The mechanical behavior of coal is the key factor affecting underground coal mining and coalbed methane extraction.In this study,triaxial compression and seepage tests were carried out on coal at different gas pressur...The mechanical behavior of coal is the key factor affecting underground coal mining and coalbed methane extraction.In this study,triaxial compression and seepage tests were carried out on coal at different gas pressures.The mechanical properties and failure process of coal were studied,as well as the acoustic emission(AE)and strain energy.The influence of gas pressure on the mechanical parameters of this coal was analyzed.Based on the conventional energy calculation formula,the pore pressure was introduced through the effective stress formula,and each energy component of coal containing gas was refined innovatively.The contribution of gas pressure to the total energy input and dissipation during loading was quantitatively described.Finally,the influence of gas pressure on coal strength was theo-retically analyzed from the perspectives of MohreCoulomb criterion and fracture mechanics.The results show that the total absorbed energy comprises the absorbed energy in the axial pressure direction(positive)and in the confining pressure direction(negative),as well as that induced by the pore pressure(initially negative and then positive).The absorbed energy in the axial pressure direction accounts for the main proportion of the total energy absorbed by coal.The quiet period of AE in the initial stage shortens,and AE activity increases during the pre-peak stage under high gas pressure.The fractal characteristics of AE in three stages are studied using the correlation dimension.The AE process has different forms of self-similarity in various deformation stages.展开更多
Based on experimental restilts of brittle, intact sandstone under uniaxial compression, the micro-parameters were firstly confirmed by adopting particle flow code (PFC2D). Then, the validation of the simulated model...Based on experimental restilts of brittle, intact sandstone under uniaxial compression, the micro-parameters were firstly confirmed by adopting particle flow code (PFC2D). Then, the validation of the simulated models were cross checked with the experimental results of brittle sandstone containing three parallel fissures under uniaxial compression. The simulated results agreed very well with the experimental results, including the peak strength, peak axial strain, and ultimate failure mode. Using the same micro- parameters, the numerical models containing a new geometry of three fissures are constructed to investigate the fissure angle on the fracture mechanical behavior of brittle sandstone under uniaxial compression. The strength and deformation parameters of brittle sandstone containing new three fissures are dependent to the fissure angle. With the increase of the fis- sure angle, the elastic modulus, the crack damage threshold, and the peak strength of brittle sandstone containing three fissures firstly increase and secondly decrease. But the peak axial strain is nonlinearly related to the fissure angle. In the entire process of deformation, the crack initiation and propagation behavior of brittle sandstone containing three fissures under uniaxial compression are investigated with respect to the fissure angle. Six different crack coalescence modes are identified for brittle sandstone containing three fissures under uniaxial compression. The influence of the fissure angle on the length of crack propagation and crack coalescence stress is evaluated. These investigated conclusions are very important for ensuring the stability and safety of rock engineering with intermittent structures.展开更多
The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood.Although laboratory creep tests have been carried out to determine the creep deformation of various rocks,thes...The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood.Although laboratory creep tests have been carried out to determine the creep deformation of various rocks,these tests are expensive and time-consuming.Nanoindentation creep tests,as an alternative method,can be performed to investigate the mechanical and viscoelastic properties of granite samples.In this study,the reduced Young’s modulus,hardness,fracture toughness,creep strain rate,stress exponent,activation volume and maximum creep displacement of common rock-forming minerals of granite were calculated from nanoindentation results.It was found that the hardness decreases with the increase of holding time and the initial decrease in hardness was swift,and then it decreased slowly.The stress exponent values obtained were in the range from 4.5 to 22.9,which indicates that dislocation climb is the creep deformation mechanism.In addition,fracture toughness of granite’s rock-forming minerals was calculated using energy-based method and homogenization method was adopted to upscale the micro-scale mechanical properties to macro-scale mechanical properties.Last but not least,both three-element Voigt model and Burgers model fit the nanoindentation creep curves well.This study is beneficial to the understanding of the long-term mechanical properties of rock samples from a microscale perspective,which is of great significance to the understanding of localized deformation processes of rocks.展开更多
