Seepage And Stress Coupling Model For Shale Reservoir And Its Application

Shale gas, an important unconventional natural gas resource, is transforming world energy and economic and political pattern. At present, shale gas production has received widespread attention by the government. But the researches on shale gas migration of in shale reservoir and influence factor analysis of shale gas production and recovery mechanism of shale gas by injection gas is immature, which restricted greatly the development of shale gas production technology in china. In this paper, aim at structure characteristics of shale reservoir, seepage characteristics of shale in coal measures and seepage and stress coupling behaviors during shale gas production processes are studied by laboratory test, and theoretical analysis and numerical simulation comprehensively, and the following main progresses and conclusions are obtained.(1) The permeability characteristics of samples with single fracture and intact samples of shale in coal measures were obtained by the permeability experiments of shale in coal measures. At constant pore pressure, the permeability of samples with single fracture and intact samples of shale in coal measures drop as confining pressure is increased. The relationship between permeability and confining pressure is exponential function, which consists of two stages, a sharp drop stage and a slow drop stage. The permeability of samples with single fracture is 3 orders of magnitude greater than that of intact samples. The sensitivity of permeability to confining pressure is higher that of permeability to pore pressure.(2) The effective stress-dependent permeability model and the effective stress-dependent porosity model of both fracture and matrix are developed, respectively. And then, the effective stress-dependent permeability model of the fracture is verified. Furthermore, the models for gas shale under uniaxial strain and constant volume conditions are developed based on the poroelasticity theory. Then, the validity of the model under uniaxial strain conditions is verified. Based on the effective stress principle and the law of mass conservation, the governing equations for the stress field of shale reservoir deformation and for the gas flow of the fracture and matrix are derived, respectively.(3) Based on the governing equations for the stress field of shale reservoir deformation and for the gas flow of both the fracture and matrix, the impacts of reservoir depth, matrix permeability, fracture permeability, fracture aperture, fracture normal stiffness and fracture spacing on shale gas recovery were analyzed by COMSOL Multiphysics. In the whole production period, the fracture aperture have the largest impact on cumulative production, the impacts of the fracture normal stiffness, the fracture permeability, the fracture spacing, reservoir depth and matrix permeability on cumulative production reduces in turn. The cumulative production of shale reservoir can not be decided by fracture development only.(4) We derived the porosity and permeability models, the seepage and diffusion equations for CH4/N2 and CO2 in the matrix and fracture systems of dual-porosity model during CO2/N2-ESGR processes. And then, the deformation governing equation for shale reservoir is developed during CO2/N2-ESGR processes. The dual-porosity model for CO2/N2-ESGR is applied to numerical simulation of injecting gas production. The simulation results demonstrate that shale gas recovery is enhanced by CO2 and N2 injection effectively. The gas production rate has the rebound phenomena during shale gas production by CO2 injection, but the gas production rate has not the rebound phenomena during shale gas production by N2 injection.(5) The impacts of Knudsen correction factor and non-Darcy effect on the calculated value of shale gas recovery were analyzed by COMSOL Multiphysics. The simulation results show that the calculated deviation of cumulative productions of shale reservoir with fracture permeability(10-15 m2 and 10-16 m2) are 23% and 40% in the whole production period considering the impacts of Knudsen correction factor, respectively, and the calculated deviation of cumulative productions of shale reservoir with fracture permeability(10-15 m2 and 10-16 m2) are 2.4% amd 0.2% in the whole production period considering the impacts of non-Darcy effect, respectively. The smaller the fracture permeability, the greater the impacts of Knudsen correction factor on the calculated value of cumulative productions, and the smaller the impacts of non-Darcy effect on the calculated value of cumulative productions. In order to make calculated value closer to actual production, it has more necessity to update dual-porosity model utilizing Knudsen correction factor when compared with non-Darcy effect.