Diffusion of Pure CH₄, CO₂, and Their Mixtures in Shale Under Various Pressures and Temperatures

Understanding the diffusion behaviors of pure CH4, CO2, and CH4/CO2 mixtures within shale formations is a prerequisite to optimize the CO2-enhanced shale gas recovery (CO2-ESGR) process. In this research, the adsorption kinetics and diffusion coefficients were determined at various pressures (0–22 MPa) and temperatures (50 °C, 60 °C, and 70 °C) for pure CH4, CO2, and their mixtures with various ratios (25% CH4 + 75% CO2, 50% CH4 + 50% CO2, 75% CH4 + 25% CO2). The results indicate that the DeCH4 (the effective diffusion coefficient of pure CH4) decreases as pressure increases, while DeCO2 (the effective diffusion coefficient of pure CO2) is seen to initially decrease and then increase with rising pressure, which can be ascribed to the alteration of CO2 properties in the supercritical state and adsorption-induced swelling. For gas mixtures, the effective diffusion coefficient of the CH4/CO2 mixtures DeCH4/CO2 shows a notable U-shaped trend with pressure, as it decreases and then increases with increasing pressure. This reflects a trend similar to that of pure CO2, indicating that CO2 has a predominant influence on the diffusion process of the gas mixtures. Moreover, De of all gases are increases with rising temperature. These findings provide valuable insights into optimizing the engineering parameters during the CO2-ESGR process.