Construction and application of coal permeability model in reverse fault area

The superposition of tectonic stress and geostress in the reverse fault area, and the mechanical characteristics formed by compressive stress lead to gas accumulation, change of coal permeability, and easy to cause abnormal gas emission during mining, even coal and gas outburst accidents. In order to master the evolution law of the permeability of coal bodies mined in the reverse fault area, firstly, the permeability test of coal bodies mined in the reverse fault area was carried out, and the stress change of coal bodies mined under the influence of reverse faults was simulated by stress loading and unloading. It was concluded that in the pre peak stage, the coal body compressed, the cracks closed, and the permeability of coal bodies decreased; in the post peak stage, the coal stress reaches the peak, the original fractures expand and connect, while new fractures are generated and damaged, and the coal permeability increases to the maximum. The first loading scheme simulates that the permeability of coal samples M1, M2 and M3 increases by 22.1%, 28.0% and 36.7% respectively when the stress concentration factor of coal in front of the workingface increases gradually; the second loading scheme simulates that the permeability of coal samples M4, M5 and M6 increases by 23.6%, 37.2% and 20.8% respectively when the stress concentration factor of coal in front of the working face increases first and then decreases. Then, combined with the permeability test results of coal, a characterization model of coal permeability under the influence of reverse fault is established, and the calculation expressions of coal permeability at the pre peak and post peak stages are derived. The function of coal volume strain caused by gas adsorption/desorption is used to express the impact of adsorption/desorption on coal fracture volume, so as to more accurately characterize the coal permeability under the influence of reverse fault. Finally, the permeability model is imported into COSMOL software, and combined with the on-site situation of F4 reverse fault in 1503 working face of Xinchun Coal Mine, the simulation calculation is carried out. It is concluded that with the decrease of the distance from the reverse fault and the increase of the stress concentration coefficient of the coal, the gas pressure and the peak permeability of the coal are gradually increasing, which is easy to cause abnormal gas emission, and the gas concentration monitoring needs to be strengthened.