Carbon dioxide flow and interactions in a high rank coal: Permeability evolution and reversibility of reactive processes

Uncertainties exist on the efficiency of CO₂ injection and storage in deep unminable coal seems due to potential reduction in the permeability of coal that is induced by CO₂ adsorption into the coal matrix. In addition, there is a limited knowledge about the stability of CO₂ stored in coal due to changes in gas partial pressure caused by potential leakage. This paper presents an experimental study on permeability evolution in a high rank coal from South Wales coalfield due to interaction with different types of gases. The reversibility of the processes and stability of the stored CO₂ in coal are investigated via a series of core flooding experiments in a bespoke triaxial flooding setup. A comprehensive and new set of high-resolution data on the permeability evolution of anthracite coal is presented. The results show a considerable reduction of permeability above 1.5 MPa CO₂ pressure that is correlated with the coal matrix swelling induced by CO₂ adsorption. Notably studied in this work, the chemically-induced strain due to gas sorption into coal, that has been isolated and quantified from the mechanically-induced strain as a result of changes in effective stress conditions. The results of post-CO₂ core flooding tests using helium (He), nitrogen (N₂) and methane (CH₄) demonstrated a degree of restoration of the initial permeability. The injection of N₂ showed no significant changes in the coal permeability and reversibility of matrix swelling. The initial permeability of the coal sample was partially restored after replacing N₂ by CH₄. Observation of permeability evolution indicates that the stored CO₂ has remained stable in coal under the conditions of the experiments.