The modeling of heat recovery from an enhanced geothermal system(EGS)requires rock thermal parameters as inputs such as thermal conductivity and specific heat capacity.These parameters may encounter significant variat...The modeling of heat recovery from an enhanced geothermal system(EGS)requires rock thermal parameters as inputs such as thermal conductivity and specific heat capacity.These parameters may encounter significant variations due to the reduction of rock temperature during heat recovery.In the present study,we investigate the effect of temperature-dependent thermal conductivity and specific heat capacity on the thermal performance of EGS reservoirs.Equations describing the relationships between thermal conductivity/specific heat capacity and temperature from previous experimental studies were incorporated in a field-scale single-fracture EGS model.The modeling results indicate that the increase of thermal conductivity caused by temperature reduction accelerates thermal conduction from rock formations to fracture fluid,and thus improves thermal performance.The decrease of specific heat capacity due to temperature reduction,on the contrary,impairs the thermal performance but the impact is smaller than that of the increase of thermal conductivity.Due to the opposite effects of thermal conductivity increase and specific heat capacity decrease,the overall effect of temperature-dependent thermal parameters is relatively small.Assuming constant thermal parameters measured at room temperature appears to be able to provide acceptable predictions of EGS thermal performance.展开更多
Traditionally,the factor of safety(FOS)is widely used to account for uncertainties in the design of slopes within the framework of working stress design.As the uncertainties involved in the design of slopes vary,the s...Traditionally,the factor of safety(FOS)is widely used to account for uncertainties in the design of slopes within the framework of working stress design.As the uncertainties involved in the design of slopes vary,the same FOS may correspond to the different levels of reliability.In this study,the advanced first order reliability method is used to determine the resistance factors for design of slopes in a homogenous soil layer.It is found that the resistance factors depend on the target reliability index,the height of the slope,and the variability of the soil strength parameters.It is difficult to suggest a unique set of resistance factors for design of slopes.Analytic solutions are developed to determine the resistance factors for design of slopes assuming the random variables are normally distributed.An approximate method based on the concept of equivalent target reliability index is also suggested to determine the resistance factors for design of the slope when the soil strength parameters are lognormally distributed.The method suggested in this paper provides a practical way to perform load and resistance factors design of slopes.展开更多
A three-dimensional thermo-hydro-mechanical numerical model has recently been enhanced with thermal capabilities to study the response of geothermal reservoirs to stimulation and production.In this paper,we present an...A three-dimensional thermo-hydro-mechanical numerical model has recently been enhanced with thermal capabilities to study the response of geothermal reservoirs to stimulation and production.In this paper,we present an effort to consider three relevant thermal mechanisms in an existing lattice code initially designed for hydraulic fracturing:a)thermal advection in the fluid;b)heat transfer by forced convection from the rock to the fluid;and c)accurate thermal conduction in the rock matrix considering the thermal boundary layer effect.A numerical implementation of the new coupled advection-forced convection logic as well as the coupling with the existing conduction logic in the commercial code XSite is summarized.The numerical solution is compared to analytical solutions for simple simulation cases.The new simulation capability is applied in a large-scale geothermal example to illustrate its performance.展开更多
In response to the existing consolidation theory for stone column composite foundations which cannot consider the time-dependent loading and the well resistance effect of stone columns under time-dependent boundaries,...In response to the existing consolidation theory for stone column composite foundations which cannot consider the time-dependent loading and the well resistance effect of stone columns under time-dependent boundaries,a consolidation model that can reflect these characteristics is developed in this study,and the corresponding analytical solutions are obtained under permeable top surface with permeable bottom surface(PTPB)and permeable top surface with impermeable bottom surface(PTIB),respectively.In addition,the reliability of the proposed solutions is verified by comparing them with existing analytical solutions.Extensive calculations are then performed by the proposed solutions to analyze the consolidation behaviors of stone column composite foundations under time-dependent boundaries,the results show that the interface parameters have a large effect on the distribution of excess pore water pressure(EPWP)along the depth;for projects with longer construction time,the permeability of the top and bottom surfaces of the composite foundation has a smaller effect on the average consolidation rate.Finally,the proposed solution is applied to the settlement calculation in an actual engineering project,and the theoretical results show a general agreement with the measured data by considering the influence of the interface parameters.展开更多
Although rock mechanical behaviour has a long record of study,attempts to understand the role of fractures on rock deformation still have unresolved issues.Due to technical and/or economic challenges,natural rock frac...Although rock mechanical behaviour has a long record of study,attempts to understand the role of fractures on rock deformation still have unresolved issues.Due to technical and/or economic challenges,natural rock fractures are often dealt with crudely,without detailed consideration of fracture geometry and heterogeneity in many geoscience applications.Veined rocks that are ubiquitous in the upper Earth crust fall in that category where sustained efforts are needed to offer key information for rock mechanics and geomechanics applications.Following on from a recent study on the rupture of veined rocks(DOI:10.1029/2019JB019052),we further examine stress path constraints on the deformation of veined rocks(i.e.,stress-path-dependent behaviour of veined rocks)under polyaxial conditions.The Discrete Element Method is used to establish a calcite veined model where constant mean stress(σm)and constant least principal stress(σ3)paths that are representative in the subsurface activities are considered.The results reveal the stress-path dependency of brittleness for models under different loading paths.Models tested under constant-σm conditions exhibit no brittleness,compared to cases where constant-σ3 is applied.Sliding along the strike of an inclined vein is evident under constant-σm deformation,irrespective of the level of stress.Shear bands along the dominated(inclined)veins exhibit apparent particle trajectory anisotropy for the constant-σm deformations which is demonstrated by the evident colour contrast of the adjacent rock matrix and the displacement dispersion of the particles forming the shear bands.We envisage that the reactivation of veins is of relevance to Enhanced Geothermal Systems(EGS)development in terms of seismicity mitigation and multiphysics control of fracture and reservoir permeability.展开更多
