Huge numbers of induced unpropped(IU)fractures are generated near propped fractures during hydraulic fracturing in shale gas reservoirs.But it is still unclear how their fracture space and conductivity evolve under in...Huge numbers of induced unpropped(IU)fractures are generated near propped fractures during hydraulic fracturing in shale gas reservoirs.But it is still unclear how their fracture space and conductivity evolve under in-situ conditions.This paper prepares three types of samples,namely,manually split vertical/parallel to beddings(MSV,MSP)and parallel natural fractures(NFP),to represent the varied IU fractures as well as their surface morphology.Laser scan and reconstruction demonstrate that the initial fracture spaces of MSVs and MSPs are limited as the asperities of newly created surfaces are wellmatched,and the NFPs have bigger space due to inhomogeneous geological corrosion.Surface slippage and consequent asperity mismatch increase the fracture width by several times,and the increase is proportional to surface roughness.Under stressful conditions,the slipped MSVs retain the smallest residual space and conductivity due to the newly sharp asperities.Controlled by the bedding structures and clay mineral hydrations,the conductivity of MSPs decreases most after treated with a fracturing fluid.The NFPs remain the highest conductivity,benefitting from their dispersive,gentle,and strong asperities.The results reveal the diverse evolution trends of IU fractures and can provide reliable parameters for fracturing design,post-fracturing evaluation,and productivity forecasting.展开更多
For figuring out the stress interference of multiphase fracture combinations and its effect on the fracture initiation pressure of subsequent hydraulic fracturing,a calculation model for the coupled stress field with ...For figuring out the stress interference of multiphase fracture combinations and its effect on the fracture initiation pressure of subsequent hydraulic fracturing,a calculation model for the coupled stress field with multiple induced fractures preexisted was established based on the calculation model for the stress field with single induced fracture preexisted,and the change laws of circumferential stress field around the wellbore under the effect of induced stress were analyzed.Then,the fracture initiation pressure of subsequent hydraulic fracturing was calculated according to the fracturing mechanics criterion.Finally,the effects of the length,phase,horizontal principal stress difference co-efficient and quantity of preexisting fractures on its initiation pressure were analyzed.And the following research results were obtained.First,the circumferential stress difference in the area near the fractures behind the preexisting fractures increase greatly and even the horizontal principal stress is reversed.Second,as the length of new preexisting fractures increases,the initiation pressure of initial preexisting fractures rises first and then drops.And when the length of new preexisting fractures is equal to that of the initial preexisting fractures,the initiation pressure of initial preexisting fractures drops quickly first and then slowly.Third,the initiation pressure of high phase fractures is lower than that of low phase fractures.Fourth,with the increase of fracture quantity,the initiation pressure of new preexisting fractures drops gradually,but the decline trend of initiation pressure difference is not obvious.Fifth,the stress interference of multiphase fractures influences the fracture initiation pressure,and high-phase long fractures and low-phase short fractures are both favorable for the simultaneous initiation of multiphase fractures.Sixth,syn-chronous propagation of multiple fractures can generate more complicated stress interference and excite the evolution of hydraulic fracture network,so as to realize uniform fracturing stimulation.In conclusion,the research results can provide a theoretical guidance for the design of fracturing operation,e.g.perforation phase of blasting induced fracturing,scale of induced fractures,and pump pressure of subsequent hydraulic fracturing.展开更多
