Dynamic permeability and porosity evolution of coal seam rich in CBM based on the flow-solid coupling theory

Based on poroelasticity theory and diffusion-seepage mechanism, the dual porosity and permeability evolution model was established by considering fracture gas pressure and pore gas pressure, respectively. By the solid and PDE modules of COMSOL Multiphysics, the protective seam mining was simulated on the base of the effective coupling of stress and gas fields. The results showed that with the mining of the protective layer, stress relief region (SRR) and stress concentration region (SCR) are formed in the protective seam, which correspond to the gas releasing region (GRR) and gas accumulative region (GAR), respectively, in the protected seam. The porosity and permeability increase in the SRR, whereas they decrease in the SCR. It is notable that the porosity cannot be restored to the original level due to the irreversibility of internal damage in coal mass. With further mining, the surrounding rock is tensioned to form the damage region, in which tensile fractures are formed, which eventually expand and connect with each other. When the destruction area gradually enlarges to link up the protected coal seam and the gob of the protective layer, the methane accumulated in the protected coal seam flows into the gob. The evolution of stress, porosity, and gas pressure is closely related. The variation of stress will cause change in porosity, which influences gas pressure, whereas the desorption of methane affects the stress of coal, all of which together form the coupling relation. This study is helpful to reveal the coupling mechanism of coalbed stress and methane, understand the dynamic porosity and permeability evolution, and learn tensile damage development.