Hot Dry Rock Geothermal Resources Research Articles

Geothermal extraction performance in fractured granite from Gonghe Basin, Qinghai province, China: Long-term injection and production experiment

The efficient exploitation of geothermal energy through enhanced geothermal systems (EGS) has been a relevant topic for hot dry rock (HDR) geothermal resources. When cryogenic fluid is injected into a thermal reservoir, improving heat exchange efficiency is key to achieving the optimal exploitation of HDR. In this paper, granite outcrops from Gonghe Basin were used as the testing sample. The natural fractures in the granite samples were relatively well developed. To simulate long-term injection and production from multi-wells in situ, physical experiments were performed in a newly-developed, in-house large-scale true triaxial experimental system. Geothermal extraction performance of an HDR was simulated for long-term injection and production operations. Simultaneously, the mode of one-injection and multiple-production wells was represented. In the paper, the effects of the production-injection well spacing, the number of production wells and the injection rate on the production temperature and flow rate are discussed. The results show that, during long-term injection and production, there are two stages of production temperature variation, namely stabilization and attenuation. When the number of the production wells is increased, the heat extraction efficiency is accelerated. Moreover, competitive diversion of fluid among fractures occurred due to different conductivities. Furthermore, under different production modes, the production flow rate contributed differently to the heat extraction. Finally, the effect of the production-injection wells spacing on the heat exchange performance was analyzed; this is mainly reflected in the change of the effective heat exchange area between the rock and the injected fluid. The results emphasize the importance of designing an appropriate production mode and optimizing the injection-production parameters to ensure efficient HDR exploitation.

A multi-objective optimization and multi-attribute decision-making analysis for technical-thermodynamic-economic evaluation considering the rock damage on production performance of hot dry rock geothermal resources

In the long-term mining of geothermal resources in hot dry rock (HDR), the change of thermal stress and pore pressure will increase fracture conductivity evolution, further improving production performance. The optimization and decision-making of the development scheme based on the impact of damage from fractures have yet to be reported. The damage to fractures is essential in designing and adjusting geothermal resource development schemes, particularly in selecting optimal schemes. Therefore, the production performances of HDR resources under different parameters are analyzed to establish a database. Then, minimizing flow resistance, maximizing net power, and maximizing economic benefits are set as optimization goals. Various injection-mining parameters and fracture characteristics are treated as decision variables. Multi-objective optimization and multi-attribute decision analysis is conducted to obtain optimal schemes. Finally, optimal schemes are evaluated and compared, considering damage and non-damage scenarios. Results show that the NSGA-II algorithm is more suitable for optimizing geothermal development questions. Net power and economic benefits of the optimal scheme considering damage increase by 45.84 % and 21.35 % compared to the control scheme with damage. For the non-damage scenario, the above values increased by 31.55 % and 5.15 %, respectively. Compared to not considering the damage, higher mass flow and well spacing of optimal scheme can be selected for the case when damaged. Moreover, the parametric design of the optimal scheme becomes more conservative as the production cycle increases.

Simulation of thermo-hydro-mechanical coupled heat extraction process of geothermal reservoir under varied geological conditions with CO2 as working fluid

The use of supercritical CO2 as a working fluid instead of H2O in an enhanced geothermal system (EGS) for extracting heat from hot dry rock geothermal resources has garnered significant attention. The objective of this study is to investigate how different geological conditions affect the heat extraction behavior of supercritical CO2 and to reveal the efficiency differences between supercritical CO2 and H2O as working fluids in each geological scenario, providing deeper insight into reservoir management. A series of 3D coupled thermo-hydro-mechanical models were established. Besides, the impact of initial temperature, pressure, porosity, permeability, and stress field on the heat extraction of EGS with different working fluids are also analyzed and compared. Results indicate that, at the same circulating volumetric flow rate, CO2-EGS generally exhibits an earlier thermal breakthrough time, lower net energy production, and reduced heat extraction rate compared to H2O-EGS. Taking into account the extended reservoir lifespan and the relatively small pressure difference between injector and producer in CO2-EGS, CO2 is a more suitable working fluid than H2O. Furthermore, the performance of CO2-EGS is primarily affected by the initial temperature, porosity, and permeability distribution pattern, rather than the initial pressure. On the one hand, a reservoir with low initial temperature, porosity and homogeneous permeability were found to be more conducive to enhancing the average net energy production. On the other hand, reservoir vertical permeability anisotropy can have a more profound impact on CO2-EGS efficiency compared to horizontal y permeability anisotropy. Additionally, this study also examined the impact of in situ stress field. It was found that, compared to H2O-EGS, both isotropic and anisotropic in situ stress fields have a weak effect on the performance of CO2-EGS, which can be attributed to the similarity of the channelization degree of the flow field under different stress conditions.