Understanding the mechanical and hydraulic properties of fractured rocks and their coupled processes is of great significance for the exploration,design,construction,operation,and maintenance of many rock engineering ...Understanding the mechanical and hydraulic properties of fractured rocks and their coupled processes is of great significance for the exploration,design,construction,operation,and maintenance of many rock engineering projects such as hydropower development,oil and gas extraction,and underground waste disposal.With the rapid advancement of global and national strategies such as the“Paris Agreement”and the“Belt and Road Initiative”,more and more projects are developed in the complex geological environment with varying geological structures.Shear failure and rock instability are prone to occur in fractured rock masses under the coupled effects of high stress,high pore pressure,and engineering disturbance,which are main sources for engineering disasters such as roof collapse and caving,water and mud inrushes,and induced earthquakes.To solve these problems,extensive research on the coupled shear-flow behavior of fractures has been conducted.However,due to the complex mechanical,hydraulic and geometrical characteristics of single fractures and fracture networks,a large number of outstanding issues related to the impact of the coupled processes on the engineering characteristics of rock masses are still unsolved.The relevant experimental apparatuses and methods remain to be further developed.Therefore,in this review,we analyze and summarize the existing shear-flow experimental apparatuses,classify apparatus configurations,specimen shapes,and testing principles,and compare their advantages and disadvantages.We also summarize the main scientific findings obtained from various experimental apparatuses,aiming to provide a reference for developing new shear-flow experimental apparatuses and conducting related scientific research in the future.展开更多
The slope stability assessment is a classical problem in geotechnical engineering.This topic have attracted many researcher’s attention and various theoretical models for predicting critical slope heights or safety f...The slope stability assessment is a classical problem in geotechnical engineering.This topic have attracted many researcher’s attention and various theoretical models for predicting critical slope heights or safety factors in the light of the limit equilibrium(LE)method and the kinematical approach of limit analysis(LA)method.Meanwhile,a large number of experimental studies have been conducted to check the slope stability.Using centrifuge testing results,this paper aims to employ Bayesian method to characterize the model uncertainties of the classical three-dimensional rotational failure mechanism proposed by Michalowski and Drescher(2009)to predict critical slope heights in frictional soils,by incorporating the test uncertainties and parameter uncertainties.The obtained results show that the LA three-dimensional rotational failure mechanism overestimates the critical slope height compared with the LE method,and the experimental observational uncertainty has negligible influences on the posterior statistics of model uncertainty.展开更多
In actual rock engineering,fissures play an important role in determining the mechanical parameters of rock mass,whereas it is very difficult to construct fissures in cylindrical specimens.Therefore,the pre-fissured r...In actual rock engineering,fissures play an important role in determining the mechanical parameters of rock mass,whereas it is very difficult to construct fissures in cylindrical specimens.Therefore,the pre-fissured rectangular rock specimens were constructed innovatively.Moreover,a series of triaxial compression experimental results on the failure mechanical behavior of rectangular solid sandstone specimens containing a single fissure were reported.The lateral strain in different directions was monitored and the experimental results show that elastic modulus and axial strain increase non-linearly with confining pressure,and the average Poisson’s ratio parallel to fissure(μ2)is larger than that vertical to fissure(μ3).The cohesion,Hoek-Brown parameters of peak strength show similar trends with that of crack damage threshold to the fissure angle(α),and the parameters of the peak strength are larger than those of crack damage threshold.However,the internal friction angles of the peak strength and crack damage threshold are almost equal.Based on the geometries and properties of cracks,ten typical crack types are identified.Cracks vertical to pre-existing fissures occur in specimens under uniaxial compression,whereas cracks parallel to pre-existing fissures occur under triaxial compression.Finally,X-ray micro-computed tomography(CT)observations are conducted to analyze the internal damage mechanism of sandstone specimens with respect to various fissure angles.Reconstructed 3-D CT images indicate obvious effects of confining pressure and fissure angle on the crack system of sandstone specimens.This research elucidates the fundamental nature of rock failure under triaxial compression.展开更多
A large number of mines are closed or abandoned every year in China.Geothermal utilization is one of the important ways to efficiently reuse underground resources in abandoned mines.How to calculate the volume and dis...A large number of mines are closed or abandoned every year in China.Geothermal utilization is one of the important ways to efficiently reuse underground resources in abandoned mines.How to calculate the volume and distribution of underground water storage space is the key to accurately evaluate the sustainable geothermal production in abandoned mines.In this paper,according to the multi-scale characteristics of the underground space in abandoned mine,the flow and heat transfer equations in the multi-scale space are sorted out systematically,and the calculation methods of different secondary space volumes are derived in detail.Taking Jiahe abandoned mine as the background,the volume and distribution of underground secondary space are calculated,and three heat storage evaluation models considering different water storage spaces are established by using COMSOL.The simulation results show that there are great differences among different models,and the results of the equivalent porous media model considering the multi-scale space are most consistent with the reality.Sensitivity analyses of key parameters model results indicated that the heat production is closely related to not only the recharge flow rate but also the recharge temperature and operating time.Furthermore,the energy saving and emission reduction benefits of geothermal utilization in abandoned mines are calculated,the results show that geothermal utilization of abandoned mines can effectively reduce energy consumption and CO_(2)emissions,and it has great economic benefits.展开更多
Hyper-gravity experiment enable the acceleration of the long-term transport of contaminants through fractured geological barriers.However,the hyper-gravity effect of the solute transport in fractures are not well unde...Hyper-gravity experiment enable the acceleration of the long-term transport of contaminants through fractured geological barriers.However,the hyper-gravity effect of the solute transport in fractures are not well understood.In this study,the sealed control apparatus and the 3D printed fracture models were used to carry out 1 g and N g hyper-gravity experiments.The results show that the breakthrough curves for the 1 g and N g experiments were almost the same.The differences in the flow velocity and the fitted hydrodynamic dispersion coefficient were 0.97–3.12%and 9.09–20.4%,indicating that the internal fractures of the 3D printed fracture models remained stable under hyper-gravity,and the differences in the flow and solute transport characteristics were acceptable.A method for evaluating the long-term barrier performance of low-permeability fractured rocks was proposed based on the hyper-gravity experiment.The solute transport processes in the 1 g prototype,1 g scaled model,and N g scaled model were simulated by the OpenGeoSys(OGS)software.The results show that the N g scaled model can reproduce the flow and solute transport processes in the 1 g prototype without considering the micro-scale heterogeneity if the Reynolds number(Re)critical Reynolds number(Recr)and the Peclet number(Pe)the critical Peclet number(Pecr).This insight is valuable for carrying out hyper-gravity experiments to evaluate the long-term barrier performance of low-permeability fractured porous rock.展开更多
