In recent years, intelligent methods represented by multi-source data fusion, machine learning and digital twins have been increasingly applied to geothermal exploration, drilling and completion, heat extraction optimization and operational regulation, providing technical support for the transition of middle-deep geothermal development from experience-based design and offline evaluation to data sensing, dynamic prediction and collaborative decision-making. To clarify the applicable conditions, major bottlenecks and development trends of intelligent exploitation technologies for middle-deep geothermal energy, and to provide references for efficient, low-risk and sustainable development, research progress in middle-deep geothermal development in China and abroad was systematically analyzed through data synthesis, case summarization and technical comparison. Progress in intelligent exploration and target-area optimization, intelligent drilling and development optimization, heat extraction by pumping and reinjection, closed-loop heat extraction, resource utilization of existing underground engineering carriers, and intelligent operational regulation and sustainable optimization was reviewed. The applicable conditions, engineering constraints and intelligent adaptation characteristics of different heat extraction methods were compared. The results show that multi-source data fusion, machine learning and deep learning can improve the integrated utilization of geological, geophysical, geochemical, remote-sensing, well-logging and historical well data, thereby supporting the identification of hidden geothermal reservoirs and target-area optimization. Real-time drilling data sensing, rate of penetration prediction, drilling condition identification, surrogate models, data assimilation and physics-constrained models can be used for drilling parameter optimization, geothermal reservoir parameter prediction and rapid comparison of heat extraction schemes. Heat extraction by pumping and reinjection is suitable for hydrothermal reservoirs with relatively clear water productivity, reinjection conditions and inter-well connectivity, but its long-term operation is constrained by reinjection clogging, thermal breakthrough, hydro chemical changes and production-reinjection balance. Closed-loop heat extraction is less dependent on natural geothermal fluids and reinjection capacity, and is more suitable for formations with limited reinjection conditions, high groundwater protection requirements or dominant conductive heat transfer in rock masses. However, its performance is controlled by well depth, geothermal gradient, rock thermal properties, well configuration, circulation parameters and wellbore thermal resistance. Enhanced geothermal systems, CO2-based heat extraction, integration of geothermal energy with carbon capture, utilization and storage, and underground thermal energy storage and recovery further extend the development scope of middle-deep geothermal energy. Existing underground engineering carriers, including abandoned oil and gas wells, abandoned mines, mine roadways and goaf areas, can be utilized according to geothermal reservoir conditions, spatial integrity, reconstruction cost, operational safety and surface heat demand. During operational regulation, geothermal reservoir pressure and temperature fields, reinjection capacity, thermal breakthrough risk, wellbore heat transfer, equipment status and user-side heat load should be comprehensively considered, so as to promote the application of intelligent prediction and scheduling, predictive maintenance, closed-loop well control optimization and digital twins in the collaborative control of the wellbore-reservoir-surface system. The key to intelligent exploitation of middle-deep geothermal energy lies in establishing a continuous feedback mechanism across resource identification, drilling and completion, heat extraction scheme optimization and operational regulation. Further efforts are needed in whole-process data standardization, integration of physical models and data-driven methods, long-term field monitoring and engineering verification.
The Jizhong Depression hosts three prominent structural transfer zones—Hengshui—Wuji, Xushui—Wenan, and Baodi—Tongbaizhen—as well as the Niudong—Maxi strike-slip structural transfer zone, where carbonate geothermal resources with substantial yields and relatively high temperatures have been identified. To reveal the controlling mechanism of structural transfer zones on carbonate geothermal resources, newly acquired data, including water temperature, hydrochemistry, hydrodynamic fields, and 14C isotopic dating results of geothermal wells in typical structural transfer zones of the Jizhong Depression, are extensively compiled and supplemented in this study. Methods such as the Na-K-Mg equilibrium diagram, saturation indices of major minerals, and sodium-chloride coefficient were adopted to comprehensively characterize the thermal, hydrodynamic, and hydrochemical fields of carbonate geothermal water within and around representative structural transfer zones, and further analyze their indicative significance for structural belts and their tectonic activity features. Results indicate that structural transfer zones exert fundamental control on the sedimentation and evolution of adjacent strata, as well as the burial depth and hydraulic connectivity of carbonate geothermal reservoirs, thereby modulating the thermal, flow, and hydrochemical characteristics of groundwater in these systems to a significant degree. Specifically, the ascent of deep geothermal fluids along large strike-slip faults and deep circulation of groundwater along major deep faults elevate water temperatures in the vicinity of deep strike-slip fractures. For instance, geothermal wells at the intersection of the Niudong—Maxi and Xushui—Wenan structural transfer zones yield wellhead temperatures of 109.0–123.4 °C. Conversely, densely distributed tensional-normal faults within transfer zones act as dominant conduits, resulting in larger geothermal reservoir storage capacities but lower water temperatures. Structural transfer zones host abundant faults with variable fault properties, leading to complex controls on carbonate geothermal water. Extensional structural transfer zones are dominated by abundant groundwater, while strike-slip structural transfer zones readily provide conduits for deep heat sources. Future studies are recommended to conduct detailed fault structural analysis on structural transfer zones with densely distributed geothermal wells, so as to systematically clarify the constraint laws of fault structures on geothermal water.
Deep carbonate karst geothermal reservoirs represent the main host of geothermal resources in coal mining areas. Owing to extensively developed karst-fracture systems and strong reservoir heterogeneity, a single geophysical method cannot accurately delineate stratigraphic architectures, fault structures and water-rich targets, which restricts efficient geothermal resource exploration and drilling site selection in coal mining areas. To address this technical challenge, this study takes Anju Coal Mine in Jining City, Shandong Province as the research object, and constructs an integrated detection system for water abundance evaluation of carbonate karst geothermal reservoirs, combining microtremor survey, wide-area electromagnetic method and high-precision method of electromagnetic frequency spectrum (MES). Through joint inversion of subsurface shear-wave velocity structure and electrical structure, the system supports systematic division of stratigraphic sequences and accurate interpretation of the location and occurrence of fault structures. Based on the comprehensive response characteristics of apparent resistivity and magnetic field components from the MES method, karst aquifer development intervals are identified and favorable water-rich zones are delineated, with corresponding proposals for geothermal drilling hole locations put forward. The results show that the roof elevation of the Ordovician Majiagou Formation limestone in the study area ranges from −1 100 m to −1 380 m, presenting a stepwise distribution with burial depth increasing gradually from east to west. Four fault structures, including F1 (Jining Fault) and F2 (Jining No.1 Branch Fault), are interpreted via comprehensive geophysical prospecting, among which F2 serves as the dominant water-conducting fault in the study area. Two favorable water-rich zones are delineated according to the anomalous response of the MES method. Accordingly, two geothermal drilling holes are optimized: ZK01 (the preferred hole with a recommended depth of 1 600 m) and ZK02 (the alternative hole with a recommended depth of 2 000 m).The combined detection mode of microtremor survey, wide-area electromagnetic method and MES method realizes technical complementarity among different approaches, improves the accuracy of stratigraphic characterization, structural interpretation and water abundance assessment for carbonate karst geothermal reservoirs, and provides a reliable technical basis for geothermal resource exploration and development.
