]

• Investigated the creep properties of granite at real-time high temperatures up to 800°C. • Investigated the thermal damage pattern of granite at real-time high temperature. • Explored the effects of temperature and stress level on the stabilized creep rate. • Explored the relationship between the long-term strength of granite and temperature. It is significant to study the mechanical properties of hot dry rock (HDR) for the development of deep geothermal energy. At present, the creep behavior of granite under real-time high temperature is not fully understood. The creep behavior of granite at 25 ∼ 800°C was investigated by real-time high-temperature uniaxial compression and graded load creep tests, and the thermal damage mechanism of granite was studied by scanning electron microscopy (SEM) experiments. The paper systematically analyzes the evolution of mechanical indexes such as uniaxial compressive strength (UCS), elastic modulus, creep deformation, steady creep rate and long-term strength of granite under thermal-force coupling. The results show that the UCS and elastic modulus of granite increase with increasing temperature in the range of 25 ∼ 200 °C, and decrease with increasing temperature in the range of 200 ∼ 800 °C. The damage speed of granite is the fastest in the temperature range of 400 ∼ 600 °C. The steady creep rate of granite increases with the increase of temperature and stress level. The ratio of long-term strength to UCS decreases with increasing temperature, from 93.6% at 25 °C to 73.2% at 800 °C. The research results provide relevant thermal damage mechanical parameters and theoretical basis for the development of HDR. [ABSTRACT FROM AUTHOR]

• The effects of distilled water and nano emulsion on the physical and mechanical properties of hot dry rocks at high temperatures are investigated. • The temperature increase promotes the formation of massive thermal damage fractures in the hot dry rock samples, and the fluid weakens the mechanical strength of hot dry rocks. • The distilled water has a more pronounced effect on weakening the mechanical strength of the samples than the nano emulsion. Hydraulic fracturing is a crucial technology for efficient heat extraction from hot dry rock (HDR). The fracturing process involves the injection of large volumes of cold fluids, which can significantly affect the petrophysical properties of the rock, particularly when temperature differences and fluid-rock interactions are taken into account. However, there is a lack of research on the evolution of HDR petrophysical properties under the influence of different fluids at high temperatures. In this paper, we utilized a high-temperature high-pressure tri-axial loading system, rock spontaneous imbibition instrument, porosity-permeability tester, and thermal conductivity meter, along with comprehensive analysis techniques such as ultrasonic wave velocity monitoring, CT scanning, and nuclear magnetic resonance (NMR), to systematically investigate the effects of distilled water and nano emulsion on the physical and mechanical properties of the HDR at high temperatures. Through comparison with the experimental results of dry samples, comprehensive characteristics of physical parameters for the HDR under the influence of distilled water and nano emulsion are obtained, such as spontaneous imbibition mass and permeability, thermal physical properties, and rock mechanical parameters. The experimental results show that temperature increase promotes the formation of massive thermal damage fractures in the HDR samples, and the fluid weakens the mechanical strength of the HDR. When the effects of temperature and saturated fluids are combined, the hydraulic fracture network in the HDR samples exhibits increased complexity, contrasting with the fracture patterns observed in the heat-treated dry samples. The distilled water has a more pronounced effect on weakening the mechanical strength of the samples than the nano emulsion. Moreover, the analyses of permeability, thermal conductivity, ultrasonic wave velocity, and NMR results are consistent with the results of mechanical experiments. As the temperature rises, the wave velocity of saturated distilled water samples exceeds that of both saturated nano emulsion samples and dry samples, the velocity of the saturated fluid front in the samples increases, and the range of the fluid in the pores expands. Overall, these findings provide valuable experimental evidence and theoretical guidance for better understanding the hydro-thermal-mechanical behaviors of the HDR, which are crucial for the efficient development and utilization of geothermal energy. [ABSTRACT FROM AUTHOR]

The investigation of the mechanical properties of hot dry rock (HDR) is of great significance to the development of deep geothermal energy. However, the effects of thermal-mechanical couplings on the mechanical characteristics of deep HDR are currently unclear. This study investigates the mechanical behavior of deep HDR through real-time high-temperature triaxial compression experiments. The paper systematically analyzes the evolution law of the triaxial compressive strength, deformation, and fracture features of granite from Gonghe Basin under different confining pressures, temperatures, and thermal-mechanical couplings. It evaluates the distinguished effects between the independent confining stress or temperature and the thermal-mechanical coupled conditions. The experimental results show that the mechanical properties of the Gonghe HDR exhibit different evolution laws under the effects of mechanical, thermal, and thermal-mechanical couplings. The triaxial compressive peak strength, elastic modulus, and axial peak strain of the Gonghe HDR are lower under the thermal-mechanical couplings than under confining pressure without high temperature. The maximum compressive volumetric strain is slightly larger under thermal-mechanical couplings than the other conditions. Moreover, the stress levels of the reservoirs are found the dominating factor affecting the HDR mechanical properties. However, temperature-induced degradation becomes significant if the temperature gradient increases to a high level. The outcomes help provide references for the safe construction of deep HDR drilling wells especially those with similar conditions to the Gonghe basin. [ABSTRACT FROM AUTHOR]

• Reservoir properties and working fluid types for EGS are summarized based on related global projects. • The dissolution or precipitation process of common minerals and its influencing factors are described in details. • Salt precipitation in saline aquifers and its effect on reservoir permeability are illustrated. • The impacts of working fluid-rock interactions on reservoir permeability, mechanical properties and heat extraction capacity are summarized. • The prospects of CO 2 -EGS and some limitations of current studies have been discussed. Geothermal energy from Hot Dry Rock (HDR) with poor permeability is recognized as a potential future energy source, and it can be exploited by Enhanced Geothermal System (EGS) technology. In this paper, geochemical mechanisms of typical working fluids interacting with the primary rock types of EGS reservoirs, and their impacts on reservoir properties are summarized. While traditional EGS primarily utilizes granite reservoirs, the technology has been gradually extended to include sandstone and carbonate rock formations. Regarding the working fluid, water is currently the only one that has been put into practice. Mineral dissolution and precipitation and salt precipitation are the two dominant geochemical mechanisms during the heat extraction process using water or CO 2. These mechanisms are influenced by fluid pH, reservoir temperature, pressure, and flow rate. The dissolution of feldspar minerals plays an important role in increasing permeability and the availability of cations for the precipitation of secondary minerals. Carbonate minerals are often the quickest to respond to changes in fluid chemistry induced by CO 2 injection, and the re-precipitation of carbonates is triggered by increasing temperature and pH. It is difficult to predict variations in permeability due to the involvement of many factors, including particle sizes and salt precipitation. The mechanical properties of rocks are significantly weakened following interactions with working fluids. The weakening effect on rock mechanical properties is more pronounced when CO 2 is injected into water-saturated formations. Although water-rock interactions can change the thermal conductivity of the rock, the characteristics of fracture networks appear to be of greater importance because working fluids mainly flow through fractures, and maximizing heat extraction from EGS depends strongly on effective circulation. Further economic evaluations for CO 2 -EGS are necessary to confirm the economic viability of using CO 2 as a working fluid. Additionally, the THMCB (Thermo-Hydro-Mechanical-Chemical-Biological) coupling effects during water/CO 2 interactions should be comprehensively studied based on both long-term field tests and truly integrated numerical models. [ABSTRACT FROM AUTHOR]