Fractures widely exist in crustal rocks and form complex networks dominating the bulk behaviour of geological media.Thus,understanding how fracture networks affect subsurface processes/phenomena is highly relevant to ...Fractures widely exist in crustal rocks and form complex networks dominating the bulk behaviour of geological media.Thus,understanding how fracture networks affect subsurface processes/phenomena is highly relevant to many rock engineering applications.However,the large-scale behaviour of a fractured rock mass consisting of numerous fractures and rocks cannot be predicted by simple applications of the knowledge of individual fractures and/or rocks,due to upscaling complexities involving the hierarchy of scales,heterogeneities,and physical mechanisms as well as the possible emergence of qualitatively different macroscopic properties.In other words,macroscopic phenomena in fractured rocks arise from the many-body effects(i.e.collective behaviour)of numerous interacting fractures and rocks,such that the emergent properties at the fracture system scale are much richer than those of individual components.Hence,more is different!This paper gives a discussion on the mechanism of emergence in fractured media from a combined statistical physics and rock mechanics perspective,and further presents a multiscale conceptual framework to link microscopic responses of single fractures/rocks to macroscopic behaviour of rock masses consisting of many fractures and rocks.This framework can serve as a useful tool to bridge experimentally-established constitutive relationships of fracture/rock samples at the laboratory scale to phenomenologically-observed macroscopic properties of fractured rock masses at the site scale.展开更多
As rainfall infiltrates into soil slopes,the hydraulic and mechanical behaviors of soils are interacted.In this study,an efficient probabilistic parameter estimation method for coupled hydro-mechanical behavior in soi...As rainfall infiltrates into soil slopes,the hydraulic and mechanical behaviors of soils are interacted.In this study,an efficient probabilistic parameter estimation method for coupled hydro-mechanical behavior in soil slope is proposed.This method integrates the Polynomial Chaos Expansion(PCE)method,the coupled hydro-mechanical modeling,and the Bayesian learning method.A coupled hydro-mechanical numerical model is established for the simulation of behaviors of unsaturated soil slope under rainfall infiltration,following by training a cheap-to-run PCE surrogate to replace it.Probabilistic estimation of soil parameters is conducted based on the Bayesian learning technique with the Markov Chain Monte Carlo(MCMC)simulation.A numerical example of an unsaturated slope under rainfall infiltration is presented to illustrate the proposed method.The effects of measurement durations and response types on parameter estimation are addressed.The result shows that with the increase of measurement duration,the uncertainties of soil parameters are significantly reduced.The uncertainties of hydraulic properties are reduced significantly using the pore water pressure data,while the uncertainties of soil strength parameters are reduced greatly using the measured displacement data.展开更多
Safe disposal of high-level radioactive nuclear waste(HLW)is crucial for human health and the environment,as well as for sustainable development.Deep geological disposal in sparsely fractured crystalline rock is consi...Safe disposal of high-level radioactive nuclear waste(HLW)is crucial for human health and the environment,as well as for sustainable development.Deep geological disposal in sparsely fractured crystalline rock is considered one of the most favorable methods for final disposal of HLW.Extensive research has been conducted worldwide and many countries have initiated their own national development programs for deep geological disposal.Significant advancements of national programs for deep geological disposal of HLW in crystalline rock have been achieved in Sweden and Finland,which are currently under site development stage,focusing on detailed site characterization,repository construction,and post-closure safety analysis.Continued research and development remain important in the site development stage to ensure long-term safety of the HLW disposal repository.This work presents an overview and discussion of the progress as well as remaining open scientific issues and possibilities related to site development for safe disposal of HLW in crystalline rock.We emphasize that developing a comprehensive and convergent understanding of the coupled thermal,hydraulic,mechanical,chemical and biological(THMCB)processes in fractured crystalline rock remains the most important yet challenging topic for future studies towards safe disposal of HLW in crystalline rock.Advancements in laboratory facilities/techniques and computational models,as well as available comprehensive field data from site developments,provide new opportunities to enhance our understanding of the coupled processes and thereby repository design for safe geological disposal of HLW in crystalline rock.展开更多
The fault is potentially vulnerability's geological structure in the working face and its vicinity,and it is also a crucial geological factor affecting coal mine safety exploitation.To investigate the unstable fai...The fault is potentially vulnerability's geological structure in the working face and its vicinity,and it is also a crucial geological factor affecting coal mine safety exploitation.To investigate the unstable failure of surrounding rock induced by fault activation under the influence of adoption,which was studied utilizing field case and numerical analysis for the deformation and failure process of surrounding rock near the fault-affected zone.Combined with field cases,this paper analyzes disturbance stress and roof abscission layer monitoring in effecting zones of fault activation.Using the discrete element 3DEC numerical analysis method,the model of surrounding rock unstable fracture induced by fault activation under adoption is established.The unstable fracture and stress variation characteristics of surrounding rock induced by fault activation during the excavation of the upper side wall and lower side wall of the faults are simulated and analyzed.Field analysis shows that as the coal working face continues to advance,the mining stress gradually increases.There is a zigzag wave on the relationship curve between coal mining and roof displacement near the fault,which reveals that the surrounding rock of the fault activation affected zone is in the superposition state of static load and dynamic load.Furthermore,the simulation results show that the stress and displacement of surrounding rock near the fault increase with the advance of coal mining face.The closer to the fault plane,the displacement gradually returns to zero,and the stress is also in a lower state.展开更多
Flow-through experiments were conducted with three permeants to determine the effect of pH,temperature,and cation concentrations on changes in permeability.Granite with a single fracture was used for each sample.Chang...Flow-through experiments were conducted with three permeants to determine the effect of pH,temperature,and cation concentrations on changes in permeability.Granite with a single fracture was used for each sample.Changes in the permeant concentrations due to pressure dissolution,free-face dissolution,and precipitation were identified by measuring the element concentrations before and after the experiments.In addition,the mineral transformation was analyzed by SEM-EDX.The results of the flow-through experiments showed a reduction in permeability in almost all the samples.This decrease in permeability may have been caused by the interaction between pressure dissolution and free-face dissolution,which occurred in the high pH water experiment,or between pressure dissolution and precipitation,which occurred in the saturated mineral water and simulated seawater experiments.When pressure dissolution and free-face dissolution occurred in the samples,the pH and temperature were seen to greatly affect the decrease in permeability,namely,the permeability decreased significantly with increasing pH and temperature.This remarkable decrease in permeability could have taken place because the dissolution rate constant of the mineral increased with the increasing pH and temperature.Moreover,when pressure dissolution and precipitation occurred in the samples,the cation concentrations and temperature were seen to greatly affect the changes in permeability,namely,the permeability decreased significantly with increasing cation concentrations and decreasing temperature.展开更多
