Mechanistic insight into the displacement of CH4 by CO2 in calcite slit nanopores: the effect of competitive adsorption

Abstract

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 calcite slit nanopores with a pore width of ∼22 Å. It was found that, in contrast to CH₄, CO₂ molecules have a much higher capacity to be adsorbed onto calcite pore surfaces. The diffusion capacity of CO₂ molecules is much less in comparison with that of CH₄ molecules, which could be attributed to diverse interactions between CO₂ gas molecules and calcite nanopore surfaces. An effective displacement process of residual adsorbed CH₄ in calcite slit nanopores by CO₂ was performed, and it was found that the displacement efficiency was enhanced with an increase in the bulk pressure. This work provides microscopic information about the adsorption and diffusion properties of CH₄ and CO₂ in calcite nanopores, and confirmed the feasibility of the displacement of adsorbed CH₄ in calcite nanopores by CO₂, with the purpose of providing useful guidance for enhancing the extraction of shale gas by injecting CO₂.