Methane Diffusion Through Nanopore-Throat Geometry: A Molecular Dynamics Simulation Study

Abstract

Summary Molecular diffusion dominates over pressure-driven convection as the major mass transport mechanism in nanoporous media with <10-nm pores, which is typical pore size for shale gas recovery. To study fluid behavior at this scale, molecular dynamics (MD) simulation has been widely applied. Nevertheless, classic capillary tube or slit models are of uniform geometry that miss the converging-diverging pore-throat feature, while more realistic models lose simplicity and generality. In this work, we propose a novel geometric model that can reproduce the realistic converging-diverging structure in subsurface porous media without any additional complexity compared to classic slit or capillary models. In this pore-throat model, we are able to identify how nonuniform geometry affects the methane diffusion for both pure methane and for methane mixtures with water, carbon dioxide, and helium. For a pure methane system, we demonstrate the fundamental impact of throat width on diffusion coefficient when the throat width is narrower than 20 Å and identify a critical throat width that determines whether methane can self-diffuse though the throat. This critical throat size is regulated by the energy barrier at the throat rather than by molecular size. We then introduce a semianalytical model to predict self-diffusion coefficient as a function of pressure, temperature, and throat width. For mixtures, we observe the key impact of spatially nonuniform fluid distribution in determining diffusion. Water or carbon dioxide can locally concentrate at the throat, which reduces methane diffusivity, while helium prefers to stay in the pore body, which mildly enhances methane diffusivity. Specifically, although residual water reduces methane diffusion (26% reduction for 20% water molar fraction), it completely blocks the throat and thus prohibits pressure-driven methane convection. By comparison, the dominance of molecular diffusion over convection can be extended to larger pores in presence of residual water. It provides an explanation on shale gas production when connate water is expected to block the flow path.