The Jinan karst aquifer system contains abundant cold groundwater and geothermal resources. However, these two resource types have long been assessed separately, and a unified water-balance analysis framework at the catchment scale has been lacking, thereby impeding the coordinated and sustainable development of both groundwater and geothermal resources. To address this critical issue, a three-dimensional coupled numerical model of the complete karst aquifer system is developed at the catchment scale, encompassing five karst water subsystems (Dongping Lake, Pingyin–Dong’e, Changxiao, Baotu Spring Catchment, and Baiquan Spring Catchment) together with the Jibei geothermal field. The model is used to quantitatively evaluate the sustainable co-exploitation yields of groundwater and geothermal resources under the constraint of maintaining perennial spring outflow, and to reveal the mutual constraints and co-exploitation thresholds between upstream cold groundwater abstraction and downstream geothermal extraction. The model is constructed using the COMSOL Multiphysics platform, integrating regional hydrogeological borehole data, long-term groundwater-level monitoring records, and field measurements of geothermal well production and reinjection. Through parameter inversion, dynamic water-level calibration, and water-balance computations, a first quantitative assessment is provided of the sustainable exploitation capacity and co-exploitation potential of the coupled system. The results indicate that, under current exploitation conditions, the subsystems of the Jinan karst aquifer are hydraulically connected to varying degrees through fault zones and karst conduits, and the entire system maintains a dynamic equilibrium between recharge and discharge. In the middle and upper reaches of the catchment, which serve as the direct recharge and runoff zone for the spring system, the sustainable groundwater exploitation yield is 473500 m3/d while simultaneously ensuring sustained spring outflow and water-supply abstraction. In the lower reaches of the catchment (Jibei geothermal field), by adopting a balanced production–reinjection development mode and optimizing the wellfield layout and extraction–reinjection rates through numerical simulation, the sustainable geothermal water exploitation yield is 43300 m3/d without inducing thermal breakthrough or affecting upstream spring outflow. After 50 years of equivalent reinjection under unpressurized conditions, the drawdown at each production well meets the regulatory requirements. These findings provide a scientific basis for the synergistic and sustainable development of groundwater and geothermal resources in the Jinan karst aquifer system under the constraint of sustained spring outflow, and offer a pathway for resolving the conflicts among spring protection, water supply, and heating demand.
Enhanced geothermal systems (EGS) offer a practical route for extracting heat from hot dry rock, yet their operation can be accompanied by seismicity or microseismicity during hydraulic stimulation, after shut-in, and throughout later circulation. This seismic response is tied to the structure of hot dry rock reservoirs. These reservoirs are usually crystalline and tight, with very low matrix permeability, so fluids tend to move through hydraulic fractures, natural fractures, and fault damage zones rather than through a homogeneous porous medium. As a result, no single mechanism, whether pore-pressure diffusion, thermoelastic stressing, or poroelastic stressing, can explain the full range of EGS-induced seismic responses. Drawing on 79 publicly reported cases, together with theoretical analyses, field observations, and numerical simulations, this review examines how seismic responses and controlling mechanisms change from hydraulic stimulation to post-shut-in and circulation stages. Hydraulic stimulation accounts for the largest share of the reported cases, about 72%, whereas circulation accounts for about 26% and post-shut-in events for about 2%. These proportions, however, should not be read as a direct measure of risk. Larger events are more closely associated with the overlap between pressure perturbations, near-critical faults, preferential flow paths, and stress transfer. During hydraulic stimulation, microseismicity is generally concentrated near the wellbore or within the stimulated reservoir volume. It is mainly linked to hydraulic fracture propagation, stress concentration at fracture tips, shear slip on small natural fractures, and local stress redistribution. The situation changes when hydraulic fractures connect with faults, because pressure transmission and Coulomb stress changes may then promote fault slip. After shut-in, the end of injection does not necessarily mean that the system has stabilized. Residual pressure can continue to migrate through fracture networks toward the far field, while poroelastic rebound, fracture closure, aseismic creep, and static stress transfer from earlier events may still alter the stability of distant faults. During circulation, induced events are often long-lasting and low in magnitude. Early responses are controlled mainly by injection-production pressure differences and reactivation of existing fractures; with the advance of the cooling front, thermal contraction, fracture aperture changes, water-rock reactions, and evolving fault friction become increasingly relevant. EGS-induced seismicity is therefore better treated as a stage-dependent process. Hydraulic stimulation is dominated by fracture growth and possible fault connection; the post-shut-in period is governed more by pressure-stress redistribution; and long-term operation gradually brings thermo-hydro-mechanical-chemical coupling into play. Risk control needs to follow this evolution. Before stimulation, fault identification, in-situ stress evaluation, and well-site optimization are needed. During stimulation, staged injection, microseismic monitoring, and downhole pressure feedback can help constrain fracture growth. After shut-in, slow shut-in, stepwise pressure reduction, pressure-holding observation, or moderate flowback may be selected according to site conditions, with monitoring continued until pressure redistribution becomes relatively stable. During circulation, injection-production balance should be maintained, and microseismic, DAS, InSAR, downhole pressure, temperature, flow-rate, and fluid-chemistry data should be integrated into risk-model updating. Future models need to represent fracture and fault networks, thermo-poroelastic responses, fault frictional slip, and chemical weakening in more detail, so that traffic-light systems can move beyond empirical thresholds toward dynamic forecasts constrained by physical mechanisms.
