Gas transport in partially-saturated sand packs

TL;DR

This study uses a physically-based model combining percolation theory and effective-medium approximation to analyze gas diffusion and permeability in sand and glass bead packs, revealing near-linear behavior at high gas contents.

Contribution

It introduces a novel application of PT and EMA to model gas transport in porous media, validated with experimental data and pore structure analysis.

Findings

Gas diffusion and permeability are nearly linear at high gas-filled porosities.

Pore-throat size distribution has minimal effect on gas transport in the studied packs.

The pore coordination number z ranges between 2.8 and 5.3, consistent with CT results.

Abstract

Understanding gas transport in porous media and its mechanism has broad applications in various research areas, such as carbon sequestration in deep saline aquifers and gas explorations in reservoir rocks. Gas transport is mainly controlled by pore space geometrical and morphological characteristics. In this study, we apply a physically-based model developed using concepts from percolation theory (PT) and the effective-medium approximation (EMA) to better understand diffusion and permeability of gas in packings of angular and rounded sand grains as well as glass beads. Two average sizes of grain i.e., 0.3 and 0.5 mm were used to pack sands in a column of 6 cm height and 4.9 cm diameter so that the total porosity of all packs was near 0.4. Water content, gas-filled porosity (also known as gas content), gas diffusion, and gas permeability were measured at different capillary pressures. The X-ray computed tomography method and the 3DMA-Rock software package were applied to determine the average pore coordination number z. Results showed that both saturation-dependent diffusion and permeability of gas showed almost linear behavior at higher gas-filled porosities, while deviated substantially from linear scaling at lower gas saturations. Comparing the theory with the diffusion and permeability experiments showed that the determined value of z ranged between 2.8 and 5.3, not greatly different from X-ray computed tomography results. The obtained results clearly indicate that the effect of the pore-throat size distribution on gas diffusion and permeability was minimal in these sand and glass bead packs.