The failure of rocks is a complicated process as the mechanical properties of the rock are governed by loading history and cumulative ruptures.The geometric aspects of fractures,such as the size and shape of the fract...The failure of rocks is a complicated process as the mechanical properties of the rock are governed by loading history and cumulative ruptures.The geometric aspects of fractures,such as the size and shape of the fractures,the spatial distribution of the fracture networks,and the relations among these aspects also depend on the loads acting on rock mass.In general,the fractures are randomly generated in space which is difficult to be described using mathematical methods.In this paper,the failure processes of rock have been analyzed using the percolation theory.The results indicate that the failure process of rock is a transition from a stable state to an unstable state.This phenomenon is essentially consistent with the phase transition in the percolation theory.Based on this consistency,a theoretical model of percolation for earthquake prediction is proposed.A large number of seismic data provided strong evidence in support of the reliability and applicability of this model.展开更多
This study analyzed the mechanical and thermal properties of various rock types found in South Korea.The results showed that both igneous and metamorphic rocks possess higher strength compared to sedimentary rocks.The...This study analyzed the mechanical and thermal properties of various rock types found in South Korea.The results showed that both igneous and metamorphic rocks possess higher strength compared to sedimentary rocks.The Young's modulus of rocks is dependent on the extent of weathering they have undergone.The average cohesion of granites was found to be relatively higher compared to other rock types,and their friction angle also exhibited a relatively high value with a considerable variance.The results of uniaxial compression strength testing with respect to depth revealed that rock strength generally increased with depth,however,there was a large variance in strength distribution in each depth interval.Based on these findings,it can be concluded that South Korea can also secure HLW disposal sites and facilities in terms of rock mechanics by using crystalline rocks,similar to countries such as Sweden and Finland where disposal facilities are being built.Regarding the thermal properties of rocks,they are influenced by the distribution of the parent rock.The thermal conductivity is highly concentrated in the southwest and central regions of South Korea,while the geothermal gradient is high in the northeast,west,and some parts of the southeast regions.The southeast region of Korea has a high geothermal heat flow,and some central northern regions also exhibit relatively high geothermal heat flow.In light of these distributional characteristics,it is crucial to continue conducting precise studies on the mechanical and thermal properties of rocks in the future disposal depths of spent nuclear fuel.展开更多
Enhanced geothermal systems(EGSs)in this study are classified as fracturing-EGS(F-EGS),pipe-EGS(P-EGS)and excavation-EGS(E-EGS)according to reservoir stimulation strategies.However,the heat extraction performances of ...Enhanced geothermal systems(EGSs)in this study are classified as fracturing-EGS(F-EGS),pipe-EGS(P-EGS)and excavation-EGS(E-EGS)according to reservoir stimulation strategies.However,the heat extraction performances of three EGSs employing different stimulation strategies are not fully understood.Here,we define the region where the pore pressure increment calculated by a hydraulic fracturing process is higher than rock tensile strength as the stimulation region for establishing a more accurate F-EGS model,and then compare three geothermal systems to select a appropriate reservoir stimulation strategy.We find that the F-EGS model assuming an entire stimulated region significantly exaggerates the heat extraction results.The optimal conditions for P-EGS are low injection rates and short operation times,which is suiTablefor seasonal heating or multi-energy co-generation projects including a thermal recovery phase.Theoretically,E-EGS has better geothermal extraction performance than F-EGS based on existing model assumptions,but its construction feasibility and economics need further exploration.H2O is more suiTableas a heat exchange fluid in E-EGS than supercritical CO_(2).This study provides a reference for geothermal mining simulation and reservoir stimulation strategy selection.展开更多
Experimental rock mechanics testing provides a controlled and effective method for measuring physical properties,their dependencies,and their evolution due to the addition of localized microcracks.To understand the co...Experimental rock mechanics testing provides a controlled and effective method for measuring physical properties,their dependencies,and their evolution due to the addition of localized microcracks.To understand the contributions of microcracks to first order changes in compliance,the behavior of initial undamaged properties of a material should be comprehensively investigated as a function of stress,load path,and load history.We perform a comprehensive study of elastic properties and their dependence on a variety of materials exhibiting nonlinearity,and varying levels of anisotropy in elastic stiffnesses.We programmatically perturb the testing environment of the specimens under triaxial stresses.Elastic moduli are measured within each test,and along multiple discrete loading paths for multistage tests as a function of stress,focusing on a set launch point.Four single stage triaxial tests per rock type are performed to calculate Mohr-Coulomb failure criteria,and ultrasonic velocities are captured during compression for establishing the upper bound of elastic behavior.Shear wave velocity for granite experiences a maximum value at a lower differential stress than maximum volumetric strain.Sandstone displays a similar trend at the highest confining pressure,while these two maxima converge under the lowest confining pressure.展开更多
The aim of the present research was to establish a case study for the prediction of the unknown EDZ(Excavation Damaged Zone)distribution using a numerical analysis calibrated by replicating the trends in the EDZ obser...The aim of the present research was to establish a case study for the prediction of the unknown EDZ(Excavation Damaged Zone)distribution using a numerical analysis calibrated by replicating the trends in the EDZ observed from one of the representative underground research fields in Japan(Horonobe URL).In this study,a 2D numerical analysis using a damage model,which can determine rock deformation and fracturing simultaneously,is presented.It was calibrated to reproduce the excavation of the gallery at the Horonobe URL at a depth of 350 m.Simulated results show an excellent agreement with the extent of the measured EDZ and capture the failure modes of EDZ fractures suggested by the in-situ observations.Finally,the calibrated numerical analysis was used to realistically estimate the EDZ formation for the geological disposal of high-level radioactive waste(HLW)under the same environment as that of the above-mentioned galley at the Horonobe URL.Consequently,it was shown that the tensile/shear hybrid fractures dominantly constituted the EDZ and propagated to a maximum extent of about 0.3 m from the cavity wall during the cavity excavation for the HLW disposal.Overall,the calibrated numerical analysis and resulting estimations,targeted for the environment at the depth of 350 m at the Horonobe URL,where mudstone is located,should be useful for predicting the trends in the EDZ distribution expected in the implementation of HLW disposal projects under deep geological conditions,such as those that exist in Japan,which are dominated by sedimentary rocks,including mudstone。展开更多
基金greatly acknowledge the National Key Research and Development Program of China(No.2021YFA0716000)the China National Petroleum Corporation-Peking University Strategic Cooperation Project of Fundamental Research.