Deep coal seams are one of the world’s most widespread deposits for carbon dioxide(C02)disposal and are generally located near large point sources of CO_(2)emissions.The injection of CO_(2)into coal seams has great p...Deep coal seams are one of the world’s most widespread deposits for carbon dioxide(C02)disposal and are generally located near large point sources of CO_(2)emissions.The injection of CO_(2)into coal seams has great potential to sequester CO_(2)while simultaneously enhancing coalbed methane(CO_(2)-ECBM)recovery.Pilot tests of CO_(2)-ECBM have been conducted in coal seams worldwide with favorable early results.However,one of the main technical barriers in coal seams needs to be resolved:Injecting CO_(2)reduces coal permeability and well injectivity.Here,using in situ synchrotron X-ray microtomography,we provide the first observational evidence that injecting nitrogen(N_(2))can reverse much of this lost permeability by reopening fractures that have closed due to coal swelling induced by CO_(2)adsorption.Our findings support the notion that injecting minimally treated flue gas-a mixture of mainly N_(2) and CO_(2)-is an attractive alternative for ECBM recovery instead of pure CO_(2)injection in deep coal seams.Firstly,flue gas produced by power plants could be directly injected after particulate removal,thus avoiding high CO_(2)-separation costs.Secondly,the presence of N_(2)makes it possible to maintain a sufficiently high level of coal permeability.These results suggest that flue-gas ECBM for deep coal seams may provide a promising path toward net-zero emissions from coal mines.展开更多
To investigate the deformation mechanisms of rock under hydrostatic stress, destructive experiments were conducted on sandstone under different levels of hydrostatic stress and stress Lode angles. The results reveal t...To investigate the deformation mechanisms of rock under hydrostatic stress, destructive experiments were conducted on sandstone under different levels of hydrostatic stress and stress Lode angles. The results reveal that the shape of the strength envelope on the π plane gradually changes from the shape of the Lade criterion to the shape of the Drucker-Prage criterion with an increase in hydrostatic stress.Normally, there exists a deviation between the strain and stress paths for porous rocks on the π plane,and the deviation decreases with an increase in stress Lode angle and hydrostatic stress. A rock failure hypothesis based on the rock porous structure was proposed to investigate the reasons for the abovementioned phenomena. It was found that the shear expansion in the minimum principal stress direction is the dominant factor affecting the Lode angle effect(LAE);the magnitude of the hydrostatic stress induces the variation of the porous structure and influences the shear expansion. Therefore, the hydrostatic stress state affects the LAE. The failure hypothesis proposed in this paper can clarify the hydrostatic stress effect, LAE, and the variation of the rock strength envelope shape.展开更多
The brittleness index plays a significant role in the hydraulic fracturing design and wellbore stability analysis of shale reservoirs.Various brittleness indices have been proposed to characterize the brittleness of s...The brittleness index plays a significant role in the hydraulic fracturing design and wellbore stability analysis of shale reservoirs.Various brittleness indices have been proposed to characterize the brittleness of shale rocks,but almost all of them ignored the anisotropy of the brittleness index.Therefore,uniaxial compression testing integrated with geophysical logging was used to provide insights into the anisotropy of the brittleness index for Longmaxi shale,the presented method was utilized to assess brittleness index of Longmaxi shale formation for the interval of 3155e3175 m in CW-1 well.The results indicated that the brittleness index of Longmaxi shale showed a distinct anisotropy,and it achieved the minimum value at β=45°-60°.As the bedding angle increased,the observed brittleness index(BI_(2_β))decreased firstly and increased then,it achieved the lowest value at β=40°-60°,and it is consistent with the uniaxial compression testing results.Compared to the isotropic brittleness index(β=0°),the deviation of the anisotropic brittleness index ranged from 10%to 66.7%,in other words,the anisotropy of brittleness index cannot be ignored for Longmaxi shale.Organic matter content is one of the main intrinsic causes of shale anisotropy,and the anisotropy degree of the brittleness index generally increases with the increase in organic matter content.The present work is valuable for the assessment of anisotropic brittleness for hydraulic fracturing design and wellbore stability analysis.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
基金This study is sponsored by the National Natural Science Foun-dation of China(Grant No.12002270)the China Postdoctoral Science Foundation(Grant Nos.2021T140553 and 2021M692600).