• Samples pre-equilibrated at 500 °C and exposed to LN 2 for a longer time showed the largest augmentation in measured permeability of approximately 90 %. • Permeability enhancements were corroborated with SEM observations that documented both freezing time and freezing-thawing processes induced new primary microscale fractures. • Values of the absorbed energy capacity follow the same trend as the compressive strength showing, progressive increases in UPC Peak and absorbed energy over the baseline value for thermal shocks with initial temperatures less than or equal to 300°C, but tests at 500°C showed dramatic loss in both UPC Peak and absorbed energy, typically only a third of expected value based upon the established trend with initial temperature. Liquid nitrogen (LN 2) is a new stimulation technology suitable for geothermal energy extraction that increases porosity, permeability, and overall contact area in candidate subsurface formations. It is considered appropriate for hot dry rock (HDR) reservoirs. Due to the significant temperature difference between hot rock and cryogenic fluid, extensive thermally induced cracking is anticipated upon contact, which is helpful in future water-steam cycling operations. To examine the degree of rock integrity failure in granite samples as a function of exposure mode, different procedures were followed for comparison. In particular, three commercial granite samples from three regions of Kazakhstan: Kapal-Arasan, Zhylgyz, and Zheltau, were heated to various elevated temperatures prior to immersion in LN 2 with variations in freezing time (FT) and numbers of freezing-thawing cycles (FTC). Samples were examined by various methods to quantify the impact of the cryogenic fracturing process. Compression, acoustic emission (AE), and permeability tests were performed on granite with different starting temperatures ranging from 200 °C to 500 °C. Permeability enhancement was generally in the range of 50-90 %. Scanning electron microscopy (SEM) was also used to document the microscopic characteristics of thermally-induced cracks and indicated most induced fractures to be microscale features. Results showed reductions in Unconfined Compressive Strength of LN 2 -treated samples by up to 76 %. The experimental outcomes demonstrate that LN 2 cooling of hot granite induces mechanical rock failure and permeability augmentation. Furthermore, the degree of thermo-fracturing increases with temperature difference and time of LN 2 treatment in both freezing time and freezing-thawing cycle methods. This is the first systematic study of its kind to test three commercially available granites from Kazakhstan with this combination of examination tools to assess alternate cryo-fracturing methods for geothermal energy prospecting and contact area enhancement. [ABSTRACT FROM AUTHOR]

• Nanoscale pore-fracture structure of HDR reservoir has been firstly reconstructed. • Significant physical properties of HDR reservoir have been revealed. • Nanoscale investigation is necessary for the physical properties of HDR reservoir. • Hydraulic and mechanical apertures of fractures in HDR reservoir have been compared. As a new type of renewable energy, Hot-Dry-Rock (HDR) is gaining increasing attention. The physical properties of rock influenced by the pore-fracture structure are vital for the hydraulic and chemical characteristics of HDR reservoirs. However, the natural pore-fracture structure in HDR has not been sufficiently studied. To fulfill this gap, we used focused ion beam-scanning electron microscopy (FIB-SEM) to construct three-dimensional (3D) digital models for four granite samples from the deep HDR reservoir in Gonghe Basin, Qinghai Province, China, and obtained the nanoscale structure of the pores and fractures in HDR reservoir for the first time. Our analysis based on the 3D models indicates that the heat transfer area in the HDR reservoir is mainly from fractures and crack-like pores, but a large amount of small but regular pores could be neglected when heat transfer in HDR is considered. The permeability of HDR reservoirs at nanoscale varies from 9.414 nD to 0.082 mD, and the high permeability in this study is accompanied by throughgoing fractures. The variation of porosity and permeability of deep HDR at nanoscale in this study is much more severe than that of shallow granites previously studied at microscale. The performance of geothermal field is significantly influenced by the size of throats and the average size and tortuosity of the interconnected pore-fracture network in HDR. The hydraulic aperture could be 32-58% lower than the mechanical aperture of the natural fractures in HDR reservoirs, which is a vital clue for the numerical simulation associated with enhanced geothermal systems. [ABSTRACT FROM AUTHOR]

• A novel technique of hydraulic fracturing assisted by the DTH-hammer-induced impact fractures is proposed. • Complex fracture networks can be formed near the wellbore. • The novel technique can significantly reduce the fracturing pressure and improve the permeability near the wellbore. Hydraulic fracturing is one of the key technologies in geothermal development of hot-dry-rock (HDR) geothermal energy. How to reduce the fracturing pressure and enhance the fractures connectivity of HDR has become a hot issue. In this paper, we propose to use a down-the-hole (DTH) hammer to impact granite, which assist to create new artificial fractures similar to natural fractures or widen the existing fractures near the wellbore, consequently enhancing the connectivity of fractures during hydraulic fracturing. A series of experiments were conducted to verify the feasibility of this stimulation technique, including the observation and quantification of impact fractures, and the determination of fracturing pressure. In addition, numerical simulation of impact stress wave was carried out to explain the distribution of fractures. The results showed that the granite rock mass damage caused by percussive rotary drilling was about 5–7 times larger than that caused by diamond rotary drilling. Meanwhile, the distribution of fractures was characterized by regionalization, which was mainly attributed to the attenuation of impact stress wave, and the numerical results were consistent with the experimental observations. Laboratory experiment results of hydraulic fracturing indicated that the fracturing pressure of granite samples near the wellbore after percussive drilling had been greatly reduced, which was nearly 50% lower than that of conventional drilling. It indicates that the proposed multi-fractured stimulation technique of hydraulic fracturing assisted by the impact fractures induced by a DTH hammer can be a potential solution to solve the problems of hydraulic fracturing. [ABSTRACT FROM AUTHOR]

