Gas Permeability Characteristics and Energy Evolution Laws of Gas-Bearing Coal under Multi-Level Stress Paths

Gas permeability in coal characterizes the migration capacity of gas in coal, and it is an important parameter influencing the gas flow in a coal seam. Energy dissipation and energy release are important causes of rock mass failure. By utilizing a THM-2-type thermo-fluid-solid coupling test system for gas-bearing coal developed at Chongqing University, China, experimental research was conducted on raw coal samples from the Jinjia Coal Mine, Guizhou Province, China, under two different cyclic loading paths. The experimental results demonstrated that the overall change trends of permeability were consistent under the different loading and unloading paths and decreased exponentially with increasing stress. As the unloading level increased, the permeability hysteresis loops changed from sparsely to densely distributed and from spindle-shaped to crescent-shaped and ladder-shaped. The change laws of the upper and lower limits of permeability were closely related to the stress paths. The change trends of the lower limit of permeability under the two paths were consistent with that of the overall permeability and decreased exponentially with the increase in energy density. When the lower limit of unloading stress was constant, the upper limit of permeability gradually increased exponentially with increasing energy density, indicating that the gas permeability in the coal samples had strong hysteresis characteristics. The energy densities under the different paths nonlinearly increased with increasing stress; these relationships could be fitted by quadratic functions. The total input energy density showed the fastest rate of increase, followed by the elastic energy density, while the dissipated energy density had the slowest rate of increase, demonstrating that the energy evolution showed nonlinear characteristics. The experimental results were in good agreement with the evolution characteristics of the permeability of the stress loading and unloading zones in front of the working face. Understanding permeability from the perspectives of different stress loading and unloading paths and energy densities provides certain insights into the safe production and extraction of coalbed methane from coal mines.