Although supercritical carbon dioxide(SC-CO_(2))fracturing shows tremendous potential for maximizing injection efficiency and enhancing storage volumes,few investigations have been reported on the SC-CO_(2) fracturing...Although supercritical carbon dioxide(SC-CO_(2))fracturing shows tremendous potential for maximizing injection efficiency and enhancing storage volumes,few investigations have been reported on the SC-CO_(2) fracturing characteristics of tight basalts and the reactions between fractured basalt and SC-CO_(2).In this study,hydraulic fracturing experiments were conducted on cylindrical basalt specimens using water and SC-CO_(2) as fracturing fluids.Geometric parameters were proposed to characterize the fracture morphologies based on the three-dimensional(3D)reconstructions of fracture networks.The rock slices with induced fractures after SC-CO_(2) fracturing were then processed for fluid(deionized water/SC-CO_(2))-basalt reaction tests.The experimental results demonstrate that SC-CO_(2) fracturing can induce complex and tortuous fractures with spatially dispersed morphologies.Other fracturing behaviors accompanying the acoustic emission(AE)signals and pump pressure changes show that the AE activity responds almost simultaneously to variation in the pump pressure.The fractured basalt blocks exposed to both SC-CO_(2) and water exhibit rough and uneven surfaces,along with decreased intensities in the element peaks,indicating that solubility trapping predominantly occurs during the early injection stage.The above findings provide a laboratory research basis for understanding the fracturing and sequestration issues related to effective CO_(2) utilization.展开更多
Considering the danger of water inrush in mining very thick coal seam under water-rich roof in Majialiang Coal Mine,the universal discrete element(UDEC)software was used to simulate the overburden fracture evolution l...Considering the danger of water inrush in mining very thick coal seam under water-rich roof in Majialiang Coal Mine,the universal discrete element(UDEC)software was used to simulate the overburden fracture evolution laws when mining 4#coal seam.Besides,this study researched on the influence of face advancing length,speed and mining height on the height of the water flowing fractured zones(HWFFZ),and analyzed the correlation of face advancing length and change rules of aquifer water levels and goaf water inflow.Based on those mentioned above,this research proposed the following water-controlling technologies:draining the roof water before mining,draining goaf water,reasonable advancing speed and mining thickness.These water-controlling technologies were successfully used in the feld,thus ensured safely mining the very thick coal seam under water-rich roof.展开更多
Since 2000, the French National Radioactive Waste Management Agency （ANDRA） has been constructing an Underground Research Laboratory （URL） at Bure （east of the Paris Basin） to perform experiments in order to obt...Since 2000, the French National Radioactive Waste Management Agency （ANDRA） has been constructing an Underground Research Laboratory （URL） at Bure （east of the Paris Basin） to perform experiments in order to obtain in situ data necessary to demonstrate the feasibility of geological repository in the Callovo- Oxfordian claystone. An important experimental program is planned to characterize the response of the rock to different drift construction methods, Before 2008, at the main level of the laboratory, most of the drifts were excavated using pneumatic hammer and supported with rock bolts, sliding steel arches and fiber shotcrete. Other techniques, such as road header techniques, stiff and flexible supports, have also been used to characterize their impacts. The drift network is developed following the in situ major stresses. The parallel drifts are separated enough so as they can be considered independently when their hydromechanical （HM） behaviors are compared. Mine-by experiments have been performed to measure the HM response of the rock and the mechanical loading applied to the support system due to the digging and after excavation. Drifts exhibit extensional （mode I） and shear fractures （modes II and III） induced by excavation works. The extent of the induced fracture networks depends on the drift orientation versus the in situ stress field. This paper describes the drift convergence and deformation in the surrounding rock walls as function of time and the impact of different support methods on the rock mass behavior. An observation based method is finally applied to distinguish the instantaneous and time-dependent parts of the rock mass deformation around the drifts.展开更多
Crushing and embedment are two critical downhole proppant degradation mechanisms that lead to a significant drop in production outputs in unconventional oil/gas stimulation projects. These persistent production drops ...Crushing and embedment are two critical downhole proppant degradation mechanisms that lead to a significant drop in production outputs in unconventional oil/gas stimulation projects. These persistent production drops due to the non-linear responses of proppants under reservoir conditions put the future utilization of such advanced stimulation techniques in unconventional energy extraction in doubt. The aim of this study is to address these issues by conducting a comprehensive experimental approach. According to the results, whatever the type of proppant, all