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The non-equilibrium transport of inhomogeneous and dense gases highlyconfined by surface is encountered in many engineering applications. Forexample, in the shale gas production process, methane is extracted fromultra-tight pores under high pressure so the gas is inhomogeneous and dense.Currently, the complex non-equilibrium transport of inhomogeneous and densegases where gas surface interactions play a key role is commonly investigatedby molecular dynamics or on a continuum-assumption basis. Here, a tractablekinetic model based on the generalized Enskog equation and the mean-fieldtheory is employed to couple the effects of the volume exclusion and thelong-range intermolecular attraction forces. The interactions between gasmolecules and confined surface are modelled by a 10-4-3 Lennard-Jonespotential, which can capture gas surface adsorption. The cross-sectionaldensity profiles of methane under different confinements are in good agreementwith the molecular dynamics results reported in the literature, and thetransport behaviors are validated by the non-equilibrium molecular dynamics.The velocity of methane flow in shale matrix is plug-like due to its densecharacteristics in nanopores. The influence of pressure, temperature, pore sizeand shale composition on density and velocity profiles is analyzedquantitatively. Our results show that the Klinkenberg correction is notapplicable to model shale gas flow in the production process; the Navier-Stokesmodel using the second-order slip boundary condition cannot produce the propervelocity profiles, and consequently fails to predict the accurate flow rate innanopores. This study sheds new light on understanding the physics ofnon-equilibrium dense gas flows in shale strata.

Non-equilibrium transport of inhomogeneous shale gas under ultra-tight confinement