Impact of gas adsorption on coal relative permeability: a laboratory study

Abstract

CO₂ geo-sequestration and compressed air energy storage in depleted coal seam gas reservoirs are promising techniques for mitigating the greenhouse effect and combating climate change. However, gas adsorption-induced swelling in coal matrices poses challenges to gas injectivity by reducing both coal absolute permeability and relative permeability. While the sorption-induced impact on absolute permeability has been extensively studied, its impact on relative permeability remains little explored. To address this gap, a suite of two-phase flow experiments was conducted with both absorbing and non-absorbing gases. A series of relative permeability curves for helium-water, nitrogen-water, and CO₂-water systems were obtained. The results show lower relative permeability for absorbing gas-water systems (nitrogen and CO₂) compared to non-absorbing gas (helium) due to the sorption-induced swelling impact. Specifically, the relative permeability of helium-water systems is more than two times higher than that of nitrogen-water systems, followed by CO₂-water injection due to differences in adsorption capacity. Finally, quantitative correlations for estimating the relative permeability of nitrogen-water and CO₂-water systems were obtained, based on four newly introduced coefficients. These coefficients enable direct estimation of absorbing gas-water two-phase flow behavior (e.g., CO₂ sequestration and compressed air storage) in coal. The applicability of these coefficients was further validated using data from other studies, providing useful insights for assessing the injectivity of CO₂ geo-sequestration and underground compressed air energy storage.