Study on the Microscopic Mechanism of CO2 Injection for CH4 Displacement in Coal Seams Based on Molecular Simulation

To obtain the microscopic mechanism of the CO 2 injection displacement of CH 4 in a coal seam, in this work, the adsorption characteristics and competitive adsorption characteristics of CH 4 and CO 2 in coal molecules were analyzed through molecular simulations and verified by CO 2 injection displacement of CH 4 experiments. The results showed that the adsorption amount of CH 4 and CO 2 in coal was positively correlated with the adsorption pressure and negatively correlated with the adsorption temperature. More CO 2 adsorption sites were found in coal, and the adsorption amount of CO 2 was stronger. In terms of bicomponent gas competitive adsorption, when CH 4 and CO 2 were competitively adsorbed, the adsorption amount of CO 2 was much higher than that of CH 4 . The coal seam also showed an obvious competitive adsorption advantage for CO 2, and the adsorption heat of CH 4 was 25.98 kJ/mol compared to that of CO 2, which was 31.44 kJ/mol. In single-factor terms, the influence of pressure and water content on CH 4 adsorption was extremely significant, the influence of temperature on CH 4 adsorption was significant, and the interaction between pressure and water content was significant in the multifactor interaction. The production efficiency of CH 4 after CO 2 injection was 76.9%, which was significantly higher than the natural emission efficiency of CH 4 (30.6%).