Gas diffusivity‐based characterization of aggregated soils linking to methane migration in shallow subsurface

Abstract

Methane transport in soil is primarily affected by soil physical conditions such as soil texture and soil structure, soil moisture, soil-gas diffusivity, permeability, and soil temperature. Aggregated soils have distinct soil structure with two pore regions characteristics (i.e., interaggregate and intraaggregate regions) and therefore show bimodal behavior with respect to soil physical properties controlling gas migration. This study characterized an aggregated soil retrieved near a natural gas (NG) extraction site at Denver-Julesburg (D-J) basin in northeast Colorado (USA) with respect to soil-water characteristic (SWC), pore-size distribution, gas diffusivity and thermal conductivity. The investigated soil exhibited distinctive two-region characteristics, which were adequately parameterized with extended, existing, and newly developed bimodal functions. We carried out an analysis with integrated model parameters to obtain a graphical insight on the correlation of properties. In addition, CH 4 concentration profiles originated from a point source representing a buried pipeline leakage at three different flow rates (6, 12, and 24 L min -1 ) were simulated with a numerical tool that can simulate the multiphase flow of gas mixture under dry and different saturation conditions of the soil. Simulated results highlighted pronounced effects of soil moisture and, to a lesser degree, of gas leakage rate on subsurface CH 4 concentrations profiles, suggesting diffusion-dominated movement of CH 4 in subsurface.

Soil-Gas Diffusivity-Based Characterization of Variably Saturated Agricultural Topsoils

“…where α 1 , β 1 , and α 2 , β 2 (dimensionless) are parameters characterizing the Region 1 (interaggregate) and Region 2 (intraaggregate), respectively, while w is a weighing factor that numerically demarcates Region 1 and Region 2. In an extended formulation, Shanujah et al [21] presented a two-region model incorporating an additional parameter ε p (cm 3 cm −3 ) , referred to as a gas percolation threshold, an air-filled porosity below which the gas diffusivity is negligibly small due to extremely high moisture-induced tortuosity in the soil-gas phase in Region 1. Thus, the model for Region 1 takes the form: For Region 1:…”