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The main objective of this study was to develop a spatial temperature distribution of the Karkonosze Pluton to indicate optimum locations for HDR systems at drillable depth.

The Hot Dry Rock (HDR) is considered as a clean and renewable energy, poised to significantly contribute to the global energy decarbonization agenda. Many HDR projects worldwide have accumulated valuable experience in efficient drilling and completion, reservoir construction, and fracture simulation. In 2019, China Geological Survey (CGS) initiated a demonstration project of HDR exploration and production in the Gonghe Basin, aiming to overcome the setbacks faced by HDR projects. Over the ensuing four years, the Gonghe HDR project achieved the first power generation in 2021, followed by the second power generation test in 2022. After establishing the primary well group in the initial phase, two directional wells and one branch well were drilled. Noteworthy progress was made in successfully constructing the targeted reservoir, realizing inter-well connectivity, power generation and grid connection, implementing of the real-time micro-seismic monitoring. A closed-loop technical validation of the HDR exploration and production was completed. However, many technical challenges remain in the process of HDR industrialization, such as reservoir fracture network characterization, efficient drilling and completion, multiple fracturing treatment, continuous injection and production, as well as mitigation of induced seismicity and numerical simulation technology.

In recent years, Enhanced Geothermal System (EGS) technologies have been applied to the geothermal resources production in the Hot Dry Rock (HDR). The core of EGS technologies is to adopt hydraulic fracturing in the reservoir to create a connected network of discrete fractures with the consideration of water as a working fluid for hydraulic fracturing and heat production. This paper investigates the characteristics of water flow behaviors through a single rough fracture under different temperature and pressure conditions. A single fracture model with rough fracture surfaces is constructed and then characterized, and influences of the anisotropic factor on the average tortuosity and frictional resistance coefficient of water flow through a single fracture with rough surfaces have been compared and analyzed. With consideration of other impacting factors (temperature, pressure, fracture roughness), the impact of mass flow rate has also been presented. Numerical simulation results present that changes of average tortuosity for water flow through a single rough facture are mainly affected by temperature. It can be observed that higher temperature leads to larger average tortuosity but the frictional resistance coefficient shows an opposite trend. As for pressure conditions, it is found that effects of pressure on average tortuosity and frictional resistance coefficient is very small, which can be neglected under high pressure conditions. Furthermore, the average tortuosity shows a progressively linear relationship with the mass flow rate. On the contrary, the frictional resistance coefficient has a negative relationship with the mass flow rate. It is revealed that when the mass flow rate reaches a critical point, the influences of temperature on the frictional resistance coefficient will be negligible. Comparisons of single rough fractures with different anisotropic factors show that values of average tortuosity and frictional resistance coefficient have positive relationships with the increase of anisotropic factors.

Hydraulic fracturing has become the main technology for the efficient development of geothermal energy in hot dry rock (HDR), however, few studies on the propagation behavior and mechanism of HDR hydraulic fractures under high-temperature conditions have investigated. In this paper, a large-size high-temperature true triaxial hydraulic fracturing physical modeling apparatus is designed, and hydraulic fracturing experiments with it are performed to investigate the fracture initiation and propagation behavior in natural granite samples collected from Gonghe Basin, the first HDR site in China. The experimental results show that the designed high-temperature apparatus provides a constant-temperature condition during the whole hydraulic fracturing process and the maximum temperature can reach 600 °C, showing its ability to simulate realistic temperatures and pressures in both ultra-deep and HDR formations. Although the tensile strength of the rock samples remains almost unchanged at a temperature of 200 °C, the cooling effects of the fracturing fluid in high-temperature rock can induce the formation of microfractures and significantly reduce the rock strength, thus lowering the breakdown pressure and increasing the complexity of the hydraulic fracture morphology. Compared with traditional oil and gas reservoirs, the hydraulic fractures in HDR are rougher and the specific surface area of a single fracture is larger, which can be helpful for heat extraction. This study provides a basis for understanding hydraulic fracture geometries and field construction design in HDRs.

Hot dry rock (HDR), as a kind of geothermal energy resource, has attracted much attention due to its wide distribution and huge reserves. This paper presents a numerical study on energy mining in the HDR by the enhanced geothermal system (EGS). The thermal-hydraulic-mechanical coupling model is employed in these simulations. The multi-field evolution process and the influence of the fracture parameters on the heat recovery performance of an EGS are analyzed comprehensively. The results show that the fractures parallel to the connecting line of the injection-production well can lead to an early thermal breakthrough, resulting in a thermal recovery performance decrease, while fractures perpendicular to the connecting line between the production and injection wells can enhance the production temperature of an EGS, when compared with the reservoir without fractures. The production temperature drop rate of the EGS with percolated fracture network is much quicker than that of the EGS with an isolated fracture network. Additionally, short fractures can lessen the potential for working fluid preferred flow channels to emerge; therefore, an EGS with short fractures may operate better than an EGS with lengthy fractures.

Hot Dry Rock (HDR) is a renewable energy source that has garnered attention due to its abundant reserves, widespread distribution, and consistent energy supply. However, the injection of cold water during the drilling and production process of HDR can alter the temperature of the rock in the HDR reservoir, leading to variations in its physical and mechanical properties near the wellbore. These changes can impact the stability and safety of the HDR wellbore. This study investigated the alterations in the physical and mechanical properties of HDR under different temperature conditions. The results revealed that there were negligible changes in the physical and mechanical properties when the temperature rose from 25 °C to 400 °C. However, noticeable changes occurred as the temperature increased from 400 °C to 800 °C, establishing 400 °C as the threshold for physical and mechanical property variations in the granite. Building upon these experimental findings, a segmented wellbore instability model was developed and validated. The model demonstrated that an increased temperature difference between the drilling fluid and the borehole corresponded to an expanded range of borehole failure. Furthermore, higher wellbore temperatures led to more pronounced disparities between the maximum and minimum principal stress of the borehole, rendering it more susceptible to instability. The research also uncovered that the optimal drilling position was influenced by temperature.