proppant packs tend to undergo significant plastic deformation under the first loading cycle.Moreover, the utilization of ceramic proppants(which retain proppant pack porosity up to 75%), larger proppant sizes(which retain proppant pack porosity up to 15.2%) and higher proppant concentrations(which retain proppant pack porosity up to 29.5%) in the fracturing stimulations with higher in-situ stresses are recommended to de-escalate the critical consequences of crushing associated issues. Similarly, the selection of resin-coated proppants over ceramic and sand proppants may benefit in terms of obtaining reduced proppant embedment.In addition, selection of smaller proppant sizes and higher proppant concentrations are suggested for stimulation projects at depth with sedimentary formations and lower in-situ stresses where proppant embedment predominates. Furthermore, correlation between proppant embedment with repetitive loading cycles was studied.Importantly, microstructural analysis of the proppant-embedded siltstone rock samples revealed that the initiation of secondary induced fractures. Finally, the findings of this study can greatly contribute to accurately select optimum proppant properties(proppant type, size and concentration) depending on the oil/gas reservoir characteristics to minimize proppant crushing and embedment effects.展开更多
Pre-existing natural fractures and other structurally weak planes are usually well-developed in unconventional reservoirs.When such fractures intersect with hydraulic induced fractures,they will redirect and propagate...Pre-existing natural fractures and other structurally weak planes are usually well-developed in unconventional reservoirs.When such fractures intersect with hydraulic induced fractures,they will redirect and propagate as an important mechanical principle of volume fracturing by the formation of complex fracture networks.Under the shadow effect of natural fractures and other structurally weak planes with hydraulic supported fracture stress,hydraulic fractures do not fully propagate in the direction of the maximum horizontal-principal-stress.This paper computed the stress intensity factors of hydraulic fracture types I and II by integrating the various interactions,established universally-applicable mechanical principles for the propagation behavior when a hydraulic fracture propagating in an arbitrary direction intersects with a natural fracture at an arbitrary angle,and demonstrated the mechanical principles of the intersection between hydraulic induced fractures and pre-existing natural fractures.This study proved the following conclusions:as the intersection angle between the hydraulic fracture and the maximum horizontal-principal-stress increased,the possibility of the hydraulic fracture being captured by the natural fracture with an identical approaching angle first increased and then decreased;as the net stress increased,the intersection behavior between the hydraulic fracture and the natural fracture transitioned from penetration to capture.展开更多
Oil exploration and production,well stability,sand production,geothermal drilling,waste-water or CO_(2) sequestration,geohazards assessment,and EOR processes such as hydraulic fracturing,require adequate information a...Oil exploration and production,well stability,sand production,geothermal drilling,waste-water or CO_(2) sequestration,geohazards assessment,and EOR processes such as hydraulic fracturing,require adequate information about in-situ stresses.There are several methods for analyzing the magnitude and direction of in-situ stresses.The evaluation of tensile fractures and shear fractures in vertical oil and gas wellbores using image logs is one of these methods.Furthermore,when image logs are run in boreholes,they can be extremely costly and possibly stop the drilling.The data for this study were gathered from seven directional wells drilled into a strike-slip fault reservoir in southern Iran.Vertical stress,minimum horizontal stress,pore pressure,Poisson's ratio of formations,and 233 mud loss reporting points make up the entire data.This is the first time maximum horizontal stress direction has been calculated without referring to image log data.In addition,the points of lost circulation were categorized into natural and induced fracture.The results revealed that,the maximum horizontal stress direction of the reservoir was calculated at 65northeast-southwest.The error rate is roughly 10when comparing the results of this investigation to those obtained from the image log.The maximum horizontal stress direction is calculated precisely.In terms of tensile fracture pressure,the in-situ stress ratio identifies the safest as well as the most critical inclination and azimuth for each well.展开更多
