Refinement Study on Pressure Relief Zone of Gas Extraction Borehole Considering Roughness

Pre-pumping of coal seam gas by gas extraction borehole is an effective measure to manage gas. The existing study concluded that the pressure relief zone of the roadway is difficult to seal, and thus the sealing depth of the gas extraction borehole should generally exceed the pressure relief zone of the roadway. Based on the relationship between roughness and GSI, a numerical simulation was carried out with the engineering background of the 22200 machine tunnel of the E8 coal seam of the Sixth Mine of Pingdingshan Tianan Coal Industry Company Limited. The results of numerical simulation and field experiments show that: (1) the quantitative standard of whether the borehole has “air leakage” or whether the surrounding rock cracks around the borehole are connected to the free surface can be characterized by permeability; (2) the variation rate of relative permeability has an inflection point at 2–4 m from the free surface of the roadway wall; (3) the dividing point of whether the surrounding rock cracks around the borehole are connected to the free surface can be set at 6 m, which is located in the pressure relief zone. (4) when the sealing depth of the coal roadway is 6 m and the sealing depth of the rock roadway is 5 m, there is no gas leakage in the borehole, that is, there is an area where a crack is not connected with the free surface of the roadway in the pressure relief zone.