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 CO2 injection displacement of CH4 in a coal seam, in this work, the adsorption characteristics and competitive adsorption characteristics of CH4 and CO2 in coal molecules were analyzed through molecular simulations and verified by CO2 injection displacement of CH4 experiments. The results showed that the adsorption amount of CH4 and CO2 in coal was positively correlated with the adsorption pressure and negatively correlated with the adsorption temperature. More CO2 adsorption sites were found in coal, and the adsorption amount of CO2 was stronger. In terms of bicomponent gas competitive adsorption, when CH4 and CO2 were competitively adsorbed, the adsorption amount of CO2 was much higher than that of CH4. The coal seam also showed an obvious competitive adsorption advantage for CO2, and the adsorption heat of CH4 was 25.98 kJ/mol compared to that of CO2, which was 31.44 kJ/mol. In single-factor terms, the influence of pressure and water content on CH4 adsorption was extremely significant, the influence of temperature on CH4 adsorption was significant, and the interaction between pressure and water content was significant in the multifactor interaction. The production efficiency of CH4 after CO2 injection was 76.9%, which was significantly higher than the natural emission efficiency of CH4 (30.6%).