Hot Dry Rock Geothermal Energy Research Articles

Laboratory investigation on fracture initiation and propagation behaviors of hot dry rock by radial borehole fracturing

Creating complex and interconnected fracture networks between injection and production wells is crucial for exploiting hot dry rock (HDR) geothermal energy. However, the simple planar fractures created by conventional hydraulic fracturing, primarily controlled by in situ stress, fail to connect directionally with the target well. This study proposes a novel stimulation method, i.e. radial borehole fracturing, which shows great potential for guiding the directional propagation of fractures. The fracture initiation and propagation behaviors of high-temperature granite under radial borehole fracturing are investigated and compared with those of conventional fracturing. Three-dimensional morphological scanning and reinjection tests are used to quantitatively evaluate fracturing performance. Additionally, the influences of key parameters, including rock temperature, in situ stress, injection rate, fluid viscosity, azimuth of the radial borehole, and the number of radial boreholes on the fracture morphology and breakdown pressure are investigated. The results show that radial borehole fracturing can effectively guide the initiation and propagation of fractures along the radial borehole. The breakdown pressure of radial borehole fracturing can be reduced by 14.1%–43.7% compared to conventional fracturing. A higher fluid-rock temperature difference reduces the directional propagation range of fractures guided by the radial borehole. Increases in the vertical density of radial boreholes, injection rate, and fluid viscosity enhance the guiding ability of radial boreholes. Furthermore, there is a competitive relationship between in situ stress and the azimuth of radial boreholes in controlling fracture propagation. This research provides a viable alternative for the directional connection of injection-production wells in HDR reservoirs.

Effects of Different Cooling Treatments on Heated Granite: Insights from the Physical and Mechanical Characteristics.

The exploration of Hot Dry Rock (HDR) geothermal energy is essential to fulfill the energy demands of the increasing population. Investigating the physical and mechanical properties of heated rock under different cooling methods has significant implications for the exploitation of HDR. In this study, ultrasonic testing, uniaxial strength compression experiments, Brazilian splitting tests, nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) were conducted on heated granite after different cooling methods, including cooling in air, cooling in water, cooling in liquid nitrogen, and cycle cooling in liquid nitrogen. The results demonstrated that the density, P-wave velocity (Vp), uniaxial compressive strength (UCS), tensile strength (σt), and elastic modulus (E) of heated granite tend to decrease as the cooling rate increases. Notably, heated granite subjected to cyclic liquid nitrogen cooling exhibits a more pronounced decline in physical and mechanical properties and a higher degree of damage. Furthermore, the cooling treatments also lead to an increase in rock pore size and porosity. At a faster cooling rate, the fracture surfaces of the granite transition from smooth to rough, suggesting enhanced fracture propagation and complexity. These findings provide critical theoretical insights into optimizing stimulation performance strategies for HDR exploitation.

Evolution mechanism of permeability of hot dry rock under coupled effect of thermal fatigue and seawater interaction during coastal geothermal development

Using seawater as a heat transfer fluid for developing hot dry rock (HDR) geothermal energy in coastal areas is a frontier of engineering worth exploring; however, only a few studies have considered this research topic. During the development of HDR based on enhanced geothermal system (EGS) technology, the thermal shock fatigue effect of low-temperature seawater on high-temperature reservoirs leads to changes in the permeability of the HDR, notably impacting the reservoir reconstruction design and heat transfer efficiency. Accordingly, the changes in pore structure, pore distribution, porosity, and permeability of HDR in the Guangdong-Hong Kong-Macao Greater Bay Area were investigated based on nuclear magnetic resonance (NMR) techniques and fractal theory after treatment to different temperatures and numbers of thermal shocks due to seawater interaction. The experimental results showed that with an increase in temperature and the number of seawater interactions, the pore structure distribution of the HDR underwent significant changes, with the proportion of mesopores and macropores increasing by 24.29 % and 10.52 %, respectively. Moreover, the porosity and permeability increased with increasing temperature and number of thermal shock cycles, especially above 300 °C. When the temperature increased from 100 °C to 500 °C, the total porosity increased from 0.98 % to 3.85 %, an increase of 292.9 %, while the permeability increased from 10−4 mD to 0.1 mD, an increase of nearly 1000 times. Changes in the pore structure, pore distribution, porosity, and permeability of HDR are caused by multiple damage mechanisms related to high-temperature nonlinear expansion, thermal shock effects, chemical erosion, and fatigue. Additionally, the permeability of the HDR after seawater treatment at the same heat treatment temperature and number of thermal shocks was higher than that after freshwater treatment. At a temperature of approximately 500 °C and 20 thermal shocks, the permeability of HDR under the action of seawater was approximately seven times that of fresh water. Therefore, these results suggest that using seawater as a heat transfer fluid has more advantages in enhancing the permeability of reservoirs.

