The impact of effective stress and gas slippage on coal permeability under cyclic loading

The determination of the effective stress coefficient of porous media (such as coal) remains a controversial issue. The purpose of this paper is to determine the effective stress coefficient of coal during gas penetration and to investigate the impact of effective stress and gas slippage on coal permeability under cyclic loading conditions. Analyzing the evolution law of coal anisotropic permeability with effective stress allows the deformation characteristics of the coal's internal structure, such as cleat or bedding, to be studied. The effective stress coefficient of long flame coal is obtained through modified permeability models based on experimental data. Test results show that the slippage effect significantly influences the permeability of coal samples, specifically in the range of low pore gas pressure, and that the effect of gas slippage is larger than that of effective stress. Permeability decreases gradually when effective stress increases, but it increases during unloading, and ascending and descending curves show significant irreversibility of permeability. Moreover, PLR (permeability loss rate) and IPLR (irreversible permeability loss rate) results indicate that the influence of effective stress on permeability perpendicular to bedding is greater than that of permeability parallel to bedding and that the ability of the cleat to resist deformation induced by effective stress is weaker than that of bedding. Under identical pressure conditions, the cleat shows more vulnerability and produces larger plastic deformation.