文摘The modeling of heat recovery from an enhanced geothermal system(EGS)requires rock thermal parameters as inputs such as thermal conductivity and specific heat capacity.These parameters may encounter significant variations due to the reduction of rock temperature during heat recovery.In the present study,we investigate the effect of temperature-dependent thermal conductivity and specific heat capacity on the thermal performance of EGS reservoirs.Equations describing the relationships between thermal conductivity/specific heat capacity and temperature from previous experimental studies were incorporated in a field-scale single-fracture EGS model.The modeling results indicate that the increase of thermal conductivity caused by temperature reduction accelerates thermal conduction from rock formations to fracture fluid,and thus improves thermal performance.The decrease of specific heat capacity due to temperature reduction,on the contrary,impairs the thermal performance but the impact is smaller than that of the increase of thermal conductivity.Due to the opposite effects of thermal conductivity increase and specific heat capacity decrease,the overall effect of temperature-dependent thermal parameters is relatively small.Assuming constant thermal parameters measured at room temperature appears to be able to provide acceptable predictions of EGS thermal performance.
基金substantially supported by the Shuguang Program from Shanghai Education Development Foundation and Shanghai Municipal Education Commission(19SG19)the Natural Science Foundation of China(42072302,52025094)the Fundamental Research Funds for the Central Universities.
文摘Traditionally,the factor of safety(FOS)is widely used to account for uncertainties in the design of slopes within the framework of working stress design.As the uncertainties involved in the design of slopes vary,the same FOS may correspond to the different levels of reliability.In this study,the advanced first order reliability method is used to determine the resistance factors for design of slopes in a homogenous soil layer.It is found that the resistance factors depend on the target reliability index,the height of the slope,and the variability of the soil strength parameters.It is difficult to suggest a unique set of resistance factors for design of slopes.Analytic solutions are developed to determine the resistance factors for design of slopes assuming the random variables are normally distributed.An approximate method based on the concept of equivalent target reliability index is also suggested to determine the resistance factors for design of the slope when the soil strength parameters are lognormally distributed.The method suggested in this paper provides a practical way to perform load and resistance factors design of slopes.
文摘A three-dimensional thermo-hydro-mechanical numerical model has recently been enhanced with thermal capabilities to study the response of geothermal reservoirs to stimulation and production.In this paper,we present an effort to consider three relevant thermal mechanisms in an existing lattice code initially designed for hydraulic fracturing:a)thermal advection in the fluid;b)heat transfer by forced convection from the rock to the fluid;and c)accurate thermal conduction in the rock matrix considering the thermal boundary layer effect.A numerical implementation of the new coupled advection-forced convection logic as well as the coupling with the existing conduction logic in the commercial code XSite is summarized.The numerical solution is compared to analytical solutions for simple simulation cases.The new simulation capability is applied in a large-scale geothermal example to illustrate its performance.
基金supported by the National Natural Science Foundation of China(Grant No.51878320).
文摘In response to the existing consolidation theory for stone column composite foundations which cannot consider the time-dependent loading and the well resistance effect of stone columns under time-dependent boundaries,a consolidation model that can reflect these characteristics is developed in this study,and the corresponding analytical solutions are obtained under permeable top surface with permeable bottom surface(PTPB)and permeable top surface with impermeable bottom surface(PTIB),respectively.In addition,the reliability of the proposed solutions is verified by comparing them with existing analytical solutions.Extensive calculations are then performed by the proposed solutions to analyze the consolidation behaviors of stone column composite foundations under time-dependent boundaries,the results show that the interface parameters have a large effect on the distribution of excess pore water pressure(EPWP)along the depth;for projects with longer construction time,the permeability of the top and bottom surfaces of the composite foundation has a smaller effect on the average consolidation rate.Finally,the proposed solution is applied to the settlement calculation in an actual engineering project,and the theoretical results show a general agreement with the measured data by considering the influence of the interface parameters.
基金support from the UK Natural Environment Research Council(NERC,NE/W004127/1).
文摘Although rock mechanical behaviour has a long record of study,attempts to understand the role of fractures on rock deformation still have unresolved issues.Due to technical and/or economic challenges,natural rock fractures are often dealt with crudely,without detailed consideration of fracture geometry and heterogeneity in many geoscience applications.Veined rocks that are ubiquitous in the upper Earth crust fall in that category where sustained efforts are needed to offer key information for rock mechanics and geomechanics applications.Following on from a recent study on the rupture of veined rocks(DOI:10.1029/2019JB019052),we further examine stress path constraints on the deformation of veined rocks(i.e.,stress-path-dependent behaviour of veined rocks)under polyaxial conditions.The Discrete Element Method is used to establish a calcite veined model where constant mean stress(σm)and constant least principal stress(σ3)paths that are representative in the subsurface activities are considered.The results reveal the stress-path dependency of brittleness for models under different loading paths.Models tested under constant-σm conditions exhibit no brittleness,compared to cases where constant-σ3 is applied.Sliding along the strike of an inclined vein is evident under constant-σm deformation,irrespective of the level of stress.Shear bands along the dominated(inclined)veins exhibit apparent particle trajectory anisotropy for the constant-σm deformations which is demonstrated by the evident colour contrast of the adjacent rock matrix and the displacement dispersion of the particles forming the shear bands.We envisage that the reactivation of veins is of relevance to Enhanced Geothermal Systems(EGS)development in terms of seismicity mitigation and multiphysics control of fracture and reservoir permeability.
基金funded by the National Natural Science Foundation of China(Grant Nos.42077252,42011530122)Natural Science Foundation of Shandong Province,China(Grant No.ZR2021QE069).
