Flow of Gases in Slit Shaped Organic Nanopores of Shale: A Boundary-Driven Molecular Simulation Study

Abstract

Abstract In modeling of fluid transport in organic nanopores of shale, particular attention should be paid to the gas-wall interactions, specifically the adsorption phenomena, due to the fact that the size of pores are comparable with the mean-free-path of the gas molecules. The objective for this work is to fulfill the need for the investigation of how much the adsorbed phase contributes to the total mass flux of organic nanopores. Molecular Dynamics (MD) is proved to be a credible technique to examine dynamics of atomic-level phenomena. In this study, transport of four different gases, Methane and Argon (adsorbing) and Helium and Neon (less-adsorbing), is studied and their transport are analyzed using dual control volume grand canonical molecular dynamics (DCV-GCMD) simulations with identical setups of graphite nano-channels. DCV-GCMD simulations are performed for different pressures, pressure gradients, and channel sizes. For each simulation, profiles of velocity, mass flux, and density across the channel height are calculated. Based on the DCV-GCMD simulation results, as the pressure of the system increases, the number of gas molecules adsorbing to the graphite walls increases to reach a state of full single-layer coverage. The absolute adsorption of a particular gas is the same for both 2 nm and 5 nm channel. However, the excess adsorption of gases in 2 nm channel are less than those in 5 nm one. Normalized velocity profiles of Argon and Methane become less concave as the pressure increase. This is in contrary to theory of slip. The normalized velocity profiles of less adsorbing gases demonstrate a plug shape type flow. As the average channel pressure increases, the contribution of the adsorbed phase to the total mass flux decreases. Furthermore, the results show that the channel length have significant impacts on transport of gases through nanochannels.