Molecular simulation of CH4/CO2/N2 ternary mixture competitive adsorption and diffusion dynamics in tight sandstone

Abstract

Injecting impure CO₂ for enhanced gas recovery (CO₂-EGR) offers a dual benefit by improving natural gas extraction while enabling CO₂ sequestration. However, the interactions between CO₂, N₂, and CH₄ under reservoir conditions require further investigation. This study employs Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations to quantify the adsorption and diffusion behaviors of CO₂, N₂, and CH₄ in quartz nanopores over a pressure range of 1–24 MPa under varying water saturations and gas compositions. The results indicate that: (1) CO₂ exhibits the broadest energy distribution and the strongest adsorption stability, occupying about 20 %–30 % more adsorption sites than CH₄ or N₂ and showing the least sensitivity to water saturation, with only a 30 % reduction at 50 % saturation, compared to 60 % for CH₄, giving CO₂ a clear competitive advantage. (2) The adsorption and desorption behaviors are strongly pressure dependent, as increasing pressure reduces the adsorption layer area and shifts gas distribution from adsorption dominated to free phase. Competitive adsorption analysis reveals that while CO₂ dominates displacement at low pressures, mixtures that contain N₂ achieve higher CH₄ desorption efficiency above 13 MPa by mitigating diffusion resistance. (3) A higher N₂ fraction improves CH₄ diffusion coefficients, thereby facilitating gas mobility and ensuring superior recovery performance under high-pressure conditions. This study advances the fundamental knowledge of microscale gas behavior in tight sandstones and supports the feasibility of impure CO₂ injection as a practical strategy for sustainable gas production.