文摘Understanding the mechanical and hydraulic properties of fractured rocks and their coupled processes is of great significance for the exploration,design,construction,operation,and maintenance of many rock engineering projects such as hydropower development,oil and gas extraction,and underground waste disposal.With the rapid advancement of global and national strategies such as the“Paris Agreement”and the“Belt and Road Initiative”,more and more projects are developed in the complex geological environment with varying geological structures.Shear failure and rock instability are prone to occur in fractured rock masses under the coupled effects of high stress,high pore pressure,and engineering disturbance,which are main sources for engineering disasters such as roof collapse and caving,water and mud inrushes,and induced earthquakes.To solve these problems,extensive research on the coupled shear-flow behavior of fractures has been conducted.However,due to the complex mechanical,hydraulic and geometrical characteristics of single fractures and fracture networks,a large number of outstanding issues related to the impact of the coupled processes on the engineering characteristics of rock masses are still unsolved.The relevant experimental apparatuses and methods remain to be further developed.Therefore,in this review,we analyze and summarize the existing shear-flow experimental apparatuses,classify apparatus configurations,specimen shapes,and testing principles,and compare their advantages and disadvantages.We also summarize the main scientific findings obtained from various experimental apparatuses,aiming to provide a reference for developing new shear-flow experimental apparatuses and conducting related scientific research in the future.
基金supported by the National Natural Science Foundation of China(52108388)the science and technology innovation Program of Hunan Province(Project No.2021RC3015).
文摘The slope stability assessment is a classical problem in geotechnical engineering.This topic have attracted many researcher’s attention and various theoretical models for predicting critical slope heights or safety factors in the light of the limit equilibrium(LE)method and the kinematical approach of limit analysis(LA)method.Meanwhile,a large number of experimental studies have been conducted to check the slope stability.Using centrifuge testing results,this paper aims to employ Bayesian method to characterize the model uncertainties of the classical three-dimensional rotational failure mechanism proposed by Michalowski and Drescher(2009)to predict critical slope heights in frictional soils,by incorporating the test uncertainties and parameter uncertainties.The obtained results show that the LA three-dimensional rotational failure mechanism overestimates the critical slope height compared with the LE method,and the experimental observational uncertainty has negligible influences on the posterior statistics of model uncertainty.
基金supported by the Fundamental Research Funds for the Central Universities(2020ZDPYMS34).
文摘In actual rock engineering,fissures play an important role in determining the mechanical parameters of rock mass,whereas it is very difficult to construct fissures in cylindrical specimens.Therefore,the pre-fissured rectangular rock specimens were constructed innovatively.Moreover,a series of triaxial compression experimental results on the failure mechanical behavior of rectangular solid sandstone specimens containing a single fissure were reported.The lateral strain in different directions was monitored and the experimental results show that elastic modulus and axial strain increase non-linearly with confining pressure,and the average Poisson’s ratio parallel to fissure(μ2)is larger than that vertical to fissure(μ3).The cohesion,Hoek-Brown parameters of peak strength show similar trends with that of crack damage threshold to the fissure angle(α),and the parameters of the peak strength are larger than those of crack damage threshold.However,the internal friction angles of the peak strength and crack damage threshold are almost equal.Based on the geometries and properties of cracks,ten typical crack types are identified.Cracks vertical to pre-existing fissures occur in specimens under uniaxial compression,whereas cracks parallel to pre-existing fissures occur under triaxial compression.Finally,X-ray micro-computed tomography(CT)observations are conducted to analyze the internal damage mechanism of sandstone specimens with respect to various fissure angles.Reconstructed 3-D CT images indicate obvious effects of confining pressure and fissure angle on the crack system of sandstone specimens.This research elucidates the fundamental nature of rock failure under triaxial compression.
基金supported by the Beijing Natural Science Foundation(8212033)the Fundamental Research Funds for the Central Universities(2021JCCXLJ05)innovation fund research project(SKLGDUEK202221).
文摘A large number of mines are closed or abandoned every year in China.Geothermal utilization is one of the important ways to efficiently reuse underground resources in abandoned mines.How to calculate the volume and distribution of underground water storage space is the key to accurately evaluate the sustainable geothermal production in abandoned mines.In this paper,according to the multi-scale characteristics of the underground space in abandoned mine,the flow and heat transfer equations in the multi-scale space are sorted out systematically,and the calculation methods of different secondary space volumes are derived in detail.Taking Jiahe abandoned mine as the background,the volume and distribution of underground secondary space are calculated,and three heat storage evaluation models considering different water storage spaces are established by using COMSOL.The simulation results show that there are great differences among different models,and the results of the equivalent porous media model considering the multi-scale space are most consistent with the reality.Sensitivity analyses of key parameters model results indicated that the heat production is closely related to not only the recharge flow rate but also the recharge temperature and operating time.Furthermore,the energy saving and emission reduction benefits of geothermal utilization in abandoned mines are calculated,the results show that geothermal utilization of abandoned mines can effectively reduce energy consumption and CO_(2)emissions,and it has great economic benefits.
基金supported by the Basic Science Center Program for Multiphase Evolution in Hypergravity of the National Natural Science Foundation of China(No.51988101)the National Key Research and Development Project China(No.2018YFC1802300)+1 种基金the National Natural Science Foundation of China(No.42007262)the National Natural Science Foundation of China(No.42277128).
文摘Hyper-gravity experiment enable the acceleration of the long-term transport of contaminants through fractured geological barriers.However,the hyper-gravity effect of the solute transport in fractures are not well understood.In this study,the sealed control apparatus and the 3D printed fracture models were used to carry out 1 g and N g hyper-gravity experiments.The results show that the breakthrough curves for the 1 g and N g experiments were almost the same.The differences in the flow velocity and the fitted hydrodynamic dispersion coefficient were 0.97–3.12%and 9.09–20.4%,indicating that the internal fractures of the 3D printed fracture models remained stable under hyper-gravity,and the differences in the flow and solute transport characteristics were acceptable.A method for evaluating the long-term barrier performance of low-permeability fractured rocks was proposed based on the hyper-gravity experiment.The solute transport processes in the 1 g prototype,1 g scaled model,and N g scaled model were simulated by the OpenGeoSys(OGS)software.The results show that the N g scaled model can reproduce the flow and solute transport processes in the 1 g prototype without considering the micro-scale heterogeneity if the Reynolds number(Re)critical Reynolds number(Recr)and the Peclet number(Pe)the critical Peclet number(Pecr).This insight is valuable for carrying out hyper-gravity experiments to evaluate the long-term barrier performance of low-permeability fractured porous rock.
