Abstract: The geothermal resource has some advantages in energy-saving and emission-reduction. They include enormous reserves, high energy utilization efficiency, and low-cost operation and have played an important role in achieving the targets of carbon peak and carbon neutrality as the only renewable-clean energy on the planet that is not affected by weather and seasons. A systematical analysis of the developing courses and new progress of foreign countries’ middle-deep high-temperature geothermal resources was carried out to analyze the occurrence characteristics and development status of high-temperature geothermal resources. Furthermore, in comparison to the development of high-temperature geothermal resources in China, several suggestions which may provide a reference for the utilization of middle-deep geothermal resources in China are put forward for the actual demands. Conventional hydrothermal geothermal resources in China generally have mature power technology and significant potential. However, in comparison to the total resources, geothermal resources in China have a low development degree and significant development potential. Significantly, a variety of rare mineral resources are associated with geothermal fluids in China, but some issues exist in the development of the associated mineral resources in high-temperature geothermal fluids, such as unclear trace element distribution and development potential, resulting in a low level of resource development. Consequently, based on the potential evaluation of the associated mineral resources, comprehensive utilization of the associated mineral resources in deep geothermal fluids should be strengthened. High-temperature heat harm has become a significant problem as engineering construction in the high-temperature geothermal area has advanced, and the mining depth of mineral resources has gradually increased. The high-temperature heat harm not only has a serious impact on the health of workers but also impedes the construction process and raises costs. However, the resource attribute of high-temperature heat harm, on the other hand, has received little attention. Hence, there is relatively little research on the “resource utilization of heat energy” in deep mines and engineering construction, resulting in the waste of geothermal resources. Based on the potential evaluation of “heat harm resources,” more attention should be paid to the utilization of “heat harm resources” in engineering construction and “ore-thermal co-mining” in deep mines, as well as actively developing high-temperature heat harm resource utilization technology. In general, more attention should be paid to the development of China’s middle-depth geothermal resources. The development of an enhanced geothermal system based on conventional hydrothermal geothermal resources could be more effective. Furthermore, geothermal resource utilization should not be limited to geothermal fluid; associated mineral resources and high-temperature heat-harm resources have enormous resource potential as well.
Abstract: The gradual increase in mining depth will inevitably lead to several problems because of mine geothermal energy. However, although mine geothermal energy poses dangers such as high temperature and heat hazards, it is also a resource that can be developed and utilized. Based on the existing research results, this paper first summarized the disaster-causing forms of mine geothermal energy. Then, the current prevention and control technologies of mine heat hazards were reviewed. Finally, the main utilization methods of mine geothermal energy were summarized. The findings show that the forms of disasters caused by mine geothermal energy can be classified into three types: aggravating the deterioration of coal and rock mass, inducing the failure of supporting structures, and creating high-temperature and high-humidity environments, including aggravating the deformation and failure of surrounding rock, inducing adsorption gas overflow, reducing the anchor pullout force, aggravating the corrosion of the anchor structure, damaging workers’ physical and mental health, reducing the labor efficiency of workers and machines, and increasing the failure rate of machinery and equipment. Two types of heat hazard control technologies are used: artificial and non-artificial cooling technologies. Non-artificial cooling technology can be divided into three categories: heat source control technology, heat-humidity environment control technology, and individual protection technology. According to various refrigerants, an artificial cooling system can be divided into three categories: air-cooled, ice-cooled, and water-cooled, including compressed air refrigeration cooling systems, ice-cooling systems, ground centralized refrigeration cooling systems, surface heat dissipation, underground centralized refrigeration cooling systems, return air exhaust heating underground centralized refrigeration cooling systems, ground cogeneration refrigeration cooling systems, and resource utilization of heat-harm systems. Extracting waste heat from mine water and mine return air for defreezing of the mine head, bath heating, and building heating is the main method for using mine geothermal energy at present, which can effectively reduce the consumption of primary energy at the same time; thus, it is of great significance for promoting green mining and sustainable development of coal mines. Using a buried tube heat exchanger to extract thermal energy from surrounding rock and realizing the coordinated use of several types of clean energy in a mining area is a future development direction for mine geothermal energy use. By drilling holes in the surrounding rock of a coal mine roadway, the buried pipe heat exchanger is arranged in the surrounding rock of the roadway, and water or organic matter is used as a heat exchange medium. The geothermal energy of roadway surrounding rock is extracted using ground source heat pump technology. In addition, for mining areas with excellent lighting conditions or sufficient wind energy, wind power generation and photovoltaic power generation/heat collection can be used simultaneously, and the produced electric energy and thermal energy can be directly used by users and for water pumps, heat pump units, and so on. The results of this paper provide a reference for mine heat hazard control and resource utilization in our country.