During the development process of deep geothermal energy (Hot dry rock, HDR), the high-temperature granite matrix in the wellbore needs to withstand thermal-cooling cycles, easily leading to wellbore instability and collapse. Considering the actual drilling engineering using different types of drilling fluid, thermal-cooling cycle experiments (cycles of 1, 2, 4, 8, 12 times) of granite were conducted using natural, water, and liquid nitrogen (LN2) cooling methods. Based on the low-field nuclear magnetic resonance measurement, the development and evolution of pore size, quantity, and distribution under different cooling ways and thermal cycles were studied, and the changes in rock mechanical parameters under different cooling methods and thermal cycling conditions were quantified. Then, the rock damage coefficients based on the variation patterns of different types of pores were calculated, and a quantitative relationship was established among T2spectrum, pore, and mechanical damage. Finally, the proton weighting method was used to analyze the changes in the internal bearing area of rocks, revealing the deterioration mechanism of rock mechanical properties under different cooling ways and thermal cycling conditions. The results showed that the porosity increased with thermal cycles during the thermal-cooling cycle treatment, and the growth rate was first fast and then slow. Under the same number of thermal cycles, the groundwater cooling porosity was the highest, followed by LN2and natural cooling. The relationship between T2spectral area and rock damage was established on this basis, and the rock damage value increased with thermal cycles in an exponential function, and the proportion of large pores significantly affected the pore damage. A relationship (positively proportional linear) between pore damage and the mechanical degradation coefficient of high-temperature cooling cycle granite and a method for predicting rock degradation through pore damage were established, the deterioration mechanism of rock mechanical properties under thermal cycle treatment was revealed.

Hot dry rock (HDR) is a kind of clean energy with significant potential. Since the 1970s, the United States, Japan, France, Australia, and other countries have attempted to conduct several HDR development research projects to extract thermal energy by breaking through key technologies. However, up to now, the development of HDR is still in the research, development, and demonstration stage. An HDR exploration borehole (with 236 °C at a depth of 3705 m) was drilled into Triassic granite in the Gonghe Basin in northwest China in 2017. Subsequently, China Geological Survey (CGS) launched the HDR resources exploration and production demonstration project in 2019. After three years of efforts, a sequence of significant technological breakthroughs have been made, including the genetic model of deep heat sources, directional drilling and well completion in high-temperature hard rock, large-scale reservoir stimulation, reservoir characterization, and productivity evaluation, reservoir connectivity and flow circulation, efficient thermoelectric conversion, monitoring, and geological risk assessment, etc. Then the whole-process technological system for HDR exploration and production has been preliminarily established accordingly. The first power generation test was completed in November 2021. The results of this project will provide scientific support for HDR development and utilization in the future.

Researchers from the Beijing Institute of Petrochemical Technology conducted an experimental study on the effects of saturated fluids on the petrophysical properties of hot dry rock (HDR) at high temperatures. The study utilized various instruments and analysis techniques to investigate the impact of distilled water and nanoemulsion on the physical and mechanical properties of HDR. The results showed that temperature increase promotes the formation of thermal damage fractures in the HDR samples, and the fluids weaken the mechanical strength of the rock. The findings provide valuable evidence for understanding the hydro-thermal-mechanical behaviors of HDR and can contribute to the development and utilization of geothermal energy. [Extracted from the article]

Due to the high strength of hot dry rock (HDR), the rate of penetration (ROP) improvement is one of the issues in deep HDR drilling; it is believed that percussive drilling is one of the methods to improve the ROP of HDR. Study shows that the specific shape of impact load can use a certain proportion of its impact energy to break rock, which indicate the impact load shape of percussive drilling tool has remarkable influence on the ROP of HDR. Therefore, a novel HDR drilling tool with sinusoidal impact load is proposed in this paper, and the theoretical analyze of the HDR drilling tool reveals that the impact load of the HDR drilling tool is sinusoidal shape. Thereafter, the energy utilization efficiency of sinusoidal impact load is discussed, show that the best energy utilization efficiency of sinusoidal impact load is 80%. Lastly, the influence laws of load characteristics from impeller vane number, impeller position, flow rate and outlet size are investigated, aiming to analyze the impact load shape and frequency of the HDR drilling tool. The results demonstrate that the impact load of the HDR drilling tool aligns with the theoretical sinusoidal shape, and the load amplitude and magnitude of the tool is increased with the increasing of flow rate. The impact load magnitude and amplitude of the tool is higher when the impeller is installed at the top right position, and the impact frequency is much higher than other positions as well. Meanwhile, the relationship between flow rate and impact load and frequency is linear, and the outlet size has remarkably influence on impact load, with larger outlet sizes resulting in a substantial decrease in impact load. Ground testing validates the simulated results under same conditions, the impact load of the HDR drilling tool shows sinusoidal wave-like fluctuations, achieving the expected design goals. The research could provide theoretical guidance to the percussive drilling tool design of HDR.

