Hydraulic fracturing,an effective method for enhancing coal seam productivity,largely determines coalbed methane(CBM)production,which is significantly influenced by geological and engineering factors.This study focuse...Hydraulic fracturing,an effective method for enhancing coal seam productivity,largely determines coalbed methane(CBM)production,which is significantly influenced by geological and engineering factors.This study focuses on the L block to investigate the mechanisms influencing efficient fracture propagation and enhanced stimulated reservoir volume(SRV)in fracturing.To explore the mechanisms influencing effective fracture propagation and enhanced SRV,the L block was selected as the research object,with a comprehensive consideration of geological background,reservoir properties,and dynamic production data.By combining the discrete lattice method with numer-ical analysis and true triaxial experimental simulation,the fracture morphology of a single cluster and the propagation patterns of multiple clusters of complex fractures were obtained.Additionally,the optimization of temporary plugging timing and the fracture map under multiple factors were innovatively proposed.Results indicate that greater flow rate and viscosity can effectively overcome the stress shadow effect of the outermost fractures(1st and 6th clusters),increasing the fracture pressure of the single cluster and the equilibrium degree of multiple fracture propagation,thus forming a more complex fracture network.Moreover,when viscosity exceeds 45 pressure concentrates at fracture mPa⋅s,tips,promoting discontinuous propagation and reducing flow resistance.Conversely,increased gangue thickness and spacing between horizontal wells increase the vertical propagation pressure,suppressing fracture growth and reducing central flow velocity.This study provides a multi-cluster fracture propagation map for optimizing volumetric fracturing in coal seams and suggests that the optimal temporary plugging time significantly enhances the SRV.展开更多
Refracturing is an importa nt technique to tap the potential of reservoirs and boost production in depleted oil and gas fields.However,fracture propagation during refracturing,including both conventional refracturing ...Refracturing is an importa nt technique to tap the potential of reservoirs and boost production in depleted oil and gas fields.However,fracture propagation during refracturing,including both conventional refracturing and temporary-plugging refracturing remains poorly understood,especially for cases with non-uniform distribution of formation pressure due to long-term oil production and water injection.Therefore,taking pilot tests of refracturing with sidetracking horizontal wells in tight reservoirs in the Changqing Oilfield,China as an example,we establish a three-dimensional numerical model of conventional refracturing and a numerical model of temporary-plugging refracturing based on the discrete lattice method.Non-uniform distributions of formation pressure are imported in these models.We discuss the effects of key operating parameters such as injection rate,cluster spacing,and number of clusters on the propagation of multi-cluster fractures for conventional refracturing.For temporaryplugging refracturing,we examine the impacts of controlling factors such as the timing and number of temporary plugging on fracture propagation.In addition,we analyze a field case of temporaryplugging refracturing using well P3 in the Changqing Oilfield.The results show that fractures during re fracturing tend to propagate preferentially and dominantly in the depleted areas.Improved stimulation effect can be obtained with an optimal injection rate and a critical cluster spacing.The proposed model of temporary-plugging refracturing can well describe the temporary plugging of dominant existingfractures and the creation of new-fractures after fracturing fluid is forced to divert into other clusters from previous dominant clusters.Multiple temporary plugging can improve the balanced propagation of multi-cluster fractures and obtain the maximum fracture area.The established numerical model and research results provide theoretical guidance for the design and optimization of key operating parameters for refracturing,especially for temporary-plugging refracturing.展开更多
The karst cave serves as the primary storage space in carbonate reservoirs.Simultaneously connecting multiple karst caves through hydraulic fracturing is key to the efficient development of carbonate reservoirs.Howeve...The karst cave serves as the primary storage space in carbonate reservoirs.Simultaneously connecting multiple karst caves through hydraulic fracturing is key to the efficient development of carbonate reservoirs.However,there is lack of systematic research on the mechanisms and influencing factors of fracture propagation in car-bonate rocks.This paper established models including karst cave models,single natural fracture-cave models,and multiple natural fracture-cave models based on the discrete lattice method.It thoroughly studied how geological and operational factors affect the fracture propagation and the connectivity of karst caves.The final step involved establishing a prototype well model and optimizing operation parameters.The research indicates that an increase in the Young's modulus and pore pressure of karst cave could facilitate hydraulic fracture connecting with caves.When the pore pressure is lower than that in the matrix,it will generate a repulsive effect on hydraulic fractures.The natural fracture along the hydraulic fracture path significantly facilitates the connection with caves.When the wellbore azimuth is less than 60℃,the fracture's diversion radius is small,and hydraulic fractures primarily connect with karst cave through natural fractures.When the wellbore azimuth exceeds 60℃,the fracture's diversion radius increases.Under the combined action of hydraulic fractures and natural fractures,the stimulated volume of the karst cave noticeably increases.Under the same liquid volume,increasing the injection rate could enhance the cave stimulated volume.Combining the findings from numerical simulation studies resulted in the development of a diagram that depicts the connectivity of karst caves,providing valuable insight for hydraulic fracturing operations in carbonate reservoirs.展开更多