• Alteration damage is consistently more severe with pressure increasing. • Damage to sandstone/granite is enhanced and to marble is weakened with temperature. • Temperature effect on mineral alteration depends on different activation energy. • Mechanical damage degree is linearly positively correlated with alteration extent. • Alteration damage variable is more accurate for insoluble mineral skeleton rocks. While high-efficiently developing hot dry rock (HDR), supercritical carbon dioxide (SC-CO 2) can cause mechanical damage to reservoir rocks by chemical alteration reactions. Accurately mastering the rock damage phenomena under different temperature and pressure is critical to directionally develop HDR in different reservoir environments. The interaction test between SC-CO 2 and rocks was performed under different temperature and pressure. By controlling the liquid environment to flow or not, the interaction test was divided into dynamic and static alteration tests to comparatively study the effect of flowing liquid on mechanical damage and chemical reactions. With pressure increasing, mechanical damage and chemical reactions both grow more severe. With temperature increasing, sandstone and granite are more severely damaged, while marble is insignificantly affected. Most reactions of rock forming minerals are catalyzed, while that of calcite is inhibited. Considering dynamic alteration effect, the damage to sandstone and marble is more severe, while that to marble is slighter. The reactions of insoluble minerals are enhanced, while those of soluble minerals are weakened. The basic reason for the differential performance of temperature effect is the activation energy E a of different minerals. With temperature increasing, the reaction rate of minerals with low E a significantly decreases, while that of minerals with high E a increases. In addition, a damage variable D ξ was established based on chemical kinetics method, showing high applicability for the rocks with insoluble mineral skeletons. [ABSTRACT FROM AUTHOR]

Highlights • Laboratory performances of cryogenic fracturing on granite under triaxial-confining stresses are investigated. • Breakdown pressures and fracture patterns of specimens under different cryogenic treatments and stress levels are compared. • A 3D fluid flow and heat transfer model for liquid nitrogen injection into hot dry rock reservoir is presented. • Field applications for liquid nitrogen cryogenic stimulation in HDR reservoir are discussed. Abstract Hot dry rock (HDR) has substantial potential as a thermal energy source. It can be exploited by hydraulic fracturing to extract heat and generate electricity. Because of its great depth (2–6 km) and consequent high temperature (150–650°C) and high pressure, the stimulation of HDR is difficult. The purpose of this study is to study the feasibility of cryogenic fracturing on HDR. We conducted a series of laboratory experiments on rock samples under triaxial-confining stresses. Two schemes were utilized: gas fracturing with and without liquid nitrogen (LN) (−196°C) treatment. For the LN treatment, high-temperature granitic specimens were immersed in LN for several hours before the fracturing tests. Three critical factors were considered, namely, rock specimen temperature (100–600°C), axial stress (5–20 MPa), and lateral stress (5–20 MPa). Subsequently, the fracture patterns were identified and correlations for both LN rapid-cryogenic and natural-cryogenic stimulations were developed to predict the breakdown pressure based on the experimental data. Finally, the field applications for LN cryogenic stimulation in HDR reservoir were discussed and a thermal-hydraulic coupled transient numerical model was established to provide suggestions of LN injection schedule for an effective cryogenic stimulation. The results show that the LN cryogenic stimulation could reduce the breakdown pressure level by 9–51% more than that of the untreated specimens. Particularly, LN provides superior fracturing efficiency on rock specimens above 200°C. This could be attributed to a sharp thermal gradient induced by LN, which can cause the strong local tensile stress to expand the micro-pore structures inside the high-temperature rock samples. Furthermore, with the assistance of LN, larger fracture apertures and better-connected fracture networks inside the rocks can be generated. Additionally, LN treatment, or rapid cryogenic fracturing, plays an increasingly important role when high-temperature rocks are subjected to higher anisotropic stresses. The key findings of this study could provide an efficient approach to explore HDR resources for geothermal development. [ABSTRACT FROM AUTHOR]

Liquid nitrogen jet fracturing is a novel stimulation technology, which is expected to be suitable for hot dry rock (HDR) reservoirs. Due to the large temperature difference between hot rock and cryogenic fluid, a great number of thermal cracks would be created during fracturing process, which is conductive to improve the penetration capacity of formation. In this study, a set of experiments were conducted to investigate the characteristics of thermal cracks. In these experiments, granite specimens with temperatures ranging from 200 ℃ to 300 ℃ were impacted by the low-pressure liquid nitrogen jet. The complexity and connectivity of cracks were quantitatively analyzed by a fractal method. The permeability and ultrasonic velocity of the granite specimens were tested in order to evaluate the damage conditions caused by thermal stress. Additionally, scanning electron microscope was adopted to analyze the microscopic characteristics of the thermal cracks. The results show that the heating process has a slight effect on thermal-crack generation compared with the liquid-nitrogen impact. The cracks mainly concentrate in the region near the impingement surface, due to the large temperature gradient there. The impacted rock breaks as the effects of tensile stress and shear stress. With an increase of initial rock temperature, the number of thermal cracks increases, and a more complex crack-network is formed in each specimen. Transient pulse evaluation and ultrasonic velocity measurements indicate that the impact of liquid nitrogen jet can improve the permeability and cause the damage of hot rock noticeably. This study demonstrates the important effect of thermal stress on crack generation during liquid nitrogen jet fracturing for HDR reservoirs, and the results shed light on the exploitation of HDR energy. [ABSTRACT FROM AUTHOR]

The Gonghe-Guide area, which is located at the northeastern edge of the Qinghai-Tibetan Plateau, has the greatest hot dry rock (HDR) geothermal resources exploration and development potential in China. Separated by the Waligong tectonomagmatic belt, the study area includes the Gonghe and Guide basins, which are characterized by distinctly different thermal state and surface thermal manifestations. Although a number of geophysical explorations have been undertaken, studies on fundamental geothermal theory remain scarce, including a lack of high-quality heat flow determinations and thermal structure studies. In this study, we obtained continuous steady-state temperature logs from three deep boreholes (GR2 and DR3 in the Gonghe basin and ZR1 in the Guide basin). The thermal conductivities and radiogenic heat production rates of the outcrop samples gathered from the two basins were measured. The heat-flow values were determined to be 116.3–122.3 mW m −2 for the Gonghe basin, yielding a mean of 119.3 mW m −2 . Two existed heat flow sites (R2 and R3 boreholes) gave an average heat flow of 75 mW m −2 for the Guide basin, which is approximately that of the regional background heat flow level of the northeastern Tibetan Plateau. The 1-D theoretical crustal thermal structures of the Gonghe and Guide basins were constructed by solving steady-state heat conduction equation. The results show that the Moho temperature and mantle heat flow in the Guide basin were 1016 °C and 27 mW m −2 , respectively. However, the temperature field in the Gonghe basin indicated partial melting in the crust. The high temperatures and thus heat flow in the Gonghe basin may suggest the existence of anomalous heat source body in crust. Successful inversions of vertical and horizontal temperature variations of boreholes in the Gonghe basin were obtained by numerically simulating a cooling rectangular magma chamber with a buried depth of 5–6 km and horizontal dimensions of 12–14 km. The study emphasizes that the anomalous heat flow in the Gonghe basin may be the joint thermal effect of: (1) the high regional background heat flow level of the northeastern Tibetan Plateau, which is mainly due to the radiogenic heat of thickened crust; (2) the cooling of a shallow magma chamber, which may be caused by the uplift and denudation of the plateau. [ABSTRACT FROM AUTHOR]

