Permeability evolution of deep coal samples subjected to energy-based damage variable

A better understanding of mining-induced permeability evolution of coals is of an importance for underground coal extraction and coalbed methane exploitation. In this paper, a laboratory study of coal permeability evolution is carried out by simulation of in-situ mining-induced stress state of coals in a deep coal seam. The permeability of coals is estimated by the fractional derivative transient pulse method. And the whole deformation and failure process of coals is studied on the basis of the energy evolution, after which a modified damage variable is defined based on accumulative dissipated energy. Moreover, the evolution of permeability is characterized with damage variable, indicating that the dissipated energy density and the damage variable increase in the post-yield stage. The permeability reduces in the process of stress-induced compaction and starts to increase after the onset of dilatancy. It is found that the permeability appears to be a negative power correlation with damage variable in compaction stage while a positive power relationship is observed after dilation. Furthermore, a correlation formula of permeability and energy-based damage variable is suggested to consider the effects of mining depth on permeability of coal.