基金supported by the National Natural Science Youth Foundation of China(No.52104003)the Open Fund of Engineering Research Center of Development and Management for Low to Ultra-Low Permeability Oil&Gas Reservoirs in West China,Ministry of Education(No.KFJJ-XB-2020-5)+2 种基金the Science and Technology Planning Project of Sichuan Province(No.22NSFSC4005)the National Natural Science Foundation of China(No.52274031 and No.52374005)the Natural Science Youth Foundation of Sichuan Province(No.2023NSFSC0930)。
文摘Huge numbers of induced unpropped(IU)fractures are generated near propped fractures during hydraulic fracturing in shale gas reservoirs.But it is still unclear how their fracture space and conductivity evolve under in-situ conditions.This paper prepares three types of samples,namely,manually split vertical/parallel to beddings(MSV,MSP)and parallel natural fractures(NFP),to represent the varied IU fractures as well as their surface morphology.Laser scan and reconstruction demonstrate that the initial fracture spaces of MSVs and MSPs are limited as the asperities of newly created surfaces are wellmatched,and the NFPs have bigger space due to inhomogeneous geological corrosion.Surface slippage and consequent asperity mismatch increase the fracture width by several times,and the increase is proportional to surface roughness.Under stressful conditions,the slipped MSVs retain the smallest residual space and conductivity due to the newly sharp asperities.Controlled by the bedding structures and clay mineral hydrations,the conductivity of MSPs decreases most after treated with a fracturing fluid.The NFPs remain the highest conductivity,benefitting from their dispersive,gentle,and strong asperities.The results reveal the diverse evolution trends of IU fractures and can provide reliable parameters for fracturing design,post-fracturing evaluation,and productivity forecasting.
基金Project supported by the National Natural Science Foundation of China,“Microscopic Damage Mechanism of Saturated Brittle Rock during Blasting Induced Fracturing and Its Control Law on Macroscopic Damage”(No.51874339)the Natural Science Foundation of Shandong Province,“Study on the Coupled Response Mechanism of Crack Propagation and Rock Damage in the fracturing condition of multistage strong pulse”(No.ZR2016EEQ04)the Fundamental Research Funds for the Central Universities,China,“Study on the Mechanical Mechanism of Multistage Blasting Induced SRV in Deep-Seated Tight Reservoirs with High Horizontal Stress Difference”(No.17CX05004).
文摘For figuring out the stress interference of multiphase fracture combinations and its effect on the fracture initiation pressure of subsequent hydraulic fracturing,a calculation model for the coupled stress field with multiple induced fractures preexisted was established based on the calculation model for the stress field with single induced fracture preexisted,and the change laws of circumferential stress field around the wellbore under the effect of induced stress were analyzed.Then,the fracture initiation pressure of subsequent hydraulic fracturing was calculated according to the fracturing mechanics criterion.Finally,the effects of the length,phase,horizontal principal stress difference co-efficient and quantity of preexisting fractures on its initiation pressure were analyzed.And the following research results were obtained.First,the circumferential stress difference in the area near the fractures behind the preexisting fractures increase greatly and even the horizontal principal stress is reversed.Second,as the length of new preexisting fractures increases,the initiation pressure of initial preexisting fractures rises first and then drops.And when the length of new preexisting fractures is equal to that of the initial preexisting fractures,the initiation pressure of initial preexisting fractures drops quickly first and then slowly.Third,the initiation pressure of high phase fractures is lower than that of low phase fractures.Fourth,with the increase of fracture quantity,the initiation pressure of new preexisting fractures drops gradually,but the decline trend of initiation pressure difference is not obvious.Fifth,the stress interference of multiphase fractures influences the fracture initiation pressure,and high-phase long fractures and low-phase short fractures are both favorable for the simultaneous initiation of multiphase fractures.Sixth,syn-chronous propagation of multiple fractures can generate more complicated stress interference and excite the evolution of hydraulic fracture network,so as to realize uniform fracturing stimulation.In conclusion,the research results can provide a theoretical guidance for the design of fracturing operation,e.g.perforation phase of blasting induced fracturing,scale of induced fractures,and pump pressure of subsequent hydraulic fracturing.
