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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 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]