Numerical simulation of permeability evolution of fractured coal under adsorption condition

Matrix-fracture interaction is a key factor affecting the non-uniform evolution of coal permeability. In order to explore the real evolution process of fracture permeability behavior under adsorption condition, a reconstruction model based on two-dimensional CT scan images of fractured coal was used to compare the long-term experimental data of permeability evolution under adsorption, and the reliability of the numerical model was verified. The visual observation of matrix-fracture interaction guided by fracture structure is realized. The results show that the adsorption caused by gas injection leads to the local expansion of the matrix near the crack. The adsorption expansion behavior extends from near fracture region to far fracture region with gas permeation. When adsorption equilibrium is reached, the local expansion effect decreases, and a new matrix-fracture equilibrium state is formed. When the matrix expands and causes the cracks to compact, the volume of the cracks decreases. The heterogeneous expansion behavior leads to a four-stage evolution process of permeability enhancement, decline, rebound and stability; the increase of injection pressure can inhibit the decrease of permeability and increase the final permeability; Langmuir strain constant is positively correlated with the fracture permeability decreasing process during expansion; the lower the initial permeability of matrix, the greater the decrease of fracture permeability caused by local expansion, and the longer the permeability rebound period.