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

This study is a review paper collecting all previous investigations on the thermal structure and the geothermal potential of the Cappadocia region. The main purpose of this review is to reveal the geothermal potential of the research area and stimulate the geothermal exploration. The scientific objective of this paper is to present a compilation of previous studies on the thermal behavior of the crust in the Cappadocia region and to indicate the geothermally prospective areas in the region. Authors of this study commenced the search of potential geothermal areas in Turkey using the aeromagnetic anomalies, starting from the Central Anatolia. They initiated the investigations of shallow crustal structure of Cappadocia, first and determined recently formed depressions down to 2 km. In the second stage of these research activities, detailed CPD calculations were performed and mapped throughout Cappadocia. They found an elliptical shallow CPD anomaly up to 7 km where the geothermal gradient is obtained as 68 °C/km in the apex of the anomaly. In connection with these values, heat flows were estimated up to 210 mW/m, while the radiogenic heat production is being observed as maximum of 0.70/μWm in this area. These calculations are different and more encouraging than the general regional heat-flow map of Turkey calculated previously from the hot springs and bottom-hole temperatures of shallow wells. A possible magma chamber beneath the caldera-like circular depression between Nevsehir–Aksaray–Nigde–Yesilhisar cities is thought to be the main reason for high heat flow and high geothermal gradient in association with the young volcanism in and around Cappadocia. Due to lack of reservoir units, hydrothermal–geothermal energy possibility is not expected in this region. On the other hand, large anomalous area between Nigde, Aksaray and Nevsehir presents very high hot dry rock (HDR) energy potential which is considered as the most prospective geothermal energy type in the future.

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.

In enhanced geothermal systems (EGS), numerous interlaced fractures exist, and the intertwined morphology of these fractures significantly influences the heat extraction performance of supercritical CO2(SCCO2) flowing through them. In a small-scale context, to investigate the heat extraction performance of SCCO2within two intersecting fractures in hot dry rock(HDR), we first constructed the geometric shapes of the main and branch fractures using a 2D self-affine Weierstrass-Mandelbrot(W-M) function, with joint roughness coefficients(JRC) of 12.75 and 8.39, respectively. Subsequently, based on the thermal properties constitutive equation of SCCO2, a thermo-hydraulic coupling(TH) numerical model was established. Finally, under different inlet flow velocities and temperatures, the effects of the intersection angle between the branch fractures and the main fracture, as well as the width and length of the branch fractures, on the outlet mean temperature (OMT) and output thermal power (OTP) were evaluated. The results indicate that: (1) At the intersection of fractures, the main fracture generates a kind of suction effect on the branch fractures, thereby influencing the internal flow field variations within the branch fractures. (2) With different intersection angles between the branch fractures and the main fracture, the OMT reaches its maximum value of 199.43°C at an intersection angle of 125°. The maximum OTP, reaching 3748.2 W/m, occurs at an intersection angle of 125°. (3) As the width of the branch fractures varies, the maximum OTP, reaching 3757.2 W/m, occurs at a width of 0.432 mm. (4) As the length of the branch fractures varies, the maximum OTP, reaching 3757.7 W/m, occurs at a length of 40 mm.

Exploring dynamic mechanical responses and failure behaviors of hot dry rock (HDR) is significant for geothermal exploitation and stability assessment. In this study, via the split Hopkinson pressure bar (SHPB) system, a series of dynamic compression tests were conducted on granite treated by cyclic thermal shocks at different temperatures. We analyzed the effects of cyclic thermal shock on the thermal-related physical and dynamic mechanical behaviors of granite. Specifically, the P-wave velocity, dynamic strength, and elastic modulus of the tested granite decrease with increasing temperature and cycle number, while porosity and peak strain increase. The degradation law of dynamic mechanical properties could be described by a cubic polynomial. Cyclic thermal shock promotes shear cracks propagation, causing dynamic failure mode of granite to transition from splitting to tensile-shear composite failure, accompanied by surface spalling and debris splashing. Moreover, the thermal shock damage evolution and coupled failure mechanism of tested granite are discussed. The evolution of thermal shock damage with thermal shock cycle numbers shows an obvious S-shaped surface, featured by an exponential correlation with dynamic mechanical parameters. In addition, with increasing thermal shock temperature and cycles, granite mineral species barely change, but the length and width of thermal cracks increase significantly. The non-uniform expansion of minerals, thermal shock-induced cracking, and water-rock interaction are primary factors for deteriorating dynamic mechanical properties of granite under cyclic thermal shock.

An abrasive water jet (AWJ) is commonly used to develop deep geothermal resources, such as drilling in hot dry rock (HDR). The influence of rock mineral properties, such as mineral types, mineral contents, and grain size, on the formation of perforation by AWJ is unclear yet. In this study, we investigate AWJ impacts on three types of granite samples with different mineral fractions using a polarizing microscope and scanning electron microscope (SEM). The results show that when the grain size is doubled, the perforation depth increases by 16.22% under the same type of structure and properties. In general, fractures are more likely to be created at the position of rough surfaces caused by abrasive impact, and the form of fractures is determined by the mineral type. In addition, microstructure analysis shows that transgranular fractures typically pass through large feldspar particles and quartz removal occurs along mineral boundaries. The longitudinal extension of perforation depends mainly on the strong kinetic energy of the jet, while the lateral extension is controlled by the backflow. The results contribute to a better understanding of the process involved in the breaking of hard rock by abrasive jets during deep geothermal drilling.

In order to more accurately understand the rock mechanics properties of hot dry rock (HDR) reservoirs under high-temperature conditions and further guide the drilling and construction of HDR wells, conducting research on rock damage evolution under high-temperature conditions plays a guiding role in the efficient development of geothermal resources. To reveal the evolution law of fracture damage of HDR under temperature influence, taking marble as an example, the thermo-mechanical damage and failure mechanism under the conditions of 30°C, 60°C, 90°C, 120°C and 150°C were studied using a rock high-temperature rheometer and an acoustic emission (AE) testing system. The numerical simulation results were compared with the indoor experimental results by combining the particulate flow program Particle Flow Code 3D (PFC3D) to verify the effectiveness of the simulation analysis model of thermo-coupled particulate flow at the micro scale. The results show that under high temperature, the marble three-axis compression failure passes through the compaction and elastic deformation stage, plastic deformation stage, ductile failure stage and instability failure stage, and the peak strength and elastic modulus of the marble decrease with the increase of temperature, and the acoustic emission b-value of the sample can judge the damage evolution process of the marble. At the micro scale, high temperature accelerates the force chain transmission between particles, the contact force between marble particles gradually decreases during the heating process, and the number of particle bonding failures increases steadily; the bearing capacity and elastic modulus attenuation of the rock are the result of microcrack evolution, and the force chain void zone and particle displacement field inside the sample are consistent with its macroscopic failure form. The development of microcracks in marble under different test loading conditions is different, and with the increase of test temperature, the total number of cracks inside the sample increases, but the proportion of shear cracks decreases, the activity of particle displacement increases, and the “plastic extension” characteristic of marble becomes more significant. The thermal damage evolution of marble was quantitatively analyzed based on the elastic modulus method and the peak stress method, and the damage evolution law of marble under different temperatures was obtained.