Research on circulating heat recovery law of single horizontal well for hot dry rock geothermal resources

Hot dry rock is essential to geothermal resources because of its advantages of clean green, good stability, large reserves, and high utilization coefficient. However, there are few large-scale and commercial mining schemes due to increased development costs, induced earthquakes, and low recovery during fracturing. Based on the above background, the concept of hot dry rock development for circulating heat recovery in the single horizontal well is put forward, injection and recovery of circulating working fluid are realized by single horizontal well, interchange of hollow pipe in well with liquid flow passage in wall annulus is realized by the fast connection of flow passage transformation, and flow direction control of thermal fluid is recognized by its unique bottom hole structure, to achieve the goal of cleanliness, low energy consumption, and high-efficiency heat recovery, to achieve the purpose of avoiding a series of problems caused by reservoir fracturing. A mathematical model has been created to calculate heat recovery efficiency in a single horizontal well. This model analyzes the impact of critical parameters in mining hot dry rock geothermal resources from single horizontal wells. The results show that the flow rate of circulating working fluid, the wellbore’s horizontal section length, and the formation’s thermal conductivity greatly influence thermal recovery. For instance, a domestic hot dry rock project can recover heat up to 3.08 MW through cyclic mining with a single horizontal well over 30 years.

Structure of the Gonghe Sedimentary Basin in the northeastern Tibetan Plateau: evidence from teleseismicPwaves recorded by a dense seismic array

SUMMARYThe Gonghe Basin in the northeast Tibetan Plateau presents significant potential for hot dry rock (HDR) geothermal resources. A 1990 Mw 6.4 earthquake in the basin furthers the need for an improved understanding of its sedimentary structure. In this study, we utilize data from a dense seismic array of 88 short-period seismometers deployed at an interstation spacing of approximately 3 km to scrutinize the sedimentary structure of the Gonghe Basin. By analysing teleseismic P waveforms, we identify P-to-S converted waves (Ps wave) originating from the sedimentary basement. We then determine the delay time between the Ps waves and the direct P waves (P wave) through waveform cross-correlation. By integrating this delay time with empirical velocity structure models, HDR borehole data and results from teleseismic receiver function analysis, we derive a sediment thickness model of the Gonghe Basin for the Qabqa geothermal area. Our findings reveal a gradual increase in sediment thickness from around 500 m in the east to approximately 3000 m in the west, which is consistent with other geophysical surveys and borehole data. The thick sediments in the basin could potentially serve as an excellent thermal storage cover for HDR. The strong ground motion simulation using our sediment thickness model shows that thick sediments can amplify seismic waves, increasing the risk of seismic hazards. Moreover, our study indicates that the clear Ps waves can be effectively extracted to construct a dependable sediment thickness model using teleseismic P waves recorded by a short-period dense seismic array.

The fragmentation mechanism of granite in electrical breakdown process of plasma channel drilling

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.

3-D Inversion of Gravity Data of the Central and Eastern Gonghe Basin for Geothermal Exploration

The Gonghe Basin is one of the most important regions for the exploration and development of hot dry rock geothermal resources in China. However, there is still some controversy about the main heat source of hot dry rock geothermal resources in the Gonghe Basin. Combined with previous research results including three-dimensional magnetotelluric imaging and linear inversion of Rayleigh wave group and phase velocity result, we obtained a high-resolution underground spatial density distribution model of the Gonghe Basin based on satellite gravity data by using 3-D gravity focusing inversion method. According to the results, there are widely distributed low density anomalies relative to surrounding rock in the middle crust of the study area. The low-density layer is speculated to be a low-velocity, high-conductivity partial melting layer in the crust of the Gonghe Basin. The inversion result confirms for the first time the existence of a partial melt layer from the gravity point of view, and this high temperature melt layer may be the main heat source of the hot dry rock geothermal resources in the Gonghe Basin. It can provide a new basis for further research on the genesis of the hot dry rock geothermal system in the Gonghe Basin.

