Analysis of reservoir permeability evolution and influencing factors during CO2-Enhanced coalbed methane recovery

CO2-ECBM is an effective energy saving and emission reduction technology. Among them, the permeability of coal seam will affect the CO2 injection efficiency, which is the key factor affecting the CH4 production. In order to reveal the evolution of reservoir permeability, effective stress, adsorption/desorption effect, and binary gas seepage and diffusion are considered. The research results indicate that the evolution of permeability can be summarized into two stages. The permeability during the CO2 undisturbed period is mainly controlled by the effective stress, which exhibits a parabolic relationship with permeability. The CO2 disturbed period is mainly controlled by the strain, which exhibits a negative correlation with permeability. Increasing the injection pressure, during the CO2 undisturbed period, the effective stress increases, the strain decreases, and the higher the permeability. During the CO2 disturbed period, the lower the effective stress, the higher the strain, and the lower the permeability. Increasing the injection temperature, the greater the effective stress and permeability in the undisturbed period. In the CO2 disturbed period, the competitive adsorption intensifies, the larger the strain, and the faster the permeability decreases. Increasing the initial water saturation, the slower the gas transportation rate. The slower the effective stress rises in the CO2 undisturbed period, the lower the permeability. In the CO2 disturbed period, the smaller the strain, and the higher the permeability. In summary, injection pressure, temperature, initial water saturation will have an impact on the permeability, in engineering practice, it is not the case that the higher the injection pressure, the higher the injection temperature, the smaller the initial water saturation is the better.