Study on the influence of coal seam permeability on gas extraction key parameters of directional long borehole

Directional long borehole drainage technology plays an important role in coal seam gas control. In this paper, the numerical model of directional long borehole is established by COMSOL, and the variation of gas extraction parameters with permeability is studied. The variation of different extraction parameters with extraction time in 0.02-0.004 25 mD permeability interval is analyzed. The results show that the gas pressure in the directional long borehole decreases with the increase of coal seam permeability, and the time required for drilling extraction to reach the standard is 400 days（permeability = 0.02 mD）, 600 days （permeability = 0.012 mD）and 1 800 days（permeability = 0.004 25 mD）, respectively. Under the same permeability of coal seam, the radial gas pressure of borehole and the fracture rate of coal seam decrease with the increase of gas extraction time, and the decrease of gas pressure and fracture rate decreases with the increase of radial depth of borehole. The evolution law of effective gas extraction radius with time is power function distribution.