Research on physical-mechanical properties and energy evolution characteristics of granite after high temperature treatment

Granite is the main carrier of geothermal energy of hot dry rock. Therefore, the study of influence of different temperatures on the physical-mechanical properties and energy evolution characteristics of granite after high temperature treatment is an important way to guide the successful exploitation of hot dry rock geothermal resources. Physical tests and uniaxial compression tests were carried out on granite after high temperature treatment ranging from 25-350?C. The results show that change rates in volume, mass, density, P-wave velocity and porosity of granite increase with increasing temperature. The research results can provide basic data support for the safe and efficient development of hot dry rock in China, and also develop the theory of high temperature rock mechanics.

Methods for geothermal resource assessment of hot dry rock: A case study in the Gonghe Basin, China

Hot dry rock (HDR) geothermal resources are renewable energy source. Many of the findings of HDR resource evaluations have been used in energy planning and EGS design. However, to assess the amount of HDR resources in different locations, a consistent classification scheme and evaluation methods are still lacking. Considering geological credibility and economic feasibility, HDR resources are separated into three categories: vision, reserve, and exploitable. Vision and reserve are stationary resources that can be evaluated using the volume technique, and the exploitable resources can be evaluated using the numerical simulation approach. The HDR vision resource of the Gonghe Basin is evaluated to be 4.076 × 1022 J, and the reserve resource of the Qiabuqia HDR mass is evaluated to be 2.11 × 1020 J. At the Qiabuqia HDR development site, a discrete fracture network (DFN) model is applied for numerical simulation computations, which is based on the notion of local thermal nonequilibrium. The K1 and K2 wells produce varying amounts of heat due to the heterogeneous features of the fractured medium model, which is primarily due to differences in fracture density, heat exchange area, and fluid migration pattern. The categorization system and assessment technique can be used as a guide for evaluating HDR resources in the future.

Energy and exergy analysis of a hot dry rock geothermal resource power generation system based on organic Rankine cycle

Abstract Hot dry rock is an abundant, stable and low-carbon geothermal resource, which has a promising prospect for power generation in China. In this paper, a hot dry rock power generation system model based on conventional organic Rankine cycle was established. The performance of the system was evaluated by thermodynamic analysis, as well as energy and exergy analyses. Four types of organic working fluids were selected to investigate their effects on system energy efficiency. R114 had much higher energy efficiency (17.50%~ 19.00%) than other three organic working fluids (R245fa, R245ca and R601) when water flow rate ranged from 80 t/h to 260 t/h. When R245fa was used as working fluid and power output was set at 4 MW, energy and exergy efficiency of the system were 15.27% and 50.42%, respectively. According to the energy flow analysis, the major energy losses (15.67 MW) in the system were attributed to the discharge temperature of turbine exhaust. However, exergy flow analysis indicated that the energy loss from turbine exhausts was not as serious as expected, since the work potential of that energy was limited. The exergy destruction of each component in the system was calculated, and the results showed that heat exchanger and the vapor separator were the weakness components in the system, and they respectively contributed to 44.8% and 29.8% of the total exergy destruction. The results obtained in this study provide possible strategies to improve the overall performance of the hot dry rock power generation system based on conventional organic Rankine cycle.

Performance analysis of pinnate horizontal well in enhanced geothermal system

• Feasibility of enhanced geothermal system with pinnate horizontal wells is studied. • Total heat production is improved by 21.4% with pinnate horizontal wells. • Effect of wells structure on heat extraction performance is evaluated. • Reliable layout of pinnate horizontal well is presented. The well layout is crucial for improving the utilization of hot dry rock geothermal resources. In present work, a three-dimensional enhanced geothermal system (EGS) with pinnate horizontal well is proposed. Furthermore, the effect of pinnate horizontal well structure on the heat extraction performance of present EGS is numerically investigated. For scCO 2 -based EGS and water-based EGS, the stable heat extraction state and a competitive heat production can be obtained by present EGS. Compared with the conventional EGS with doublet system, the production temperature and the total heat production of EGS with pinnate horizontal well increase by 21.54 K and 21.4 %, respectively. The heat extraction performance of EGS with pinnate horizontal well is positively correlated with the main well bore number and length. In comparison, the branch well structure and number have less effect on the heat extraction performance. Consequently, main wellbore number with 3 or main wellbore length with 300 m can be taken as criterion for the design of the main wellbore. Besides, the branch well with staggered pattern, inclination angel of 60° and the number of 4, is proved to be the optimum arrangement.

