Distribution of CO₂-CH₄ in a nanoslit medium and the efficiency of displacement of CH₄ by CO₂

The replacement of CH4 with CO2 in reservoirs is crucial for natural gas exploitation and sequestration of CO2. Therefore, the states and distribution patterns of coexisting CO2 and CH4 and the dynamic patterns of CH4 displacement by CO2 in mineral slits must be understood. Here, we simulate the molecular dynamics and displacement efficiency of CH4 in gypsum, quartz, and illite mineral slit models. The results demonstrate that gas density is not uniform throughout the slits and prove that gypsum and quartz have a greater adsorption capacity for CO2 than for CH4 and illite has a greater adsorption capacity for CH4. We find that (1) the application of the same force to different mineral slits leads to dissimilar CH4 displacement efficiencies, whereas an increase in the applied force in the same mineral slit is positively correlated with the CH4 displacement efficiency; (2) the main factor affecting CH4 displacement efficiency is CO2 adsorption; (3) the greater the adsorption capacity of mineral slits for CO2 (such as for gypsum), the more difficult it is to displace the CH4 in the slit despite large applied force; and (4) CH4 displacement efficiencies of illite, quartz, and gypsum decrease under the same conditions. These results provide a reference for theoretical research and industrial applications related to geologic carbon sequestration.