Transport Diffusion Behaviors and Mechanisms of CO2/CH4 in Shale Nanopores: Insights from Molecular Dynamics Simulations

The understanding of gas adsorption and transport behaviors in nanoscaled pores plays a critical role in evaluating unconventional gas exploitation from tight gas reservoirs. In the present work, the transport diffusion mechanisms and behaviors of a CO2/CH4 mixture in shale inorganic and organic nanopores are explored by employing grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. Kaolinite and kerogen slit nanopores are first constructed to represent the inorganic and organic nanopores of the shale matrix. Then, the effects of temperature, pressure, and pore size on the adsorption and diffusion characteristics of CO2/CH4 are examined. The diffusion trajectories clearly show different diffusion mechanisms for gas molecules near the surface and middle area of the pore. Both surface diffusion and Knudsen diffusion have been observed for CO2/CH4 diffusion in shale nanopores. The surface diffusion of CH4 has been found weakened with the presence of CO2. Simulation results indicate that the conditions of higher temperature and lower pressure are beneficial to the efficiency of CH4 diffusion. With the increasing pore size, the impact of surface diffusion on gas transport gradually weakens, leading to the stronger diffusion of CH4 over CO2 in shale nanopores. The obtained results could provide insights into the diffusion mechanism of CO2/CH4 in shale nanopores and offer fundamental data for CO2 sequestration with enhanced gas recovery (CS-EGR) in shale reservoirs.