Joint interpretation technology of favorable HDR geothermal resource exploration in Northern Songliao Basin

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 500km2, its thickness 150–330m, its static formation temperature reaching 152° in the depth of 3360m, 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.

Petrological and geochemical constraints on the petrogenesis of granitoids in the Gonghe geothermal basin, western Qinling (China)

The Gonghe geothermal basin is situated in the westernmost part of the western Qinling orogen and possesses the supreme potentials to explore and develop hot dry rock (HDR) geothermal resources in China. The basal rock of HDR reservoir in Gonghe basin is composed of granitoids. In this contribution, in order to constrain the formation mechanism of the Gonghe HDR, granitic samples from outcropping have been collected and drilling core samples data from the archives also been compiled. The studied results show that the obtained zircon U–Pb ages of 255.1 ± 1.9 Ma and 253.9 ± 2.6 Ma for granodiorite and monzogranite, respectively. All granitoids from both outcropping and drilling core have the similar rock associations and their major and trace elements possess the consistent evolved patterns, such as relative enrichment of LILEs (e.g., K, Rb), depletion of HFSEs (e.g., Nb, Ti), and negative Eu anomalies (0.32–0.66), indicating that they share the same magma sources and evolutional processes. And they have the affinity of I-type granites. They were derived from the mixture of partial melting of metabasaltic rocks in the lower crust and a small amount of mantle-derived magma and formed in a subduction-related setting relative to the southward subduction of the Zongwulong oceanic crust beneath the west Qinling terrane. The heat production values of rocks around the Gonghe basin were calculated and all basal granitic rocks ranges from 0.35 to 8.51 μW/m3, yielding an arithmetic mean value of 1.97 μW/m3, indicating that the radioactive heat generation capacity of granitoids in the Gonghe basin is slightly lower. The HDR reservoir in the Gonghe basin was probably due to an allied thermal effect of the radiogenic heat production in the thickening continental crust and the heat contributed by a deep magma chamber below the basin. The early Triassic granitoids with a higher thermal conductivity serve as the conductive medium of the geothermal heat; whilst the sedimentary rocks marked by lower thermal conductivity above the granitic batholiths act as the reservoir cap. This contribution provides a relatively rational understanding for the mechanism of the HDR reservoir in Gonghe geothermal basin from the petrological and geochemical perspectives, which is conducive to the geothermal potential assessment and estimation, and to-be implementation of the Enhanced Geothermal System demonstration project in China.

Catalog of Enhanced Geothermal Systems based on Heat Sources

AbstractIt is common sense that a deeper well implies higher temperature in the exploration of deep geothermal resources, especially with hot dry rock (HDR) geothermal resources, which are generally exploited in terms of enhanced geothermal systems (EGS). However, temperature is always different even at the same depth in the upper crust due to different heat sources. This paper summarizes the heat sources and classifies them into two types and five sub‐types: crust‐origin (partial melting, non‐magma‐generated tectonic events and radiogenic heat production), and mantle‐origin (magma and heat conducted from the mantle). A review of global EGS sites is presented related to the five sub‐types of heat sources. According to our new catalog, 71% of EGS sites host mantle‐origin heat sources. The temperature logging curves indicate that EGS sites which host mantle‐origin magma heat sources have the highest temperature. Therefore, high heat flow (>100 mW/m2) regions with mantle‐origin magma heat sources should be highlighted for the future exploration of EGS. The principle to identify the heat source is elucidated by applying geophysical and geochemical methods including noble gas isotope geochemistry and lithospheric thermal structure analysis. This analytical work will be helpful for the future exploration and assessment of HDR geothermal resources.

Thermodynamic and Techno‐economic Analysis of a Triple‐pressure Organic Rankine Cycle: Comparison with Dual‐pressure and Single‐pressure ORCs

AbstractInvestigation of a triple‐pressure organic Rankine cycle (TPORC) using geothermal energy for power generation with the net power output of the TPORC analyzed by varying the evaporation pressures, pinch temperature differences (tpp) and degrees of superheat (tsup) aimed to find the optimum operation conditions of the system. The thermodynamic performance of the TPORC was compared with a dual‐pressure organic Rankine cycle (DPORC) and a single‐pressure ORC (SPORC) for geofluid temperatures ranging from 100°C to 200°C, with particular reference to the utilization of a hot dry rock (HDR) geothermal resource. Thermodynamic performances of the TPORC system using eight different organic working fluids have also been investigated in terms of the net power outputs. Results show that a higher geofluid mass flow rate can make a considerable contribution to shortening the payback period (PBP) as well as to decreasing the levelized electricity cost (LEC), especially when the geofluid temperature is low. For the temperature range investigated, the order from high to low based on thermodynamic and techno‐economic performances is found to be TPORC > DPORC > SPORC. In terms of using geothermal resources within the given temperatures range (100°C–200°C), the TPORC system can be a better choice for geothermal power generation so long as the wellhead geofluid temperature is between 140°C and 180°C.

