Creep-based permeability evolution in deep coal under unloading confining pressure

Abstract

To better understand the effect of creep deformation on gas permeability, creep experiments under unloading radial stress (URS) were carried out based on the in-situ stress state in coal at depth. The gas permeability was measured using a transient pulse decay method along with the axial and radial strains during all creep tests. The evolution of coal permeability was analysed with time and creep deformation. The results show that gas permeability evolution is sensitive to the deformation process in such coal and can be used as an indicator of microstructural change such as the growth of micro-cracks. During the creep tests, the permeability decreases in the primary creep stage, remains stable in the steady creep stage and increases rapidly in the accelerated creep stage. It can be further generalized that the permeability curves are concave upward in the time or strain space. Then, an improved permeability model was suggested by assuming that percolating network in correlation with permeability provides the minimum potential energy of coal during the evolution of pore or fracture in coal. The model developed in the study provides a better agreement with the entire process of permeability evolution, which first decreases, maintains a constant level, and then increases during creep tests of URS in deep coal.