Molecular dynamics simulations of shale gas transport in rough nanopores

Abstract

For the effective exploration and development of shale gas, it is important to understand the transport and adsorption mechanisms of methane, the main composition of shale gas. The wide variety of mineral components in shale matrices makes the pore surface complex, causing difficulty to understand the mass transport and adsorption behaviors within shale pores. Molecular dynamics (MD) have high fidelity to simulate the domains with complex geometries at nanoscale. However, most of the previous MD studies focused on the mechanisms of methane in slit nanopores, which could not reflect the real adsorption and transport behaviors of shale gas in nanopores with complex structures. This work studied the fluid properties in shale nanopores with high relative roughness and complex boundary geometries. The graphene sheets with different geometrical structures are used to characterize the complex boundary of shale pores. In addition, the effects of temperature, pressure, driving force, relative roughness, and absolute roughness on the adsorption and transport behaviors are investigated. In this study, shale gas diffusion is developed by the Einstein method to calculate the spatial diffusion coefficient D, planar diffusion coefficient Dxy, and vertical diffusion coefficient Dz of gas molecules to quantify and analyze the effects of temperature and pressure on shale gas diffusion. The results indicate that the main diffusion mechanism of methane is planar diffusion. Diffusion coefficients are sensitive to temperature and pressure changes for rarefied gases. Then, Grand Canonical Monte Carlo (GCMC) method is carried out to investigate the adsorption behavior. The simulation results show that the adsorption density increases with the increase of pressure and the decrease of temperature. The pressure-driven flow behavior of methane in nanopores is investigated by using non-equilibrium molecular dynamics (NEMD). The results indicate that the flow velocity of methane increases with the driving force. Moreover, the rough elements near wall dampen the transport of methane molecules.