Performance analysis of enhanced geothermal system under thermo-hydro-mechanical coupling effect with different working fluids

Hot dry rock (HDR) geothermal energy is an effective alternative to traditional fossil fuels and can be exploited through enhanced geothermal systems (EGS). Compared to water, CO2 may have better performance in EGS as a working fluid. In this paper, the thermo-hydro-mechanical (THM) coupling mechanism inside the HDR reservoir was analyzed, and a dual-medium coupling model was constructed and validated using a saturated soil column problem. A numerical model of an EGS containing four different fracture networks was simulated using CO2 and water as working fluids, and the physical field evolution and heat extraction performance were compared under different working fluid conditions. The results indicate that reducing the fracture spacing and having a fracture network that is more intricate and inter-connected is more conducive to improving EGS heat extraction efficiency. When the HDR matrix is initially at temperatures of 423.15 K and 473.15 K, using CO2 as the working fluid leads to a faster decrease in outlet temperature and a sustained higher heat extraction ratio compared to water. When the initial temperature of the HDR matrix is 453.15 K, using CO2 as the working fluid results in a more stable outlet temperature. Using CO2 as the working fluid and intermittent exploitation mode may be more favorable for obtaining higher output thermal power. The findings of the study offer valuable guidance for choosing appropriate working fluids during EGS operations.

Effect of heating–water cooling cycle treatment on the pore structure and shear fracture characteristics of granite

In hot dry rock (HDR) geothermal energy extraction systems, jointed rock masses are often subjected to heating–water cooling (H-WC) cycles. In this process, HDR is at risk of failure and instability due to the change in pore structure and fracture characteristics. To identify the shear fracture characteristics of rocks after H-WC cycle treatment. First, shear fracture mechanical tests were carried out on granite specimens treated with H-WC cycles by using the short core compression (SCC) test method. The experimental results show that cyclic H-WC treatment leads to a decrease in fracture toughness values. It is especially noteworthy that the weakening effect of fracture toughness decreases with increasing cycles. Second, the NMR analysis results show that the porosity of granite samples increases with the number of H-WC cycles. At 400℃ and 600℃, the pore size distribution changes under H-WC. The slope of the fitting curve between fracture toughness and porosity indicates that the higher the temperature is, the lower the sensitivity of fracture toughness to porosity. Then, acoustic emission characteristics in the mechanical experimental process were analysed by using the b-value analysis method. The results indicated that the b-value of acoustic emission decreased with increasing cycles at 200 °C, so the crack size and energy release increased; however, at T = 400 °C and 600 °C, the b-value increased with an increasing number of cycles, so the crack size and energy release decreased. Finally, the fracture plane morphology of the failure specimens was obtained by a 3D optical scanner. The calculation results of the fractal dimension DF indicate that the cyclic H-WC treatment enhanced the fracture plane roughness, and the average increase in fractal dimension DF increases with increasing temperature, also indicating that the increasing effect of fracture plane roughness with increasing temperature becomes increasingly significant. At the same time, the fracture toughness under cyclic H-WC action was negatively correlated with the fracture plane roughness of granite.

