A preliminary investigation of CH4 diffusion through gas shale in the Paleozoic Longmaxi Formation, Southern Sichuan Basin, China

Diffusion is one of the key factors influencing the resource evaluation and gas production prediction of shale gas; however, our understanding of diffusion processes is far from complete. This study focuses on a high-maturity organic-rich shale sample in the Paleozoic Longmaxi Formation in the Southern Sichuan Basin of China. The matrix pore distribution was measured using a low-temperature liquid nitrogen adsorption method, followed by an isothermal adsorption experiment at different temperatures. Finally, based on the pressure vs. time data recorded in the experiment, the diffusion parameters under different temperature-pressure conditions were calculated using a unipore diffusion model. The results suggest that the mesopores are well developed in the gas shale sample, whereas only a few micropores were measured due to the limitations of the measurement method. The results suggest that methane diffusion through shale is adsorbate concentration-dependent, and the effective diffusivity increases with a rise in pressure but decreases with a rise in temperature. The factors that influence the methane diffusion through shale and the reliability of the unipore-diffusion modeling method were addressed.