As one of the key technologies for hot dry rock (HDR) geothermal exploitation, artificial reservoir is giant hydraulic fracturing at its core and takes the theory of hydraulic fracturing of rocks under thermo-mechanical coupling as its key scientific problem. With large samples of Shandong grey granite being the experimental subject, this paper conducted hydraulic fracturing experiment under triaxial stress at 20 °C, 100 °C, 200 °C, 300 °C and 400 °C, analyzed the characteristic of water pressure loading curves in the experiment, and found that crack initiation pressure decreased remarkably after 300 °C. In order to verify the factors controlling the influence of temperature on crack initiation, this paper established a strong transient thermal stress model, carried out numerical computation on this model and compared it with the experimental results. The results showed that the cooling effect of fracturing fluid for high temperature borehole can lead to thermal shock phenomenon and cause tensile stress near the borehole surface; the area near the borehole experienced two impact shocks, namely, elastic wave and thermal wave; the mechanism for the effect of temperature on the hydraulic fracturing of granite is not changes of rock mechanical parameters, but the thermal shock generated by the action of fracturing fluid of rock at high temperature. [ABSTRACT FROM AUTHOR]

• A new drilling method combining HVEI drilling with mechanical drilling is proposed. • A combined HVEI-PDC bit is designed in this paper. • The HVEI-PDC rock fragmentation model is established in this paper. • Electrode spacing significantly affect the efficiency of the combined bit. Low rock breaking efficiency of drilling bit is one of the key factors restricting the large-scale development of oil gas in deep formations and geothermal energy resources in hot dry rock (HDR). The high-voltage electric impulse (HVEI) drilling technology has unique advantages in deep formation and HDR drilling. However, existing electrode bits form low-quality borehole walls and are not capable of performing directional drilling. To broaden the scope of engineering application of HVEI drilling, a new method combining HVEI drilling with mechanical drilling is proposed in this paper. Based on the structure of the existing electrode bits, a combined HVEI- Polycrystalline Diamond Compact (PDC) bit is designed, and the rock breaking mechanism of the bit is studied. A dynamic damage model (DDM) of rock containing an electrode bit is established on the basis of HVEI drilling's basic process. Furtherly, the HVEI-PDC rock fragmentation model is established by coupling the DDM with the heterogeneous granite model which is based on Voronoi subdivision. Through the analysis of granite fragmentation in HVEI-PDC drilling, relevant conclusions are obtained: Compared with conventional PDC bit, there is no significant difference in the torque variation of the combined bit during drilling. When the electrode spacing L e < 40mm, the energy consumption of the combined bit is higher than that of the conventional PDC bit at low rotating speed, and the efficiency is reduced. When L e ≥40mm, the larger the L e , the more obvious the speed-up effect of the combined bit compared with that of the conventional PDC bit, and the lower the drilling cost. The conclusions of this paper are expected to provide a reference for promoting HVEI drilling to achieve wider industrial applications. [ABSTRACT FROM AUTHOR]

• Development of a comprehensive methodology to assess the risk of earthquakes induced by hot dry rock mining. • Build DFN and hydro-thermal coupled numerical models. • The magnitude of earthquakes during hydraulic fracturing is generally less than 2.0. • The maximum earthquake magnitude during a 30-year development period is less than 1.28. Earthquakes induced during hot dry rock (HDR) geothermal development have attracted extensive attention and constrained the further exploitation of HDR resources. Seismic risk evaluation of HDR projects is essential. The first HDR demonstration project in China is located in the Gonghe Basin in Qinghai. The EGS (enhanced geothermal systems) site is located near Gonghe County and is very seismically sensitive. The seismic risk induced by hydraulic fracturing and long-term cycling during the HDR development process was evaluated using a multi-field coupling method based on analysis of the geothermal geological conditions, ground stress, and natural fracture characteristics. During hydraulic fracturing, the predicted magnitude of earthquake events is generally below 2.0, with a range of 0.7–1.3. During a 30-year development period, the predicted maximum earthquake magnitude is less than 1.28. The HDR development will not have an impact on the local geological safety. Considering the uncertainty of the geology, a further discussion of measures to reduce the risk of earthquakes is presented. An entire induced seismic risk evaluation process method is developed, which can provide a reference for related research. [ABSTRACT FROM AUTHOR]

• The initial water saturation condition has a significant impact on the fracture conductivity post-stimulation. • Polymer-alternating-CO 2 (PAG) injection strategy was found to consistently generate fractures with the highest conductivity values (up to two orders of magnitude) independently of injection flow rate for hot dry rock and 1-day water pre-flooded rock. • Higher water saturation in rock (3-day water preflooded rock) will lead to lower breakdown pressure and lower permeability values post-stimulation independently of the fracturing fluid used. • Nevertheless, PAG was the only fluid that attained a relatively higher conductive fracture under these high water-saturation conditions. StimuFrac (US Patents 9,873,828 B2 and 9,447,315 B2), a CO 2 -reactive polymer aqueous solution [polyallylamine (PAA) 1wt% in water] combined with CO 2 , can potentially be used as a less water-intensive fracturing fluid for enhanced geothermal systems (EGS). Our previous results show that in hot dry rock (HDR), PAA/CO 2 fracturing fluids outperformed other fluids such as water, CO 2 , and CO 2 /water in generating large fractures with less fluid consumed. The objective of this work is to investigate the effect of initial water saturation on the performance of StimuFrac by conducting hydraulic fracturing tests with ½ foot cubic rock samples held under representative EGS stress/temperature conditions and by using cyclic injection strategies (under constant injection rate). The resulting fracture hydraulic conductivities, breakdown pressures, and volumes of fluids required are compared. To simulate geothermal reservoir conditions, in all tests, the rock sample was held under triaxial confinement and at 200 °C, and different volumes of water were initially injected into the rock sample before any fracturing processes were initiated. For the single-cycle PAA (or water) alternating CO 2 (PAG or WAG) injection fracturing experiments, one complete cycle consisted of two steps: (1) injecting a PAA slug (or water slug) followed by (2) injecting CO 2 to initiate and propagate the fracture. For experiments involving multiple injection cycles, the CO 2 injection pressure is increased until it peaks and begins to decline (indicating fracture initiation at this moment), and then continued being injected for another 30 s to propagate the fracture. Then, these two-step cycles [injection of PAA (or water) followed by CO 2 injection (up to 2–4 mL/min)] are repeated. The results of this study suggest that water saturation significantly affects the fracturing fluid transmission into the rock pore space, thus affecting fracture initiation and propagation. In this study, fracturing tests via a single injection cycle or multiple injection cycles were performed. When the rock samples are split in half following testing, it is evident that fracture propagation is considerably restricted under high water saturation conditions (where a three-day initial water injection was conducted) in comparison to stimulation experiments carried out in hot and dry rock. The fractures propagate less than 1/3 of the distance from the wellbore to the outer rock surface, and in some cases, no fracture is generated. This may be caused by leak-off dominating the fracturing process and the fluid injection rate is insufficient to overcome leak-off, even under high injection rate conditions. Additionally, CO 2 could be leaking off into the wellbore annulus and this may be making it more difficult to generate sufficiently high-pressure gradients away from the near-wellbore region. Under low water saturation conditions (dry rock or after 1-day initial water injection), PAA/CO 2 consistently generated significantly larger fractures compared with the other fluids. CO 2 generated large fractures only in the hot dry rock and only when using high injection rates, though data variability is high. [ABSTRACT FROM AUTHOR]

