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

Understanding the diffusion behaviors of pure CH 4, CO 2, and CH 4 /CO 2 mixtures within shale formations is a prerequisite to optimize the CO 2 -enhanced shale gas recovery (CO 2 -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 CH 4, CO 2, and their mixtures with various ratios (25% CH 4 + 75% CO 2, 50% CH 4 + 50% CO 2, 75% CH 4 + 25% CO 2 ). The results indicate that the D e C H 4 (the effective diffusion coefficient of pure CH 4 ) decreases as pressure increases, while D e C O 2 (the effective diffusion coefficient of pure CO 2 ) is seen to initially decrease and then increase with rising pressure, which can be ascribed to the alteration of CO 2 properties in the supercritical state and adsorption-induced swelling. For gas mixtures, the effective diffusion coefficient of the CH 4 /CO 2 mixtures D e C H 4 / C O 2 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 CO 2, indicating that CO 2 has a predominant influence on the diffusion process of the gas mixtures. Moreover, D e of all gases are increases with rising temperature. These findings provide valuable insights into optimizing the engineering parameters during the CO 2 -ESGR process.