In this work,the microscopic behaviors of CH4 and CO2 in montmorillonite slitnanopores were conducted to give out guidance for enhancing shale gas extraction by injecting CO2(CO2-EGR).The adsorption and diffusion properties of CH4 and CO2,along with the displacement property of the adsorbed CH4 by CO2 in montmorillonite slit-nanopores were performed by the grand canonical Monte Carlo(GCMC)and molecular dynamics(MD)simulation methods,at the constant temperature of 323 K and various pressures.It was found that,the adsorption of CH4 and CO2 could be well described by the Langmuir model.The CH4 and CO2 molecules preferred to adsorb onto the pore surface to achieve a saturated adsorption layer,with a thickness of almost one molecular layer during the whole adsorption process.The diffusion properties,including the self and outward-diffusion process,of both CH4 and CO2 decrease with the enlarged equilibrium pressure.And comparing with CH4,the CO2 has a relative weak capacity in the diffusion process due to the strong interactions of vdW and electrostatic interaction between the gas molecules and the pore surface.The residual adsorbed CH4 in montmorillonite nanopores could be well displaced by CO2,and the displacement efficiency increases with the enhanced bulk pressure,especially when the state of the bulk CO2 changes from the gas into supercritical fluid.