Mechanism and effect analysis of enhanced gas drainage with large diameter double boreholes

To study the mechanism of enhanced gas drainage with large diameter double boreholes, numerical models of single holes with different diameters and double ø350 mm holes were established by using the RFPA^2D-Gas software in working face 30110 of Shiquan Coal Mine, Shanxi Province. Stress distribution, damage and fracture, gas permeability evolution and gas migration law of the coal around the hole were analyzed, and then the extraction effect of double ø350 mm holes was investigated. Simulation results show that the pressure relief radii of the ø100 mm, ø250 mm, ø350 mm and double ø350 mm holes are 0.25 m, 0.9 m, 1.2 m and 2.2 m, respectively. The coal damage extends to the upper left and upper right around the holes of ø250 mm, ø350 mm and double ø350 mm, and the final fracture shape is like spreading wings of a butterfly. The permeability coefficient of the coal at 1.5 m above the double ø350 mm holes can increase from 0.25 m²/(MPa²·d) to 240 m²/(MPa²·d). The gas migration rate increases with the increase of hole diameter, and gradually decreases in the drainage. However, when the coal around the hole is damaged, it will increase to a certain extent. The field investigation results show that the gas extraction effect is enhanced by the collapse of double ø350 mm holes. For the double ø350 mm hole, the drainage radius is 2 m, the gas flow can be 21 times that of the ø100 mm hole, and the total volume of extraction is more than 10 times and even 40 times of the ø100 mm hole.