The displacement process of CH4 by the CO, injection in the shale micropores plays a dominant role in the CO2 enhanced shale gas recovery (CO2-ESGR). In this paper, we have addressed the displacement of CH4 by CO, in the micropores, and particularly, we have investigated the contribution of each specific pore size from 0.50 to 2.0 nm to the competitive adsorption of CH4 and CO2 in terms of the CH4 recovery and residual CH4 and CO2 adsorption after the displacement. The results showed that the micropores have different contributions to the CH4 recovery depending on the pore size, CO2 ratio, temperature, and pressure. The pores below 0.61 nm make no contribution to CH4 recovery, but the 0.55-0.60 nm pores are beneficial for CO2 storage. The 0.65-0.70 nm pores show the highest CH4 storage capacity and a high selectivity for CO2. As a result, the CH4 recovery reaches the maximum and is not affected by CO2 ratio. Besides, the pores above 1.3 nm provide little to the CH4 recovery at lower pressures, and the injected CO2 ratio changes the optimum pore size in terms of the maximum CH4 recovery. The pore size for the maximum CH4 recovery decreases slightly with the increase of pressure. In addition, the CH4 recovery density is higher at lower temperatures due to higher preadsorption of CH4 and lower residual CH4 capacity. Furthermore, the distribution of the adsorbed CH4 and CO2 after the displacement showed that the residual CH4 distribution is not affected by the injected CO2 and is randomly located among the adsorbed CO2 molecules.