Fluid flow through fractured rock masses is a key process controlling the safety and performance of deep geoengineering systems,shaped by the complex interactions of thermal,hydraulic,mechanical and chemical(THMC)fiel...Fluid flow through fractured rock masses is a key process controlling the safety and performance of deep geoengineering systems,shaped by the complex interactions of thermal,hydraulic,mechanical and chemical(THMC)fields.This paper presents a systematic review of this subject with special emphasis on the multi-physics governing it.First,we elucidate the interdependent mechanisms and governing equations,highlighting the nonlinear,path-dependent,and evolving nature of the relationship between stress and permeability.Next,mainstream modeling approaches,including equivalent continuum,discrete fracture network(DFN),and dual-porosity/dual-permeability methods,are critically evaluated,and a strategy for model selection based on project scale and geological context is proposed accordingly.Moreover,experimental insights from single-fracture and triaxial flow studies are synthesized,revealing how effective stress,shear displacement,and fracture roughness control permeability evolution.In particular,the practical significance of THMC coupling is demonstrated through case studies on nuclear waste disposal,Enhanced Geothermal Systems,and tunneling projects.The reviewfurther explores AI-and machine learning-driven innovations,particularly physics-informed neural networks and hybrid modeling,which address limitations in computational efficiency,data scarcity,and physical consistency.Finally,persistent challenges,including multi-scale coupling,parameter uncertainty,and complex fracture network representation are identified and critically discussed while paying attention to future developments.展开更多
Hydraulic stimulation technology is widely employed to enhance the permeability of geothermal reservoirs.Nevertheless,accurately predicting hydraulic fracture propagation in complex geological conditions remains chall...Hydraulic stimulation technology is widely employed to enhance the permeability of geothermal reservoirs.Nevertheless,accurately predicting hydraulic fracture propagation in complex geological conditions remains challenging,thereby hindering the effective utilization of existing natural fractures.In this study,a phase field model was developed utilizing the finite element method to examine the influence of fluid presence,stress conditions,and natural fractures on the initiation and propagation of hydraulic fractures.The model employs Biot's poroelasticity theory to establish the coupling between the displacement field and the fluid field,while the phase field theory is applied to simulate fracture behavior.The results show that whenσ_(x0)/σ_(y0)<3 or qf<20 kg/(m^(3)·s),the presence of natural fractures can alter the original propagation direction of hydraulic fractures.Conversely,in the absence of these conditions,the propagation path of natural fractures is predominantly influenced by the initial stress field.Furthermore,based on the analysis of breakdown pressure and damage area,the optimal intersection angle between natural fractures and hydraulic fractures is determined to range from 45°to 60°.Finally,once a dominant channel forms,initiating and propagating hydraulic fractures in other directions becomes increasingly difficult,even in highly fractured areas.This method tackles the challenges of initiating and propagating hydraulic fractures in complex geological conditions,providing a theoretical basis for optimizing Enhanced Geothermal System(EGS)projects.展开更多
The available heat content (stored heat energy) of hot dry rock (HDR) at a depth of 1–10 km in the global land crust is estimated to be 5.06×10~8 EJ,attracting considerable global attention.This paper presents a...The available heat content (stored heat energy) of hot dry rock (HDR) at a depth of 1–10 km in the global land crust is estimated to be 5.06×10~8 EJ,attracting considerable global attention.This paper presents a comprehensive analysis of the geological framework,HDR resource potential,exploration advancements,and the development of enhanced geothermal systems (EGSs) in China.HDR resources are extensively distributed across China.Within the depth range of 3–10 km,China’s estimated potential approximates2.29×10~7 EJ,with a theoretical power generation capacity of approximately 1.67×10^(16) k Wh.Replacing coal power with HDR can help to achieve a net emission reduction of 1.34×10^(16) kg CO_(2) (approximately1.34×10^(13) t),representing an emission reduction efficiency of 94.4%.Based on a development cycle of100 years,the average annual emission reduction reaches 1.34×10^(10) t CO_(2),equivalent to 117%of China’s annual carbon emissions in 2022.Furthermore,in the context of global warming,the development and utilization of HDR,which is feasible in virtually any region worldwide,offers significant potential to support global carbon reduction efforts.China has made substantial progress in HDR exploration in recent years.This paper systematically classifies China’s HDR resources into four genetic types—highly radioactive heat-producing,sedimentary