Abstract: Geothermal resources are renewable new energy sources. They have the characteristics of large reserves, wide distribution, good stability, and recyclability and are clean and environmentally friendly. Using geothermal energy represents a new direction for China’s sustainable development through achieving green, clean, low-carbon, and sustainable energy. After more than forty years of development, China’s geothermal industry, including power generation and heating, has made remarkable achievements and played an essential role in helping defend the blue sky and achieve carbon peaking and carbon neutrality goals. Metal mines, especially those with large mining depths, often contain plenty of geothermal resources. These deep mines often have a large amount of high-temperature rock mass or geothermal water. These resources have great mining and utilization values. However, during the mining process of mineral resources, especially in the process of deep development of underground mines, the mine temperature is too high due to factors such as geothermal gradients. The high temperature has become an important factor restricting mine production. To maintain daily production, mines often adopt enhanced ventilation or artificial cooling. How to turn geothermal energy into a usable resource in the process of mine development and achieve a win-win situation in resource development and utilization is a difficult problem worth exploring. The Jiaodong Peninsula is located on the continental margin where the Pacific plate subducts beneath the Eurasian plate. The Jiaodong Peninsula is the largest gold production area in China. Because of the preferable geodynamic setting, the Jiaodong Peninsula is also one of the regions with the most abundant geothermal resources in eastern China. The geothermal and mineral resources in the Jiaodong Peninsula share similar geodynamic settings. Therefore, the spatial distribution of geothermal and mineral resources is highly overlapping. Many large metal mines have large reserves of mineral resources and abundant geothermal resources. However, geothermal resources are often considered detrimental to underground mining activities. This paper analyzed the spatial distribution of geothermal and mineral resources in the Jiaodong Peninsula and summarized the current low-temperature geothermal resource utilization in China. The burial depth and mining of gold resources in the Jiaodong Peninsula are relatively large. Accordingly, it is recommended to use mature low-temperature geothermal heating technology to control the heat damage of mines and realize the co-mining of mineral and geothermal resources at gold mines such as the Sanshandao, Jinqingding, Xincheng, and Linglong gold deposits.
Abstract: With the depletion of shallow mineral resources, China’s mineral mining is gradually developing into deep rock mining, which will become an important source of mineral resources in China. The mineral – geothermal co-mining technology can reduce the heat disaster and utilize the geothermal resources simultaneously, thus being a deep mining technology with wide applications in the future. The co-mining of deep mineral and geothermal energies is an important means to ensure the sustainable utilization of deep resources. However, the construction of deep roadways and chambers faces many new challenges and technical issues. High temperature and stress are the two major characteristics of deep rock, causing completely different mechanical characteristics of deep strata to those of ordinary strata. Technical solutions are required to resolve these two problems in the development of mineral–geothermal co-mining. This study analyzed the strategic position and significance of roadway and chamber construction in deep high-temperature strata and introduced the basic theory of roadway and chamber construction technology in a deep high-temperature environment. Current research is insufficient for application in practical engineering; thus, in the future, diagenetic rock characteristics of rocks under high temperatures should be studied, and the stress–strain characteristics of deep strata under multifield coupling should be described. Targeting the problems of heat disaster and surrounding rock stability control in the construction of deep roadways and chambers, this study summarized and introduced the existing techniques and analyzed the shortcomings in the construction of deep roadways and chambers with co-mining. The traditional deep roadway and chamber construction technology does not fully utilize the resources and fails to provide enough safety guarantees. The construction of deep roadways and chambers should understand the obscure fundamental, physical, and mechanical properties of rock under high temperature and stress and deal with the backward control technology of surrounding rock stability. In addition, new technologies and materials should be used to improve the utilization rate of geothermal energy and achieve carbon neutralization. Finally, establishing a technical system for fine geological survey, optimization of surrounding rock cooling and stability control technology, and roadway and chamber risk monitoring is discussed in this paper.