• A new idea is proposed to improve the fracability of hot dry rock under alternating temperature loads. • Effects of alternating temperature loads on mechanical properties of hot dry rock are experimentally investigated. • Microstructure analysis under alternating temperature loads are given in detail. Inspired by the physical process of thermal expansion and cold shrinkage, this study proposes a new idea to improve the capacity to form complex fractures (i.e., fracability) in hot dry rock (HDR) under alternating temperature loads. However, the evolution of the HDR mechanical properties under alternating temperature loads is rarely reported. This study analyzed the evolution mechanism of the HDR rock mechanics properties under alternating temperature loads by using the high-temperature high-pressure rock mechanics triaxial test system. The results show that mechanical parameters such as compressive strength, dilatancy stress, and elastic modulus are significantly reduced after alternating temperature loading, compared with those after simple heat treatment. After alternating temperature loading, compared with the case of simple heat treatment, the HDR rock samples reach the dilatancy stress earlier; its stress-strain curve features a visible concave arc at the micro-fracture closure segment; the axial loading ends up with the shear failure and enhanced complexity. The microstructural analysis shows that cyclic thermal stress occurs inside the HDR as the temperature increases and extensive mineral metasomatism is triggered, which is dominated by quartz penetrating plagioclase via metasomatism. Therefore, the decrease of quartz content and the HDR mineral metasomatism under alternating temperature loads are the fundamental causes for the great changes in the HDR mechanical properties. [ABSTRACT FROM AUTHOR]

• A numerical model of high pressure water flowing through a rough rock was established. • A compensating method was proposed for optimize flow rate to postpone thermal breakthrough. • The influence of thermo-physical properties on heat extraction performance were clarified. 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. [ABSTRACT FROM AUTHOR]

• Induced seismicity mitigation methods for EGS development are investigated based on laboratorial acoustic emissions. • Acoustic emission characteristics are analyzed by true-triaxial hydraulic fracturing experiments under different injection flow rates, temperatures and confining stresses conditions. • Induced seismic risk is positively correlated with the injection flow rate, negatively correlated with the confining stress. Geothermal energy has attracted more attention lately. During geothermal energy extraction from Enhanced Geothermal Systems (EGS), hydraulic fracturing has become a standard reservoir stimulation technology. Induced seismicity during this stimulation process is inevitable and has been documented worldwide. Sometimes, large magnitude earthquakes may be induced during hydraulic fracturing process, such as the EGS engineering project failure in Pohang and Basel. The induced seismicity has already become the main obstacle for the development of EGS. Therefore, it is necessary to investigate the characteristics of induced seismicity to find seismic mitigation strategy. To advance this mitigation research, large-size Hot Dry Rock (HDR) formation environment is established to analyze induced seismicity characteristics. Acoustic emission (AE) is used to simulate induced seismicity in field, the AE characteristic parameters are analyzed under different experimental conditions. The results of AE indicate that high injection flow rate and high confining stress could increase the maximum induced earthquake magnitude, the induced seismic risk will increase in these two circumstances. The increase of temperature do not obviously affect AE occurrence and slightly decreases the maximum AE energy which means that hydraulic fracturing in high temperature reservoir will not increase seismic risk. These results of this study can provide some reasonable guidance on the application of induced earthquake mitigation in field-scale HDR hydraulic fracturing operation. [ABSTRACT FROM AUTHOR]

Multiple-well Enhanced Geothermal Systems (EGS) can enable economic recovery of energy from underutilized hot dry rock (HDR) reservoirs. Hydraulic fracturing is a promising stimulation method for improving fluid flow and heat extraction in EGS. Laboratory simulations of EGS with hydraulic fracture stimulation have recently been completed in two large 300 × 300 × 300 mm 3 granite block specimens to better understand this complex process of geothermal energy recovery. The first experiment implemented a binary well layout with an injector and producer. The second experiment used a triplet well layout with one injector and two producers. Selection of production well trajectory so as to intersect the hydraulic fractures was guided by acoustic emission (AE) events collected during stimulation. Both model reservoirs were subjected to heating and true-triaxial stress confinement throughout a series of drilling, stimulation, and flow and heat circulation tests. Stimulated thermal reservoir flow was characterized by a series of constant pressure, constant flow rate, stepped constant pressure and stepped constant flow rate injection tests. Tested blocks were cross-sectioned to characterize final locations and 3D geometries of the induced fractures. Insights and lessons learned from these experiments are presented with focus on application to field-scale EGS. [ABSTRACT FROM AUTHOR]

• The uncertainty of nature permeability is randomly simulated through TBM. • Both heterogeneity of nature permeability and permeability enhancement are considered. • A comprehensive index is proposed to comparatively evaluate the EGS performance. • Three typical modes of EGS performance are identified. An enhanced geothermal system (EGS) has two critical features in the stimulated reservoir volume (SRV): random distribution of permeability and definite diminishing enhancement of permeability. However, the effects of these two critical features on heat extraction simulations of hot dry rock (HDR) reservoirs have been rarely studied. This paper will propose a theoretical framework to incorporate these two features into an uncertainty study of heat extraction performance. First, five sets of random fluctuations are generated with turning bands method (TBM) to characterize the uncertainty feature of permeability. Five descend trends in an elliptic SRV are constructed to characterize the definite diminishing feature of permeability. Then, nine hundred stimulated virtual reservoirs are constructed in two groups by assigning the superposition of random fluctuations and descend trends to the permeability. Third, a thermal-hydraulic-mechanical coupling model is established to simulate the heat extraction from these virtual reservoirs. Finally, several indices are defined for the elevation of EGS performance. The statistical analysis on production flux and thermal breakthrough time of these simulations identified three modes of EGS performances. Larger correlation length and variance result in higher uncertainty of EGS performance. Larger permeability enhancement in the SRV zone consistently increases the production flux of a stimulated reservoir. Small flux induced by large impedance is the first barrier to a satisfactory EGS performance of low permeability HDR reservoirs with current reservoir stimulation techniques. [ABSTRACT FROM AUTHOR]