basin,active volcanic,and intensely tectonic zones—and offers detailed exploration insights for each category.Each classification exhibits distinct geological and tectonic characteristics that influence heat source mechanisms and resource distribution.Furthermore,this paper documents significant advances in EGS construction,particularly in the Gonghe Basin on the northeastern margin of the Qianghai-Xizang Plateau and the Matouying uplift in the North China Basin,where successful reservoir stimulation,microseismic monitoring,and experimental power generation have been achieved.Despite these developments,challenges persist,including technical adaptability under complex geological conditions and the economic viability of large-scale HDR development.This paper suggests that future initiatives should emphasize resource exploration,technological research,and policy support to foster sustainable HDR resource development in China,thereby contributing to the global energy transition and environmental sustainability.展开更多
An enhanced geothermal system(EGS)represents a promising approach to sustainable energy generation by harnessing subsurface heat from deep geological formations with low natural permeability.Sedimentary basins-such as...An enhanced geothermal system(EGS)represents a promising approach to sustainable energy generation by harnessing subsurface heat from deep geological formations with low natural permeability.Sedimentary basins-such as the Williston Basin in North Dakota-are considered viable candidates for EGS development due to their broad geographic extent and moderate geothermal potential.Notably,depleted or non-productive oil wells within these basins offer a cost-effective opportunity for EGS implementation as they can be repurposed,thereby significantly reducing the need for new drilling.This study evaluates the feasibility of EGS deployment in McKenzie County,North Dakota.Core samples from five partially abandoned or dry oil wells associated with production from the Red River Formation were obtained from the Core Library of the North Dakota Geological Survey.These samples,spanning the entire thickness of the formation,were sectioned and polished at defined depth intervals for detailed analyses and precise measurements of key reservoir properties critical to geothermal assessment.Several parameters were analyzed to assess the geothermal viability of these wells,including formation temperature,temperature gradient,porosity,thermal conductivity,energy storage potential,and estimated power output via the Organic Rankine Cycle(ORC).The results demonstrate significant depth-dependent variations in thermal and petrophysical properties.Specifically,the depth range of 4000-4500 m is identified as a promising target for EGS stimulation since it is characterized by elevated temperatures,high thermal conductivity,favorable temperature gradients,and sufficient porosity-all essential properties for enhancing permeability through hydraulic fracturing.Furthermore,the calculated energy content and potential ORC power output at these depths indicate that effective geothermal energy extraction is technically feasible.This suggests a compelling opportunity to repurpose existing fossil energy infrastructure-such as abandoned oil wells-for renewable geothermal applications.Overall,the findings of this study underscore the potential of sedimentary formations for EGS development and contribute to advancing low-carbon,diversified energy solutions in alignment with national decarbonization goals.展开更多
In modal logic,topological semantics is an intuitive and natural special case of neighbourhood semantics.This paper stems from the observation that the satisfaction relation of topological semantics applies to subset ...In modal logic,topological semantics is an intuitive and natural special case of neighbourhood semantics.This paper stems from the observation that the satisfaction relation of topological semantics applies to subset spaces which are more general than topological spaces.The minimal modal logic which is strongly sound and complete with respect to the class of subset spaces is found.Soundness and completeness results of some famous modal logics(e.g.S4,S5 and Tr)with respect to various important classes of subset spaces(eg intersection structures and complete fields of sets)are also proved.In the meantime,some known results,e.g.the soundness and completeness of Tr with respect to the class of discrete topological spaces,are proved directly using some modifications of the method of canonical mode1,without a detour via neighbourhood semantics or relational semantics.展开更多