Possessing exceptional geothermal resource endowments and superior development conditions, the Niutuozhen geothermal field is one of the leading geothermal fields in Xiong’an New Area. Geothermal exploitation historically concentrated on reservoirs above 2 000 m, yet the differentiation rules, tectonic evolution control mechanisms, and geothermal enrichment patterns above 4 000 m carbonate reservoirs of the Wumishan and Gaoyuzhuang formations of are still poorly understood. There was a lack of understanding of deep carbonate rock thermal reservoirs. Spatial distribution, karst development and productivity characteristics of Jixian system carbonate thermal reservoirs are revealed in this study through comprehensive research based on authentic data acquired from deep geophysical exploration, geothermal drilling, well logging and productivity tests: the Jixian system carbonate buried hill internal structure is generally characterized by high in the south and low in the north, high in the east and low in the west, and the high point is located in the south of the uplift and gradually sinks to the northeast. The highest point is located at the southern end of the uplift, gradually dipping towards the northeast. The Wumishan formation in the south of Xiongxian County is partially missing, and the Gaoyuzhuang formation is unconformably overlaid on the Mesozoic strata. The interpretation of geothermal well reservoirs shows that the Wumishan formation is characterized by developed karst fractures and dissolution pores within a range of 200 m near the top weathering crust. These intervals are the development sites of geothermal reservoirs, characterized by good porosity and permeability and excellent geothermal productivity. In contrast, the Gaoyuzhuang formation shows significant inter-well variability, with only a few wells near the 150-meter vicinity of the weathering crust unconformity displaying developed karst features, while most wells exhibit overall underdeveloped karst conditions. As the depth of the strata increases, the degree of karst development deteriorates, the water output decreases, and the geothermal production capacity is poor. Against the backdrop of regional tectonic evolution, analysis on the typical section evolution of the Langgu depression—Niutuozhen uplift—Baxian depression reveals three evolutionary phases of sedimentation and tectonism for the geothermal field: Neoproterozoic—Mesozoic reservoir formation, Paleogene uplift construction, and Neogene—Quaternary reservoir burial. Such evolution controls the spatial distribution, karst growth and physical properties of the two thermal reservoir sequences, determining their modern geometry and petrophysical features. Analyses of steady borehole temperature and terrestrial heat flow data demonstrate that the superposition of heat accumulation by thermal refraction and fluid convection along deep major faults and high-permeability karst conduits jointly governs the spatial distribution of present-day subsurface temperature in the research area.
Researching methods for modifying and enhancing the permeability of fractured reservoirs in enhanced geothermal systems (EGS) is of significant strategic importance for promoting efficient thermal energy utilization and achieving green and sustainable energy development. To investigate the permeability enhancement mechanisms of carbon-oxygen thermochemical modification of granite gneiss, Carbon-oxygen thermochemical modification experiments were conducted on single-fracture granitic gneiss at modification temperatures of 225, 250, 275, 300, and 325 ℃. By comparing the fracture seepage parameters and fracture surface morphology before and after modification, the evolution of seepage caused by carbon-oxygen thermochemical modification and its modification mechanism were systematically revealed. Studies have shown that: Carbon-oxygen thermochemical modification technology can significantly improve the permeability of single-fracture granite gneiss, and the permeability of the sample gradually increases with the increase of modification temperature, and shows a significant increase at 275 and 325 ℃. With the increase of volume stress, the permeability growth rate of the sample before and after modification shows an increasing trend. The structural weakening induced by thermochemical action is more likely to promote the connection of fracture surfaces under high volume stress condi-tions, showing the characteristics of “high pressure promoting permeability”. After carbon-oxygen thermochemical modification, the fracture surface tends to be flatter, the elevation fluctuation is reduced, the overall roughness is reduced, joint roughness coefficient (CJR) decreases by 1.50%–8.27%, and the surface roughness ratio Rs decreases by 0.10%–1.27%. Through fitting, it was found that there is a significant positive correlation between the CJR reduction rate and the permeability growth rate. Carbon-oxygen thermochemical modification enhances the erosion effect of micro-protrusions on the fracture surface. As the modification temperature increases, the erosion effect of micro-protrusions on the fracture surface gradually evolves from a local and non-uniform distribution to a large-scale and relatively uniform distribution. The area of the elevation zone gradually transforms into the low and medium elevation zones. Carbon-oxygen thermochemical modification can effectively modify reservoir fractures through exothermic reactions under high temperature conditions, thereby achieving reservoir permeability enhancement. Scanning electron microscopy (SEM) results showed that as the modification temperature increased from 225 ℃ to 325 ℃, the microscopic damage characteristics of transgranular fractures, intergranular fractures, particle breakage, and local detachment on the fracture surface gradually increased. The micro-protrusion damage and structural reconstruction induced by carbon-oxygen thermochemical heat release effectively improved the flow space of the fracture surface, thereby improving the conductivity of the fracture. Carbon-oxygen thermochemical modification can effectively transform reservoir fractures through exothermic reactions under high temperature conditions, thereby achieving reservoir permeability enhancement. The research results provide a new idea and technical approach for the efficient diversion and sustainable utilization of fractured reservoirs in enhanced geothermal systems (EGS).
Mining efficiency is effectively improved through the adoption of super-long fully-mechanized longwall faces. However, when the face length is increased and single-shearer operation is retained, equipment idle time is increased and face-advance efficiency is reduced. To improve the mining efficiency of fully-mechanized longwall faces, two-shearer mining processes are designed for 600 m super-long faces. Coal-flow intensity on the armoured face conveyor (AFC) and the associated process parameters are evaluated under various mining processes. Leveraging the cooperative mining of two shearers, mining processes of two shearers traveling in the same direction and opposite direction are proposed. A previously developed spatio-temporal distribution model of coal flow is used to simulate the spatio-temporal distributions of coal flow of the AFC under mining processes of single shearer, two-shearer in the same-direction and opposite-direction. Three mining processes are compared using process cycle time, no-load rate, peak cross-sectional area and volume of coal flow, average transport volume, volumetric fluctuation coefficient, and rated power required for the AFC. Under mining processes of two shearers, the AFC no-loading rate is reduced and resource utilization is improved. Coal-flow intensity is also decreased, and the risk of scraper chain fatigue fracture is reduced. Compared with the same-direction two-shearer mining process, a smaller conveyed coal volume is obtained under the opposite-direction two-shearer mining process. Lower coal flow intensity, a smaller volumetric fluctuation coefficient, and a more stable AFC load are also achieved. Effects of traction speed under two-shearer mining process in the opposite-direction and the coal-flow intensity on the AFC are investigated through seven sets of comparative simulation tests. When the traction-speed ratio is increased, the AFC no-load rate is reduced and operational efficiency is improved. When the ratio is decreased, the AFC no-load rate is increased and operational efficiency is reduced. When the traction-speed ratio is either increased or decreased, the maximum cross-sectional area of the conveyed coal flow is increased. Maximum conveyed coal volume and the average conveyed coal volume are also increased. Meanwhile, the volumetric fluctuation of coal flow is intensified, the AFC rated power is increased, and the mining cycle is shortened. However, the minimum center distance between two shearers remains unaffected.