文摘The mechanical behavior of coal is the key factor affecting underground coal mining and coalbed methane extraction.In this study,triaxial compression and seepage tests were carried out on coal at different gas pressures.The mechanical properties and failure process of coal were studied,as well as the acoustic emission(AE)and strain energy.The influence of gas pressure on the mechanical parameters of this coal was analyzed.Based on the conventional energy calculation formula,the pore pressure was introduced through the effective stress formula,and each energy component of coal containing gas was refined innovatively.The contribution of gas pressure to the total energy input and dissipation during loading was quantitatively described.Finally,the influence of gas pressure on coal strength was theo-retically analyzed from the perspectives of MohreCoulomb criterion and fracture mechanics.The results show that the total absorbed energy comprises the absorbed energy in the axial pressure direction(positive)and in the confining pressure direction(negative),as well as that induced by the pore pressure(initially negative and then positive).The absorbed energy in the axial pressure direction accounts for the main proportion of the total energy absorbed by coal.The quiet period of AE in the initial stage shortens,and AE activity increases during the pre-peak stage under high gas pressure.The fractal characteristics of AE in three stages are studied using the correlation dimension.The AE process has different forms of self-similarity in various deformation stages.
基金supported by the Fundamental Research Funds for the Central Universities (China University of Mining and Technology) (Grant 2014YC10)the National Basic Research 973 Program of China (Grant 2014CB046905)
文摘Based on experimental restilts of brittle, intact sandstone under uniaxial compression, the micro-parameters were firstly confirmed by adopting particle flow code (PFC2D). Then, the validation of the simulated models were cross checked with the experimental results of brittle sandstone containing three parallel fissures under uniaxial compression. The simulated results agreed very well with the experimental results, including the peak strength, peak axial strain, and ultimate failure mode. Using the same micro- parameters, the numerical models containing a new geometry of three fissures are constructed to investigate the fissure angle on the fracture mechanical behavior of brittle sandstone under uniaxial compression. The strength and deformation parameters of brittle sandstone containing new three fissures are dependent to the fissure angle. With the increase of the fis- sure angle, the elastic modulus, the crack damage threshold, and the peak strength of brittle sandstone containing three fissures firstly increase and secondly decrease. But the peak axial strain is nonlinearly related to the fissure angle. In the entire process of deformation, the crack initiation and propagation behavior of brittle sandstone containing three fissures under uniaxial compression are investigated with respect to the fissure angle. Six different crack coalescence modes are identified for brittle sandstone containing three fissures under uniaxial compression. The influence of the fissure angle on the length of crack propagation and crack coalescence stress is evaluated. These investigated conclusions are very important for ensuring the stability and safety of rock engineering with intermittent structures.
基金financial support provided by the China Scholarship Council(No.201706420055)funded by various projects,namely,China Postdoctoral Science Foundation(No.2020M673142)+3 种基金Chongqing Science and Technology Bureau Foundation(No.cstc2020jcyj-bsh0059)Chongqing Human Resources and Social Security Bureau Foundation(No.cx2019100)Chongqing Science and Technology Bureau Foundation(No.cstc2020jcyj-zdxm0122)National Natural Science Foundation of China(No.U19B2009)。
文摘The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood.Although laboratory creep tests have been carried out to determine the creep deformation of various rocks,these tests are expensive and time-consuming.Nanoindentation creep tests,as an alternative method,can be performed to investigate the mechanical and viscoelastic properties of granite samples.In this study,the reduced Young’s modulus,hardness,fracture toughness,creep strain rate,stress exponent,activation volume and maximum creep displacement of common rock-forming minerals of granite were calculated from nanoindentation results.It was found that the hardness decreases with the increase of holding time and the initial decrease in hardness was swift,and then it decreased slowly.The stress exponent values obtained were in the range from 4.5 to 22.9,which indicates that dislocation climb is the creep deformation mechanism.In addition,fracture toughness of granite’s rock-forming minerals was calculated using energy-based method and homogenization method was adopted to upscale the micro-scale mechanical properties to macro-scale mechanical properties.Last but not least,both three-element Voigt model and Burgers model fit the nanoindentation creep curves well.This study is beneficial to the understanding of the long-term mechanical properties of rock samples from a microscale perspective,which is of great significance to the understanding of localized deformation processes of rocks.