Thermal damage mechanisms are crucial in reservoir stimulation for enhanced geothermal system(EGS).This study investigates the thermal damage mechanisms in granite samples from the Gonghe Basin,Qinghai,China.The grani...Thermal damage mechanisms are crucial in reservoir stimulation for enhanced geothermal system(EGS).This study investigates the thermal damage mechanisms in granite samples from the Gonghe Basin,Qinghai,China.The granite samples were heated to 400℃ and then cooled in air,water,or liquid nitrogen.The physical and mechanical properties of the thermally treated granite were evaluated,andmicrostructural changes were analyzed using a scanning electron microscope(SEM)and computed tomography(CT).The results indicate that cooling with water and liquid nitrogen significantly enhancespermeability and brittleness while reducing P-wave velocity,strength,and Young's modulus.Specifically,liquid nitrogen cooling increased granite permeability by a factor of 5.24 compared to the untreatedsamples,while reducing compressive strength by 13.6%.After thermal treatment,the failure mode of thegranite shifted from axial splitting to a combination of shear and tension.Microstructural analysisrevealed that liquid nitrogen-cooled samples exhibited greater fracture complexity than those cooledwith water or air.Additionally,acoustic emission(AE)monitoring during damage evolution showed thatliquid nitrogen cooling led to higher cumulative AE energy and a lower maximum AE energy rate,withnumerous AE signals detected during both stable and unstable crack growth.The results suggest thatliquid nitrogen induces a stronger thermal shock,leading to more significant thermal damage andpromoting the development of a complex fracture network during EGS reservoir stimulation.This enhancesboth the heat exchange area and the permeability of the deep hot dry rock(HDR)in EGS reservoirs.The insights from this study contribute to a deeper understanding of thermal damagecharacteristics induced by different cooling media and provide valuable guidance for optimizing deepgeothermal energy extraction.展开更多
Creating complex and interconnected fracture networks between injection and production wells is crucial for exploiting hot dry rock(HDR)geothermal energy.However,the simple planar fractures created by conventional hyd...Creating complex and interconnected fracture networks between injection and production wells is crucial for exploiting hot dry rock(HDR)geothermal energy.However,the simple planar fractures created by conventional hydraulic fracturing,primarily controlled by in situ stress,fail to connect directionally with the target well.This study proposes a novel stimulation method,i.e.radial borehole fracturing,which shows great potential for guiding the directional propagation of fractures.The fracture initiation and propagation behaviors of high-temperature granite under radial borehole fracturing are investigated and compared with those of conventional fracturing.Three-dimensional morphological scanning and reinjection tests are used to quantitatively evaluate fracturing performance.Additionally,the influences of key parameters,including rock temperature,in situ stress,injection rate,fluid viscosity,azimuth of the radial borehole,and the number of radial boreholes on the fracture morphology and breakdown pressure are investigated.The results show that radial borehole fracturing can effectively guide the initiation and propagation of fractures along the radial borehole.The breakdown pressure of radial borehole fracturing can be reduced by 14.1%–43.7%compared to conventional fracturing.A higher fluid-rock temperature difference reduces the directional propagation range of fractures guided by the radial borehole.Increases in the vertical density of radial boreholes,injection rate,and fluid viscosity enhance the guiding ability of radial boreholes.Furthermore,there is a competitive relationship between in situ stress and the azimuth of radial boreholes in controlling fracture propagation.This research provides a viable alternative for the directional connection of injection-production wells in HDR reservoirs.展开更多
文摘Fluid flow through fractured rock masses is a key process controlling the safety and performance of deep geoengineering systems,shaped by the complex interactions of thermal,hydraulic,mechanical and chemical(THMC)fields.This paper presents a systematic review of this subject with special emphasis on the multi-physics governing it.First,we elucidate the interdependent mechanisms and governing equations,highlighting the nonlinear,path-dependent,and evolving nature of the relationship between stress and permeability.Next,mainstream modeling approaches,including equivalent continuum,discrete fracture network(DFN),and dual-porosity/dual-permeability methods,are critically evaluated,and a strategy for model selection based on project scale and geological context is proposed accordingly.Moreover,experimental insights from single-fracture and triaxial flow studies are synthesized,revealing how effective stress,shear displacement,and fracture roughness control permeability evolution.In particular,the practical significance of THMC coupling is demonstrated through case studies on nuclear waste disposal,Enhanced Geothermal Systems,and tunneling projects.The reviewfurther explores AI-and machine learning-driven innovations,particularly physics-informed neural networks and hybrid modeling,which address limitations in computational efficiency,data scarcity,and physical consistency.Finally,persistent challenges,including multi-scale coupling,parameter uncertainty,and complex fracture network representation are identified and critically discussed while paying attention to future developments.