Abstract: With the continuous increase in mining depths for mineral resources, the high-temperature thermal damage caused by deep earth temperatures has become a critical factor that restricts the safe and efficient mining of mineral resources. High-temperature environments directly affect the health of underground operators and reduce the service performance and lifetime of underground facilities and equipment. These high temperatures not only restrict mining efficiencies but also are a major safety hazard. However, the existing shaft facilities and abundant heat in the deep layers of mines provide favorable conditions for the large-scale development and utilization of geothermal energy. As clean and renewable energy, geothermal energy has significant advantages and great potential to reduce the cost of the deep mining of mineral resources. Making full use of geothermal energy stored in deep rock masses can not only effectively alleviate heat damage in mineral resource mining but also can promote the green, low-carbon, and sustainable development of the energy industry. To categorize mineral resources that may be associated with geothermal resources, here, we review and summarize existing mineral–geothermal co-mining technologies, i.e., the mine water source heat pump system, high-temperature exchange machinery system, deep salt mine geothermal extraction system, and oil and gas field geothermal energy comprehensive utilization project. Future modes, i.e., co-mining based on a brine circulation system, excavation technology, the filling mining method, in situ leaching method, and reuse of abandoned mines, are also analyzed. Moreover, the main challenges faced during the co-mining are discussed, including strengthening the exploration of the co-mining areas with mineral–thermal resources, developing rock breaking and tunneling technology for deep high-temperature hard rocks, strengthening the theoretical and experimental research of deep multifield coupled environmental rock mechanics, and establishing a graded utilization system of thermal energy for the co-mining of mineral–thermal resources. These research results are aimed at promoting geothermal energy development in the mining of mineral resources to benefit the large-scale utilization of geothermal resources and can provide a useful reference for the mining of deep mineral resources and the development of geothermal resources in China. Furthermore, promoting mineral–geothermal co-mining can promote the development of China’s deep resources to achieve the double carbon goal of “carbon peaking and carbon neutralization.”
Abstract: Coal resources are non-renewable one-time energy. With the increase in mining depth, thermal energy is released with the exploitation of deep coal resources. It is renewable and clean, unaffected by environmental factors, and is an important part of geothermal resources. The Chinese government attaches great importance to and encourages the development and utilization of clean energy. Therefore, this paper summarizes the development potential of geothermal resources in deep mines and the current situation of geotherm and coal mining, discusses the necessity and feasibility of geothermal development in deep mines, innovates a collaborative mining of geothermal energy and coal resources, and expounds the scientific and technical problems of the coordinated development of geothermal and coal resources in mines. The scientific problems primarily include the distribution characteristics and the supply law of geothermal sources in deep mines, the law of energy conduction and evolution in stopes, and the heat and mass transfer mechanism of low-grade geothermal energy. The technical problems mainly include the optimization method of the system for the coordinated development of geothermal and coal resources in deep mines, the method for the coordinated development of geothermal and coal resources in deep mines, etc. Focusing on the theme of the development of rock heat and hydrothermal resources in deep mines and considering the post-mining space and production system, this paper introduces four heat recovery methods: buried pipe heat recovery method in the backfilled stopes, water storage and heat recovery method in the goaf, heat recovery method in the closed fracture and caving zones, and in-situ drilling for the heat recovery method in deep aquifers. Keys and difficulties of this paper include the detection and evaluation of geothermal energy in deep mines, heat-absorbing functional materials of coal-based solid waste in deep and large spaces, the characteristics and controlling methods of the rock stratum movement in a multi-field environment in mine stopes, efficient transmission and stepped utilization system of low-grade heat energy, and the intelligent monitoring of geothermal energy and coal collaborative mining system. The results of this paper will provide technical support for the collaborative mining of geothermal energy and coal resources in deep mines in China, as well as provide a theoretical and practical reference for the development of a deep mine resources system in China and promote the construction of green mines and multi-economic development of deep mines in China.
