Molecular insight into the micro-behaviors of CH4 and CO2 in montmorillonite slit-nanopores

Abstract

The Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to investigate the adsorption and diffusion properties of CH₄ and CO₂ in montmorillonite slit-nanopores. It is found that, both CH₄ and CO₂ could adsorb closely onto the pore surface, while different adsorption states occur for CH₄ and CO₂, respectively, in montmorillonite slit-nanopores. Competitive adsorption of CO₂ over CH₄ exists in montmorillonite slit-nanopores, especially at the lower pressures, which is attributed to the different interaction intensity between the CH₄–CO₂ molecules and the pore surface. The diffusion coefficients of CH₄ and CO₂ both decrease with the enhanced pressures, while the CO₂ has a relative weak diffusion coefficient comparing with CH₄. A well displacement of the residual CH₄ by CO₂ in montmorillonite slit-nanopores was investigated, which is found that the displacement efficiency increases with the enhanced bulk pressures. It was determined that, the CO₂ can be captured and reserved in the montmorillonite slit-nanopores during the displacement, and the sequestration amount of CO₂ gets enhanced with the bulk pressure increasing. This study provides micro-behaviours of CH₄ and CO₂ in montmorillonite slit-nanopores, for the purpose to give out useful guidance for enhancing shale gas extraction by injecting CO₂.