基金supported by the National Key Research and Development Program(2021YFB150740401)National Natural Science Foundation of China(42202336)the CAS Pioneer Hundred Talents Program in China(Y826031C01)。
文摘Hydraulic stimulation technology is widely employed to enhance the permeability of geothermal reservoirs.Nevertheless,accurately predicting hydraulic fracture propagation in complex geological conditions remains challenging,thereby hindering the effective utilization of existing natural fractures.In this study,a phase field model was developed utilizing the finite element method to examine the influence of fluid presence,stress conditions,and natural fractures on the initiation and propagation of hydraulic fractures.The model employs Biot's poroelasticity theory to establish the coupling between the displacement field and the fluid field,while the phase field theory is applied to simulate fracture behavior.The results show that whenσ_(x0)/σ_(y0)<3 or qf<20 kg/(m^(3)·s),the presence of natural fractures can alter the original propagation direction of hydraulic fractures.Conversely,in the absence of these conditions,the propagation path of natural fractures is predominantly influenced by the initial stress field.Furthermore,based on the analysis of breakdown pressure and damage area,the optimal intersection angle between natural fractures and hydraulic fractures is determined to range from 45°to 60°.Finally,once a dominant channel forms,initiating and propagating hydraulic fractures in other directions becomes increasingly difficult,even in highly fractured areas.This method tackles the challenges of initiating and propagating hydraulic fractures in complex geological conditions,providing a theoretical basis for optimizing Enhanced Geothermal System(EGS)projects.
基金supported by the National Key Research and Development Program of China (2021YFB1507401)Qinghai Province Clean Energy Minerals Special Project(2022013004qj004)Geological Survey Project of China Geological Survey (DD20221676, DD20230019)。
文摘The available heat content (stored heat energy) of hot dry rock (HDR) at a depth of 1–10 km in the global land crust is estimated to be 5.06×10~8 EJ,attracting considerable global attention.This paper presents a comprehensive analysis of the geological framework,HDR resource potential,exploration advancements,and the development of enhanced geothermal systems (EGSs) in China.HDR resources are extensively distributed across China.Within the depth range of 3–10 km,China’s estimated potential approximates2.29×10~7 EJ,with a theoretical power generation capacity of approximately 1.67×10^(16) k Wh.Replacing coal power with HDR can help to achieve a net emission reduction of 1.34×10^(16) kg CO_(2) (approximately1.34×10^(13) t),representing an emission reduction efficiency of 94.4%.Based on a development cycle of100 years,the average annual emission reduction reaches 1.34×10^(10) t CO_(2),equivalent to 117%of China’s annual carbon emissions in 2022.Furthermore,in the context of global warming,the development and utilization of HDR,which is feasible in virtually any region worldwide,offers significant potential to support global carbon reduction efforts.China has made substantial progress in HDR exploration in recent years.This paper systematically classifies China’s HDR resources into four genetic types—highly radioactive heat-producing,sedimentary basin,active volcanic,and intensely tectonic zones—and offers detailed exploration insights for each category.Each classification exhibits distinct geological and tectonic characteristics that influence heat source mechanisms and resource distribution.Furthermore,this paper documents significant advances in EGS construction,particularly in the Gonghe Basin on the northeastern margin of the Qianghai-Xizang Plateau and the Matouying uplift in the North China Basin,where successful reservoir stimulation,microseismic monitoring,and experimental power generation have been achieved.Despite these developments,challenges persist,including technical adaptability under complex geological conditions and the economic viability of large-scale HDR development.This paper suggests that future initiatives should emphasize resource exploration,technological research,and policy support to foster sustainable HDR resource development in China,thereby contributing to the global energy transition and environmental sustainability.
文摘An enhanced geothermal system(EGS)represents a promising approach to sustainable energy generation by harnessing subsurface heat from deep geological formations with low natural permeability.Sedimentary basins-such as the Williston Basin in North Dakota-are considered viable candidates for EGS development due to their broad geographic extent and moderate geothermal potential.Notably,depleted or non-productive oil wells within these basins offer a cost-effective opportunity for EGS implementation as they can be repurposed,thereby significantly reducing the need for new drilling.This study evaluates the feasibility of EGS deployment in McKenzie County,North Dakota.Core samples from five partially abandoned or dry oil wells associated with production from the Red River Formation were obtained from the Core Library of the North Dakota Geological Survey.These samples,spanning the entire thickness of the formation,were sectioned and polished at defined depth intervals for detailed analyses and precise measurements of key reservoir properties critical to geothermal assessment.Several parameters were analyzed to assess the geothermal viability of these wells,including formation temperature,temperature gradient,porosity,thermal conductivity,energy storage potential,and estimated power output via the Organic Rankine Cycle(ORC).The results demonstrate significant depth-dependent variations in thermal and petrophysical properties.Specifically,the depth range of 4000-4500 m is identified as a promising target for EGS stimulation since it is characterized by elevated temperatures,high thermal conductivity,favorable temperature gradients,and sufficient porosity-all essential properties for enhancing permeability through hydraulic fracturing.Furthermore,the calculated energy content and potential ORC power output at these depths indicate that effective geothermal energy extraction is technically feasible.This suggests a compelling opportunity to repurpose existing fossil energy infrastructure-such as abandoned oil wells-for renewable geothermal applications.Overall,the findings of this study underscore the potential of sedimentary formations for EGS development and contribute to advancing low-carbon,diversified energy solutions in alignment with national decarbonization goals.
