Pore Structure and CO2/CH4 Adsorption-Diffusion Characteristics in Tight Sandstone: Experimental Characterization and Molecular Simulation

Tight sandstone gas reservoirs are characterized by complex pore-throat structures, and the occurrence and transport of natural gas are jointly controlled by pore-throat size, mineral composition, water saturation, and other factors. Clarifying the intrinsic relationships between pore-throat structure and CO2/CH4 adsorption and diffusion is therefore important for understanding the microscopic mechanisms of CO2-enhanced gas recovery (CO2-EGR). In this study, three types of tight sandstone cores with different permeabilities from the Yan’an Gas Field were selected. Cast thin-section analysis, scanning electron microscopy, X-ray diffraction, nuclear magnetic resonance (NMR), and high-pressure mercury intrusion were employed to characterize the pore structure, mineral composition, and movable-fluid occurrence. Fractal analysis was further used to distinguish intragranular and intergranular pores. Based on experimentally determined pore-throat scales and mineral compositions, quartz and illite pore models were constructed, and grand canonical Monte Carlo (GCMC) and molecular dynamics (MDs) simulations were performed to investigate CO2/CH4 competitive adsorption and diffusion. The results show that larger pore throats become better developed and pore-throat connectivity improves markedly with increasing core permeability. The proportion of movable fluid in intergranular pores increases from 63.76% to 98.61%, while that in intragranular pores increases from 10.87% to 70.33%. Molecular simulation results indicate that CO2 exhibits a substantially stronger competitive adsorption capacity than CH4 in both quartz and illite pores. Increasing pressure favors gas adsorption, whereas increasing temperature, pore size, and water saturation alter the occurrence and transport characteristics of both gases. Under the same conditions, the diffusion coefficient of CH4 is consistently higher than that of CO2. Larger pore sizes and higher temperatures enhance gas diffusion, whereas increasing pressure and water saturation suppress diffusion. These findings provide a theoretical basis for evaluating CO2-EGR and geological CO2 storage in tight sandstone reservoirs.