Plasma jet rock-breaking technology, characterized by its high rock-breaking efficiency, low specific energy, and absence of mechanical wear, is one of the most promising methods for efficiently fracturing hard rock formations such as hot dry rock (HDR). This study designed a novel high-energy pulsed plasma power supply and pulsed gas supply system to achieve rock fracturing through the instantaneous high-temperature and thermal stress impact of the high-energy pulsed plasma jet on the rock surface. This paper conducted comparative experiments with a traditional constant plasma jet. The impact of crucial electrical parameters (peak current, valley current, frequency, and duty cycle) and rock initial temperature on the damage and fracturing patterns of granite was extensively investigated. Comprehensive evaluation criteria, including removal mass, diameter, depth, rate of penetration, and specific energy, were employed to study the effects meticulously. The results indicate that the pulsed plasma jet significantly enhances energy utilization, effectively reducing the occurrence of thermal melting. The specific energy decreased significantly by 54.74%, while the rate of penetration increased by 17.81%. Additionally, electrode wear was reduced by 50.06%, extending the lifespan of the plasma bit. Parametric analysis revealed that the peak current is the primary factor influencing the rock-breaking performance and efficiency. The valley current has minimal impact on hole morphology but significantly affects specific energy. Frequency and duty cycle can be adjusted to optimize the single-pulse energy, achieving the best specific energy and efficiency. Additionally, the rock temperature has a great influence on the breaking effect. The fracturing characteristics of granite were also analyzed from a microscopic level. Based on the experimental results, recommended parameters of pulsed plasma jet are provided for breaking granite. These findings offer guidance for high-energy pulsed plasma jet drilling of HDR.

Conventional hydraulic fracturing techniques are often found problematic for extracting geothermal energy in hot dry rock (HDR). As an alternative, employing the less viscous gas to replace water as the fracturing fluid showed great potential for more effective fracturing of HDR. In this work, the failure behavior and mechanism of granite during gas fracturing under different confining pressures and gas injection rates are comprehensively examined. It is shown that the breakdown pressure increases with the increase of confining pressure, whereas higher gas injection rate can result in evident decrease of the breakdown pressure. As the confining pressure grows, the acoustic emission (AE) event increases rapidly, with much higher AE counts observed at high gas injection rates than at low injection rates. Comparatively, the AE energy decreases under high confining pressure, due probably to granite transitioning from brittle to ductile. It is interesting that the b-value of AE varies dramatically as the gas injection rate becomes higher with significant fluctuations, indicating the ratio of large fracture and small fracture changes drastically during gas fracturing. In addition, the length of the induced fractures decreases with the increase of confining pressure during gas fracturing, and the length and width of vertical fractures are evidently larger when at high gas injection rate. Last, a novel theoretical predictive model is proposed for estimating breakdown pressure during gas fracturing based on the average tensile stress criteria, which is featured by considering the effect of confining pressure and gas flow behaviors. The theoretical prediction agrees with the experimental results. The present study can provide valuable results for theoretical analysis and engineering applications of gas fracturing in stimulating the HDR reservoirs.

Hot dry rock (HDR) has great development potential because of its advantages of clean, environmental protection and renewable. The study of physical and mechanical properties of HDR is one of the important links in the process of geothermal energy development. Previous studies have mainly focused on the granite thermal reservoir but research on the sandstone thermal reservoir, especially the physical properties of sandstone after multiple thermal shocks, is scarce. Therefore, in this study, cyclic thermal shock experiments of sandstone at different temperatures are carried out, and the variation law of P-wave velocity of sandstone after heat treatment is revealed. It is found that the P-wave velocity of red sandstone decreases with the increase of temperature and cycle times, especially when the temperature is higher than 400 °C, the maximum change rate of wave velocity reaches 52.6%. It is particularly noteworthy that the P- wave velocity of sandstone increases abnormally at about 600 °C. And this article puts forward three hypotheses to explain the wave velocity anomaly. In addition, the nonlinear relationship model among P-wave velocity, temperature and number of cycles is established for the first time, and the correlation coefficient R2is more than 0.9. This study serves as a reference for the development and utilization of the sandstone geothermal reservoir.

Low rate of penetration (ROP) and high drilling cost are the key problems that encountered in hot dry rock (HDR) geothermal resources exploitation and utilization. Plasma channel drilling (PCD) is a potential technology which has the advantages of high rock-breaking efficiency and ROP. A detailed understanding of the rock-breaking of PCD is essential to optimize the electrical parameters and bit structure. Therefore, this paper uses the probabilistic evolution model and thermo-mechanical coupling model in PFC2D to reveal the generation of plasma channel trajectory and cracks in heterogeneous granite under the high-voltage electric pulse. The results show that increasing temperature on the plasma channel will not alter the trend of initiation and propagation of cracks, but simply reduces the time consumption of crack initiation. The quantity of intragranular shear cracks is always the largest, and they are mainly distributed around the channel. In addition, compared with plasma channels formed when complete electrical breakdown occurs, the maximum depth of the channels of partial electrical breakdown is larger under the same electrode gap. This study also includes an experimental program to examine the granite fragmentation and borehole generation caused by an electrode bit. The research results can provide new ideas and methods for improving the drilling efficiency of HDR formation, and help to design the structural of drill bits based on PCD technology.

