Theoretical and experimental validation of mining-enhanced permeability for simultaneous exploitation of coal and gas

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Simultaneous recovery of coal and methane is a promising solution for making reasonable use of energy resources and preventing gas disasters. Coal mining produces an effect that enhances the permeability of coal and thus paradoxically enhances the ability to collect methane. Hence, existing knowledge about permeability enhancement needs to be improved. Therefore, a new parameter, referred to as mining-enhanced permeability, is proposed to characterize the evolution of permeability for coal and other rocks. A theoretical expression for the new parameter is deduced from the Tablet Fluid model. To analyze the mining-enhanced permeability of coal mass quantitatively, a laboratory test was conducted to simulate the stress environment of the protected layers in protected coal seam mining. At the same time, a field test was conducted at a working face of Mine No.10 at the Pingdingshan Coal Mining Group in China. The evolution of the volumetric strain and mining-enhanced permeability for both protective and protected layers were analyzed quantitatively. The loading-induced volumetric strain and permeability enhancement in the laboratory showed improvements of 1.59 and 72.26 %, respectively. The mining-induced volumetric strain and permeability enhancement also showed improvements of 0.14 and 7.77 %, respectively. The evolution of enhanced-permeability agreed with gas flow data observed in both laboratory and in situ tests. Namely, a higher mining-enhanced permeability led to high gas flow, demonstrating the plausibility of mining-enhanced permeability theory based on the Tablet Fluid model. The theory describes the impact of mining on coal permeability and quantitatively captures the evolution of coal permeability. In conclusion, the proposed theory provides a strong foundation for designing systems to simultaneously recover coal and gas.