基金supported by the National Social Science Fund of China(No.20CZX048)。
文摘In modal logic,topological semantics is an intuitive and natural special case of neighbourhood semantics.This paper stems from the observation that the satisfaction relation of topological semantics applies to subset spaces which are more general than topological spaces.The minimal modal logic which is strongly sound and complete with respect to the class of subset spaces is found.Soundness and completeness results of some famous modal logics(e.g.S4,S5 and Tr)with respect to various important classes of subset spaces(eg intersection structures and complete fields of sets)are also proved.In the meantime,some known results,e.g.the soundness and completeness of Tr with respect to the class of discrete topological spaces,are proved directly using some modifications of the method of canonical mode1,without a detour via neighbourhood semantics or relational semantics.
基金support from the National Natural Science Foundation of China(Grant Nos.52192622 and 52304003)the National Key Research and Development Program of China(Grant No.2023YFF0614102).
文摘Thermal damage mechanisms are crucial in reservoir stimulation for enhanced geothermal system(EGS).This study investigates the thermal damage mechanisms in granite samples from the Gonghe Basin,Qinghai,China.The granite samples were heated to 400℃ and then cooled in air,water,or liquid nitrogen.The physical and mechanical properties of the thermally treated granite were evaluated,andmicrostructural changes were analyzed using a scanning electron microscope(SEM)and computed tomography(CT).The results indicate that cooling with water and liquid nitrogen significantly enhancespermeability and brittleness while reducing P-wave velocity,strength,and Young's modulus.Specifically,liquid nitrogen cooling increased granite permeability by a factor of 5.24 compared to the untreatedsamples,while reducing compressive strength by 13.6%.After thermal treatment,the failure mode of thegranite shifted from axial splitting to a combination of shear and tension.Microstructural analysisrevealed that liquid nitrogen-cooled samples exhibited greater fracture complexity than those cooledwith water or air.Additionally,acoustic emission(AE)monitoring during damage evolution showed thatliquid nitrogen cooling led to higher cumulative AE energy and a lower maximum AE energy rate,withnumerous AE signals detected during both stable and unstable crack growth.The results suggest thatliquid nitrogen induces a stronger thermal shock,leading to more significant thermal damage andpromoting the development of a complex fracture network during EGS reservoir stimulation.This enhancesboth the heat exchange area and the permeability of the deep hot dry rock(HDR)in EGS reservoirs.The insights from this study contribute to a deeper understanding of thermal damagecharacteristics induced by different cooling media and provide valuable guidance for optimizing deepgeothermal energy extraction.
基金supported by the National Science Fund of China for Major International(Regional)Joint Research Project(Grant No.52020105001)the National Natural Science Foundation of China(Grant Nos.52304053 and 52204019).
文摘Creating complex and interconnected fracture networks between injection and production wells is crucial for exploiting hot dry rock(HDR)geothermal energy.However,the simple planar fractures created by conventional hydraulic fracturing,primarily controlled by in situ stress,fail to connect directionally with the target well.This study proposes a novel stimulation method,i.e.radial borehole fracturing,which shows great potential for guiding the directional propagation of fractures.The fracture initiation and propagation behaviors of high-temperature granite under radial borehole fracturing are investigated and compared with those of conventional fracturing.Three-dimensional morphological scanning and reinjection tests are used to quantitatively evaluate fracturing performance.Additionally,the influences of key parameters,including rock temperature,in situ stress,injection rate,fluid viscosity,azimuth of the radial borehole,and the number of radial boreholes on the fracture morphology and breakdown pressure are investigated.The results show that radial borehole fracturing can effectively guide the initiation and propagation of fractures along the radial borehole.The breakdown pressure of radial borehole fracturing can be reduced by 14.1%–43.7%compared to conventional fracturing.A higher fluid-rock temperature difference reduces the directional propagation range of fractures guided by the radial borehole.Increases in the vertical density of radial boreholes,injection rate,and fluid viscosity enhance the guiding ability of radial boreholes.Furthermore,there is a competitive relationship between in situ stress and the azimuth of radial boreholes in controlling fracture propagation.This research provides a viable alternative for the directional connection of injection-production wells in HDR reservoirs.