Enhance liquid nitrogen fracturing performance on hot dry rock by cyclic injection

Producing complex fracture networks in a safe way plays a critical role in the hot dry rock (HDR) geothermal energy exploitation. However, conventional hydraulic fracturing (HF) generally produces high breakdown pressure and results only in single main fracture morphology. Furthermore, HF has also other problems such as the increased risk of seismic events and consuption of large amount of water. In this work, a new stimulation method based on cyclic soft stimulation (CSS) and liquid nitrogen (LN 2 ) fracturing, known as cyclic LN 2 fracturing is explored, which we believe has the potential to solve the above issues related to HF. The fracturing performances including breakdown pressure and fracture morphology on granites under true-triaxial stresses are investigated and compared with cyclic water fracturing. Cryo-scanning electron microscopy (Cryo-SEM) tests and X-ray computed tomography (CT) scanning tests were used for quantitative characterization of fracture parameters and to evaluate the cyclic LN 2 fracturing performances. The results demonstrate that the cyclic LN 2 fracturing results in reduced breakdown pressure, with between 21% and 67% lower pressure compared with using cyclic water fracturing. Cyclic LN 2 fracturing tends to produce more complex and branched fractures, whereas cyclic water fracturing usually produces a single main fracture under a low number of cycles and pressure levels. Thermally-induced fractures mostly occur around the interfaces of different particles. This study shows the potential benefits of cyclic LN 2 fracturing on HDR. It is expected to provide theoretical guidance for the cyclic LN 2 fracturing application in HDR reservoirs.

Experimental Investigation on Failure Mechanism of Hot Dry Rock under Microwave Irradiation

Hot dry rock (HDR) geothermal energy is clean energy that can meet people’s demand for low carbon. The traditional hydraulic fracturing method is challenging to break HDR dominated by granite, mainly because of HDR’s high temperature and hard characteristics. Microwave irradiation is considered a proven approach to breaking granite. This study investigates the failure mechanism of HDR at 500°C–800°C under microwave irradiation. The experiments cover preheating, uniaxial compression, granulometric analysis, binocular vision monitoring (BVM) technique, and X-ray powder diffraction. The result shows that the uniaxial compression strength of granite decreases the most at 500°C–600°C, which is 62.77% on average. The failure form is from the brittle failure (untreated) to ductile failure (800 °C) with the rise of the microwave irradiation temperature. The microwave makes a particle size in the range of 0.6–2.36 mm the most apparent particle size of granite after uniaxial compression. The BVM technique reconstructs the surface deformation with a 0.7% error. Microwaves cause heat accumulation near the rock near the magnetron, resulting in the formation of crack networks and a molten cavity. The possibility of microwave-assisted fracturing of HDR is discussed at the end of the article.

A 3D analysis of the occurrence of fractures in hot dry rock reservoirs based on the spatial distribution of natural fractures

Hot dry rock (HDR) geothermal energy is a kind of widely distributed clean energy with huge reserves. However, its commercial development has been constrained by reservoir stimulation. In the early stage of HDR geothermal energy development, properly determining spatial distribution patterns of natural fractures in HDR reservoirs can effectively guide reservoir stimulation. This study analyzes the spatial distribution of natural fractures by using FracMan software based on the actual geological data and log data of well M − 2 in the Matouying Uplift area, Hebei Province. The fracture parameters are counted and Monte Carlo simulation technique is introduced to optimize the parameters, which makes the natural fracture model more accurate and reliable. Furthermore, this study simulates hydraulic fracturing using the model combined with the actual in-situ stress parameters and the construction scheme. As verified by fitting the changes in simulated wellhead pressure during hydraulic fracturing with the actual wellhead pressure data detected during construction, the methods for natural fracture modeling used in this study are scientific and reasonable. The preliminary prediction results show that the displacement design scheme with a pump displacement of 2.0-3.0 m3/min, 4.0-5.5 m3/min and 6-7 m3/min in the early, middle and late stages, respectively, has good fracturing effect. The results of this study can be utilized as a reference for preparing development schemes for HDR reservoirs.

