Measurement of Gas Diffusion Coefficient in Highly Permeable Porous Media

Abstract

Core Ideas A modified method was used to quantify gas diffusion within coarse media. All data were highly correlated to an inverse solution of the diffusion equation. DP/D0 results were in the range of common DP/D0 predictive models. Observed increase in gas transport with permeability was named enhanced diffusion. Gas diffusion is a fundamental transport process in porous media. However, to date, there is no method for experimentally estimating the gas diffusion coefficient of coarse porous media. A modified method is proposed here and was validated against sand media. The method was tested using particle sizes ranging between 1 and 4 cm: ∼10−3 cm2 < intrinsic permeability < ∼10−2 cm2. Laboratory experiments were conducted in large horizontal columns filled with different homogeneous spherical particles inside a climate‐controlled laboratory under isothermal (± <0.2°C) and windless conditions. Diffusion coefficients were similar for particles sizes of 1 and 2 cm, which indicates that pure diffusion governs gas transport. Above 2 cm, diffusion coefficients were higher, suggesting an enhanced gas transport, most likely initiated by small, local advective components. Because the enhanced transport can be modeled by the diffusion equation using a larger diffusion coefficient, it is associated with the enhanced diffusion concept.

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“…The spatial and temporal distribution of CO 2 at the earth-atmosphere interface and at the boundary layer above it is of great importance for soil, agricultural and atmospheric sciences. In general, the source of elevated CO 2 concentrations at this boundary layer depends mostly upon root respiration and soil microbial activity [2] , [3] , [4] and on atmospheric transport (diffusion, dispersion, and advection) [5] , [6] , [7] , [8] .…”