A two-dimensional coupled lattice Boltzmann immersed boundary discrete element method is introduced for the simulation of polygonal particles moving in incompressible viscous fluids. A collision model of polygonal par...A two-dimensional coupled lattice Boltzmann immersed boundary discrete element method is introduced for the simulation of polygonal particles moving in incompressible viscous fluids. A collision model of polygonal particles is used in the discrete element method. Instead of a collision model of circular particles, the collision model used in our method can deal with particles of more complex shape and efficiently simulate the effects of shape on particle–particle and particle–wall interactions. For two particles falling under gravity, because of the edges and corners, different collision patterns for circular and polygonal particles are found in our simulations. The complex vortexes generated near the corners of polygonal particles affect the flow field and lead to a difference in particle motions between circular and polygonal particles. For multiple particles falling under gravity, the polygonal particles easily become stuck owing to their corners and edges, while circular particles slip along contact areas. The present method provides an efficient approach for understanding the effects of particle shape on the dynamics of non-circular particles in fluids.展开更多
This paper investigates the effect of initial volume fraction on the runout characteristics of collapse of granular columns on slopes in fluid. 2-D sub-grain scale numerical simulations are performed to understand the...This paper investigates the effect of initial volume fraction on the runout characteristics of collapse of granular columns on slopes in fluid. 2-D sub-grain scale numerical simulations are performed to understand the flow dynamics of granular collapse in fluid. The discrete element method（DEM） technique is coupled with the lattice Boltzmann method（LBM）, for fluid-grain interactions, to understand the evolution of submerged granular flows. The fluid phase is simulated using multiple-relaxation-time LBM（LBM-MRT） for numerical stability. In order to simulate interconnected pore space in 2-D, a reduction in the radius of the grains（hydrodynamic radius） is assumed during LBM computations. The collapse of granular column in fluid is compared with the dry cases to understand the effect of fluid on the runout behaviour. A parametric analysis is performed to assess the influence of the granular characteristics（initial packing） on the evolution of flow and run-out distances for slope angles of 0 °, 2.5°, 5 ° and 7.5 °. The granular flow dynamics is investigated by analysing the effect of hydroplaning, water entrainment and viscous drag on the granular mass. The mechanism of energy dissipation, shape of the flow front, water entrainment and evolution of packing density is used to explain the difference in the flow characteristics of loose and dense granular column collapse in fluid.展开更多
基金the project of the State Key Laboratory of Petroleum Resources and Engineering(No.PRE/open-2307)the CNOOC Research Institute(No.2020PFS-03).
文摘Hydraulic fracturing,an effective method for enhancing coal seam productivity,largely determines coalbed methane(CBM)production,which is significantly influenced by geological and engineering factors.This study focuses on the L block to investigate the mechanisms influencing efficient fracture propagation and enhanced stimulated reservoir volume(SRV)in fracturing.To explore the mechanisms influencing effective fracture propagation and enhanced SRV,the L block was selected as the research object,with a comprehensive consideration of geological background,reservoir properties,and dynamic production data.By combining the discrete lattice method with numer-ical analysis and true triaxial experimental simulation,the fracture morphology of a single cluster and the propagation patterns of multiple clusters of complex fractures were obtained.Additionally,the optimization of temporary plugging timing and the fracture map under multiple factors were innovatively proposed.Results indicate that greater flow rate and viscosity can effectively overcome the stress shadow effect of the outermost fractures(1st and 6th clusters),increasing the fracture pressure of the single cluster and the equilibrium degree of multiple fracture propagation,thus forming a more complex fracture network.Moreover,when viscosity exceeds 45 pressure concentrates at fracture mPa⋅s,tips,promoting discontinuous propagation and reducing flow resistance.Conversely,increased gangue thickness and spacing between horizontal wells increase the vertical propagation pressure,suppressing fracture growth and reducing central flow velocity.This study provides a multi-cluster fracture propagation map for optimizing volumetric fracturing in coal seams and suggests that the optimal temporary plugging time significantly enhances the SRV.