Capacity Allocation of Hybrid Power System with Hot Dry Rock Geothermal Energy, Thermal Storage, and PV Based on Game Approaches

This study utilizes hot dry rock (HDR) geothermal energy, which is not affected by climate, to address the capacity allocation of photovoltaic (PV) -storage hybrid power systems (HPSs) in frigid plateau regions. The study replaces the conventional electrochemical energy storage system with a stable HDR plant assisted by a flexible thermal storage (TS) plant. An HPS consisting of an HDR plant, a TS plant, and a PV plant is proposed. Game approaches are introduced to establish the game pattern model of the proposed HPS as the players. The annualized income of each player is used as the payoff function. Furthermore, non-cooperative game and cooperative game approaches for capacity allocation are proposed according to the interests of each player in the proposed HPS. Finally, the proposed model and approaches are validated by performing calculations for an HPS in the Gonghe Basin, Qinghai, China as a case study. The results show that in the proposed non-cooperative game approach, the players focus only on the individual payoff and neglect the overall system optimality. The proposed cooperative game approach for capacity allocation improves the flexibility of the HPS as well as the payoff of each game player. Thereby, the HPS can better satisfy the power fluctuation rate requirements of the grid and increase the equivalent firm capacity (EFC) of PV plants, which in turn indirectly guarantees the reliability of grid operation.

The situation analysis of hot dry rock geothermal energy development in China-based on structural equation modeling

In order to combat climate change, China proposed a dual carbon target in 2020. (China's 2030 and 2060 Goals). If the goals can be achieved as scheduled, it will help the world achieve the carbon neutrality goal earlier and improve the increasingly serious global climate. Reducing the utilization of fossil fuels and increasing the share of clean energy in primary energy are major ways to achieve China's 2030 and 2060 Goals. As a geothermal resource with large reserves, high energy storage, clean and pollution-free, dry hot rocks can effectively contribute to the goals. To promote the utilization of dry hot rock, this paper quantitatively studies the factors affecting the development and utilization of dry hot rock, and first summarizes five major influencing factors, namely, resources, environment, market, exploration and development technology, and effective use of technology. Then questionnaire survey based on these five factors was designed and distributed by email to geothermal energy experts in universities (China University of Geosciences, etc.) and research institutions (Chinese Academy of Sciences, etc.). Finally, the questionnaires are subjected to a reliability credibility test and validity analysis to remove the non-conforming items, and the structural equation model was constructed to analyze the data. The results show that environment, market and exploration and development technology have significant effects on dry hot rocks, while resources and effective use of technology have insignificant effects on dry hot rocks. Finally, some suggestions to promote the development of dry hot rocks in China are proposed.

UNet–Based Temperature Simulation of Hot Dry Rock in the Gonghe Basin

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

Numerical study on the induced seismicity through hydraulic fracturing in the deep heat mining project in Basel, Switzerland using a pseudo dynamic model and considering fully hydro-mechanical coupling

The induced seismicity associated with fluid injection (hydraulic fracturing) or forced circulation has become a key issue in the safe exploitation of hot dry rock geothermal energy. In this paper, the induced seismicity in the Deep Heat Mining Project in Basel, Switzerland was numerically studied by using a developed 3D numerical code with considering the fully hydro-mechanical coupling and the dynamic stress transfer effects. The numerical study was based on history matching of the injection pressure and the statistic characteristics of the induced seismicity. Further, sensitivity analysis of some parameters relating to injection operation, slip behavior and strength-stress ratio was conducted. According to this study, the following conclusions were obtained: 1) the temporal and spatial distribution of the seismic events can be divided into an indirectly induced zone, a directly induced zone, and a quiet zone; 2) large seismic events can be found within the indirectly induced zone; 3) the released energy is not exactly proportional to the injection volume; 4) when the geological condition is well-known, the maximum induced seismicity by fluid injection can be approximately determined; 5) the slip behavior and the strength-stress state are dominate factors for inducing large seismic events.