Electronics are not only the foundation of intelligent coal mining but also its core driving force for continuous evolution. As coal mine intelligence advances, it imposes stringent challenges on the functions and performance of general purpose electronics. Addressing the technical bottleneck whereby general purpose electronics cannot safely, systematically, comprehensively, and reliably adapt to the complex underground operating conditions of coal mines, this work draws on advanced concepts from automotive, defense, and aerospace electronics, and is devoted to the development and research of coal mine specific electronic technologies. The concept of “coal mining electronics” is proposed for the first time; its definition is provided; the four essential attributes including environmental adaptability, intrinsic safety, real time determinism, and whole life cycle maintainability are clarified; and a four in one functional classification and system architecture encompassing sensing, communication, control, and storage is established, with the aim of laying the foundation for defining the research direction of “coal mining electronics”. The research scope of coal mining electronics is distilled; its core contents are elaborated from the four aspects of safety characteristics, functional characteristics, maintainability characteristics, and energy characteristics, with the aim of indicating the direction for determining the research content of “coal mining electronics”. The key generic technologies of coal mining electronics are extracted; The connotations of seven key generic technologies, comprising intrinsic safety, high reliability, sensing and detection, intelligent collaborative control, intelligent storage of multi source heterogeneous data, real time anti interference communication, and whole life cycle health maintenance are expounded, with the aim of providing basic ideas for tackling key common technologies of “coal mining electronics”. The prospects of coal mining electronics are envisioned; a priority development direction is identified that takes “systemicity, adaptability, reliability, and safety” as the principal connotations and “standardization, modularization, intelligence, and green development” as the principal goals, the “four attributes and four izations”, the implementation pathway targeting electronic components, circuits, and systems within underground coal mine environments is proposed, with the aim of clarifying the orientation for the rapid and sound development of this interdisciplinary field. From the proposal of the basic concept, the construction of the theoretical system, the tackling of key technologies, and the formation of standards and specifications to the implementation of engineering applications, the development of “coal mining electronics” should learn from the experience of successful sectoral electronics. Rooted in the scientific and technological frontier and aligned with the intelligentization strategy of the coal industry, conducting systematic and in depth research is the essential pathway for “coal mining electronics” to move from concept to implementation and from nascency to strength. The research and application of “coal mining electronics” will provide important support for addressing the key core technical challenges of intelligent coal mining, consolidating the specialized electronic foundation for coal mine intelligence, building the hardware foundation for intelligent mines, and promoting the high end, intelligent, and green high quality development of the coal industry.
Driven by the destruction of the North China Craton, large-scale crust-mantle mixed magmatism and concomitant upwelling of deep-seated high-temperature heat sources collectively shaped the anomalously high geothermal background in its eastern margin—Shandong Province, where abundant geothermal resources are preserved. However, the regional geothermal field characteristics, the spatial distribution pattern of terrestrial heat flow, and the heat source-water source driving mechanisms and accumulation patterns remain poorly constrained. Tectonic analysis, measurements of rock thermal conductivity and radiogenic heat production, borehole temperature logging, and calculations of geothermal gradient as well as conductive and advective heat flow were integrated to systematically characterize the geothermal field and the spatial distribution of heat flow across Shandong. Several high-heat-flow zones were identified, including the Jiaodong Uplift, the Tan−Lu fault zone, Qiguang fault zone, Lanliao fault zone, the basin–range fault zones in the central-southern Shandong Uplift, and the buried hills within the Jiyang Depression of northwestern Shandong. On this basis, a complete causal chain—lithospheric thinning → mantle heat flow uplift → fault-controlled thermal conduit → shallow heat flow focusing—was established which, from a deep geodynamic perspective, reveals the transport mechanisms of deep-seated heat through deep-rooted faults and magmatic activity, and further elucidates the physical processes governing shallow crustal heat accumulation. The results demonstrate that lithospheric-scale active fault zones reaching the mantle provide preferential pathways for the upwelling of mantle-derived magmatic and thermal materials. The heat flow values in these zones are significantly higher than the regional background values of their respective tectonic units, forming high-heat-flow geothermal anomalies, in which the advective heat flow component accounts for 29% to 70% of the total heat flow, confirming that deep convective heat upwelling is the dominant heat source for shallow high-heat-flow anomalies. At the shallow level, heat accumulation is governed by dual physical mechanisms: the thermal refraction effect caused by the high thermal conductivity of hard rocks in buried hill uplift zones—i.e., the lateral focusing of heat flow lines by the high-conductivity basement, and heat flow upwelling driven by high pressure gradients and high temperature gradients of compaction water and acidic fluids in depression centers. This study further demonstrates that incorporating the advective-dominated heat flow component into regional heat flow mapping can more accurately highlight high-heat-flow geothermal anomaly zones, thereby providing high heat flow as a critical and direct line of evidence for optimizing geothermal exploration target selection.