基金supported by the National Key Research and Development Project(Grant No.2023YFE0110900)the National Natural Science Foundation of China(Grant No.42320104003)the Shanghai Pujiang Programme(Grant No.23PJD105).
文摘Although supercritical carbon dioxide(SC-CO_(2))fracturing shows tremendous potential for maximizing injection efficiency and enhancing storage volumes,few investigations have been reported on the SC-CO_(2) fracturing characteristics of tight basalts and the reactions between fractured basalt and SC-CO_(2).In this study,hydraulic fracturing experiments were conducted on cylindrical basalt specimens using water and SC-CO_(2) as fracturing fluids.Geometric parameters were proposed to characterize the fracture morphologies based on the three-dimensional(3D)reconstructions of fracture networks.The rock slices with induced fractures after SC-CO_(2) fracturing were then processed for fluid(deionized water/SC-CO_(2))-basalt reaction tests.The experimental results demonstrate that SC-CO_(2) fracturing can induce complex and tortuous fractures with spatially dispersed morphologies.Other fracturing behaviors accompanying the acoustic emission(AE)signals and pump pressure changes show that the AE activity responds almost simultaneously to variation in the pump pressure.The fractured basalt blocks exposed to both SC-CO_(2) and water exhibit rough and uneven surfaces,along with decreased intensities in the element peaks,indicating that solubility trapping predominantly occurs during the early injection stage.The above findings provide a laboratory research basis for understanding the fracturing and sequestration issues related to effective CO_(2) utilization.
基金provided by the Priority Academic Program Development of Jiangsu Higher Education Institutions of China(No.SZBF2011-6-B35)the Fundamental Research Funds for the Central Universities of China(No.2012LWB42)
文摘Considering the danger of water inrush in mining very thick coal seam under water-rich roof in Majialiang Coal Mine,the universal discrete element(UDEC)software was used to simulate the overburden fracture evolution laws when mining 4#coal seam.Besides,this study researched on the influence of face advancing length,speed and mining height on the height of the water flowing fractured zones(HWFFZ),and analyzed the correlation of face advancing length and change rules of aquifer water levels and goaf water inflow.Based on those mentioned above,this research proposed the following water-controlling technologies:draining the roof water before mining,draining goaf water,reasonable advancing speed and mining thickness.These water-controlling technologies were successfully used in the feld,thus ensured safely mining the very thick coal seam under water-rich roof.
文摘Since 2000, the French National Radioactive Waste Management Agency （ANDRA） has been constructing an Underground Research Laboratory （URL） at Bure （east of the Paris Basin） to perform experiments in order to obtain in situ data necessary to demonstrate the feasibility of geological repository in the Callovo- Oxfordian claystone. An important experimental program is planned to characterize the response of the rock to different drift construction methods, Before 2008, at the main level of the laboratory, most of the drifts were excavated using pneumatic hammer and supported with rock bolts, sliding steel arches and fiber shotcrete. Other techniques, such as road header techniques, stiff and flexible supports, have also been used to characterize their impacts. The drift network is developed following the in situ major stresses. The parallel drifts are separated enough so as they can be considered independently when their hydromechanical （HM） behaviors are compared. Mine-by experiments have been performed to measure the HM response of the rock and the mechanical loading applied to the support system due to the digging and after excavation. Drifts exhibit extensional （mode I） and shear fractures （modes II and III） induced by excavation works. The extent of the induced fracture networks depends on the drift orientation versus the in situ stress field. This paper describes the drift convergence and deformation in the surrounding rock walls as function of time and the impact of different support methods on the rock mass behavior. An observation based method is finally applied to distinguish the instantaneous and time-dependent parts of the rock mass deformation around the drifts.