Abstract: The abundant metal minerals and geothermal resources reserved in the deep earth can provide key support for global economic development and human survival. As a subversive and unconventional mining method, the fluidized mining of deep metal ore provides an essential idea for the efficient, low-carbon, and safe development of deep resources. In view of this, we focus on metal mineral resources in the deep earth based on the uranium in-situ leaching technology; by combining the technical characteristics of the “metal mineral fluidization mining” and “deep geothermal development,” we innovatively propose the process concept of strengthening the fluidized leaching process of deep metal ore-geothermal co-mining, The idea structure and potential techniques to realize the process concept were discussed, and preliminary assumptions were given. This study mainly includes three steps: 1) investigation (such as investigating mineral properties and geothermal conditions), 2) preparation of systems (including drilling and pipeline transportation system, leaching solution and strain preparation system, geothermal utilization system, metal precipitation system, and production assistance system), 3) operation (including experimental results, optimize process parameter, and industrial utilization). Key systems, such as a dense solid mineral fluidization system, were proposed from the perspectives of mineral leaching, environmental perception, process control, energy replacement, and synergistic correlation. Key discussions were performed in five aspects: 1) the intelligent perception system for deep earth resources to solve the problems of low permeability, low penetration, premature solution preferential flow, low leaching rate, and excessive blockage, thereby decreasing the percentage of unsaturated leaching areas and effectively recycling the lower-grade minerals; 2) the seepage control system for solution in deep mining areas, divided into data acquisition, data analysis, and decision-making parts, thereby realizing efficient coordination between ground immersion environment and production information, avoiding system slowdown, and reducing heat/electric energy consumption; 3) the energy replacement system for geothermal–leaching solution, including the geological exploration and production monitoring system and ecological reclamation monitoring system, resulting in targeted adjusting the fluid flow behavior, improving capillary penetration and mass transfer; 4) the coupling system for thermal energy replacement–leaching solution circulation, which has three requirements—one is high temperature resistance, corrosion resistance and heat insulation, the second is good thermal conductivity and corrosion resistance, and the third is to be equipped with an intelligent monitoring system; 5) the thermal energy replacement-solution circulation coupling system that is based on large-scale pregnant solution container and equipped with thermal energy replacement, metal precipitation, leaching solution and strain preparation devices. Besides, the basic theoretical bottlenecks, key technical problems, and future development trends in the process of fluidized leaching–geothermal cooperative co-mining of metal ores are carefully discussed in this study. This study will provide ideas and references for enhancing the fluidized leaching process of deep earth metal ores and geothermal co-mining.
Abstract: Massive low-grade thermal energy and water vapor are stored in the hot and humid airflow of mines, typically resulting in a poor underground working environment and posing threats to worker safety and health. The direct discharge of exhausted ventilation air causes an enormous waste of resources as well as pollution problems to the surrounding environment. Therefore, the extraction and utilization of mine ventilation heat and humidity have become some of the most important ways to solve thermal damage problems in deep mines, thereby boosting their low-carbon transformation development. Affected by the changes in the surface atmospheric and downhole heat and humidity sources, the hot and humid airflow parameters in mines change with time. Real-time determination of the hot and humid airflow characteristics in mines is key to extracting low-grade thermal energy from the underground environment efficiently. In this paper, the distribution law and variation characteristics of key heat and humidity joints are determined based on the real-time calculation of the hot and humid air network. A calculation model of condensation heat and humidity extraction is established, and the technology of low-grade condensation waste heat extraction from heat and humidity airflow is also developed, which, in composition, forms a low-grade heat in situ utilization system combined with refrigeration and dehumidification. Furthermore, the centralized and distributed thermal and humidity extraction and resource utilization methods of coal mine ventilation are put forward. The effects of heat extraction and water recovery are also analyzed using examples. The results show that around 224 t of moisture is wasted every day in ventilation air emission, whose recycling has economic benefits. Thousands of kilowatts of thermal energy are stored in the ventilation air emission, which can be used as direct heat energy or converted electricity. An approximately linear relationship is revealed between the temperature decrease and theoretical moisture recovery from ventilation air emission, providing a rapid way for engineering estimation. The analysis of the heat recovery shows that heat extraction is favored by high initial temperature and humidity due to high values and efficiencies. The application of heat–moisture recovery in underground nodes can effectively moderate the in situ working environment and simultaneously recover some energy as a supply to running costs. This work provides significant construction ideas and a theoretical basis for the extraction and utilization of low-level thermal energy and heat damage control in mines.