Key to stability of gob-side entry retaining lies in stable bearing capacity of roadside backfill body (roadside support body). Traditional roadside backfill bodies exhibit inadequate adaptability to the roof structure in terms of mechanical properties such as strength and toughness, making them prone to brittle failure and the formation of through-going gas-conducting channel, thereby affecting safe mining at working face. This study proposes a method for mechanical modification of roadside backfill materials using fibers. A mechanical model for crack initiation and propagation in fiber-modified backfill body was established, elucidating the intrinsic mechanism by which fibers enhance material strength and toughness by improving the stress field at matrix crack tip. Experimental tests determined the influence characteristic of fiber content, length, and type on the mechanical properties of the fiber-modified high-water materials backfill body, and polypropylene fibers with a length of 6 mm and a volumetric fraction of 0.3% were selected as the optimal modification parameters, increasing the peak strength of the fiber-modified high-water materials backfill body by 21% while significantly enhancing its toughness and deformation capacity. Meso-mechanical simulations show that fibers exert a bridging effect within the fiber-modified high-water materials backfill body, effectively maintaining the mechanical connection between particles, suppressing the total amount and propagation rate of cracks, improving the spatial distribution morphology of cracks, and enhancing the uniformity, integrity, and stress transfer efficiency of the force chain network. Using the 150110 tailgate in Ping’an Coal Mine as a case study, a mechanical interaction model between the roadside backfill body and the roof was established considering the roof fracture and stability characteristics of gob-side entry retaining. It was clarified that the roadside backfill body should possess sufficient support strength (support resistance not less than 11.5 MPa) and deformation capacity (peak strain reaching 6.5×10−2) to accommodate roof rotation and subsidence (deformation amount reaching 195.0 mm). A discrete element numerical model of the gob-side entry retaining surrounding rock was constructed, and a time-dependent simulation method for the mechanical properties of the fiber-modified roadside backfill body was developed. A comparative analysis was conducted on the roof caving characteristics, crack propagation and damage degree of the roadside backfill body, and mining-induced stress evolution under the unmodified and fiber-modified backfill body conditions. The simulation results show that high-strength and high-toughness properties of fiber-modified backfill body significantly shorten rotation process of key block B, reduce its rotation subsidence, optimize the surrounding rock stress distribution, and reduce the tensile and total damage of roadside backfill body by 27.1%−30.8% and 19.5%−22.3%, respectively. Based on these findings, a synergistic control technology for gob-side entry retaining integrating high-strength and high-toughness roadside backfill body, roof bearing reinforcement, and zonal and time-phased support was proposed and successfully applied in 150110 working face of Ping’an Coal Mine. Field monitoring demonstrated that after adopting the fiber-modified backfill body for gob-side entry retaining, the roof-to-floor and rib-to-rib convergences are reduced by 135 mm (a reduction of 21.8%) and 96 mm (a reduction of 15.9%), respectively, significantly improving surrounding rock stability and meeting the requirements for safe mining in the subsequent panel. This research provides a novel approach for the design and performance optimization of roadside backfill materials for gob-side entry retaining and holds significant theoretical significance and engineering value for advancing non-pillar coal mining technology.
Against the background of the energy transition, the importance of geothermal resources has become increasingly prominent. The northern part of the Jizhong Depression, where the Xiong'an New Area is located, hosts the most abundant hydrothermal geothermal resources in eastern China. The carbonate reservoirs in this region feature moderate burial depth, relatively high temperature, large water yield, and favorable reinjection conditions. However, the hydrothermal circulation processes in these reservoirs remain poorly understood, the renewal rate and sustainable utilization capacity of the fluids are still unresolved. Taking the Taihang Mountains−Xiong'an New Area as the study area, based on structural geology and isotope hydrogeology, systematically analyzed the δD, δ18O and 14C isotopic characteristics of fluids in different zones and strata. Recharge sources were identified, fluid circulation pathways were clarified, fluid residence times in different subregions were calculated, and the fluid renewal capacity was quantitatively evaluated. The results indicate that geothermal reservoir fluids within the uplifted areas of the depression (where Xiong'an is centered) are recharged by paleo-meteoric water originating from the western the Taihang Mountains. The recharge elevation ranges from 317 m to 717 m, with a recharge temperature of 5.07−8.94 °C. The 14C ages of geothermal fluids were corrected and calculated via the Tamers and other models, revealing that the fluid residence time exceeds 14 000 years, with a sharp increase in groundwater age after the fluids enter the depression. Integrated with tectonic settings, temperature logging data, and field test results, the fluid circulation pattern was determined: meteoric water infiltrates through bedrock outcrops in the Taihang Mountain, converges into piedmont faults and stratigraphic interiors, migrates eastward along faults via multiple pathways to the deep depression, and finally ascends through fault zones to the Jixian System strata in the Xiong'an New Area. The regional fluid renewal rate is calculated to be 0.002 6%～0.148 8% per year, which is significantly controlled by fault distribution and tectonic conditions. The recharge rates of different circulation pathways were quantified, and the groundwater runoff velocity along flow paths ranges from 1.1 m/a to 23.5 m/a. Spatially, the runoff velocity is the highest in uplifted mountainous areas, moderate near faults within the new area, and the lowest in local uplifts and sags.
Deep mine geothermal resources constitute a significant component of China’s geothermal resources. The large-scale, efficient development and utilization of mine geothermal resources are crucial for ensuring energy supply in mining areas, optimizing the energy structure in mining areas, and promoting green and low-carbon transformation, and are therefore of great importance for achieving China’s strategic goals of carbon peaking and carbon neutrality. The coal-geothermal co-mining technology is an effective method for developing and utilizing mine geothermal energy, which not only reduces the risk of mine water inrush but also fully unlocks resource development potential. To further advance this technology and provide engineering application references for mines with geothermal development potential, an integrated technical approach to mine geothermal water development was proposed, comprising analysis of geothermal genesis, resource evaluation, delineation of geothermal reservoirs and water-rich zones, optimization of the production-system layout, resource utilization and benefit evaluation. China’s first industrial trial on the development and utilization of mine geothermal water was conducted at Pingdingshan Tianan Coal Industry Co., Ltd. No. 10 Mine. This study analyzed the genesis of geothermal resources in the mining area by investigating regional structures and hydrogeological conditions, calculated the resource reserves and development potential using the geothermal reservoir volume method and numerical simulation, designed a mine geothermal resource development and utilization system based on underground transient electromagnetic exploration data and the existing mine production-system layout, and evaluated the comprehensive benefits of geothermal development along with an economic sensitivity analysis. The results indicate that, the geothermal resources in Pingdingshan No. 10 Mine are primarily controlled by regional geological structures, with atmospheric precipitation, surface rivers, and reservoir water serving as the main recharge sources of geothermal fluids. Heat transfer from the Earth’s deep interior due to basement uplift acts as the primary heat source, while surface limestone outcrops and faults constitute the main groundwater flow pathways. The coal-bearing sandstone and mudstone assemblage forms a water-resisting caprock, and the axis of the Likou Syncline forms a local water and heat accumulation center, ultimately creating a geothermal reservoir. The total geothermal resources in the mining area amount to 11.85 × 107 J, equivalent to 40.44 million tons of standard coal. Underground transient electromagnetic technology was employed to accurately delineate water-rich zones in the geothermal reservoir, and geothermal production wells were installed, achieving stable extraction of geothermal water at 52 ℃ and 190 m3/h. A dedicated multistage lifting and transportation system for geothermal water was established by retrofitting the existing mine production system. A geothermal water heat-pump system was designed to meet the heating demand of 80 000 m2 of buildings in the mining area and the requirement for 3 800 m3 of chilled water for underground use. No significant variation in the produced-water temperature was observed during the long-term operation of the geothermal wells. The total investment in the mine geothermal water development project was 27.1627 million yuan, with an annual comprehensive economic benefit of 10.1798 million yuan. The net present value of the project was 28.9887 million yuan. Life cycle assessment calculations showed an average annual profit of 6.0683 million yuan. Compared with conventional coal-fired heating, geothermal water heating can reduce carbon emissions by 9 970 t/a, demonstrating significant safety, economic, and environmental benefits. Sensitivity analysis indicates that the project’s profitability is highly dependent on steam prices and that project implementation requires stable and sustained heat demand and a reasonable heat-pricing mechanism. Challenges in the coal-geothermal co-mining mode were systematically summarized across five aspects, economics, extraction, transportation, utilization, and safety. Future research directions for mine geothermal development and utilization were outlined to promote the large-scale application of mine geothermal energy, facilitate the green and clean transformation of mining areas, and support their long-term sustainable development.
