Gas transport in partially-saturated sand packs

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

Gas Diffusivity and Thermal Properties of Compost-mixed Soils under Variable Water Saturation

Gas and heat transport through compostmixed landfill cover soils affect the emission of toxic gases and methane oxidization processes. In this study, we mixed soils with three different composts in the ratio of either 1:5 or 1:10 (compost:soil) to understand the effect of compost mixing for gas diffusivity and thermal properties. The gas diffusion coefficient (Dp), thermal conductivity ( ), and heat capacity (HC) were measured for soils, composts, and compost-mixed soils at different soil-water matric potentials ( ) starting from nearly saturated to = -10,000 cm H2O and dry conditions. Data were fitted to the Brooks-Corey soil-water retention curve model to estimate the bubbling pressure ( b). For all materials, Dp increased linearly with increased air content ( ), and the Penman-Call linear Dp( ) model with the model slope (C) and threshold soil-air content ( th) fitted the data well. The th values increased with increasing compost content, relating non-linearly to the Brooks-Corey b but highly linearly to the soil macroporosity. Analogous to the Dp( ) model, Penman-Call type linear ( ), and HC( ) models with slopes (C′ and C′′) and intercepts ( 0 and HC0, thermal conductivity and heat capacity at a volumetric water content of = 0) captured reasonably well the data measured from dry to wet conditions. The C′ for varied depending on the compost ratio and decreased with increasing compost ratio. The C′′ for HC, on the other hand, had less effect on the compost mix. The thermal properties under the dry condition, 0 and HC0, were well correlated to the volumetric solid content. The results from this study will be helpful towards designing compost-mixed landfill cover soils with optimal heat and gas transport characteristics

X-ray CT-derived soil characteristics explain varying air, water, and solute transport properties across a loamy field

The characterization of soil pore space geometry is important for explaining fluxes of air, water, and solutes through soil and understanding soil hydrogeochemical functions. X‐ray computed tomography (CT) can be applied for this characterization, and in this study CT‐derived parameters were used to explain water, air, and solute transport through soil. Forty‐five soil columns (20 by 20 cm) were collected from an agricultural field in Estrup, Denmark, and subsequently scanned using a medical CT scanner. Nonreactive tracer leaching experiments were performed in the laboratory along with measurements of air permeability (Ka) and saturated hydraulic conductivity (Ksat). The CT number of the matrix (CTmatrix), which represents the moist bulk density of the soil matrix, was obtained from the CT scans as the average CT number of the voxels in the grayscale image excluding macropores and stones. The CTmatrix showed the best relationships with the solute transport characteristics, especially the time by which 5% of the applied mass of tritium was leached, known as the 5% arrival time (t0.05). The CT‐derived macroporosity (pores >1.2 mm) was correlated with Ka and log10(Ksat). The correlation improved when the limiting macroporosity (the minimum macroporosity for every 0.6‐mm layer along the soil column) was used, suggesting that soil layers with the narrowest macropore section restricted the flow through the whole soil column. Water, air, and solute transport were related with the CT‐derived parameters by using a best subsets regression analysis. The regression coefficients improved using CTmatrix, limiting macroporosity, and genus density, while the best model for t0.05 used CTmatrix only. The scanning resolution and the time for soil structure development after mechanical activities could be factors that increased the uncertainty of the relationships. Nevertheless, the results confirmed the potential of X‐ray CT visualization techniques for estimating fluxes through soil at the field scale