In the newly-proposed excavation-enhanced geothermal system (E-EGS), the massive hot dry rock (HDR) can be preconditioned by hydraulic fracturing to promote spontaneous block caving. A crucial problem in E-EGS is how to obtain controlled hydraulic fractures that intersect the natural fractures to crack the HDR matrix into blocks of suitable size. Here, we proposed a novel two-stage fracturing approach in which first-stage fracturing is used to modulate the in-situ stress state, rather than to introduce additional joints, and create favorable stratigraphic conditions for the second-stage fracturing to create desired fractures. We found that the minimum in-situ stress (σmin) in the region between fractures is significantly rotated even reaching nearly 90° over first-stage fracturing. In this region, the hydraulic fractures propagate approximately parallel to the initial σmin(σy) during second-stage fracturing and intersect with the natural fractures perpendicular to the initial σmin. The optimum in-situ stress state is achieved via the first-stage fracturing with similar fracture length and spacing. Compared to conventional fracturing, the fractures propagate asymmetrically and are shorter and wider in the second-stage fracturing. Two-stage fracturing can effectively control the fracture trajectory. This study supplies new insights for HDR preconditioning and benefits the effective construction of E-EGS reservoirs.

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

This paper presents an improved understanding of coupled hydro-thermo-mechanical (HTM) hydraulic fracturing of quasi-brittle rock using the bonded particle model (BPM) within the discrete element method (DEM). BPM has been recently extended by the authors to account for coupled convective–conductive heat flow and transport, and to enable full hydro-thermal fluid–solid coupled modeling. The application of the work is on enhanced geothermal systems (EGSs), and hydraulic fracturing of hot dry rock (HDR) is studied in terms of the impact of temperature difference between rock and a flowing fracturing fluid. Micro-mechanical investigation of temperature and fracturing fluid effects on hydraulic fracturing damage in rocks is presented. It was found that fracture is shorter with pronounced secondary microcracking along the main fracture for the case when the convective–conductive thermal heat exchange is considered. First, the convection heat exchange during low-viscosity fluid infiltration in permeable rock around the wellbore causes significant rock cooling, where a finger-like fluid infiltration was observed. Second, fluid infiltration inhibits pressure rise during pumping and delays fracture initiation and propagation. Additionally, thermal damage occurs in the whole area around the wellbore due to rock cooling and cold fluid infiltration. The size of a damaged area around the wellbore increases with decreasing fluid dynamic viscosity. Fluid and rock compressibility ratio was found to have significant effect on the fracture propagation velocity.

Geothermal systems have a big draw as a provider for free thermal energy for electrical generation. The resource based on fracture networks that permit fluid circulation, and allow geothermal heat to be extracted. Most geothermal resources occur in rocks that posses lack fracture permeability and fluid circulation. Hence, the fluid will be heated due to the Hot Dry Rock (HDR). The flow is circulated through the cracks, and extracts the heat to the ground. The emphasis of the simulators is on the HDR and on the development of methods that produce the hydraulic fractures. Linear elastic fracture mechanics approach (LEFM) was used to predict the crack propagation for initial crack. Finite element method (FEM) is used to predict the maximum stress areas, hence, determining the crack initiation.

Hot dry rock (HDR) is deep geothermal reservoir rock predominantly containing granite, whose physical and mechanical properties are governed by heterogeneous crystal structures. However, the fluid conductivity of intact granite with ultra-low permeability cannot be reformed using existing techniques. Complex fracture networks must be created by connecting hydraulic fractures with natural fractures developed in reservoirs. In this study, we introduce the wall barrier method into the novel hydro-grain-based model (hydro-GBM) to build a prefractured granite model at the mineral grain scale. This model enables investigation of the effects of the stress environment, mineral spatial distribution, and fluid injection rate on hydraulic fracturing characteristics. Microcracks around the inner tips of pre-existing fractures reproduce the initiation, propagation, and coalescence behaviors similar to experimental results. Mineral spatial distributions induce random changes in the propagation paths of hydraulic fractures. Increasing the fluid injection rate delays the deflection of hydraulic fractures and extends the propagation distance along the major axes of pre-existing fractures. The average activity level and proportion of large seismic events are reduced by injecting low-rate fluid. In summary, our results reveal the interactions between hydraulic and pre-existing fractures and provide a valuable reference for constructing an efficient enhanced geothermal system (EGS) for deep reservoirs.

Enhanced geothermal system (EGS) is a primary method to develop geothermal resources stored in hot dry rock (HDR), but it faces several key problems, such as unreasonable hydraulic fracture networks at high reservoir temperature, unclear multi-scale and multi-field coupling regularity, low heat extraction efficiency caused by the flashing flow in geothermal wells, and low thermoelectric conversion efficiency of geothermal fluid, which restricts the large-scale commercial development of geothermal resources. To resolve these major bottleneck problems, systematically reviews and analysis of the research progress and development trend of EGS are conducted in this paper. Particular attentions are devoted to four key technologies involved in the development of HDR geothermal resource by EGS: (1) the hydraulic fracturing technology for HDR reservoirs, including reservoir reconstruction methods, hydraulic fracturing network forming mechanisms and fracture propagation prediction models is illustrated in detail; (2) the fracture characterization methods, mathematical models and solution methods are described from three aspects including pore-scale multi-field coupled models, reservoir-scale multi-field coupled models and upscaling methods; (3) the efficient extraction technology of wellbore thermal fluid, involving the mechanism of flashing flows in geothermal wells and the experimental and numerical methods for investigating the characteristics offlashing flows are discussed; (4) the HDR geothermal power generation technologies, considering the principles of geothermal power generation, the types of power generation systems and the main application markets are introduced. In conclusion, EGS is a technology-intensive system, however, due to the complex working conditions of the underground reservoirs and the instability of the ground equipment, theoretical research tends to be separated from the practice. For the purpose of promoting the applicability of EGS, intimate combination and mutual guidance with the pilot tests are necessary to develop a production-research combined mode, and to raise the awareness and break through the key points in a constant back-and-forth.

