Research on damage-based coal permeability evolution for the whole stress–strain process considering temperature and pore pressure

In order to understand the permeability evolution of coal under different temperature and pore pressure conditions, a series of experiments are carried out by simulating stress state and temperature conditions of coal in depth, meanwhile the permeability is obtained by calculation using fractional derivative calculation method. A damage-induced permeability model for the complete stress–strain process of deep coal considering temperature and pore pressure is proposed by introducing the non-uniform coefficient. The model is validated by experimental data from different perspectives. Results show that the initial permeability decreases with increasing temperature while it increases with pore pressure rising. Furthermore, the developed permeability model is proved to agree better with the experimental data of permeability evolution with pore pressure in comparison with the traditional models. The experimental results of permeability evolution caused by temperature and axial stress can be characterized by the proposed permeability model well. Finally, the model can accurately characterize the piecewise permeability evolution that it decreases in compaction stage and increases in dilation stage with axial strain. Discussions on the effects of non-uniform deformation coefficient and permeability-damage coefficient on the permeability model indicate that the increasing non-uniform deformation coefficient will accelerate permeability evolution. The permeability trend in the volumetric dilation stage is found to be more sensitive to the permeability-damage coefficient in comparison with the compaction stage.