Permeability Evolution of Coal Seam Roof Sandstone Under Thermal Treatment

The permeability evolution for coal seam roof overburden subjected to high temperature plays a significant role in efficient, clean, and safe production of underground coal gasification. This paper proposes an elastic–plastic behavior-based permeability model, which depicts the impacts of non-isothermal gas adsorption, total effective stress (mechanical and thermal stress), as well as the damage of matrix in rock on permeability. In this work, an idealized fractal geometry model known as equivalent fractured cube model was presented to quantitatively characterize the structure of rock with new matrix damage generation. Then, the availability of the elastic–plastic behavior-based permeability model was validated by the measured permeability data from the unchanged stress and pore pressure tests through three typical post-high-temperature rocks with different compactness. The experiment results indicate that with the elevation in heating treatment temperature, both the density and mass loss rate exhibit similar evolutionary trends for these three specimens, while there are significant differences in permeability evolution for them. Furthermore, the parameters (λ and η) in the elastic–plastic behavior-based permeability model possess specific physical and mechanical implications and are closely related to rock properties, which will improve the accuracy of permeability prediction. Later, the density-dependent damage variable D_ρ was introduced into a classical damage-dependent permeability model, and the results show that although the D_ρ is the apparent damage variable, it can describe the rock damage to a certain extent, which is consistent with the permeability evolution. The leading significances for this study are to provide a method of simulating the permeability evolution in coal seam roof overburden during the process of underground coal gasification.