The flocculation and sedimentation of coal slime water is one of the key factors affecting the efficient and stable of coal preparation plants, while the fluid flow of the flocculation device exerts a significant influence on the flocculation process and its effectiveness. To develop a high-efficiency coal slime water flocculation device, a down-flow grid flocculator with cylindrical grid bar was designed with reference to grid flocculators in the water treatment field. Computational Fluid Dynamics (CFD) and the CFD-Discrete Element Method (CFD-DEM) were employed to simulate the distribution of hydrodynamic parameters in the flow field of the grid and the collision-adhesion behaviors of particles in the flow field, thereby identifying the most critical parameters affecting particle collision. Taking these key parameters as evaluation indices, the CFD method was further used to optimize the structure of the grid flocculation device. Finally, CFD-DEM simulations and experiments of coal slime water flocculation were carried out on the optimized grid device to verify the rationality of the research. The results show that in the grid flocculation device, the grid bar induces turbulent fluctuations of the fluid and generates trailing vortices, which increase the turbulent kinetic energy (TKE) and turbulent kinetic energy dissipation rate (TKEDR) of the fluid, thereby enhancing the collision probability of coal slime water particles. Among the influencing factors, TKE and TKEDR are the two key parameters most closely related to particle collision probability. However, the TKEDR caused by friction between the fluid and the grid surface is ineffective dissipation and exerts no significant promotion effect on particle collision. The grid diameter (d), intra-row distance-diameter ratio (l/d), and inter-row distance-diameter ratio (h/d) have important impacts on TKE and TKEDR. Within the research range, the optimal grid diameter is 4 mm. When l/d ranges from 2.0 to 2.3, both TKE and TKEDR increase with the rise of l/d and reach their maximum values at l/d = 2.3; with a further increase in l/d, both parameters decrease accordingly. As the inter-row distance-diameter ratio h/d increases from small to large values, TKE and TKEDR also increase first and then decrease, achieving the optimal values at h/d = 3.5. In the optimized grid flocculation device, the variation in the number of particles contained in the flocs obtained from CFD-DEM simulations shows good consistency with the variation in floc particle size measured in experiments, indicating the rationality of the simulation results. The flocculation growth rate in the grid region of the flocculation device is nearly linear, which is significantly better than that in the non-grid region, demonstrating that the optimized grid structure has a favorable promotion effect on particle flocculation.
Vibrating fluidized bed separator, leveraging the synergistic effect of vibration and airflow, enhances gas-solid contact and mitigates particle agglomeration, making it particularly suitable for the dry separation of fine coal. During the separation process, however, the inefficient passage of 1 mm particles through dry screening, coupled with collisions within the mechanical structure and inter-particle abrasion, leads to the incorporation of both inherent and newly generated 1−0 mm fine coal particles into the separation bed. Therefore, it is imperative to investigate the mixing and separation processes of fine coal and magnetite powder within the vibrating fluidized bed, and to elucidate both the underlying mechanisms of these processes and the synergistic mechanism between vibration and airflow. This understanding is crucial for guiding the steady-state control of the vibrating fluidized bed separator towards highly efficient fine coal separation. This study employs a binary dense medium composed of 1−0 mm fine coal and 0.30−0.15 mm magnetite powder. The mixing and separation processes of this medium were investigated to determine the influence of bubble fluctuations on these processes. It was found that in a conventional gas-solid fluidized bed for separation, bubbles serve to agitate the bed and entrain particles. While they can induce the segregation of fine particles through entrainment, their stirring action simultaneously disrupts the very conditions required for stable segregation. Under the synergistic action of vibration and airflow, low vibration intensity transforms particles from irregular random motion into periodic oscillations, squeezing bubbles and driving them to rupture rapidly. The transmission and dissipation of vibrational characteristics from bottom to top create a quasi-homogeneous fluidization state, where 0.5−0 mm coal fines are carried upward by the trailing vortices of micro-bubbles, enhancing segregation. At high vibration intensity, vibration induces particle levitation, facilitating airflow aggregation into large bubbles and attenuating the propagation of vibrational energy. Slugging bubbles drive the migration and back-mixing of binary dense media, achieving uniform mixing. Furthermore, a study was conducted on the evolution of the fluidized bed density under both mixing and separation states of the binary dense medium, quantifying the energy distribution of density fluctuations across different bed zones. It was found that in the conventional gas-solid fluidized bed, bubble motion induced density fluctuations in a relatively uniform manner across all zones. Under low-intensity vibration, however, the dominant factors for fluctuations differed significantly: vibrational energy prevailed in the middle and bottom layers, whereas micro-bubbles dominated at the top. When the vibrational energy was increased, it substantially loosened the particle bed, causing airflow to coalesce into large bubbles which became the primary source of bed fluctuations. Consequently, the energy distribution became more consistent across the various bed regions.
