The author writes that the generally accepted Knudsen diffusion in shales is based on a mistranslation of the flow physics and may give theoretically unsound predictions of the increased permeability of shales to gas flow. This increase of permeability comes from the micropores, fine-scale microfractures, and cracks. The nanopores in shales provide gas storage by sorption and capillary condensation of heavier gas components. In the smallest nanopores, even methane molecules are increasingly ordered and resemble liquid more than gas. These nanopores feed the macroscopic flow paths in ways that are not captured well by generally accepted equations.

Introduction

For gas pressures below 1 bar, gas permeability can exceed that of liquid substantially. Size distribution of a single pore is a distribution of radii of the largest spheres that can be fitted at each point along this pore. “Pore size” or “pore-body radius” is the radius of maximum sphere that can be inscribed into a pore, while “pore throat” refers to the radius of a minimum inscribed sphere common to two adjacent pores. In slit-like pores, pore throats and bodies are the same and pore widths are often reported to account for gas sorption. Pore sizes—whatever this term means to different authors—in the crushed samples of mudrocks are often inferred from small-angle and ultrasmall-angle neutron scattering, multistage desorption measurements, and molecular or statistical physics calculations; these sizes are not directly measured. A specific definition of pore size is provided in the complete paper.