Establishment of dynamic permeability model of coal reservoir and its numerical simulation during the CO2-ECBM process

The permeability evolution during the CO2-ECBM process is the key factor affecting the CH4 production. Therefore, it is necessary to clarify the evolution law and its influencing factor of the matrix-permeability (M-P) and fracture-permeability (F-P). The evolution model of the M-P and F-P was derived by analyzing the change of the matrix shrinkage and swelling and the effective stress. Furthermore, the stress field model and seepage field model were derived, and the numerical simulation of permeability evolution was carried out in COMSOL software. The results show that the change of the effective stress caused by the pressure relief and the CO2 pressure injected, and the change of the matrix shrinkage and swelling are the key factors affecting the permeability. Under different CH4 production conditions, the M-P and F-P have the same evolution law, and the F-P is greater than the M-P at the same location and time. The increase of CO2 pressure injected will improve the variation range of permeability, which is more obvious near the injection well (IW) than that near the production well (PW), and the downtrend of permeability is more obvious. When CH4 is directly mined, the permeability gradually increases with the production time, and the closer to the PW, the higher the permeability. During the CO2-ECBM process, the permeability near the IAN gradually decreases, while the permeability near the PW first increases and then decreases. The permeability evolution during the CO2-ECBM process can be divided into three stages according to the difference between M-P and F-P, and the change of reservoir permeability.