基金support from the Swiss National Science Foundation (Grant No.189882)the National Natural Science Foundation of China (Grant No.41961134032).
文摘Fractures widely exist in crustal rocks and form complex networks dominating the bulk behaviour of geological media.Thus,understanding how fracture networks affect subsurface processes/phenomena is highly relevant to many rock engineering applications.However,the large-scale behaviour of a fractured rock mass consisting of numerous fractures and rocks cannot be predicted by simple applications of the knowledge of individual fractures and/or rocks,due to upscaling complexities involving the hierarchy of scales,heterogeneities,and physical mechanisms as well as the possible emergence of qualitatively different macroscopic properties.In other words,macroscopic phenomena in fractured rocks arise from the many-body effects(i.e.collective behaviour)of numerous interacting fractures and rocks,such that the emergent properties at the fracture system scale are much richer than those of individual components.Hence,more is different!This paper gives a discussion on the mechanism of emergence in fractured media from a combined statistical physics and rock mechanics perspective,and further presents a multiscale conceptual framework to link microscopic responses of single fractures/rocks to macroscopic behaviour of rock masses consisting of many fractures and rocks.This framework can serve as a useful tool to bridge experimentally-established constitutive relationships of fracture/rock samples at the laboratory scale to phenomenologically-observed macroscopic properties of fractured rock masses at the site scale.
基金supported by the National Key Research and Development Program of China(2021YFB2600700)the National Natural Science Foundation of China(Project Nos.52025094,52088102,51979158)+1 种基金support from Shanghai Municipal Education Commission(Project No.2021-01-07-00-02-E00089)Key Projects for Intergovernmental Cooperation in International Science,Technology and Innovation(Grant No.2018YFE0125100).
文摘As rainfall infiltrates into soil slopes,the hydraulic and mechanical behaviors of soils are interacted.In this study,an efficient probabilistic parameter estimation method for coupled hydro-mechanical behavior in soil slope is proposed.This method integrates the Polynomial Chaos Expansion(PCE)method,the coupled hydro-mechanical modeling,and the Bayesian learning method.A coupled hydro-mechanical numerical model is established for the simulation of behaviors of unsaturated soil slope under rainfall infiltration,following by training a cheap-to-run PCE surrogate to replace it.Probabilistic estimation of soil parameters is conducted based on the Bayesian learning technique with the Markov Chain Monte Carlo(MCMC)simulation.A numerical example of an unsaturated slope under rainfall infiltration is presented to illustrate the proposed method.The effects of measurement durations and response types on parameter estimation are addressed.The result shows that with the increase of measurement duration,the uncertainties of soil parameters are significantly reduced.The uncertainties of hydraulic properties are reduced significantly using the pore water pressure data,while the uncertainties of soil strength parameters are reduced greatly using the measured displacement data.
文摘Safe disposal of high-level radioactive nuclear waste(HLW)is crucial for human health and the environment,as well as for sustainable development.Deep geological disposal in sparsely fractured crystalline rock is considered one of the most favorable methods for final disposal of HLW.Extensive research has been conducted worldwide and many countries have initiated their own national development programs for deep geological disposal.Significant advancements of national programs for deep geological disposal of HLW in crystalline rock have been achieved in Sweden and Finland,which are currently under site development stage,focusing on detailed site characterization,repository construction,and post-closure safety analysis.Continued research and development remain important in the site development stage to ensure long-term safety of the HLW disposal repository.This work presents an overview and discussion of the progress as well as remaining open scientific issues and possibilities related to site development for safe disposal of HLW in crystalline rock.We emphasize that developing a comprehensive and convergent understanding of the coupled thermal,hydraulic,mechanical,chemical and biological(THMCB)processes in fractured crystalline rock remains the most important yet challenging topic for future studies towards safe disposal of HLW in crystalline rock.Advancements in laboratory facilities/techniques and computational models,as well as available comprehensive field data from site developments,provide new opportunities to enhance our understanding of the coupled processes and thereby repository design for safe geological disposal of HLW in crystalline rock.
基金supported by the National key Research and development program for young scientists(2021YF2900400)Supported by Youth Foundation of National Natural Science Foundation of China(52104077)Major collaborative innovation project of Guizhou's mineral prospecting breakthrough strategic action[2022]ZD001-02-02,which are all gratefully appreciated.
文摘The fault is potentially vulnerability's geological structure in the working face and its vicinity,and it is also a crucial geological factor affecting coal mine safety exploitation.To investigate the unstable failure of surrounding rock induced by fault activation under the influence of adoption,which was studied utilizing field case and numerical analysis for the deformation and failure process of surrounding rock near the fault-affected zone.Combined with field cases,this paper analyzes disturbance stress and roof abscission layer monitoring in effecting zones of fault activation.Using the discrete element 3DEC numerical analysis method,the model of surrounding rock unstable fracture induced by fault activation under adoption is established.The unstable fracture and stress variation characteristics of surrounding rock induced by fault activation during the excavation of the upper side wall and lower side wall of the faults are simulated and analyzed.Field analysis shows that as the coal working face continues to advance,the mining stress gradually increases.There is a zigzag wave on the relationship curve between coal mining and roof displacement near the fault,which reveals that the surrounding rock of the fault activation affected zone is in the superposition state of static load and dynamic load.Furthermore,the simulation results show that the stress and displacement of surrounding rock near the fault increase with the advance of coal mining face.The closer to the fault plane,the displacement gradually returns to zero,and the stress is also in a lower state.
基金supported by JSPS KAKENHI Grant Numbers 26249139 and 19H02237.
文摘Flow-through experiments were conducted with three permeants to determine the effect of pH,temperature,and cation concentrations on changes in permeability.Granite with a single fracture was used for each sample.Changes in the permeant concentrations due to pressure dissolution,free-face dissolution,and precipitation were identified by measuring the element concentrations before and after the experiments.In addition,the mineral transformation was analyzed by SEM-EDX.The results of the flow-through experiments showed a reduction in permeability in almost all the samples.This decrease in permeability may have been caused by the interaction between pressure dissolution and free-face dissolution,which occurred in the high pH water experiment,or between pressure dissolution and precipitation,which occurred in the saturated mineral water and simulated seawater experiments.When pressure dissolution and free-face dissolution occurred in the samples,the pH and temperature were seen to greatly affect the decrease in permeability,namely,the permeability decreased significantly with increasing pH and temperature.This remarkable decrease in permeability could have taken place because the dissolution rate constant of the mineral increased with the increasing pH and temperature.Moreover,when pressure dissolution and precipitation occurred in the samples,the cation concentrations and temperature were seen to greatly affect the changes in permeability,namely,the permeability decreased significantly with increasing cation concentrations and decreasing temperature.