Against the backdrop of the carbon peaking and carbon neutrality goals, the demand for the development and utilization of medium- and deep geothermal resources continues to increase, making the identification of deep geothermal reservoirs and target selection an important research focus in geothermal exploration. However, thick overburden, complex geological structures, and anthropogenic interference impose significant limitations on conventional single exploration methods in terms of investigation depth, resolution, and interpretation accuracy, making them inadequate for evaluating deep geothermal reservoirs under complex geological conditions. Focusing on deep geothermal reservoir target selection, recent advances in the application of seismic exploration, electromagnetic exploration, and hydrogeochemistry are systematically reviewed in representative geothermal fields worldwide. The applicability and limitations of these methods for identifying deep heat sources, characterizing reservoir structures, detecting fault-controlled fluid pathways, and analyzing geothermal fluid migration are comprehensively evaluated. On the basis of integrated exploration case studies from representative geothermal fields, the collaborative mechanisms of seismic imaging, electrical resistivity inversion, and hydrogeochemical constraints are summarized, and the technical characteristics and applicability of multi-source data integration under different geological settings are synthesized. The results indicate that seismic methods are effective in delineating fault distributions and deep structural features, electromagnetic methods exhibit a strong capability for identifying deep low-resistivity geothermal reservoirs and fluid-enriched zones, and hydrogeochemical methods provide effective constraints on reservoir temperature, fluid origin, and circulation processes. Individual methods are inherently constrained by geological conditions and their own detection capabilities, whereas integrated multi-method exploration fully exploits the complementary strengths of different techniques. By using seismic imaging to constrain the structural framework, electrical resistivity inversion to delineate the spatial distribution of geothermal reservoirs, and hydrogeochemical investigations to verify fluid circulation characteristics, comprehensive identification of the deep heat source–reservoir–fluid conduit system can be achieved. For geological settings characterized by thick overburden and strong interference, an integrated target selection strategy centered on the collaborative application of seismic exploration, electromagnetic exploration, and hydrogeochemical constraints is summarized. Corresponding combinations of exploration techniques and their applicable geological conditions are also synthesized, providing a practical technical framework for deep geothermal reservoir exploration in complex geothermal systems.
The Red River Fault Zone in western Yunnan is endowed with abundant geothermal resources. Systematic investigations of its geochemical signatures and genetic mechanisms are essential for the sustainable development and utilization of regional geothermal energy. Multi-media geochemical surveys with machine learning approaches—including a coupled self-organizing map and K-means (SOM-KM) clustering model and Mantel test-validated positive matrix factorization (PMF) modeling, were employed to elucidate the response of geothermal fluids and associated travertine deposits to deep hydrothermal processes. The results demonstrate that SOM-KM clustering effectively distinguishes the dominant factors controlling the formation and evolution of geothermal fluid chemistry. Quantitative source apportionment identifies five primary processes governing fluid solute compositions: evaporite dissolution and mixing with paleo-saline water within fault-bounded basins (38.05%), carbonate dissolution coupled with high-temperature decarbonation (23.50%), atmospheric precipitation, surface water, and shallow groundwater recharge (19.41%), water-rock interaction with silicate rocks (9.76%), and upwelling of deep primary geothermal fluids (9.28%). Notably, geothermal fluids in the Eryuan Niujie–Sanying Basin exhibit geochemical affinities with mid-ocean ridge hydrothermal systems, driven by intensive water–rock interactions with oceanic island tholeiites and pelagic hydrothermal carbonates. Such geochemical characteristics are consistent with the alkaline high-temperature fluids of the Tengchong Rehai geothermal field, indicating fluid origins dominated by deep crustal circulation rather than direct magmatic input. Collectively, geochemical evidence confirms that the typical geothermal systems along the Red River Fault are non-magmatic, deep-circulation hydrothermal systems. The geothermal accumulation models can be classified as deep-circulation convective type in uplifted mountainous fault zones, and convective-conductive composite type in fault-depression basins. The thermal regime is characterized by a layered heat convergence mechanism involving mantle-derived heat supply at depth, frictional shear heating along fault planes, convective heat transfer in shallow aquifers, and radiogenic heat production within concealed intrusive and metamorphic crystalline basements. Estimated reservoir temperatures in this region range from 61.89 ℃ to 186.40 ℃, with circulation depths varying between 1 231.67 m and 6 308.35 m. Furthermore, the core segment of the Ailaoshan-Red River Fault Zone, characterized by extensive Cenozoic potassic magmatism and lithospheric delamination, provides favorable structural conditions for deep fluid infiltration and circulation. Intersections between primary faults, secondary branch faults, and extensional rift-basin faults are highlighted as high-potential targets for medium- and high-temperature geothermal exploration.
The Chenjiazhuang buried hill geothermal field is an important carbonate karst geothermal reservoir area in Shandong Province. The evolution of the geothermal field under large-scale production-reinjection conditions remains unclear, and there is a particular lack of criteria for determining the sustainable exploitation resource threshold. Numerical simulation is performed to determine sustainable exploitation thresholds under the dual constraints of thermal breakthrough and water-level safety, covering critical production-reinjection rate, critical well spacing, and recoverable resource amount. Based on regional structural and stratigraphic conditions, measured geothermal gradients, terrestrial heat flow, and full-hole temperature logging data, the main controlling factors of geothermal anomalies are systematically analyzed. A three-dimensional hydrothermal coupling numerical model is established using COMSOL Multiphysics. Under balanced production-reinjection constraints, multiple groups of production-reinjection parameters and well layout schemes are designed to simulate the long-term evolution of temperature and hydrodynamic fields in both doublet and multi-well systems, and the sustainable recoverable geothermal resources are quantitatively calculated to determine the threshold. The results show that: Low-temperature reinjected geothermal water accumulates around reinjection wells and forms a cold front that migrates toward production wells driven by density differences, water-level potential differences, and hydrothermal convection within the reservoir. In the doublet system, the thermal breakthrough time t of production wells decreases in a power function with the increase of production-reinjection rate Q, expressed as t=4819Q−1.06, R2=0.9997. It increases exponentially with the rise of well spacing R0, with the fitting formula \begin{document}${t_{{R_0}}} = a{{\rm{e}}^{b{R_0}}} $\end{document}, where a and b are constants. R2 > 0.97. Within a 100-year production-reinjection period, under the dual constraints of thermal breakthrough (average reservoir temperature drop ≤1 ℃) and maximum water table burial depth (Dmax≤170 m), the sustainable exploitation threshold is determined as a well spacing of 600 m and a single-well production-reinjection rate of 80 m3/h, which are the critical safe production-reinjection parameters. Multi-well simulations show that the checkerboard well pattern forms a relatively uniform flow field, with an average reservoir temperature drop of only 0.47 ℃ and a maximum water table burial depth of 159.08–169.41 m over 100 years, meeting both constraints; the orbital pattern results in an uneven flow field and a higher thermal breakthrough risk; the centralized pattern causes severe imbalance of the geothermal field and has the poorest sustainability. Based on the checkerboard layout, a total of 144 production-reinjection well pairs can be theoretically deployed in the whole area. The calculated recoverable geothermal water yield is 27.65×104 m3/d, the annual recoverable geothermal energy is 57.11×1014 J/a, equivalent to 19.49×104 t/a of standard coal, which represents the resource threshold for sustainable exploitation. The findings provide a theoretical basis for the design of production-reinjection well networks and resource assessment in similar karst reservoirs with buried hill settings, and also offer a reference method for determining sustainable exploitation thresholds in comparable geothermal fields.