High-hardness and poor-drillability of hot dry rock (HDR) impose great challenges on drilling-rate improvement, limiting the economic exploitation of HDR geothermal. Axial-torsional coupled percussive drilling (ATCPD), as a potential drilling-rate improvement method, might provide solutions for achieving efficient breaking of HDR. To determine the rock breaking performance of ATCPD in HDR, we investigate the failure behaviors of high temperature rocks under percussion loads. Three loading modes, i.e., static loading, axial percussion loading, and torsional percussion loading, are compared with axial-torsional coupled percussion loading (A-TCPL) mode, with respect to rock breaking capacity and failure behaviors. The effect of some important engineering parameters, e.g., rock temperature, impact amplitude and frequency, is put particular emphasis to obtain the great rock breaking performance. Results show that ATCPD is characterized by the higher amplitude force, lower torque and larger cracking volume as compared to other loading modes, thereby resulting in a growth drilling efficiency. A-TCPD shows greater performance to high temperature rocks in stress response and material damage. The maximum principal stress, tensile and compressive damage factors are positively correlated with rock temperatures under A-TCPL mode. Increasing the impact amplitude and frequency are beneficial for inducing damage on rocks with temperatures ranging from 200 to 600 °C. By using the rock breaking total displacement as the evaluation index, the optimal ratio of axial to torsional impact frequencies for greater rock breaking capacity is obtained. For rocks at 200 °C, 400 °C, and 600 °C, the optimal ratio of axial to torsional impact frequencies is 1:2, 1:3, and 2:3, respectively. The results are expected to provide new insights to enhance drilling rate for geothermal well and HDR excavation.

The use of sequestered carbon dioxide (CO2) as the heat exchange fluid in enhanced geothermal system (EGS) has significant potential to increase their productivity, contribute further to reducing carbon emissions and increase the economic viability of geothermal power generation. Coupled CO2sequestration and geothermal energy production from hot dry rock (HDR) EGS were first proposed 15 years ago but have yet to be practically implemented. This paper reviews some of the issues in assessing these systems with particular focus on the power generation and CO2sequestration capacity. The Habanero geothermal field in the Cooper Basin of South Australia is assessed for its potential CO2storage capacity if supercritical CO2is used as the working fluid for heat extraction. The analysis suggests that the major CO2sequestration mechanisms are the storage in the fracture-stimulation damaged zone followed by diffusion into the pores within the rock matrix. The assessment indicates that 5% of working fluid loss commonly suggested as the storage capacity might be an over-estimate of the long-term CO2sequestration capacity of EGS in which supercritical CO2is used as the circulation fluid.

AbstractHigh heat flow associated with the tectonic spreading of the Red Sea make western Saudi Arabia a region with high potential for geothermal energy development. The hydraulic properties of the Precambrian-age rocks occurring in this region are not conducive to direct production of hot water for heat exchange, which will necessitate use of the hot dry rock (HDR) heat harvesting method. This would require the construction of coupled deep wells; one for water injection and the other for steam recovery. There are some technological challenges in the design, construction, and operation of HDR geothermal energy systems. Careful geotechnical evaluation of the heat reservoir must be conducted to ascertain the geothermal gradient at the chosen site to allow pre-design modeling of the system for assessment of operational heat flow maintenance. Also, naturally occurring fractures or faults must be carefully evaluated to make an assessment of the potential for induced seismicity. It is anticipated that the flow heat exchange capacity of the system will require enhancement by the use of horizontal drilling and hydraulic fracturing in the injection well with the production well drilled into the fracture zone to maximum water recovery efficiency and reduce operating pressure. The heated water must be maintained under pressure and flashed to steam at surface to produce to the most effective energy recovery. Most past evaluations of geothermal energy development in this region have been focused on the potential for solely electricity generation, but direct use of produced steam could be coupled with thermally driven desalination technologies such as multi-effect distillation, adsorption desalination, and/or membrane distillation to provide a continuous source of heat to allow very efficient operation of the plants.

High heat flow associated with the tectonic spreading of the Red Sea make western Saudi Arabia a region with high potential for geothermal energy development. The hydraulic properties of the Precambrian-age rocks occurring in this region are not conducive to direct production of hot water for heat exchange, which will necessitate use of the hot dry rock (HDR) heat harvesting method. This would require the construction of coupled deep wells; one for water injection and the other for steam recovery. There are some technological challenges in the design, construction, and operation of HDR geothermal energy systems. Careful geotechnical evaluation of the heat reservoir must be conducted to ascertain the geothermal gradient at the chosen site to allow pre-design modeling of the system for assessment of operational heat flow maintenance. Also, naturally occurring fractures or faults must be carefully evaluated to make an assessment of the potential for induced seismicity. It is anticipated that the flow heat exchange capacity of the system will require enhancement by the use of horizontal drilling and hydraulic fracturing in the injection well with the production well drilled into the fracture zone to maximum water recovery efficiency and reduce operating pressure. The heated water must be maintained under pressure and flashed to steam at surface to produce to the most effective energy recovery. Most past evaluations of geothermal energy development in this region have been focused on the potential for solely electricity generation, but direct use of produced steam could be coupled with thermally driven desalination technologies such as multi-effect distillation, adsorption desalination, and/or membrane distillation to provide a continuous source of heat to allow very efficient operation of the plants.