Abstract: Geothermal resources are abundant in deep mines. Functional backfill technology combines deep mining and deep geothermal mining to achieve a win-win situation for mineral and geothermal resource development and is an important measure for extending the life of deep mines. In this paper, on the basis of an analysis of the research status of geothermal resource extraction through tubes embedded in backfill bodies in mines, a horizontal square-spiral-type backfill heat exchangers (S-S BHE) is proposed. Considering the significant influence of groundwater advection on the heat extraction of backfill heat exchangers (BHE) in mines and the relative scarcity of previous studies, a verified three-dimensional unsteady BHE model coupling heat transfer and seepage are established using COMSOL software. Based on this model, a mathematical model of the backfill heat exchanger coupled heat pump (BHECHP), and four comprehensive evaluation indicators are established. Firstly, the performance of the S-S BHE is compared with that of two typical serpentine BHEs under the same geometric and physical conditions. The results show that the S-S BHE performs better than the two serpentine BHEs across the board, and the advantage is more substantial under situations of higher permeability flow. Secondly, the characteristics of the S-S BHE and its coupled heat pump are examined in relation to the in-tube flow rate, tube spacing, seepage velocity, and inlet water temperature. The in-tube flow rate and seepage velocity are found to have the most significant effects on the comprehensive evaluation indicators. The average heat transfer power per unit of tube length increases with flow rate, but the heating seasonal performance factor (HSPF) decreases obviously. The analysis revealed an optimum interval of 0.4–0.6 m·s?1 for the flow rate in the tube, where the flow of circulating water in the tube is in transition from the transition zone to the fully turbulent flow zone. The effect of seepage velocity is negligible at less than 10–6 m·s?1, and all comprehensive evaluation indicators present linearly increasing trends in the usual seepage range of 10–6 to 10–5 m·s?1. Finally, an ecological evaluation of the S-S BHECHP was conducted. A comparison with traditional heating methods reveals that the heating method using S-S BHECHP has a significant energy saving and carbon reduction effect. The primary energy consumption and carbon emissions of the S-S BHECHP are reduced by 83.39%, 61.57%, and 56.84% compared to the regenerative electric boiler, coal-fired boiler, and air-source heat pump, respectively. The findings of this study show how well the S-S BHE and the S-S BHECHP performance, and they also provide some theoretical recommendations for the application and exploration of heat storage/energy storage functional backfill in deep mines.
Abstract: The joint exploitation of deep mineral and geothermal resources strategy provides an effective way to realize the economic exploitation of “resources-thermal” and achieve a win-win situation. The technology for deep high-temperature stratum shaft and roadway construction is an important support and guarantee for the safe and efficient implementation of the “ore-thermal co-mining” strategy. The necessity and urgency of this deep “ore-thermal co-mining” strategy to the technical requirements of shaft and roadway construction are analyzed. Combined with the analysis of the status quo of mine construction technology, it is clear that the non-blasting rock breaking technology represented by mechanical rock breaking is an important direction in the development of deep shaft and roadway construction technology at present. Mechanical rock breaking technology is a technical approach to solve the existing problems in the process of drilling and blasting excavation, such as too many underground workers, complex working procedures, serious occupational injury, and environmental pollution. In the hard rock stratum, partial section roadheader equipment has low boring efficiency, large tool consumption, and high economic cost, whereas full-section boring machines used for shaft or roadway have advantages in detection, rock breaking, slag discharge, surrounding rock supporting, and other aspects. Therefore, the development direction of intelligent shaft and roadway construction is proposed. It analyzes the difficulties and challenges faced by shaft and roadway construction in the deep high-temperature stratum, such as precise formation exploration, formation reinforcement and water plugging, high ground temperature prevention and control, high ground stress prevention and control, deep shaft lifting, and manufacturing of shaft and roadway boring machines. Three priority development tasks are proposed: 1) a geological guarantee system for deep high-temperature strata shaft and roadway construction; 2) a construction mode and planning of shaft and roadway in deep high-temperature strata; 3) a complete set of technology and equipment for deep high-temperature stratum shaft and roadway construction. Eight basic theories and key technologies are summarized based on the three priority development tasks: in situ exploration and transparent reconstruction of stratum, suitability of shaft and roadway construction methods in the high-temperature stratum; non-blasting rock breaking in the high-temperature stratum; continuous lifting of deep shaft; modification of deep bad stratum and long-term stability control of surrounding rock; thermal damage treatment of deep shaft; intelligent perception of shaft and roadway equipment; intelligent control of shaft and roadway boring equipment. Based on the above content, the basic theory and technology research system of shaft and roadway construction in a deep high-temperature stratum is preliminarily constructed to provide a reference for deep resource clean mining and the large-scale geothermal clean energy development.
