Abstract
The apparent gas permeability of a porous medium is an important parameter in the prediction of unconventional gas production, which was first investigated systematically by Klinkenberg in 1941 and found to increase with the reciprocal mean gas pressure (or equivalently, the Knudsen number). Although the underlying rarefaction effects are well known, the reason that the correction factor in Klinkenberg’s famous equation decreases when the Knudsen number increases has not been fully understood. Most of the studies idealize the porous medium as a bundle of straight cylindrical tubes; however, according to the gas kinetic theory, this only results in an increase of the correction factor with the Knudsen number, which clearly contradicts Klinkenberg’s experimental observations. Here, by solving the Bhatnagar–Gross–Krook equation in simplified (but not simple) porous media, we identify, for the first time, two key factors that can explain Klinkenberg’s experimental results: the tortuous flow path and the non-unitary tangential momentum accommodation coefficient for the gas–surface interaction. Moreover, we find that Klinkenberg’s results can only be observed when the ratio between the apparent and intrinsic permeabilities is ${\lesssim}30$; at large ratios (or Knudsen numbers) the correction factor increases with the Knudsen number. Our numerical results could also serve as benchmarking cases to assess the accuracy of macroscopic models and/or numerical schemes for the modelling/simulation of rarefied gas flows in complex geometries over a wide range of gas rarefaction. Specifically, we point out that the Navier–Stokes equations with the first-order velocity-slip boundary condition are often misused to predict the apparent gas permeability of the porous medium; that is, any nonlinear dependence of the apparent gas permeability with the Knudsen number, predicted from the Navier–Stokes equations, is not reliable. Worse still, for some types of gas–surface interactions, even the ‘filtered’ linear dependence of the apparent gas permeability with the Knudsen number is of no practical use since, compared to the numerical solution of the Bhatnagar–Gross–Krook equation, it is only accurate when the ratio between the apparent and intrinsic permeabilities is ${\lesssim}1.5$.
Highlights
Unconventional gas reservoirs have recently received significant attention due to the shale gas revolution in North America (Wang et al 2014), but the accurate prediction of unconventional gas production remains a grand research challenge
By solving the linearized Bhatnagar–Gross–Krook equation in simplified but not simple porous media via the discrete velocity method, for the first time in the last 76 years we have pinpointed two key ingredients that lead to his discovery: the tortuous flow path and the coexistence of the diffuse and specular scatterings of the gas molecules when impinging on solid surfaces
We have found that Klinkenberg’s results can only be observed when the ratio between the apparent and intrinsic permeabilities is 30; at large ratios the correction factor increases with the Knudsen number
Summary
Unconventional gas reservoirs have recently received significant attention due to the shale gas revolution in North America (Wang et al 2014), but the accurate prediction of unconventional gas production remains a grand research challenge. It is necessary to investigate how the permeability changes with gas pressure, properties of the porous media and gas–surface interactions. For laminar flows in highly permeable porous media, Darcy’s law states that the flux q (i.e. discharge per unit area, with units of length per time) is proportional to the pressure gradient ∇p: q = − k∞ μ ∇p, (1.1). Where μ is the shear viscosity of the fluid and k∞ is the permeability of a porous medium that is independent of the fluid. For this reason, k∞ is known as the intrinsic permeability
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