Experimental and Theoretical Investigation of Diffusion Characteristics of CH₄ and CO₂ in Shales

The diffusion characteristics of CH₄ and CO₂ in micro-nanopores of shale gas reservoirs exert a significant impact on the production performance of supercritical CO₂-enhanced shale gas recovery. High-temperature and high-pressure experimental investigations on shale gas diffusion are still relatively limited compared with abundant laboratory studies under conventional conditions. To investigate the diffusion kinetic behavior of CH₄ and CO₂ in shale micro-nanopores under high-temperature and high-pressure conditions, a series of isobaric diffusion experiments across variable pressure and temperature conditions were performed on marine shale samples from the Longmaxi Formation in the Sichuan Basin, China. A diffusion coefficient model that incorporates both bulk diffusion and surface diffusion mechanisms was adopted to fit the experimental data, and the influences of shale pore size, temperature, and pressure on the diffusion behaviors of CH₄ and CO₂ in shale reservoirs were systematically analyzed. The results indicate the following: (1) Under identical high-temperature and high-pressure conditions, the diffusion coefficient of CH₄ in shale nanopores is larger than that of CO₂. (2) The diffusion coefficients of CH₄ and CO₂ exhibit a positive correlation with temperature and a negative correlation with pressure. (3) Under the coupled influence of temperature and pressure, the diffusion coefficients of CH₄ and CO₂ generally exhibit a decreasing trend as both temperature and pressure increase simultaneously. (4) The pore size distribution of shale samples has a significant effect on the diffusion characteristics of both gases. The larger the overall pore size of the shale, the greater the diffusion coefficients of CH₄ and CO₂ under identical temperature and pressure conditions. The research findings provide valuable insights for the dynamic prediction of supercritical CO₂ enhanced shale gas recovery.