Abstract: Hot dry rock (HDR) is an underground rock with high temperatures (usually above 180 °C), low porosity, and low permeability. The extraction of geothermal energy from HDR generally requires the stimulation of man-made reservoirs. In the enhanced geothermal system (EGS) project, high-pressure water is usually injected into the deep HDR reservoir from the injection well, and the artificial fracture network is stimulated via fracking. The ultimate goal is to enhance fluid flow and heat exchange between injection and production wells. During this period, thermal shock induced by the injected cold water, also known as thermal stimulation, leads to thermal fracture of the HDR, which contributes to the formation of fractures near the injection well. However, this process results in a series of rock damage problems to the high-temperature rock mass, such as borehole collapse and microseismicity. To analyze the mechanical properties and damage evolution of high-temperature granite after thermal shock, the uniaxial compression test of granite specimens at different temperatures in the range of 25 °C–600 ℃ was conducted, and the stress–strain relationship of the specimens was obtained. Based on the theory of damage mechanics, a thermal–mechanical coupled damage constitutive model considering the combination of the initial thermal shock damage and the microelement fracture damage during loading was proposed, and the relevant parameters of the statistical damage constitutive model were theoretically solved. Furthermore, given the effect of pore structure deterioration caused by thermal shock, the constitutive relationship of thermal shock granite was modified by introducing a compaction coefficient. The statistical damage constitutive model was also verified by the experimental results. The influence of temperature on the damage evolution of thermal shock granite under uniaxial compression was discussed. Results showed that with the increase in thermal shock temperature, the initial thermal damage of the granite specimen increases continuously, resulting in a nonlinear compaction stage in the stress–strain curve. The statistical damage constitutive model modified by the compaction coefficient can accurately characterize the nonlinear compaction characteristics of thermal shock granite specimens in the initial loading stage. When the thermal shock temperature is low, the damage variable evolution curve rises steeply. However, with the increase in the thermal shock temperature, the increase rate of the curve gradually slows down and changes from nonlinear to linear. The research results not only help elucidate the deterioration process of the mechanical properties of thermal shock granite but also provide important theoretical guidance for the construction of accurate numerical calculation models and engineering scheme demonstrations.
Abstract: To exploit the geothermal energy from low penetration rocks at a depth of 3–10 kilometers below the ground surface, an artificial geothermal system usually must be built. Thermal and chemical stimulation can be used as auxiliary means of hydraulic fracturing for artificial geothermal reservoir reconstruction, which is conducive to reducing the risk of earthquakes. However, thermal shock and chemical corrosion can also cause changes in physical parameters such as density, porosity, longitudinal wave velocity, and the thermal conductivity of high-temperature rock mass, which brings great uncertainty to the service life of a geothermal system. To study the evolution of the physical properties and thermal conductivity of thermal shock granite under chemical modification, long-term acid and neutral solution immersion tests were performed on granite specimens subjected to thermal shock at temperatures ranging from 25 ℃ to 600 ℃. Using ultrasonic testing, nuclear magnetic resonance, thermal constant testing, and scanning electron microscopy, the evolution of the physical parameters of thermal?chemical modified specimens with thermal shock temperature was quantitatively characterized, the internal correlation among physical parameters was established, and the microscopic mechanism of the change in physical properties was revealed. The results show that with increasing thermal shock temperature, the volume of thermal–chemical? modified specimens increases gradually, the mass and density decrease gradually, the longitudinal wave velocity decreases linearly, the porosity increases by a power function, and the thermal conductivity and thermal diffusivity decrease exponentially and linearly, respectively. At the same thermal shock temperature, the volume growth fraction, longitudinal wave velocity, and thermal conductivity of the modified specimens are in the order of non-immersion > water immersion > acid immersion, while the mass loss fraction and porosity are in the order of acid immersion > water immersion > non-immersion. The increase in porosity and the deterioration of thermal conductivity are accompanied by a decrease in longitudinal wave velocity, so the porosity and thermal conductivity can be estimated by measuring the longitudinal wave velocity. The pore structure of the modified specimens is more sensitive to temperatures in the range of 150–450 ℃, while the solid particle skeleton is more sensitive to temperatures above 450 ℃, and the deterioration of the particle skeleton will further cause the transformation of the pore structure. The thermal-chemical modification results in the development of pore structure and phase transformation, which are the fundamental reasons for the changes in the physical properties of granite. High-temperature thermal shock plays a leading role in the process of thermal-chemical modification, while chemical corrosion plays an auxiliary role. At the selected test temperature levels, 300 ℃ can be considered the temperature threshold for severe thermal shock.
