The seepage properties and permeability enhancement mechanism in coal under temperature shocks during unloading confining pressures

Abstract

To understand the gas penetration properties during unloading confining pressures and permeability enhancement mechanism in coal, temperature shocks, seepage tests, NMR, CT and SEM scanning tests were carried out with trimmed coal samples. The results showed that the coal permeability increased with the decrease of confining pressures after temperature shocks performed, and that the coal permeability fully followed an exponential change law. To achieve a higher gas permeability, heating-cooling shocks should be executed in low stress regions and stress-releasing areas in coal. According to the NMR results, the increase in pore volume improved the gas diffusion coefficient of coal. The CT and SEM scanning results declared that the growth and extension of internal and surface cracks eventually enhanced the coal permeability. After temperature shocks performed, it can be concluded that coal anisotropy and excessive thermal stresses are the essential causes of the coal damage and gas permeability enhancement.