• The geothermal potential of the Spanish region of Ávila is evaluated. • The temperature of the ground is measured at different levels through available drillings. • Two main areas are identified for potential heating uses or even HDR purposes. • An interactive visualization tool includes the principal results of the work. • Results will be essential for planning future geothermal uses in the region. Exploring and exploiting a geothermal resource has become one of the most prolific tasks for contributing to the global sustainable development. Despite this fact, several countries, such as Spain, are still far from achieving a generalized use of these renewable systems. The reason for this underuse often derives from the lack of information and characterization of the geothermal energy source. Considering this, the present research aims to provide relevant data about the geothermal potential of the Spanish region of Ávila. The geological context of this province lays the foundations for considering the region as a promising site for different geothermal uses. In order to estimate the geothermal energy potential of the region, the existing geological information has been complemented with thermal surveys carried out in the study area. The experimental measuring has consisted of the register of the underground temperature in piezometers of variable depth and natural springs distributed throughout the province. The processing of these records has allowed knowing the thermal evolution of the subsoil at the different levels evaluated in the research. Results show that there are two main potential areas in the province that could be successfully used for heating purposes (maybe as part of district heating systems) and for future deeper evaluations in the sense of Hot Dry Rock (HDR) techniques. Final conclusions have also been included in an interactive and open-source tool that allows visualizing the thermal findings with the aim of planning future geothermal uses in the region. [ABSTRACT FROM AUTHOR]

• Provide a novel perspective to optimize the arrangement of PDC bits. • Evaluate the performance of PDC bit in high-temperature formations. • Investigate the penetration process of high-temperature granite. • Mechanical properties of granite at different temperatures are determined. • The relationship between penetration force and rock failure modes is validated. Hot dry rock (HDR) is a renewable and clean energy resource, its efficient exploration has attracted widespread attention from scholars. However, the rock encountered during drilling is characterized by poor drillability, high abrasiveness and great hardness, which aggravates the wear of bits and increases drilling costs. To address such an issue, much research has been conducted to investigate the failure mechanism of high-temperature rocks. But most of them focused on the rock cutting process by PDC cutters and the penetration of rocks was ignored. In the present study, a series of experiments were performed to determine the penetration efficiency of PDC cutters under different temperatures and back rake angles. The penetration force and 3D topography of penetration pits were measured to analyze rock failure characteristics. Besides, uniaxial compressive experiments were carried out to study the effects of temperature on rock properties. The results indicate that as the rock is heated from 25 to 240 ℃, its properties change slightly. Analogous rock failure characteristics can be found in rock indentation process, where chipping and crushing of rocks, on following another, are presented, respectively. Moreover, the occurrence of chipping is the major contributor to the increase in penetration efficiency. The further increase in rock temperature (greater than 360 ℃) results in a sharp decrease in the strength of rocks, which is accompanied by the enhancement of rock plasticity. At his moment, the plastic failure dominates rock indentation process under different back rake angles. The induced horizontal force cannot promote brittle failure and the penetration efficiency is hard to be improved, significantly affecting the performance of conventional PDC bits in high-temperature formations. Based on the results, the low penetration efficiency at high temperatures is a major obstacle that restricts the exploration of HDR. Increasing the cutting depth should be an important target in optimizing drilling parameters and the arrangement of PDC bits. [ABSTRACT FROM AUTHOR]

• Four fracturing fluids (water, CO 2 , CO 2 /water, CO 2 /PAA) were evaluated in ½-foot side cubic granite samples at 200°C and under geothermal relevant stress conditions. • For CO 2 -based stimulation fluids, i.e., CO 2 injected in hot dry rock, CO 2 injected in rock partially saturated with water, and CO 2 injected in rock partially saturated with aqueous polyallylamine (PAA), the fracture conductivity is larger than that of water. • The leakoff rate of CO 2 can be significantly reduced using PAA due to the CO 2 -triggered crosslinking reaction of PAA building up pressure faster and creating larger fractures as compared to the other fracturing fluids. • CO 2 /PAA fracturing fluid system was found to consistently generate fractures with the highest conductivity values and independently of injection flow rate. The objective of this work is to study the performance of 1wt% StimuFrac fluid [polyallylamine (PAA) in water] in ½-foot size rock samples under thermal and fluid phase conditions representative of enhanced geothermal systems (EGS) and reveal the mechanisms governing the fracturing process. Four fracturing fluids including water, CO 2 , CO 2 with water, and CO 2 with aqueous PAA were used. For all the water "only" fracturing tests, the conductivity of the rock fractured is quite low (less than 2 μ m 3 based on radial flow assumption), regardless of the constant flow rate or discrete pressure increments injection approach. For all three CO 2 -based fracturing fluids, granite was fractured at higher breakdown pressures, higher transient flow rates, and generated higher-conductivity fractures as compared to water. When partially saturating the rock sample with 1wt% PAA aqueous solution followed by fracturing with CO 2 , the volume expansion and viscosity increase triggered by CO 2 -induced cross-linking PAA leads to a faster pressure increase than CO 2 in water saturated or initially dry. This faster pressurization rate is caused by a decrease in relative permeability of CO 2 in the presence of the high-viscosity cross-linked fluid compared to the un-crosslinked CO 2 /water. It was also found that CO 2 as a fracturing fluid can generate high fracture conductivity only when injected at very high flow rates in the presence of water or in hot dry rock (HDR). However, the conductivity of CO 2 fracturing in HDR is highly variable. CO 2 injected in the presence of 1wt% aqueous PAA generates fractures with the highest conductivity independently of injection flow rates and using 1/6 of the mass of CO 2 as compared to CO 2 fracturing in HDR. The results of this study suggest CO 2 /PAA is the best performing stimulation fluid under the studied P/T conditions. CO 2 /PAA offers the following three additional advantages over waterless CO 2 , and CO 2 /water fracturing fluids: i) it requires significantly lower volumes of CO 2 due to the reduced leak off; ii) large fractures can be generated reproducibly at both low and high CO 2 injection flow rate, and iii) the reversible (previously reported) viscosity increase could be beneficial to transport proppants when they become available for EGS. [ABSTRACT FROM AUTHOR]

