Shale gas transport through the inorganic cylindrical and conical nanopores: A density gradient driven molecular dynamics

Pores heterogeneity is an essential part of porous media, however, the effect of the changeable pore is usually overlooked in studies on the shale gas transport in inorganic nanopores. Research currently focuses on inorganic nanoslits that fail to simulate the realistic methane transport through cylindrical and conical inorganic nanopores because variable apertures are not captured in the simulation. However, the impact of such varied apex angles and diameters on methane transport capacity in nanopores has not been studied yet in inorganic shale nanopores. To compensate for the defect, density gradient-driven molecular dynamics (DGD-MD) are used to study the methane transport behavior through the cylindrical and conical inorganic nanopores. The clay material (Ca-montmorillonite) is used to establish the conical nanopores with different apex angles and the cylindrical inorganic shale nanopores with different diameters. Main results show that the permeability of methane increases with enlarged nanopores diameter in cylindrical inorganic nanopores and can be described by the Hagen-Poiseuille (HP) equation with the effective viscosity. However, the flux of methane through cylindrical nanopores is greater than that obtained by using the HP equation under high pressure drop owing to the intensive surface diffusion of gas and positive slippage. Methane is most readily absorbed in the conical nanopores with an apex angle of 9.6 degrees. The flow behavior of methane molecules gradually changes from negative slippage to positive slippage through the conical nanopores and this will result in the overestimation flux obtained by the HP equation. The flow capacity of confined methane predicted by the modified HP equation combining with apex angle is in good agreement with the simulation results. This study provides a molecular-level understanding of the key relationship between variable apex angles and permeability through the cylindrical and conical nanopores, and it can apply to the flow behavior of gas at the nanoscale in biological, chemical, medical, and physical fields. (c) 2021 Elsevier Ltd. All rights reserved.