Interfacial transport of CO2/CH4 in hydroxylated silica nanopores: Implications for CO2-enhanced gas recovery

CO2-enhanced gas recovery (CO2-EGR) requires knowledge of how interfacial interactions regulate gas transport in nanoporous reservoirs. Molecular dynamics simulations of a 6 nm hydroxylated silica slit pore at 20 MPa were conducted to assess the coupled effects of surface hydroxyl density (2.30–9.58 OH/nm2), water saturation (17%–50%), and temperature (313.15–363.15 K) on CO2/CH4 transport. A consistent three-layer organization emerges: bound water films adjacent to pore walls, interfacial enrichment of CO2, and CH4 occupying the pore center with greater mobility. Moderate hydroxylation stabilizes water films while maintaining accessible pathways, whereas higher hydroxylation enhances CO2 binding but restricts transport channels. Water saturation exerts a nonmonotonic influence: at intermediate levels, laterally continuous interfacial pathways enhance CO2 diffusivity and CO2/CH4 selectivity; at high saturation, bulk-like aqueous domains form, weakening selectivity and suppressing gas mobility. Temperature raises diffusivities more strongly for CO2 than for CH4 and follows Arrhenius behavior with activation energies ordered CH4 < CO2 < H2O. Concurrently, heating reduces CO2 adsorption, reflecting a competition between interfacial residence and in-plane migration. These findings, while bounded by the studied geometry, pressure, and force fields, elucidate general mechanisms of nanoconfined transport and provide a physical basis for understanding the trade-off between displacement efficiency and storage security in CO2-EGR.