Numerical simulation on gas drainage and borehole arrangement parameters of bedding borehole in a coal mine

Based on comprehensive consideration of gas flow inertia and slippage effect, a gas drainage flow model was established. Based on engineering background of 12150 coal face of the No.15 coal seam in Songhe Coal Mine, the distribution law of gas pressure and the change of permeability under condition of single borehole and multi—borehole were analyzed by numerical simulation, and the reasonable diameter and the layout parameters of borehole were obtained combined with the effective radius of borehole drainage. When gas is extracted with single borehole for 240 days, it is more appropriate to select 65 mm diameter through comparing gas extraction effect of borehole with diameter among 40, 65, 75, 94 mm. When three boreholes are spaced at 3, 4 and 5 m respectively, it is concluded that when the pre—extraction time is more than 180 days, the boreholes spacing of 5 m is more appropriate on the basis of analysis of effective radius of gas extraction at different time periods. When the extraction time is between 120 days and 150 days, it is more appropriate to select the 4 m borehole spacing. When the extraction time is less than 120 days, it is more appropriate to select the 3 m borehole spacing. The permeability of coal seam increases gradually with the increasing of extraction time, but the increasing amplitude gradually decreases. At the initial stage of extraction, the gas pressure gradient is larger, a large amount of adsorption gas is desorbed, and the gas pressure is greater than the adsorption expansion stress, and the fracture—pore channel is opened, at this time the permeability is relatively large. In the middle and late stages of extraction, the gas pressure continues to decrease, the gas pressure has little advantage over the adsorption expansion stress, and the increase value of fracture—pore is small, resulting in a small increase of coal seam permeability during the middle and late stages of extraction.