Selective adsorption and transport of CO2–CH4 mixture under nano-confinement

CO2-enhanced gas recovery (EGR) is a promising technology to sequestrate CO2 while enhancing CH4 recovery simultaneously in shale reservoirs. During the process, the mixture of injected CO2 and desorbed CH4 of varied compositions flows within nanopores of shale. The nanoconfinement is known to affect single-component gas flow and transport significantly but has not yet been properly addressed for non-equimolar gas mixtures. Herein, we use molecular dynamics to systematically investigate the selective adsorption and transport of CO2–CH4 mixture in kerogen slit nanopores. Results show that the gas mixture velocity decreases logarithmically with increasing CO2 molar ratio. The CO2/CH4 adsorption and transport selectivities are generally greater than one and have a strong negative correlation with the total pore gas pressure and pore size. The transport selectivity becomes rather important (i.e., much greater than one) when pore size is below 20 Å. Analyses indicate that surface adsorption and diffusion are primarily responsible for the selective transport, with bulk diffusion also playing a role. These findings provide nanoscale insights into the CO2-EGR in shale's organic matrix and suggest that the selective transport of CO2–CH4 mixture should be considered in large-scale simulations under certain pore size and pressure conditions.