• Two types of high-temperature geothermal resources are identified in the Gonghe-Guide area. • The current knowledge of the geology, geophysics, geochemistry and geothermal investigations are described. • Comparative analysis of the characteristics of the geothermal resources between the Gonghe-Guide area and other geothermal fields is conducted. The new discoveries of the high-temperature geothermal resources in the Gonghe-Guide area mark the significant breakthrough in the exploration of geothermal resources in the northeastern Tibetan Plateau. In this paper, we systematically reviewed all the available data presented in the geological, geophysical, geochemical and fundamental geothermal works that have been undertaken, in order to fully understand the characteristics of the geothermal resources in this area. The main features of the geothermal resources in this area can be summarized as follows: two distinct types of high-temperature geothermal resources have been identified, i.e., the hot dry rock (HDR) and hydrothermal geothermal resources in the Qiabuqia and Zhacang geothermal areas, respectively. The HDR reservoir in the Qiabuqia geothermal area is mainly composed of the Indosinian granitic rocks with the temperature gradient of 39 to 45.2 °C/km, thermal conductivity of 2.07 to 3.10 W/(m•K), and radiogenic heat production rate (RHPR) of 1.17 to 5.81 μW/m3. The geothermal water in the Zhacang geothermal area is meteoric origin with the relative enrichment of δ18O caused by the fluid-rock interaction under the high temperature condition. The chemical type of the geothermal water is SO 4 •Cl-Na. The local crustal partial melting or magma chamber at the 15 ∼ 35 km beneath the Gonghe basin, which is characterized with a relatively high geothermal background induced by the continental collision between the Indian and Eurasian plates, is the main cause of the HDR resources, whereas the hydrothermal resources in the Zhacang geothermal area are related to the deep circulation of the meteoric water in the damage zone of the west-east trending conductive Zhacang fault and upward transporting at the intersection of the Zhacang and the NNW-trending impermeable Reguang faults. Such a detailed review of the both geothermal resources can provide a glimpse of the geothermal resources of the northeastern Tibetan plateau as well as provide the direction for the subsequent searches for high-temperature resources elsewhere in China. [ABSTRACT FROM AUTHOR]

Abstract: The Hijiori hot dry rock (HDR) system, Japan, consists of a shallow and a deep reservoir, both in fractured granitic rocks. During a long-term circulation test (LTCT) lasting approximately 18 months and which tested different fluid production scenarios, large changes were observed in output fluid temperatures, pressures, and flow rates. A multi-reservoir, numerical model of the Hijiori HDR system was developed using the finite element heat and mass (FEHM) transfer code and applied to simulations of the LTCT. The model reproduced the pressure, temperature, and flow data observed during the test. Based on the modeling study, it was concluded that most of the produced fluid came from the shallow reservoir, that the permeability of the deep reservoir changed during the initial part of the LTCT, and that the redistribution of injected water between the two reservoirs had little impact on the relative amounts of deep and shallow fluid production. After validating the model on the LTCT, it was used to optimize injection rates in both reservoirs. The model was also used in simulations of reservoir performance where an additional heat transfer surface area has been created in the subsurface through hydraulic fracturing. [Copyright &y& Elsevier]

Abstract: Mineral scales were formed in wells and surface installations during a 2-year circulation test performed at the Hijiori Hot Dry Rock (HDR) site, Yamagata, Japan. Anhydrite deposited in the deeper parts of the production wells, while silica and calcium carbonate precipitated at the surface, downstream of the wellhead, in amounts that depended on the temperature and chemical composition of the produced fluid. In well HDR-2a, closer to the injector, most of the scale was calcium carbonate; in HDR-3, further away from the injection well, there was slight precipitation of amorphous silica. As fluid circulation progressed and temperature decreased, scaling in the flow line of well HDR-2a changed from amorphous silica to calcium carbonate. [Copyright &y& Elsevier]

Abstract: The 2000 and 2003 hydraulic stimulations carried out at the Soultz-sous-Forêts Hot Dry Rock (HDR) site, Alsace, France, generated a continuous, albeit of small magnitude, seismic activity. For each experiment, the Ecole et Observatoire des Sciences de la Terre (EOST) installed a seismic network of 18 stations (in 2000) and 23 stations (in 2003). More than 3500 seismic events were detected and located in both cases. Changes in the physical properties of the reservoir due to fluid circulation have been estimated by means of a 4D tomography in which the 3D seismic velocity structure is calculated for different time windows. The entire data set has been apportioned to different temporal subsets with a fixed number of events. We used the tomographic algorithm of [Thurber, C.H., 1983. Earthquake locations and 3D crustal structure in the Coyote Lake area, central California. J. Geophys. Res. 88, 8226–8236] for the 2000 data and a new, more accurate method that is based on a double-difference tomography [Zhang, H., Thurber, C.H., 2003. Double-difference tomography: the method and its application to the Hayward fault, California. Bull. Seismol. Soc. Am. 93 (5), 1875–1889] for the 2003 set. The latter approach gives a good resolution within the reservoir volume, and one that is slightly less so in its vicinity. This tomographic study points out the changes in reservoir velocity structure occurring during the stimulation. We discuss the variation of the seismic velocity with time during each of the hydraulic stimulations and compare the velocity changes observed in 2000 and 2003. This analysis has allowed us to define the region of influence of hydraulic activity and determine the effects of water circulation on geothermal reservoir properties. [Copyright &y& Elsevier]

Production in geothermal reservoirs often leads to observable surface displacement. As shown in this paper, there is a direct relationship between such displacement and reservoir dynamics. This relationship is exploited in order to image fluid flow at two geothermal field sites. At the first locality, the Hijiori Hot Dry Rock (HDR) test site, 17 tilt meters record deformation associated with a 2.2 km deep injection experiment. Images of fluid migration along a ring fracture system of the collapsed Hijiori caldera are obtained. At the Okuaizu geothermal field, leveling and tilt meter data provide constraints on long- and short-term fluid movement within the reservoir. A set of 119 leveling data suggest that the north-to-northeast trending Takiyagawa fault acts as a barrier to flow. The northwesterly oriented Chinoikezawa and Sarukurazawa faults appear to channel fluid from the southeast. The tilt data from Okuaizu indicate that a fault paralleling the Takiyagawa fault zone acts as a conduit to transient flow, on a time scale of several weeks. The volume strain in a region adjacent to the injection wells reaches a maximum and then decreases with time. The transient propagation of fluid along the fault may be due to pressure build-up, resulting from the re-initiation of injection. [Copyright &y& Elsevier]

