Characteristics of Elastoplastic Consolidation by Compaction and Its Effects on Coal Permeability

This paper presents a combined experimental-analytical investigation of coal strain development under fluctuating applied hydrostatic stress. The laboratory setup mimics the isotropic volumetric compaction of coal under burial-uplift cycles in the absence of tectonic stress. Special emphasis is placed on the corresponding permeability evolution of the coal strata. Our results show that the stress-strain path is exponential, approaching a linear relation in the logarithmic stress-strain space with the monotonic increase in stress. A similar behavior is found for the strain-permeability path in the logarithmic strain-permeability space. The permeability recovery undergoes hysteresis with respect to the stress in a stress loading-unloading cycle, but the hysteresis is not manifest with respect to the strain. A theoretical geomechanical consolidation analysis was performed using an elastoplastic modelling framework. The analysis suggests that plastic strain is the cause of the hysteresis of the strain recovery in a stress loading-unloading cycle. The closed hysteresis loops manifested in stress loading-unloading-reloading cycles are promoted by the plastic strain during stress unloading and the difference in evolution rates of the elastic core between loading and unloading. The results of this study are helpful for understanding the mechanism of permeability evolution and optimizing water and coal seam gas production.