Numerical Simulation on Heat Recovery Efficiency of Different Working Fluids in High-Temperature Rock Mass

It is of great significance for the sustainable development of global energy to develop hot dry rock (HDR) geothermal resources by using enhanced geothermal system (EGS) technology. Different working fluids in EGS have different heat recovery efficiencies. Therefore, this paper takes water and CO2 as the heat-carrying media and establishes a thermal hydraulic mechanical coupling model to simulate the heat recovery process in high-temperature rock mass. By considering the different confining pressures, rock temperature, and injection pressure, the advantages of H2O-EGS and CO2-EGS are obtained. The results show that with the increase of confining pressure, the heat recovery efficiency of water is significantly higher than that of CO2, but at higher reservoir temperature, CO2 has more advantages as a heat-carrying medium. The net heat extraction rate will increase with the increase of injection pressure, which indicates that the mass flow rate plays a leading role in the heat recovery process and increases the injection pressure of fluid which is more conducive to the thermal recovery of EGS. This study will provide a technical guidance for thermal energy exploitation of hot dry rock under different geological conditions.

An investigation on the deterioration of physical and mechanical properties of granite after cyclic thermal shock

As one of the main thermal storage medium of hot dry rock geothermal resources, granite is often subject to frequent thermal shock during the geothermal exploitation. Grasping the response regularity of granite to thermal shock is of great significance to the construction and long-term reliability evaluation of enhanced geothermal system (EGS). This paper investigated the physical and mechanical properties of granite after 0∼20 thermal shocks, and discussed its internal deterioration mechanism. Combined with ANSYS numerical simulation analysis, the mechanical formation mechanism of thermal cracks that lead to the deterioration of physical and mechanical properties of granite was analyzed. The results show that the first thermal shock tends to cause the severest deterioration to both physical and mechanical properties of granite. But when the number of thermal shocks exceeds 10, the deterioration slows down. The increase in contact thermal resistance between mineral particles after cyclic thermal shock results in a decrease in the thermal conductivity of granite. The enhancement of local stress concentration effect and the increase of local bearing capacity difference of granite after cyclic thermal shock result in the decrease of peak strength and the enhancement of ductility characteristics of granite. Due to the existence of thermal cracks, the granite after cyclic thermal shock has one more path to release the energy stored during the loading process than the natural granite. Therefore, the maximum counts at the beginning of crack propagation and the entire crack propagation stage are smaller than that of the natural granite. During the thermal shock, the huge instantaneous temperature difference between the interior and exterior of granite leads to the formation of thermal stress, which increases rapidly and then disappears gradually. As a result, the interior of granite mainly bears the effect of the vertical compressive thermal stress up to 7.59 MPa, while the exterior of granite mainly bears the effect of vertical tensile thermal stress up to 9.18 MPa. The propagation of some pre-existing micro cracks with proper orientation in the interior of granite and the initiation and propagation of secondary thermal cracks on granite surface are identified as two main causes to the deterioration of the physical and mechanical properties of granite after cyclic thermal shock.

Design and productivity evaluation of multi-lateral well enhanced geothermal development system