Abstract: Exploiting geothermal resources, especially hot dry rock (HDR), is essential to reduce carbon emissions to build an acceptable energy structure. The enhanced geothermal system (EGS) for mining HDR has experienced more than 50 years since it was proposed in 1970, obtaining rich research results and construction experience. It is of great significance to review the EGS history, which includes discussing the project site selection and thermal storage stimulations, summarizing the reasons for the shutdown of demonstration projects, and indicating the key factors restricting EGS development. Based on this, the future development direction of EGS is clarified, which can help explore deep geothermal energy and construct associated demonstration projects in China. The overall development of EGS is divided into two stages, namely, the research and development stage before 2000 (a total of 14 EGS projects) and the demonstration and quasi-commercialization stage since 2000 with a rapid development speed (a total of 27 EGS projects). By the end of 2021, the cumulative number of EGS worldwide has increased to 41. However, the cumulative installed capacity of power generation only reaches 37.41 MW. EGS is still on the learning curve, resulting in a long way to go to realize the large-scale commercialization of HDR geothermal energy. The factors restricting the commercialization of EGS are the lack of policy support and capital investment, the limitations of technical difficulty, and the unpredictability of the geological condition of the thermal reservoir, which weakens EGS development and even causes its suspension or termination. Because of the complex geological environment of thermal reservoirs, the fracture network and associated reservoir quality induced by hydraulic stimulations are uncontrollable, causing the fractured quality of the thermal reservoir to be lower than its critical value. It results in numerous adverse problems in most EGS projects, including insufficient thermal reservoir volume, an unstable fracture network, associated heat exchange area, severe fluid loss, and induced unacceptable earthquakes. Thus, the fundamental reason for EGS’s inability to commercialize is that it is challenging to form a reproducible thermal reservoir stimulation model induced by the difference in thermal reservoir geological conditions and the dependence of the existing stimulation technologies on the in situ reservoir geological environment. Establishing the database of HDR and EGS plays an urgent role in EGS development by forming an accurate quantitative system of reservoir geological conditions to explore the relationship between geological conditions and reservoir reconstruction and then build a replicable thermal reservoir reconstruction technology. Focusing on new and demonstration stimulations for the thermal reservoir, such as the enhanced geothermal system based on caving technology (EGS-E), FORGE, and DEEPEGS projects, may provide an acceptable way to break through the dependence of thermal reservoir stimulation on in-situ geological conditions and form the “reproducible” deep-geothermal resource mining system to realize the large-scale commercialization of deep-geothermal resources.
Abstract: In mineral and geothermal resource co-mining, the underground rock is often affected by mining stress, and fractures of different shapes, such as single fractures, T-shaped fractures, and Y-shaped fractures, are generated. To increase the reservoir permeability, the existing fractures need to be reactivated, causing them to expand under force and propagate in shear and tension modes, generating new fractures and finally forming a fracture network to increase permeability. Waterjet cutting and wire cutting equipment are used to prefabricate sandstone samples with different inclinations and single, T-shaped, and Y-shaped fractures on standard samples. This paper conducts hydromechanical coupling experiments to investigate the possibility of increasing permeability by expanding and merging fractures in prefabricated fractured sandstone samples under triaxial conditions. In addition, the focus is on mechanical properties, such as critical thresholds (crack closure stress, crack initiation stress, damage stress, and peak strength), elastic moduli, and Poisson's ratio, and the failure modes of multiple-shape prefabricated fracture sandstone samples are mainly studied. Simultaneously, the evolution law of acoustic emission and permeability during the progressive failure of fractured rock is studied, and the mechanism of permeability enhancement of fractured rocks under the action of hydraulic coupling is analyzed. The results show that under the action of hydromechanical coupling, all multi-shape prefabricated fracture specimens form secondary cracks that expand in tensile, shearing, or mixed modes through the existing fractures and generate new fractures or fracture networks, which can effectively increase the flow rate. All single-fracture specimens are shear failures, and the T-shaped and Y-shaped fracture specimens have two types of shear failure and tension-shear failure. Furthermore, the weakening effect of water has a smaller effect on strength than the effect of multiple-shape prefabricated fractures. With increasing axial pressure, the rock permeability first decreases and then increases in the pre-peak stage, and the jump coefficient increases when reaching the strength failure. When the stress suddenly drops after the peak of the sample, the permeability reaches the maximum value, and the permeability enhancement effect is the best. The change in the prefabricated fracture angles and shapes has a small influence on the jump coefficient. The average value of the jump coefficients of a single fracture is larger than that of a Y-shaped fracture, which is larger than that of a T-shaped fracture, and the jump coefficients are more than doubled. These observational and experimental results will help to understand fracture failure and fluid flow behavior, which will guide the engineering applications of mineral and geothermal resource co-mining.
