Using stress path-dependent permeability law to evaluate permeability enhancement and coalbed methane flow in protected coal seam: a case study

Pressure-relief mining is the primary method to remove gas and coal outbursts and realize coal and gas simultaneous extraction in deep, high-gas, and low-permeability coal seams. Numerical modeling has been widely used to evaluate the effectiveness of protective coal seam (PCS) longwall mining, which usually employs experiment-derived permeability models to describe permeability changes and coalbed methane (CBM) flow. Due to the difficulty of tests and data available, published models and parameters were commonly utilized in simulations, which, however, may limit the accuracy of the results because of their case-dependent nature. In this paper, a series of laboratory tests were performed on different damage degree rock mass, i.e., broken rock samples (caving zone), fracture coal samples (fracture zone and swelling deformation zone), and raw coal samples (original zone), to obtain permeability models for different damage zones surrounding the longwall face. Besides the stress states, the experiments also highlight that the damage degree and stress path play vital roles in the permeability changes. The stress sensitivity of permeability in unloading stages is found to be significantly lower than that in loading stages, indicating the importance of the stress path when simulating longwall mining-induced permeability changes. A stress path-dependent permeability law was proposed, and the associated algorithm was designed and embedded into the FLAC3D code. For the first time, the stress path-dependent permeability law was employed to simulate the pressure relief, permeability enhancement, and CBM flow in the protected coal seam (PDCS) when longwall mining is executed in the upper PCS. Field measurements proved the feasibility and accuracy of the new numerical method. Numerical results also suggest the common loading permeability model overestimates the effectiveness of the pressure relief mining. This study highlights the significant role of stress path-dependent permeability laws to accurately predict the permeability changes and CBM flows in pressure relief mining.