Heat extraction from hot dry rock by super-long gravity heat pipe: A field test

Recent theoretical studies dealing with the novel scheme for earth-deep hot dry rock geothermal energy extraction based on the use of a super-long heat pipe indicate its superior technical viability. The present work reports on a field test using a 3000-m heat pipe to extract hot dry rock geothermal energy conducted with the aim of demonstrating its practical feasibility. The in-house developed heat pipe uses deionized water as working fluid and produces steam with a maximum temperature of ∼90 °C, when the average underground formation temperature around the subsurface heat pipe is 95.6 °C. During the 30-day continuous heat mining process, the heat extraction rate achieved an average value of 190 kW with no obvious downward trend. After one month of continuous heat production, a one-week heat recovery can restore the system temperature to just slightly lower than the initial value. The thermal response of the system to different condensing temperatures was also tested. The results show that the heat extraction rate can be increased by decreasing the condensing temperature, but this improvement is insignificant when the condensing temperature is 55 °C or lower. Furthermore, the experimental results show overall good agreement with the model predictions, which indicates that the heat pipe is performing close to the desired operating conditions considered in the simulation model, in particular: no liquid accumulation, no local dry-out, and no vapor-liquid entrainment. This performance is a major departure from that of the typical heat pipes, and it makes the super-long gravity heat pipe a leading technological candidate for earth-deep geothermal heat extraction.

Thermodynamic performance of subcritical double-pressure organic Rankine cycles driven by geothermal energy

In the utilization of hot dry rock geothermal energy, the temperature of production well varies with the injection temperature of the recharge well and operation time. Taking into account of the variation of production well temperature, three kinds of double-pressure Organic Rankine Cycle (DPORC) with different layouts are constructed in present work, and the effects of different layouts on the thermodynamic performance of DPORC are probed. The results indicate that present DPORCs can effectively improve the thermodynamic performance of the system, and the net output power of the systems can be improved by at least 5.68%, 6.42% and 15.75%, compared with those of single-pressure Organic Rankine Cycle (SPORC). In addition, one of proposed DPORCs can reduces the irreversible loss in system and has better thermodynamic performance. When the injection temperature of the recharge well remains constant, the net output power, thermal efficiency and exergy efficiency reach maximum at the same k (the ratio of absorbed heat in high-pressure stage cycle to that in DPORC). After 40 years of operation, the net output power of the three arrangements and SPORC decrease by 434.07 kW, 438.04 kW, 463.94 kW and 465.72 kW, respectively. Therefore, it’s significant to reduce the temperature drop of hot dry rock production well to maintain the stable output of the system. It’s found that the exergy loss in system is mainly concentrated in evaporator, condenser and turbine. Reasonable layout can reduce the exergy loss of the system. The variation trend of cycle perfection with k is the same as other parameters do, and the maximum cycle perfection is achieved when the net output power of system is near the maximum. With the increase of running time, the cycle perfection of DPORC increases slightly. Specifically, the cycle perfection of one proposed DPORC rises in a wavy manner.

A Numerical Investigation of the Hazardous Injection Area of Induced Earthquake during Hydraulic Fracturing

Hot dry rock (HDR) geothermal energy has many advantages, such as being renewable, clean, widely distributed, and without time and weather limitations. Hydraulic fracturing is usually needed for the exploitation of HDR geothermal energy. It has many hidden faults in the reservoir/caprock sequences. Injecting fluid into underground formations during hydraulic fracturing often induces fault slip and leads to earthquakes. Therefore, to well understand the induced fault slip and earthquakes is important for the applications and development of HDR geothermal exploitation. In this study, we investigated the hazardous injection area of the induced earthquakes during hydraulic fracturing. The study was based on a hydraulic fracturing test in Qiabuqia geothermal field in China. According to the field, a fault-surrounding rock-fracturing region system was developed to study the influences of fluid injection on the stability of the specific fault. A total of 60 hydraulic fracturing regions and 180 numerical experiments were designed. The results revealed that the hazardous injection regions that threaten the fault’s stability were near to the fault and concentrated on the following four areas: (a) above the top of the fault in underlying strata; (b) above the top of the hanging wall of the fault in underlying strata; (c) near to the fault planes in both footwall and hanging wall; (d) at the bottom of the footwall of the fault in underlying strata. The hazardous injection area can be controlled effectively by adjusting the injection pressure.