• The discretization and implementation of Finite Volume Method for displacement field are proposed. • A local grid refinement algorithm for displacement field is presented. • A unified technical framework for coupled fluid flow and geomechanics simulation is proposed. • Thermal-Hydraulic-Mechanical coupling model is solved totally with Finite Volume Method. The hot dry rock (HDR) development is a hot research field in recent years. Numerical simulation based on thermal-hydraulic-mechanical coupling model is a key technique for the HDR development evaluation. The calculation of geomechanics is an important part of the coupling thermal-hydraulic-mechanical simulation, and it is usually implemented by finite element method. However, meshing methods of geological modeling software are designed to serve seepage simulation, and grid quality cannot meet the requirements of finite element method. In this paper, finite volume method has been proposed to calculate the distribution of underground stress field, in order to solve the problem of discontinuous grid vertexes and calculation failure caused by finite element method. In addition, the mesh of geological modeling software is for finite difference method or finite volume method, which cannot be used directly by finite element method. In this paper, spatial discretization pattern of quasi static stress balance equation based on the finite volume method is obtained, the local grid refinement algorithm is proposed, the solution workflow of thermal-hydraulic-mechanical (THM) coupling model is presented, and the solution of displacement discretization equations and THM one way coupling model are implemented by C++ programming language. Finally, the feasibility and accuracy of the finite volume method of stress field calculation are verified through a series of numerical test examples compared with commercial finite element software. Several THM coupling simulation cases of enhanced geothermal system are presented by finite volume method. Numerical test results show that the calculation error between finite volume method and finite element method is less than 1%. Moreover, the variation of principal stress, induced by pore pressure and thermal stress, can reach 8 to 20 MPa. The finite volume method provides a unified technical framework for geomechanics and thermal-hydraulic calculation. Moreover, the method can be directly combined with geological modeling software without using two sets of grids. The results prove that the finite volume method would be a promising approach for thermal-hydraulic-mechanical simulation. [ABSTRACT FROM AUTHOR]

• A novel method to improve the drilling efficiency of HDR is proposed. • Thermal coupled FSI analysis of rock failure process. • Laws of heat transfer under the impact of water jet and LN2 jet is obtained. • A novel method was introduced to solve issues involving multi-physics. • Rock cutting experiments were conducted to investigate the drilling of HDR. Mechanical cutting assisted by high-pressure jet is a common and efficient drilling method in the exploration of hot dry rock (HDR). The drilling efficiency can be further improved with an in-depth understanding in the rock-breaking mechanism, which is a complex process involving the coupling of thermal stress, jet impact pressure and cutting force. To address such a multi-physics coupling issue, a loosely coupling method was adopted in this work to accomplish the exchange of data, including temperature, heat flux, nodes deformation and impinging pressure of jet in every time increment. In the fluid subdomain, the liquid nitrogen (LN2) jet and water jet were modeled and compared to demonstrate the potential of the novel jet in improving the drilling rate. In the solid subdomain, cutting force monitored in the experiment was loaded, and the concrete damage plastic model was employed to evaluate damage behaviors of the rock. Relative experiments were conducted to validate the reliability of simulation results. Results indicate that a great temperature difference develops on the coupling surface under the impingement of LN2 jet due to its cryogenic feature and lower heat transfer efficiency. Thereby, irreversible damage of the rock can be induced even at room temperature (298 K). As for the water jet, tensile damage is not observed until the rock temperature increases to 473 K. The assistance of jet impact on rock breaking is mainly reflected in two aspects: (1) the occurrence of tensile damage on the coupling surface facilitates the subsequent rock cutting; (2) the enhancement of tensile damage around the cutter promotes the initiation of cracks. Higher rock temperatures significantly improve the heat transfer rate and enhance the advantage of jet assistance. The present study can provide a new sight for the exploration of HDR. [ABSTRACT FROM AUTHOR]

• A novel thermal-hydraulic-mechanical coupling model for EGS is proposed. • Grid number can be significantly reduced based on the EDFM and XFEM. • Distributions of pressure, temperature and displacement fields in EGS are analyzed. • Thermal recovery performance of EGS under different key variables is discussed. Simulation of thermal hydraulic-mechanical (THM) coupling process plays an important role in exploiting the hot dry rock (HDR) resources by enhanced geothermal systems (EGS). At present, the simulations are mostly performed by using the traditional finite element method (FEM) with very dense grids. Because of the considerable size differences between the fractures and EGS, a large number of unstructured grids should be adopted, leading to a significant computational burden. To address this challenge, the embedded discrete fracture model (EDFM) and the extended finite element method (XFEM) are combined in this paper. By doing this, the grid number can be dramatically reduced, which improves the computational efficiency while maintaining relatively high accuracy. The proposed model is verified by comparing it with two typical analytic solutions and COMSOL. Moreover, the new model is applied to analyze the pressure, temperature and displacement evolutions in a hypothetical EGS. The influences of some key variables on thermal extraction performance are also investigated. [ABSTRACT FROM AUTHOR]

• A 3D THM coupled model for both hydraulic fracturing and heat extraction in hot dry rock. • The enhancement in permeability of the hydraulic fracturing stimulated fracture network can be demonstrated in this model. • The hydraulic fracturing and heat extraction over 10 years were carried on the planned Dikili EGS in Turkey. • The optimized re-injection rate was determined at 100 L/s with the average thermal capacity of 31.75 MWth. Geothermal energy attracts extensive attention for its excellent features like large capacity and lack of dependency on weather conditions. This paper presents a development of numerical model to study the hydraulic fracturing and heat extraction from geothermal reservoir, especially from brittle hot dry rock (HDR) reservoir. In this numerical model, an anisotropic damage model has been introduced based on Thermal-Hydraulic-Mechanical (THM) coupling code THOUGH2MP-FLAC3D and this damage-permeability model has also been integrated to demonstrate the enhancement in permeability of stimulated fracture network. Furthermore, numerical studies on planed Dikili enhanced geothermal system (EGS) were carried out by using newly developed model. As the simulated results show, a roughly circular fracture network is created, wherein simulated reservoir volume and area reaches to 49 million m3 and 1.41 million m2, respectively. The maximum permeability increases from initial permeability of 4 × 10−18 m2 to 7.52 × 10−14 m2 in z-direction. Based on these results, heat extraction calculations are performed for 10-years at different injection rates. The accumulated amount of produced net energy increases with the rising injection rate, while growth rate of the average thermal capacity over 10-year slows down. The production results suggest that an injection rate of 100 L/s can recover approximately 1.00 × 1016 J of thermal energy of over a 10-year period, with an average thermal capacity of 31.75 MWth. [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]