Analysis of permeability evolution mechanism during CO2 enhanced coalbed methane recovery based on impact factor method

Abstract To studying the influence of CO2 injection on CBM recovery and reservoir permeability, a hydraulic-mechanical-thermal coupling numerical model considering gas flow, competitive adsorption, temperature, and coal deformation was established. The model validation was carried out first, then the CO2 enhanced CBM recovery (CO2-ECBM) processes under different injection pressure and temperature were simulated, and the characteristics of CH4 output and reservoir permeability evolution were analyzed. Finally, the impact factor method for quantitative evaluating the contribution of critical parameters was proposed based on the porosity model. The results show that the production increases with the increase of injection pressure. When the injection pressure is 4 MPa, the production increases by 27,860 m3. When the injection pressure was not higher than the initial reservoir pressure, the coal matrix's deformation was dominated by CH4 desorption. When the injection pressure was higher than the initial reservoir pressure, the deformation was dominated by CH4 desorption, and the permeability increased at the preliminary stage. As time increased, the relative impact factor of CH4 desorption (RIFD) decreased, and the relative impact factor of CO2 adsorption (RIFA) increased; the deformation was dominated by CO2 adsorption, so the permeability decreased. The relative impact factor of pore pressure (RIFP) decreases with the increase of injection pressure, and its maximum value is only 0.014. The microscopic mechanism of permeability evolution was analyzed in this paper from the perspective of coal matrix deformation. The results can provide a theoretical reference for the parameter optimization of CO2-ECBM engineering applications.