Study of Characteristics of Fault Slip and Induced Seismicity during Hydraulic Fracturing in HDR Geothermal Exploitation in the Yishu Fault Zone in China

Hot dry rock (HDR) geothermal energy has become promising resources for relieving the energy crisis and global warming. The exploitation of HDR geothermal energy usually needs an enhanced geothermal system (EGS) with artificial fracture networks by hydraulic fracturing. Fault reactivation and seismicity induced by hydraulic fracturing raise a great challenge. In this paper, we investigated the characteristics of fault slip and seismicity by numerical simulation. The study was based on a hydraulic fracturing project in the geothermal field of Yishu fault zone in China. It revealed that fluid injection during hydraulic fracturing can cause the faults that exist beyond the fluid-pressurized region to slip and can even induce large seismic event. It was easier to cause felt earthquakes when hydraulic fracturing was carried out in different layers simultaneously. We also examined the effects of the location, permeability, and area of the fracturing region on fault slip and magnitude of the resulting events. The results of the study can provide some useful references for establishing HDR EGS in Yishu fault zone.

Destruction Law of Borehole Surrounding Rock of Granite under Thermo-Hydro-Mechanical Coupling

The destruction of the rock that surrounds boreholes under thermo-hydro-mechanical coupling is an important factor for borehole stability in hot dry rock (HDR) geothermal energy extraction. Failure experiments for granite under triaxial stress ( σ 1 > σ 2 > σ 3 ) were conducted as 500°C superheated steam was transported through the borehole. High-temperature steam leads to large thermal cracks in the surrounding rock, which are randomly distributed around the borehole and gradually expand outwards. The randomly distributed thermally induced microcracks increase the complexity of the initial fracture morphology around the borehole and contribute to the appearance of multiple branch fractures. Fracture development is negligibly affected by ground stresses during the initial stages. However, fractures are deflected towards the maximum horizontal principal stress under ground stresses during later periods. During fracture propagation, high-temperature steam more easily penetrates the rock because its viscosity is lower than water. Towards the end of the crack expansion, the steam loses heat and liquefies, which increases the elongation resistance, and results in the arrest and intermittent expansion of the cracks.

Non-contaminating cryogenic fluid access to high-temperature resources: Liquid nitrogen fracturing in a lab-scale Enhanced Geothermal System

Hot dry rock (HDR) geothermal energy exploitation is essential to meet the energy demand and achieve low-carbon solutions. Creation of complex fracture networks in HDR by hydraulic fracturing to enhance the thermodynamic efficiency and heat production is significant. However, the economic and environmental burden of freshwater sourcing, transportation, and treatment raises concerns. Here, we investigate liquid nitrogen, an environmental-friendly and super-cooling fluid, as an alternative fracturing fluid for potential applications in Enhanced Geothermal System. We studied the fracturing performances of liquid nitrogen in high-temperature granites under true triaxial-confining stresses. Cryo-scanning electron microscopy and 3D X-ray micro-computed tomography were applied to illuminate the fracture-network patterns. Results show that liquid nitrogen fracturing exhibits the lowest breakdown pressure compared with water fracturing and nitrogen gas fracturing. The fracture morphology displays a 3-D volumetric pattern comprised of branched fractures and thermally-stimulated zones. Higher fluid-rock temperature difference and lower stress anisotropy cause more complex fracture networks. This study, for the first time, shows potential benefits of liquid nitrogen fracturing in high-temperature crystalline rocks, paving the road towards waterless fracturing in Enhanced Geothermal System. It is expected to provide a viable alternative for the sustainable development of deep geothermal resources in an efficient and clean way.

