Flow capacity characterization of unconventional natural gas bearing rocks using digital rock physics: A comparison between advection and diffusion

Abstract

Experimental flow capacity characterization of unconventional rocks is a nontrivial task, and the relative contribution of advective vs. diffusive flow mechanisms remains illusive. Herein we apply a digital rock physics workflow to systematically compare the advection and diffusion behavior of supercritical methane in two 3D reconstructed unconventional shale and coal core samples. Nitrogen sorption experiments and scanning electron microscope observations are combined to inform the construction of representative pore network models. For advection modeling, we consider the nanometer-scale confinement effect and pore shape effect, while for diffusion modeling the contributions of Fick diffusion, transitional diffusion, and Knudsen diffusion are considered separately based on the Knudsen number. The effect of pore size, shape, pressure, and temperature on methane flow capacity in a single nanopore is first studied. Then the apparent permeability and effective diffusivity of the two reconstructed nanoporous media are calculated and compared. We demonstrate that at reservoir pressure and temperature conditions, diffusion contributes to mass flux (above 10%) only in micropores and small mesopores (< 10 nm). For most realistic unconventional rocks, the contribution of bulk diffusion can be neglected when estimating the flow capacity.