Investigation of CO2-CH4-H2O Diffusion in Gas Reservoirs: Combining Experimental Measurement and Molecular Dynamics Simulation

Accurate prediction of CO2 diffusion in multi-component fluids is crucial for efficient EGR and CCUS operations. Conventional experiments struggle to simulate the diffusion of oil and gas under reservoir conditions, and it is difficult to provide insight into the diffusion mechanism at the molecular level. Therefore, a microscopic understanding of CO2 diffusion in complex fluids is urgent needed. In this study, the diffusion behavior and underlying mechanism of the CO2-CH4-H2O system under reservoir temperature and pressure conditions were explored using both experimental techniques and molecular dynamics method. The results show that at 354.15K, the diffusion coefficients follow the order DCH4 > DCO2 > DFick and decrease with increasing pressure. Higher CO2 concentrations and water content lead to a reduction in DCO2. Gravity has a relatively minor effect but tends to enhance DCO2 while slightly reducing DCH4. Near the critical point, a significant decrease in the thermodynamic factor indicates a drastic changes in thermodynamic properties. Furthermore, the presence of water promotes CO2 enrichment at the gas-water interface, consequently reducing both DCO2 and DCH4. This work provides valuable insight into bulk-phase transport in multicomponent aquifer systems under reservoir conditions and offers theoretical support for optimizing gas injection strategies and improving the efficiency of EGR and CCUS processes.