Numerical simulation of thermo-hydro-mechanical coupling effect in mining fault-mode hot dry rock geothermal energy

Hot dry rock geothermal energy is a type of renewable energy with great development prospects in deep strata. However, it is quite difficult to construct an artificial reservoir. In this paper, deep large-dip-angle fault zone in the Yangbajing geothermal field in Tibet of China is used as a natural artificial reservoir, and a set of hot dry rock heat extraction schemes for such fault modes is proposed. A three-dimensional thermo-hydro-mechanical coupling model is established for the scheme to study the distributions of temperature, stress and seepage during the process of mining fault-mode hot dry rock geothermal energy. The temperature of #1 production well remains 445 °C after 22-year operation and it decreases from 445 °C to less than 200 °C after 15-year operation in #2 production well. The initial vertical stress near the injection well is 199 MPa, which decreases to 193 MPa after 1 year and remains unchanged. The specific water flow in the fault zone between the wells increases negatively and exponentially with the extraction time. The seepage resistance of the fault rock mass gradually decreases. The total effective heat production is 13130 MWa after 22-year operation.

Experimental research on the rupture characteristics of fractures subsequently filled by magma and hydrothermal fluid in hot dry rock

Hot dry rock (HDR) geothermal energy is a green and renewable resource, and its efficient development has always been a hot research topic worldwide. Using the natural fracture system of deep HDR as a reservoir not only reduces the difficulty and developmental cost of artificial reservoir construction but also greatly increases the efficiency of water–rock heat exchange. In a study of the characteristics of the natural fracture system of deep HDR, through field observations, it was found that structural fractures in deep granite bodies filled by magma or hydrothermal fluid are common phenomena. Through observations with a polarising microscope, it was found that under the high-temperature and hydrothermal fluid action of the backfill, changes in the number of thermal-cracking fractures of fracture-filled granite versus the distance from the cementation interface can be divided into three zones: a zone in which the number of fractures in the backfill fluctuates gently, a zone showing a sharp increase in the number of fractures in the parent rock, and a zone in which the fluctuation in the number of fractures in the parent rock decreases. The position of the maximum number of thermal-cracking fractures is located in the parent rock at a certain distance away from the cementation interface. This maximum number has an exponential relation with the thickness of the backfill, and its position is also related to the thickness of the backfill. The universality of the fracture backfill and its large-range influence on the granite parent rock caused by thermal action result in the formation of a huge weak-plane structure in the granite parent rock. This weak-plane structure is very likely to become a natural reservoir in HDR geothermal development or an easy-to-rupture position when an artificial reservoir is constructed by hydraulic fracturing, which is beneficial for the construction of artificial reservoirs. Based on this result, we studied a new technology for the construction of artificial reservoirs for HDR geothermal exploitation, which constitutes a new research direction for HDR geothermal development. The shorter versionThe study of the characteristics of the natural fracture system of deep hot dry rock (HDR) constitutes a new research direction for HDR geothermal development. Through field observations, it was found that structural fractures in deep granite bodies filled by magma or hydrothermal fluid are common phenomena. Through micro-scale observations, it was found that under the high-temperature and hydrothermal fluid action of the backfill, the changes in the number of thermal-cracking fractures of fracture-filled granite versus the distance from the cementation interface can be divided into three zones. The position of the maximum number of thermal-cracking fractures is located in the parent rock at a certain distance away from the cementation interface. This maximum number and its position are both related to the thickness of the backfill. The huge weak-plane structure caused by thermal action is very likely to become a natural reservoir or an easy-to-rupture position when an artificial reservoir is constructed by hydraulic fracturing, which is beneficial for the construction of artificial reservoirs. Based on this, we studied a new technology for the construction of artificial reservoirs for HDR geothermal exploitation.