文摘Crushing and embedment are two critical downhole proppant degradation mechanisms that lead to a significant drop in production outputs in unconventional oil/gas stimulation projects. These persistent production drops due to the non-linear responses of proppants under reservoir conditions put the future utilization of such advanced stimulation techniques in unconventional energy extraction in doubt. The aim of this study is to address these issues by conducting a comprehensive experimental approach. According to the results, whatever the type of proppant, all proppant packs tend to undergo significant plastic deformation under the first loading cycle.Moreover, the utilization of ceramic proppants(which retain proppant pack porosity up to 75%), larger proppant sizes(which retain proppant pack porosity up to 15.2%) and higher proppant concentrations(which retain proppant pack porosity up to 29.5%) in the fracturing stimulations with higher in-situ stresses are recommended to de-escalate the critical consequences of crushing associated issues. Similarly, the selection of resin-coated proppants over ceramic and sand proppants may benefit in terms of obtaining reduced proppant embedment.In addition, selection of smaller proppant sizes and higher proppant concentrations are suggested for stimulation projects at depth with sedimentary formations and lower in-situ stresses where proppant embedment predominates. Furthermore, correlation between proppant embedment with repetitive loading cycles was studied.Importantly, microstructural analysis of the proppant-embedded siltstone rock samples revealed that the initiation of secondary induced fractures. Finally, the findings of this study can greatly contribute to accurately select optimum proppant properties(proppant type, size and concentration) depending on the oil/gas reservoir characteristics to minimize proppant crushing and embedment effects.
基金Sponsored by National Science and Technology Major Projects(2016ZX05052,2016ZX05014).
文摘Pre-existing natural fractures and other structurally weak planes are usually well-developed in unconventional reservoirs.When such fractures intersect with hydraulic induced fractures,they will redirect and propagate as an important mechanical principle of volume fracturing by the formation of complex fracture networks.Under the shadow effect of natural fractures and other structurally weak planes with hydraulic supported fracture stress,hydraulic fractures do not fully propagate in the direction of the maximum horizontal-principal-stress.This paper computed the stress intensity factors of hydraulic fracture types I and II by integrating the various interactions,established universally-applicable mechanical principles for the propagation behavior when a hydraulic fracture propagating in an arbitrary direction intersects with a natural fracture at an arbitrary angle,and demonstrated the mechanical principles of the intersection between hydraulic induced fractures and pre-existing natural fractures.This study proved the following conclusions:as the intersection angle between the hydraulic fracture and the maximum horizontal-principal-stress increased,the possibility of the hydraulic fracture being captured by the natural fracture with an identical approaching angle first increased and then decreased;as the net stress increased,the intersection behavior between the hydraulic fracture and the natural fracture transitioned from penetration to capture.
文摘Oil exploration and production,well stability,sand production,geothermal drilling,waste-water or CO_(2) sequestration,geohazards assessment,and EOR processes such as hydraulic fracturing,require adequate information about in-situ stresses.There are several methods for analyzing the magnitude and direction of in-situ stresses.The evaluation of tensile fractures and shear fractures in vertical oil and gas wellbores using image logs is one of these methods.Furthermore,when image logs are run in boreholes,they can be extremely costly and possibly stop the drilling.The data for this study were gathered from seven directional wells drilled into a strike-slip fault reservoir in southern Iran.Vertical stress,minimum horizontal stress,pore pressure,Poisson's ratio of formations,and 233 mud loss reporting points make up the entire data.This is the first time maximum horizontal stress direction has been calculated without referring to image log data.In addition,the points of lost circulation were categorized into natural and induced fracture.The results revealed that,the maximum horizontal stress direction of the reservoir was calculated at 65northeast-southwest.The error rate is roughly 10when comparing the results of this investigation to those obtained from the image log.The maximum horizontal stress direction is calculated precisely.In terms of tensile fracture pressure,the in-situ stress ratio identifies the safest as well as the most critical inclination and azimuth for each well.