Abstract: Cooling shock is a key technical means to improve rock breakage rate and drilling rate. Studying the acoustic emission characteristics of high-temperature granite under cooling shock is of great significance for analyzing the damage of hot dry rock (HDR) reservoirs and improving the heat extraction efficiency. This paper analyses the temperature change and acoustic emission (AE) characteristics of the granite heating process (25–600 ℃), air cooling, pore water injection, surface water spraying, and liquid nitrogen cooling, exploring the heat treatment method and the microstructural changes of granite. The results show that (1) During the cooling process, the violent cooling impact on the high temperature granite is the main reason for the AE signal. (2) In the cooling process, the effect of rapid cooling on high temperature granite damage is much greater than slow cooling. (3) When the high-temperature granite is cooled, the cumulative acoustic emission count is significantly smaller than that during the heating process. Therefore, there is a general law of greater heating damage and less cooling damage. This article has certain research significance for the engineering applications of geothermal energy mining. Article Highlights: In this paper, the acoustic emission characteristics of high-temperature granite under different cooling impacts are investigated. During the heating process of granite, its crack density gradually increases. Granite in the first heating and then cooling process, thermal damage is obvious, while the cooling damage changes less.

Hot dry rock (HDR) geothermal energy, almost inexhaustible green energy, was first put forward in the 1970s. The development and testing of HDR geothermal energy are well reported in USA, Japan, UK, France and other countries or regions. In this paper, the geological characters of Yangbajing basin were first analyzed, including the continental dynamic environments to form HDR geothermal fields in Tibet, the tectonic characteristics of south slope of Nyainqêntanglha and Dangxiong-Yangbajing basin, and the in-situ stresses based on the investigations conducted, and then the site-specific mining scheme of HDR geothermal resources was proposed. For the potential development of HDR geothermal energy, a series of experiments were conducted on large-scale granite samples, 200mm in diameter and 400mm in length, at high temperature and high triaxial pressure for cutting fragmentation and borehole stability. For the borehole stability test, a hole of 40mm in diameter and 400mm in length was aforehand drilled in the prepared intact granite sample. The results indicate that the cutting velocity obviously increases with temperature when bit pressure is over a certain value, while the unit rock-breaking energy consumption decreases and the rock-breaking efficiency increases with temperature at the triaxial pressure of 100MPa. The critical temperature and pressure that can result in intensive damage to granite are 400–500 °C and 100–125MPa, respectively.

Auf der Exkursion werden drei sehr unterschiedliche Themenbereiche aus den Aufgabenschwerpunkten des Landesamts für Geologie, Rohstoffe und Bergbau Baden-Württemberg an ausgewählten Fallbeispielen behandelt. Eine der Aufgaben der staatlichen Geologischen Dienste ist der Schutz der Bevölkerung vor Georisiken. Zu den häufigsten geologisch bedingten Gefahren gehören Rutschungen, Steinschläge und Erdfälle. Am Beispiel der landschaftlich reizvollen „Felsengärten“ bei Hessigheim werden diese Risiken aufgezeigt und auch Möglichkeiten diskutiert, solche Gefahren zu minimieren. Das Geothermie-Projekt Bad Urach liegt im Zentrum der bedeutendsten Temperaturanomalie von Baden-Württemberg und ist in Deutschland das erste Hot-Dry-Rock (HDR)-Verfahren im kristallinen Grundgebirge zur Stromproduktion und zur örtlichen Wärmeversorgung. Das kristalline Grundgebirge wurde unterhalb einer 1600 m mächtigen sedimentären Bedeckung erbohrt. Die hydraulischen und hydrochemischen Eigenschaften dieser Sedimentabfolge und des Kristallinen Grundgebirges werden dargestellt. Als weiterer Programmpunkt wird das für die Landeshauptstadt Stuttgart sehr bedeutsame Großbauvorhaben „Stuttgart 21“ in Verbindung mit dem Heilquellenschutz erörtert.

Hydraulic fracturing or hydraulic stimulation is one of the most effective methods of enhancing of the hot dry rock HDR geothermal system productivity. The 3D structure of the fractured rock is approximated with the network models of “fractal geometry”. The models of fracture networks are generated by distributing fractures randomly in space and adopting the fractal correlation NrCr−Dthat incorporates the number of fractures Nr, fractal length r, fractal dimension D, and fracture density within the rock mass C. This procedure makes possible to characterize the geothermal reservoirs by parameters measured from the field data. On the basis of this approach the mathematical model of the hydraulic rock fracturing is proposed. The model incorporates approximations of the fracture mechanical behavior drawn from the rock mechanics literature, a very simplified analysis of the operative physical processes, and mapping of the connectivity of fracture network to a cubic regular grid. Taken together, these permit the approximate engineering resolution of the multiparametric highly complex mechanical problem. The model has shown itself capable of reproducing many facets of the data collected during the field tests of stimulation of the Hijiori geothermal system Yamagata, Japan. The reliability of the developed model is validated by comparison with the experimentally determined data for the Hijiori Deep Reservoir. This implicitly justifies the numerical results and conclusions drawn in the present research. In particular, a series of computations indicates that the connectivity of the fracture network is greatly affected by the fractal dimension of the fracture network. The strong effect of the fractal dimension on the reservoir's size is also observed. The numerical results illustrate the controlling effect of the pressure and flow rate in the stimulating well for reservoir growth.

The role of shear dilation as a mechanism of enhancing fluid flow permeability in naturally fractured reservoirs was mainly recognized in the context of hot dry rock (HDR) geothermal reservoir stimulation. Simplified models based on shear slippage only were developed and their applications to evaluate HDR geothermal reservoir stimulation were reported. Research attention is recently focused to adjust this stimulation mechanism for naturally fractured oil and gas reservoirs which reserve vast resources worldwide. This paper develops the overall framework and basic formulations of this stimulation model for oil and gas reservoirs. Major computational modules include: natural fracture simulation, response analysis of stimulated fractures, average permeability estimation for the stimulated reservoir and prediction of an average flow direction. Natural fractures are simulated stochastically by implementing ‘fractal dimension’ concept. Natural fracture propagation and shear displacements are formulated by following computationally efficient approximate approaches interrelating in situstresses, natural fracture parameters and stimulation pressure developed by fluid injection inside fractures. The average permeability of the stimulated reservoir is formulated as a function of discretized gridblock permeabilities by applying cubic law of fluid flow. The average reservoir elongation, or the flow direction, is expressed as a function of reservoir aspect ratio induced by directional permeability contributions. The natural fracture simulation module is verified by comparing its results with observed microseismic clouds in actual naturally fractured reservoirs. Permeability enhancement and reservoir growth are characterized with respect to stimulation pressure, in situstresses and natural fracture density applying the model to two example reservoirs. Copyright © 2002 John Wiley & Sons, Ltd.