To clarify the differences in seepage and heat transfer between water and supercritical carbon dioxide (ScCO2) in deep carbonate geothermal reservoirs and to identify the evolution of the seepage field in complex fracture networks under the combined effects of temperature, confining pressure, fluid properties, and fracture structure, carbonate rocks from the Honghuayuan, Lunshan, and Guanyintai formations in Jurong, Jiangsu Province, were prepared as 100 mm × 100 mm × 100 mm cubic specimens. Comparative experiments were conducted using a high-temperature and high-pressure true-triaxial integrated apparatus for fracturing, seepage, and heat transfer. In the confining-pressure tests, the temperature was maintained at 100 ℃, and four triaxial stress states of 5/10/15, 8/14/20, 10/16/22, and 14/20/26 MPa were applied. In the temperature tests, the triaxial stress state was fixed at 5/10/15 MPa, and five temperature levels of 60, 80, 100, 120, and 140 ℃ were adopted. Water was injected at a constant flow rate of 15 mL/min, whereas ScCO2 was injected at a constant pressure of 7.4–7.7 MPa. The equivalent hydraulic aperture, apparent convective heat-transfer coefficient, and heat-transport efficiency were calculated from the measured flow rate, pressure difference, and inlet and outlet temperatures. Acoustic-emission localization, three-dimensional computed-tomography reconstruction, and permeability-tensor analysis were combined to determine the effects of fracture number, orientation, and connectivity on the seepage field. Increasing confining pressure reduced the equivalent hydraulic aperture, flow velocity, apparent convective heat-transfer coefficient, and heat-transport efficiency of both fluids, with the rate of reduction gradually decreasing at higher stress levels. The equivalent hydraulic aperture of ScCO2 was more sensitive to changes in confining pressure. For specimen HHY-8, the hydraulic aperture of water decreased by 1.4%, 0.3%, and 0.1% during successive loading stages, whereas that of ScCO2 decreased by 10.5%, 3.5%, and 1.5%, respectively. As the temperature increased from 60 ℃ to 140 ℃, mineral thermal expansion continuously reduced the equivalent hydraulic aperture, while the increased temperature difference between the rock and the working fluid generally enhanced the apparent convective heat-transfer coefficient and heat-transport efficiency. The temperature-induced reduction in hydraulic aperture increased with dolomite content. ScCO2 exhibited a higher apparent convective heat-transfer capacity than water under most temperature and pressure conditions. At 140 ℃, however, decreases in the density and isobaric specific heat capacity of ScCO2 caused the heat-transport efficiency of water to become generally higher than that of ScCO2. Permeability-tensor calculations showed that, for specimen LS-1, the permeability coefficient of water decreased from 12.71 m/d at 60 ℃ to 4.88 m/d at 140 ℃, whereas that of ScCO2 decreased from 8.39 m/d to 6.22 m/d. Fracture number exerted a stronger control on flow capacity at low confining pressure, whereas fracture-orientation combinations, connectivity, and hydraulic communication with the borehole became more important at high confining pressure. Temperature had a weaker effect on the principal seepage direction and the overall seepage field than confining pressure. Seepage and heat transfer in fractured carbonate geothermal reservoirs are jointly controlled by mechanical fracture closure, mineral thermal expansion, and the thermophysical properties of the working fluid. ScCO2 provides higher apparent heat-transfer performance at low and moderate temperatures, but its heat-transport advantage weakens with increasing temperature. Working-fluid selection should therefore comprehensively account for reservoir temperature, stress state, mineral composition, and fracture-network structure.
The flow with phase transition of geothermal fluids containing multi-component non-condensable gases in the wellbore is an important basis for evaluating the productivity, flash depth, and scale inhibition/removal of medium-to-high temperature geothermal production wells. Based on fugacity models for the gas mixtures and activity models for the liquid mixtures, a mutual solubility model and property calculation models (including density, viscosity, and enthalpy) for geothermal fluid containing non-condensable gases of CO2 and CH4 were established for temperatures up to 300 ℃ and pressures up to 60 MPa, and were validated against previous results. Then, according to the conservation laws of mass, energy, and momentum, combined with the two-phase thermodynamic equilibrium model and the drift-flux model for gas-liquid velocity, a one-dimensional steady-state wellbore flow model with phase change for the CO2–CH4–H2O system is developed. A robust “bottom-up” stepwise iterative solution method was proposed to accurately predict the phase behavior, flow process with phase change, and flash depth of geothermal fluids containing multi-component non-condensable gases during the production of medium-to-high temperature geothermal wells. Based on the conditions of three typical geothermal wells in Hungary, Turkey, and China, the phase-change flow processes in the wellbore were evaluated. The results show that non-condensable gases have a significant influence on the phase-change location and phase-change flow of geothermal fluids in the wellbore. For the same content of CO2 and CH4, due to the low solubility of CH4, CH4 is more likely to exsolve from the liquid phase to form a gas phase, thereby significantly increasing the flash depth. The wellbore pressure and temperature distributions are strongly affected by phase change: the deeper the phase-change location and the higher the degree of phase change, the greater the average wellbore pressure and temperature. The bottom-hole temperature, pressure, and the total and relative contents of CO2 and CH4 determine the characteristics of the phase-change flow throughout the wellbore. The wellbore phase-change flow model can accurately predict the flash depth, while the commonly used pressure-gradient identification method can approximately estimate the depth range of two-phase flow. Only in geothermal wells with a high degree of vaporization can the flash depth be identified more accurately.