基金funded by the National Natural Science Foundation of China(41772286,42077247)the Fundamental Research Funds for the Central UniversitiesOpen Research Fund of State Key Laboratory of Geomechanics and Geotechnical Engineering,Institute of Rock and Soil Mechanics,Chinese Academy of Sciences(Z020009)。
文摘Refracturing is an importa nt technique to tap the potential of reservoirs and boost production in depleted oil and gas fields.However,fracture propagation during refracturing,including both conventional refracturing and temporary-plugging refracturing remains poorly understood,especially for cases with non-uniform distribution of formation pressure due to long-term oil production and water injection.Therefore,taking pilot tests of refracturing with sidetracking horizontal wells in tight reservoirs in the Changqing Oilfield,China as an example,we establish a three-dimensional numerical model of conventional refracturing and a numerical model of temporary-plugging refracturing based on the discrete lattice method.Non-uniform distributions of formation pressure are imported in these models.We discuss the effects of key operating parameters such as injection rate,cluster spacing,and number of clusters on the propagation of multi-cluster fractures for conventional refracturing.For temporaryplugging refracturing,we examine the impacts of controlling factors such as the timing and number of temporary plugging on fracture propagation.In addition,we analyze a field case of temporaryplugging refracturing using well P3 in the Changqing Oilfield.The results show that fractures during re fracturing tend to propagate preferentially and dominantly in the depleted areas.Improved stimulation effect can be obtained with an optimal injection rate and a critical cluster spacing.The proposed model of temporary-plugging refracturing can well describe the temporary plugging of dominant existingfractures and the creation of new-fractures after fracturing fluid is forced to divert into other clusters from previous dominant clusters.Multiple temporary plugging can improve the balanced propagation of multi-cluster fractures and obtain the maximum fracture area.The established numerical model and research results provide theoretical guidance for the design and optimization of key operating parameters for refracturing,especially for temporary-plugging refracturing.
基金supported by the Natural Science Foundation of China(Grant No.52074332).
文摘The karst cave serves as the primary storage space in carbonate reservoirs.Simultaneously connecting multiple karst caves through hydraulic fracturing is key to the efficient development of carbonate reservoirs.However,there is lack of systematic research on the mechanisms and influencing factors of fracture propagation in car-bonate rocks.This paper established models including karst cave models,single natural fracture-cave models,and multiple natural fracture-cave models based on the discrete lattice method.It thoroughly studied how geological and operational factors affect the fracture propagation and the connectivity of karst caves.The final step involved establishing a prototype well model and optimizing operation parameters.The research indicates that an increase in the Young's modulus and pore pressure of karst cave could facilitate hydraulic fracture connecting with caves.When the pore pressure is lower than that in the matrix,it will generate a repulsive effect on hydraulic fractures.The natural fracture along the hydraulic fracture path significantly facilitates the connection with caves.When the wellbore azimuth is less than 60℃,the fracture's diversion radius is small,and hydraulic fractures primarily connect with karst cave through natural fractures.When the wellbore azimuth exceeds 60℃,the fracture's diversion radius increases.Under the combined action of hydraulic fractures and natural fractures,the stimulated volume of the karst cave noticeably increases.Under the same liquid volume,increasing the injection rate could enhance the cave stimulated volume.Combining the findings from numerical simulation studies resulted in the development of a diagram that depicts the connectivity of karst caves,providing valuable insight for hydraulic fracturing operations in carbonate reservoirs.
基金This study was funded by the National Science Foundation of China （Grant No. 11272176）.
文摘A two-dimensional coupled lattice Boltzmann immersed boundary discrete element method is introduced for the simulation of polygonal particles moving in incompressible viscous fluids. A collision model of polygonal particles is used in the discrete element method. Instead of a collision model of circular particles, the collision model used in our method can deal with particles of more complex shape and efficiently simulate the effects of shape on particle–particle and particle–wall interactions. For two particles falling under gravity, because of the edges and corners, different collision patterns for circular and polygonal particles are found in our simulations. The complex vortexes generated near the corners of polygonal particles affect the flow field and lead to a difference in particle motions between circular and polygonal particles. For multiple particles falling under gravity, the polygonal particles easily become stuck owing to their corners and edges, while circular particles slip along contact areas. The present method provides an efficient approach for understanding the effects of particle shape on the dynamics of non-circular particles in fluids.
基金the Cambridge Commonwealth, Overseas Trust and the ShellCambridge-Brazil collaboration for the financial support to pursue this research
文摘This paper investigates the effect of initial volume fraction on the runout characteristics of collapse of granular columns on slopes in fluid. 2-D sub-grain scale numerical simulations are performed to understand the flow dynamics of granular collapse in fluid. The discrete element method（DEM） technique is coupled with the lattice Boltzmann method（LBM）, for fluid-grain interactions, to understand the evolution of submerged granular flows. The fluid phase is simulated using multiple-relaxation-time LBM（LBM-MRT） for numerical stability. In order to simulate interconnected pore space in 2-D, a reduction in the radius of the grains（hydrodynamic radius） is assumed during LBM computations. The collapse of granular column in fluid is compared with the dry cases to understand the effect of fluid on the runout behaviour. A parametric analysis is performed to assess the influence of the granular characteristics（initial packing） on the evolution of flow and run-out distances for slope angles of 0 °, 2.5°, 5 ° and 7.5 °. The granular flow dynamics is investigated by analysing the effect of hydroplaning, water entrainment and viscous drag on the granular mass. The mechanism of energy dissipation, shape of the flow front, water entrainment and evolution of packing density is used to explain the difference in the flow characteristics of loose and dense granular column collapse in fluid.