Following the increase in oil prices in the mid-1970s, Britain assessed its geothermal resources. Low-temperature, hot-water resources, in the range 40°C to 100°C, occur in Permo-Triassic sandstones in several deep sedimentary basins. In total these resources are estimated at 69.1× 10¹⁸ joules (J) (2576 million tonnes coal equivalent). Resources also occur in Upper Palaeozoic aquifers but, as the permeability of these aquifers depends upon fissures, exploitation is difficult. The only surface manifestations of hot water at depth are the warm springs at Bath and Bristol and in the Peak District and Taff Valley, which issue from the Carboniferous Limestone. The potential of radiothermal granites for Hot Dry Rock (HDR) development has also been investigated, particularly in the Carnmenellis granite in Cornwall. Three boreholes drilled in the granite to depths of over 2 km have been connected by developing natural fractures. Water circulation between the boreholes and through the fractured rock has been successful.

Geothermal prospects in the UK are represented by low enthalpy resources in deep sedimentary basins and 'Hot Dry Rock' (HDR) resources in radiothermal granites, and possibly in deep basement rocks where they are overlain by thick low conductivity sediments. The low enthalpy resources are in Permo-Triassic sandstones at temperatures of more than 40°C. Four deep exploration wells have been drilled to investigate the potential of these sedimentary aquifers. The main HDR resource potential is associated with major granite batholiths in southwest and northern England where temperatures are predicted to be 200°C at about 5.4 and 6 km respectively. The HDR potential is being investigated by the Camborne School of Mines at their test site in Cornwall where three boreholes have been drilled to depths of between 2 and 2.5 km.
The Hot Dry Rock Accessible Resource Base at temperatures of more than 100°C and depths of less than 7 km is 36×10²¹ joules (equivalent to 130×10⁴ million tons of coal). The low enthalpy Geothermal Resource of the Permo-Triassic sandstones at temperatures of more than 40°C is 200×10¹⁸ joules (equivalent to about 8000 million tons of coal). If only a small fraction of these resources could be developed, it would be significant in terms of the UK's energy balance.

—We investigate the deep subsurface structure below the artificial reservoir at the Soultz Hot Dry Rock (HDR) site in France by a reflection method which uses acoustic emission (AE) as a wave source. In this method, we can detect reflected waves by examining the linearity of a three-dimensional hodogram. Additionally for imaging a deep subsurface structure, we employ a three-dimensional inversion with a restriction of wave polarization angles and with a compensation for a heterogeneous source distribution.¶We analyzed 101 AE wave forms observed at the Soultz site during the hydraulic testing in 1993. Some deep reflectors were revealed by this method. The bottom of the artificial reservoir that is presumed from all of the AE locations in 1993 was delineated at the depth of about 3900 m as a reflector. Other deeper reflectors were detected below the reservoir, which would not have been detected using conventional methods. Furthermore these reflectors agreed with the results of the tri-axial drill-bit VSP (Asanuma et al.,1996).

Immense amounts of energy can be obtained from the hot dry rock (HDR) of the earth, as an extension/expansion of the hydrothermal resources. The extraction of usable energy from a HDR reservoir made by hydraulically fracturing the hot, but essentially dry rock between two deep drill holes has been successfully demonstrated at Fenton Hill, New Mexico, by the Los Alamos National Laboratory. Depending on the location and depth of future HDR reservoirs, the extracted heat may be either high grade (for generation of electricity), or low grade (for direct‐use space heating, food processing, etc.). The circulating hot water can also be used to augment energy production from other energy systems, such as boiler feedwater preheat, process heat for synfuel production, or stimulating bacteria growth in cold climates (for instance, more rapid digestion in sewage treatment plants or landfill dumps). When the HDR technology of drilling and fracturing in crystalline rock is coupled with solar energy production, excess summertime heat from solar collection facilities can be transferred and stored in manmade underground reservoirs for wintertime withdrawal and utilization. The same technology can provide huge, but easily accessible, heat sinks for reject industrial heat, creating many options for industry, municipalities, and district heating organizations to integrate energy demands with heat disposal requirements.

The extraction of heat or thermal energy from the Earth - heat mining - has the potential to play a major role as an energy supply technology for the 21st century. This paper looks specifically at the potential for hot dry rock (HDR) geothermal energy in the United States. A generalized multiparameter economic model was developed for optimizing the design and performance of HDR geothermal systems. Assuming reservoir productivity comparable to existing hydrothermal systems, high-grade HDR systems (∇T > 60 °C / km) produce electricity in the 6-7c/ kWhe range. For low-grade systems (∇T < 40 ° C / km) to become commercially attractive, much higher reservoir productivity levels and / or substantially lower drilling costs are required.

The extraction of heat or thermal energy from the Earth - heat mining - has the potential to play a major role as an energy supply technology for the 21st century. However, even if reservoir productivity goals are achieved, the role of heat mining with today's energy prices and development costs is limited to only a small fraction of the Earth's surface. A generalized multi-parameter economic model was developed for optimizing the design and performance of hot dry rock (HDR) geothermal systems. Key technical and institutional obstacles to universal heat mining are discussed in a more general context. Advanced concepts in drilling technology are reviewed in light of their potential impact on overcoming some of these obstacles to universal heat mining