基金financial support was provided by National Natural Science Foundation of China(Grant No.52122405)provided by Shanxi major research program for science and technology(Grant No.202101060301024).
文摘The failure of rocks is a complicated process as the mechanical properties of the rock are governed by loading history and cumulative ruptures.The geometric aspects of fractures,such as the size and shape of the fractures,the spatial distribution of the fracture networks,and the relations among these aspects also depend on the loads acting on rock mass.In general,the fractures are randomly generated in space which is difficult to be described using mathematical methods.In this paper,the failure processes of rock have been analyzed using the percolation theory.The results indicate that the failure process of rock is a transition from a stable state to an unstable state.This phenomenon is essentially consistent with the phase transition in the percolation theory.Based on this consistency,a theoretical model of percolation for earthquake prediction is proposed.A large number of seismic data provided strong evidence in support of the reliability and applicability of this model.
基金supported by the Institute for Korea Spent Nuclear Fuel(iKSNF)and Korea Foundation of Nuclear Safety(KOFONS)funded by the Korea government(Nuclear Safety and Security Commission,NSSC)(No.2109092-0222-SB110).
文摘This study analyzed the mechanical and thermal properties of various rock types found in South Korea.The results showed that both igneous and metamorphic rocks possess higher strength compared to sedimentary rocks.The Young's modulus of rocks is dependent on the extent of weathering they have undergone.The average cohesion of granites was found to be relatively higher compared to other rock types,and their friction angle also exhibited a relatively high value with a considerable variance.The results of uniaxial compression strength testing with respect to depth revealed that rock strength generally increased with depth,however,there was a large variance in strength distribution in each depth interval.Based on these findings,it can be concluded that South Korea can also secure HLW disposal sites and facilities in terms of rock mechanics by using crystalline rocks,similar to countries such as Sweden and Finland where disposal facilities are being built.Regarding the thermal properties of rocks,they are influenced by the distribution of the parent rock.The thermal conductivity is highly concentrated in the southwest and central regions of South Korea,while the geothermal gradient is high in the northeast,west,and some parts of the southeast regions.The southeast region of Korea has a high geothermal heat flow,and some central northern regions also exhibit relatively high geothermal heat flow.In light of these distributional characteristics,it is crucial to continue conducting precise studies on the mechanical and thermal properties of rocks in the future disposal depths of spent nuclear fuel.
文摘Enhanced geothermal systems(EGSs)in this study are classified as fracturing-EGS(F-EGS),pipe-EGS(P-EGS)and excavation-EGS(E-EGS)according to reservoir stimulation strategies.However,the heat extraction performances of three EGSs employing different stimulation strategies are not fully understood.Here,we define the region where the pore pressure increment calculated by a hydraulic fracturing process is higher than rock tensile strength as the stimulation region for establishing a more accurate F-EGS model,and then compare three geothermal systems to select a appropriate reservoir stimulation strategy.We find that the F-EGS model assuming an entire stimulated region significantly exaggerates the heat extraction results.The optimal conditions for P-EGS are low injection rates and short operation times,which is suiTablefor seasonal heating or multi-energy co-generation projects including a thermal recovery phase.Theoretically,E-EGS has better geothermal extraction performance than F-EGS based on existing model assumptions,but its construction feasibility and economics need further exploration.H2O is more suiTableas a heat exchange fluid in E-EGS than supercritical CO_(2).This study provides a reference for geothermal mining simulation and reservoir stimulation strategy selection.
文摘Experimental rock mechanics testing provides a controlled and effective method for measuring physical properties,their dependencies,and their evolution due to the addition of localized microcracks.To understand the contributions of microcracks to first order changes in compliance,the behavior of initial undamaged properties of a material should be comprehensively investigated as a function of stress,load path,and load history.We perform a comprehensive study of elastic properties and their dependence on a variety of materials exhibiting nonlinearity,and varying levels of anisotropy in elastic stiffnesses.We programmatically perturb the testing environment of the specimens under triaxial stresses.Elastic moduli are measured within each test,and along multiple discrete loading paths for multistage tests as a function of stress,focusing on a set launch point.Four single stage triaxial tests per rock type are performed to calculate Mohr-Coulomb failure criteria,and ultrasonic velocities are captured during compression for establishing the upper bound of elastic behavior.Shear wave velocity for granite experiences a maximum value at a lower differential stress than maximum volumetric strain.Sandstone displays a similar trend at the highest confining pressure,while these two maxima converge under the lowest confining pressure.
基金supported by the JST FOREST Program(Grant no.JPMJFR216Y,Japan)JSPS KAKENHI(Grant nos.20K14826,22H01589,and 22H00229).
文摘The aim of the present research was to establish a case study for the prediction of the unknown EDZ(Excavation Damaged Zone)distribution using a numerical analysis calibrated by replicating the trends in the EDZ observed from one of the representative underground research fields in Japan(Horonobe URL).In this study,a 2D numerical analysis using a damage model,which can determine rock deformation and fracturing simultaneously,is presented.It was calibrated to reproduce the excavation of the gallery at the Horonobe URL at a depth of 350 m.Simulated results show an excellent agreement with the extent of the measured EDZ and capture the failure modes of EDZ fractures suggested by the in-situ observations.Finally,the calibrated numerical analysis was used to realistically estimate the EDZ formation for the geological disposal of high-level radioactive waste(HLW)under the same environment as that of the above-mentioned galley at the Horonobe URL.Consequently,it was shown that the tensile/shear hybrid fractures dominantly constituted the EDZ and propagated to a maximum extent of about 0.3 m from the cavity wall during the cavity excavation for the HLW disposal.Overall,the calibrated numerical analysis and resulting estimations,targeted for the environment at the depth of 350 m at the Horonobe URL,where mudstone is located,should be useful for predicting the trends in the EDZ distribution expected in the implementation of HLW disposal projects under deep geological conditions,such as those that exist in Japan,which are dominated by sedimentary rocks,including mudstone。
