Modeling of Methane Flow through Nanopores: Insights from Molecular Dynamics Simulations

Methane transport through nanoscale shale pores occurs under high Knudsen number conditions, where the velocity distribution deviates from the no-slip boundary assumption, making it challenging to determine the flow rate accurately. In this study, molecular dynamics (MD) simulations were performed to investigate the flow of methane within nanoscale quartz slits. The effects of pressure, pressure gradient, temperature, and pore width on methane transport and characteristic velocities (centerline and boundary velocities) were systematically examined to elucidate the mechanisms governing the boundary slip. The results show that pressure and pressure gradient determine the external force acting on methane molecules. The centerline velocity increases linearly with the applied force and scales with the square of the pore width. The boundary velocity exhibits a linear relationship with both the applied force and pore width under low-force conditions but becomes proportional to the square of the applied force and shows a nonlinear dependence on pore width at high forces. Temperature has a negligible effect on the centerline velocity but significantly enhances boundary velocity. Boundary slip originates from the collective motion of methane molecules, reflecting the combined influence of external forces, methane-wall interactions, and intermolecular forces. Finally, this study developed a model to predict the mass flow rate of methane transport through nanopores, which shows good agreement with MD simulation results and greater accuracy than the existing models.