Modeling and simulation of gas transport in carbon-based organic nano-capillaries

Modeling of transport of fluids in organic nanopores of shale is associated with complexities due to existence of ultratight pores and strong fluid-solid interactions. In this study, adsorption and transport of three different gases (argon, methane, and neon) are investigated by performing molecular simulations of identical setups of nano-scale carbon capillaries made up of smooth graphite sheets. The simulations are performed for capillaries with 2, 4, 6, and 8 nm diameters for a wide range of pressures and pressure gradients. The velocity, density, diffusion coefficients, and viscosities of the gases are computed and compared. Based on the molecular simulation results, as the pressure of the system increases, amount of the adsorbed gas molecules on the capillary walls increases to reach the saturation state. Computed velocity profiles show that for all gases in the capillaries, velocity profiles are plug-shaped. Computed gas velocities from molecular simulations are compared with the velocities predicted by regularized 13-moment model by adjusting the tangential momentum accommodation coefficients (TMAC). The calculated TMAC values are found to be between 0.03 to 0.18 for the pressure range tested. It is found that as the Knudsen number increases, the molecules reflect more specularly in the capillaries. We also investigated the effect of adsorption on TMAC values and found that the TMAC values can be predicted by including the effect of adsorption surface coverage into consideration. Furthermore, it is found that the Knudsen diffusion model underestimates the molecular fluxes in the capillaries by at least one order of magnitude. The corrected and transport diffusivity coefficients decrease as pressure increases and approach one another at low pressures. The viscosity of the gases are calculated for different fugacities and Knudsen numbers based on self-diffusivity coefficients for two capillary sizes of 2 and 4 nm. The computed viscosities of all gases decrease as pressure increases. For all gases, the viscosity under confinement are lower than their nominal values. Viscosity values for argon and methane increase approximately linearly with fugacity for both capillaries. The viscosity values of neon for the capillaries are almost the same for fugacities less than 90 atm. It is also shown that the viscosity models for gases under confinement are not good candidates to be used in organic nanopores due to adsorption of gases to pore surface. (C) 2017 Elsevier Ltd. All rights reserved.