  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.
