Experimental investigation of anisotropy ratio evolution in coal permeability: Implications for underground compressed air energy and CO₂storage

Reliable forecasting of coal seam gas production and gas injectivity (e.g., CO₂ or air) requires an accurate understanding of coal’s anisotropic permeability, which governs the directional flow of gas. Although the anisotropic nature of coal permeability is well recognized, little attention has been paid to how this ratio evolves with changes in effective stress or with the injection of gases that have different affinities to coal. In this work, more than 600 permeability tests were conducted on eight cubic Australian coal samples using He, N₂ and CO₂ gases under varying effective stresses, providing a comprehensive dataset that allows the combined effects of effective stress and gas adsorption on permeability anisotropy to be robustly assessed on the same samples. The results demonstrated that all coal samples exhibited evident permeability anisotropy, with ratios ranging from 1.11 to 6.55. For the first time, quantitative relationships between the anisotropy ratio, effective stress, and initial permeability were established for each of the three injection gases, highlighting how gas adsorption and effective stress changes both anisotropic permeability magnitude and ratio. These findings provide new insights into the directional flow behavior of gases in coal seams, with implications for underground compressed air energy storage and CO₂ sequestration.