Adsorption and Diffusion Properties of Gas in Nanopores of Kerogen: Insights from Grand Canonical Monte Carlo and Molecular Dynamics Simulations

Investigating the adsorption and diffusion processes of shale gas within the nanopores of kerogen is essential for comprehending the presence of shale gas in organic matter of shale. In this study, an organic nanoporous structure was constructed based on the unit structure of Longmaxi shale kerogen. Grand canonical Monte Carlo and molecular dynamics simulation methods were employed to explore the adsorption and diffusion mechanisms of pure CH₄, CO₂, and N₂, as well as their binary mixtures with varying mole fractions. The results revealed that the physical adsorption characteristics of CH₄, CO₂, and N₂ gases on kerogen adhered to the Langmuir adsorption law. The quantity of adsorbed gas molecules increased with rising pressure but decreased with increasing temperature. The variation in the heat of adsorption was also analyzed. Under identical temperature and pressure conditions, the adsorption of CH₄ increased with higher mole fractions of CH₄, whereas it decreased with greater mole fractions of CO₂ and N₂. Notably, CO₂ molecules exhibited a robust interaction with kerogen molecules compared to the adsorption properties of CH₄ and N₂. Furthermore, the self-diffusion coefficient of gas within kerogen nanopores gradually decreased with increasing pressure or decreasing temperature. The diffusion capacity of gas molecules followed the descending order N₂ > CH₄ > CO₂ under the same pressure and temperature conditions.