Competitive adsorption and diffusion of CH4/CO2 binary mixture within shale organic nanochannels

Molecular dynamics simulations were performed to study adsorption and dynamics properties of pure CO2, pure CH4, and their mixtures confined to a graphene nanochannel with width of 3.25 nm at different loadings, temperature and molar ratios. Preferential adsorption CO2 over CH4 on the graphene nanochannel surface is observed by the density profile, the residence time of adsorbed molecules, CO2/CH4 selectivity, and the potential energy surface (PES). The CO2 preferentially adsorbed on the surface reduces the activation energy for CH4 diffusibility, and thus improves CH4 mobility. The increasing temperature leading to the decrease of CO2/CH4 selectivity and the enhancement of CO2/CH4 diffusion. Surface diffusion cannot be neglected on CO2/CH4 diffusion, especially at low loadings. The steric hindrance weakens the diffusion of both species at high loadings. The microscopic insight provided with this study is helpful to estimate and exploit the shale gas and evaluate CO2 sequestration capacity.