文摘Deep coal seams are one of the world’s most widespread deposits for carbon dioxide(C02)disposal and are generally located near large point sources of CO_(2)emissions.The injection of CO_(2)into coal seams has great potential to sequester CO_(2)while simultaneously enhancing coalbed methane(CO_(2)-ECBM)recovery.Pilot tests of CO_(2)-ECBM have been conducted in coal seams worldwide with favorable early results.However,one of the main technical barriers in coal seams needs to be resolved:Injecting CO_(2)reduces coal permeability and well injectivity.Here,using in situ synchrotron X-ray microtomography,we provide the first observational evidence that injecting nitrogen(N_(2))can reverse much of this lost permeability by reopening fractures that have closed due to coal swelling induced by CO_(2)adsorption.Our findings support the notion that injecting minimally treated flue gas-a mixture of mainly N_(2) and CO_(2)-is an attractive alternative for ECBM recovery instead of pure CO_(2)injection in deep coal seams.Firstly,flue gas produced by power plants could be directly injected after particulate removal,thus avoiding high CO_(2)-separation costs.Secondly,the presence of N_(2)makes it possible to maintain a sufficiently high level of coal permeability.These results suggest that flue-gas ECBM for deep coal seams may provide a promising path toward net-zero emissions from coal mines.
文摘To investigate the deformation mechanisms of rock under hydrostatic stress, destructive experiments were conducted on sandstone under different levels of hydrostatic stress and stress Lode angles. The results reveal that the shape of the strength envelope on the π plane gradually changes from the shape of the Lade criterion to the shape of the Drucker-Prage criterion with an increase in hydrostatic stress.Normally, there exists a deviation between the strain and stress paths for porous rocks on the π plane,and the deviation decreases with an increase in stress Lode angle and hydrostatic stress. A rock failure hypothesis based on the rock porous structure was proposed to investigate the reasons for the abovementioned phenomena. It was found that the shear expansion in the minimum principal stress direction is the dominant factor affecting the Lode angle effect(LAE);the magnitude of the hydrostatic stress induces the variation of the porous structure and influences the shear expansion. Therefore, the hydrostatic stress state affects the LAE. The failure hypothesis proposed in this paper can clarify the hydrostatic stress effect, LAE, and the variation of the rock strength envelope shape.
基金supported by the post-doctoral project of Petrochina Southwest Oil&Gas Field Company“Research on Deep Shale Geomechanics and Effective Fracturing Factors”(Grant No.20210302-31)the Program of Introducing Talents of Discipline to Chinese Universities(111 Plan)(Grant No.D18016)+2 种基金the Sichuan Science and Technology Program(Grant No.2020JDJQ0055)the Nanchong-SWPU Science and Technology Strategic Cooperation Foundation(Grant No.SXHZ033)the Youth Scientific and Technological Innovation Team Foundation of SWPU(Grant No.2019CXTD09).
文摘The brittleness index plays a significant role in the hydraulic fracturing design and wellbore stability analysis of shale reservoirs.Various brittleness indices have been proposed to characterize the brittleness of shale rocks,but almost all of them ignored the anisotropy of the brittleness index.Therefore,uniaxial compression testing integrated with geophysical logging was used to provide insights into the anisotropy of the brittleness index for Longmaxi shale,the presented method was utilized to assess brittleness index of Longmaxi shale formation for the interval of 3155e3175 m in CW-1 well.The results indicated that the brittleness index of Longmaxi shale showed a distinct anisotropy,and it achieved the minimum value at β=45°-60°.As the bedding angle increased,the observed brittleness index(BI_(2_β))decreased firstly and increased then,it achieved the lowest value at β=40°-60°,and it is consistent with the uniaxial compression testing results.Compared to the isotropic brittleness index(β=0°),the deviation of the anisotropic brittleness index ranged from 10%to 66.7%,in other words,the anisotropy of brittleness index cannot be ignored for Longmaxi shale.Organic matter content is one of the main intrinsic causes of shale anisotropy,and the anisotropy degree of the brittleness index generally increases with the increase in organic matter content.The present work is valuable for the assessment of anisotropic brittleness for hydraulic fracturing design and wellbore stability analysis.
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.