The whole process coal permeability model of non-equilibrium evolution under the influence of gas pressure difference

In the process of coalbed methane (CBM) extraction, gas pressure difference will exist for a long time due to the different permeability between fracture and matrix, which makes coal in non-equilibrium state. The interaction between matrix and fracture in non-equilibrium state is constantly changing. At present, most studies have not fully explained the relationship between matrix adsorption deformation and fracture aperture change. Therefore, a new non-equilibrium factor related to gas pressure difference is defined to describe the influence of matrix adsorption deformation on the change of fracture aperture and coal bulk deformation at different times. A coal permeability theoretical model of non-equilibrium evolution is established, which includes the whole process from initial equilibrium to final equilibrium Furthermore, the model is verified by experimental data, and the spatial and temporal distribution of gas pressure and evolution of coal permeability during gas injection and extraction are analyzed by numerical simulation. Results show that: (1) The change of matrix pore pressure lags behind fracture pressure. The gas pressure difference exists for a long time, and its size increases rapidly from 0 and then slowly decreases to 0. (2) The permeability evolution can be divided into five stages, the first and the last stages represent the equilibrium state, the middle three stages represent the non-equilibrium state, and the permeability changes non-monotonically in the non-equilibrium evolution process. (3) The ratio of matrix width to matrix bridge width will affect the variation amplitude of permeability, and the different boundary conditions used in coal reservoir will affect the variation amplitude and the final value of permeability. The results have certain guiding significance for CBM extraction reasonably.