Molecular Dynamics Simulation of Methane Adsorption and Diffusion in Limestone Pores in the Taiyuan Formation of the Ordos Basin, China: Effects of Pore Shapes, Apertures, and Formation Water

The Taiyuan Formation limestone in the Ordos Basin of China holds significant gas-bearing potential, making it a key target for unconventional natural gas exploration. Clarifying the microscopic occurrence mechanism of gas in limestone is necessary. The effects of pore morphology, aperture, and formation water were systematically studied in this paper through MD and GCMC. The results indicate that specific surface area, pore volume, tortuosity, and interaction synergistically influence methane adsorption and diffusion. Pore shape is intricately linked to these factors, and variations in pore width impact pore volume and interaction, with a slit pore being most conducive to gas diffusion. Formation water mainly forms water films and clusters in organic–inorganic pores. Water molecules preferentially form a water film, while increasing moisture content, expanding aperture, and introducing ions promote cluster formation. Formation water can enhance surface diffusion, reduce the adsorbed phase proportion, and decrease interaction, but it also occupies flow space and forms clusters that hinder gas diffusion. At low moisture content, gas diffusion is promoted in 2 nm and 4 nm pores, while high moisture content inhibits it. In contrast, 6 nm pores consistently curb diffusion.