Enhanced geothermal development system is an effective means to develop hot dry rock geothermal resources, and its reasonable structural design is crucial to the efficient exploitation of hot dry rocks. In this paper, a multi-lateral well enhanced geothermal development system, which is composed of one low-permeability thermal reservoir, three multi-lateral injection wells, three multi-lateral production wells and three artificial fractures, was designed based on the traditional dual-well development model. Then, a 3D hydrothermal coupling numerical evaluation model was established by using the local heat balance method, and its accuracy and reliability were verified according to Lauwerier analytical theory of fractured flow and heat transfer. Finally, the influence of reservoir parameters, well layout parameters, and injection-production parameters on the heat production performance of this enhanced geothermal development system were explored based on the water-rock coupling mechanism inside the reservoir in the process of thermal production. And the following research results were obtained. First, the “rock invasion effect” of the cold front in the fractures of the multi-lateral well development system is stronger than that in the matrix. Increasing the reservoir permeability can improve the internal heat transfer effect, but also enhances the “invasion performance” of cold front. Second, the annual thermal energy extraction of the system gradually decreases over time and it also decreases with the increase of the injection temperature. It increases with the increase of the reservoir permeability in the early stage of the operation, but decreases greatly after the completion of the thermal breakthrough. Third, the system's thermal energy extraction rate and production mass flow rate are less influenced by reservoir porosity and injection temperature, but they decrease with the increase of the vertical spacing of injection-production wells. Fourth, the production temperature decreases greatly with the increase of the reservoir permeability, increases with the increase of the reservoir porosity, and decreases with the decrease of the vertical spacing of injection-production wells. Fifth, the heat production performance of the system in the early stage can be improved by increasing the length of the production well, and the heat production capacity of the system can be enhanced to a certain degree by increasing the injection-production pressure difference, but excessive pressure difference will impact the service life of the reservoir seriously.

Evaluation of favourable hot dry rock areas in the east of the Yishu fault zone in China

Hot dry rock (HDR) geothermal resources have great development potential and prospects, owing to their wide distribution and high thermal storage temperature. HDR resources are abundant in China, but their exploration and exploitation remain in the early stages. The Yishu fault zone in Shandong Province in eastern China is in a high-temperature geothermal zone of the Pacific Rim. Three craters and 15 hot springs are in this area, which possesses abundant geothermal resources and favourable conditions for the occurrence of HDR resources. Nevertheless, HDR resources in this area have not been studied in detail, and the index-evaluation system for HDR is still imperfect. In this paper, five index layers are selected, including: the gravity and magnetic fields, geothermal field, tectonic activity, reservoir and cover characteristics, and remotely sensed features. Thirteen index elements were also selected as evaluation factors. In combination, these index layers and index elements have established a favourable evaluation index system for HDR resources in this region, and the analytic hierarchy process has been used to determine the weight of each factor. Then, the GIS spatial analysis function is used to carry out spatial superposition analyses for each index element, and finally the distribution characteristics of HDR resources east of the Yishu fault zone were determined, with four favourable selection areas being delineated. The occurrence conditions of HDR resources are analysed in detail with evaluation indexes. It is believed that HDR resources are closely related to neotectonic movement, volcanism and rocks with high heat-generation rates. Searching for areas with geothermal anomalies is indispensable for the evaluation of HDR resources. This study provides a basis for the exploration and development of HDR resources in the region, and it can also provide a reference for similar work in other parts of the country. KEY POINTS A set of evaluation indexes to determine favourable areas for HDR resources, including gravity, geothermal field, geological structure and crustal stability, heat storage and heat cover and features on remotely sensed images, is established. East of Yishu fault zone, there is a great potential of HDR resources, with an area of 19 399 km2 of favourable and more favourable areas, accounting for approximately 38.8% of the total area. HDR resources are likely closely associated with neotectonics, igneous activity, geothermal anomalies and rocks with a high heat-generation rate such as granites.

Conceptualization and evaluation of the exploration and utilization of low/medium-temperature geothermal energy: a case study of the Guangdong-Hong Kong-Macao Greater Bay Area

Geothermal energy is one of the most promising renewable energies due to its high load factor. This work is devoted to presenting the conceptualizations and research advances made at Shenzhen University on the exploration and utilization of low/medium-temperature geothermal energy based on the geothermal resource potential and characteristics of the Guangdong-Hong Kong-Macao Greater Bay Area. The results indicate that the geological structure and lithology of this area are conducive to the formation of geothermal conditions. The geothermal flow and geothermal gradient in this area are higher than the average values in southern China. The total hydrothermal resources and hot dry rock geothermal resources amount to 1.8 × 1017 kJ and 4.0 × 1017 kJ, respectively. To enhance the permeability of geothermal reservoirs, especially hot dry rock reservoirs, the volumetric fracturing technique, which is based on three-dimensional fracture mechanics, and the fatigue pneumatic fracturing technique are proposed to stimulate geothermal reservoirs. The advances in theoretical and experimental research and some noteworthy future research topics on volumetric fracturing and fatigue pneumatic fracturing techniques are summarized. To increase the efficiency of low/medium-temperature geothermal power generation, conceptualizations of magnetic levitation power generation technology and thermovoltaic power generation technology for geothermal power generation are proposed. This study may provide new insights into geothermal exploration and low/medium-temperature geothermal power generation.