Abstract: Geothermal energy, as a clean and renewable resource distributed worldwide, has received extensive focus in recent years. With the improvement in drilling and logging technology, the depth of geothermal exploration has gradually increased. Carbonate reservoirs are presently the main layer for geothermal development and use in China that have the characteristics of wide distribution, large reserves, and easy reinjection. The current use is limited to the strong karst development zone, approximately 200 m at the top of the reservoirs. Because of the low permeability and strong heterogeneity, the deep carbonate geothermal reservoirs cannot be commercially developed. This study aims to solve the key technical problems of efficiently developing deep carbonate geothermal reservoirs with extreme thickness. The target section was selected by analyzing comprehensive logging and fracture imaging logging data. An innovative simulation technology combining hydraulic jetting and acid fracturing is developed, which has the characteristics of fixed-point fracturing, effective sealing, strong penetration, and a large stimulation range. A production enhancement test was conducted for carbonate geothermal wells in the following order: comprehensive logging, imaging logging, casing cementing, perforation, pumping test, small pressure test, hydraulic injection acid fracturing, pumping test (after fracturing), and other construction processes. Comprehensive logging is an effective means to interpret the macroscopic pore and permeability properties of a reservoir and can be used to initially select the target geothermal reservoir. Fracture imaging logging can provide a more intuitive understanding of fracture development and distribution characteristics. The results show that the fracture density of geothermal well D22 does not decrease substantially with increasing depth, and the fracture width tends to decrease with depth clearly. The experimental geothermal well D22, which has the largest thickness of carbonate geothermal reservoir exposed in the Xiong'an New Area, was selected to perform a pilot field test of stimulation. The results show that the water inflow of the target section at 3024–3174 m increased from 4.72 m3·h?1 before stimulation to 44.10 m3·h?1 after stimulation, increasing by 8.3-fold. The unit water inflow increased from 0.024 m3·(h·m)?1 before stimulation to 0.745 m3·(h·m)?1 after stimulation, increasing by 30-fold. The reservoir permeability coefficient increased from 4.4×10?3 m·d?1 to 146.3×10?3 m·d?1. The wellhead water temperature increased from 60.0 °C before stimulation to 66.5 °C after stimulation. Therefore, the development potential of deep and thick carbonate geothermal reservoirs can be substantially improved through the developed stimulation. This research can provide technical support for the large-scale development of geothermal resources in China.
Abstract: Geothermal energy has recently attracted substantial attention due to its abundant reserve, cleanness, and sustainability. Geothermal reservoirs can be stimulated via different approaches/techniques that lead to different heat extraction efficiencies and production through heat transfer between the working fluid and the reservoir network. Typical reservoir stimulation strategies include hydraulic fracturing, which is employed in conventional geothermal systems based on drilling, namely, EGS-D; indirect heat exchange using U-shaped pipes, namely, EGS-P; and block caving, which is based on the well-developed mining excavation framework, namely, EGS-E. Although the above three reservoir stimulation modes have been made available, their heat extraction performances for a certain reservoir over the operation lifespan have been unexplored. Selecting the appropriate reservoir stimulation approach and assessing the corresponding heat extraction performance are crucial for the design and subsequent operation of geothermal systems. Here, we systematically compared the heat extraction efficiencies of different stimulated reservoir networks under four typical stimulation modes, including a high-permeability reservoir (representing a reservoir stimulated by EGS-E), a connected fracture (representing a reservoir stimulated by EGS-P) reservoir, a reservoir with randomly distributed fractures (representing a reservoir simulated by EGS-D), and a reservoir with randomly distributed fractures and connected fractures (representing a reservoir simulated by the combination of EGS-D and EGS-P). The mechanical, hydraulic, and thermal coupling among the rock matrix, fracture network, and working fluid was realized in COMSOL Multiphysics. We found that the heat extraction efficiency of the high-permeability reservoir was the highest and that of the reservoir with randomly distributed fractures and connected fractures was the lowest. Crack aperture evolution was modulated by the competition between matrix contraction and hydraulic enhancement. The total crack aperture can be increased by increasing the matrix contraction and the hydraulic pressure of the working flow. Injection capability improved when the matrix contraction (thermal effect) prevailed but decreased when the working flow pressure (hydraulic effect) dominated. We also found that the smaller the matrix spacing, the larger the thermal effect-induced crack aperture and thus the total aperture. When the matrix spacing was reduced to 50 m, the thermal effect-induced crack aperture was nearly five times the hydraulic effect-induced crack aperture. The above findings have the following implications for EGS-E: first, the reservoir should be caved into fractured blocks that are as small as possible to increase permeability. Heat extraction efficiency and heat production can thus be highly promoted. Second, for the EGS-E with multiple reservoir slices, the slice spacing should be appropriately optimized to ensure high crack apertures and thus commensurate heat extraction efficiency.
Monthly, started in 1955 Supervising institution：Ministry of Education Sponsoring Institution：University of Science and Technology Beijing Editorial office：Editorial Department of Chinese Journal of Engineering Publisher：Science Press Chairperson：Ren-shu Yang Editor-in-Chief：Ai-xiang Wu ISSN 2095-9389CN 2095-9389
