Gas Migration around Large Diameter Borehole with the Protective Screen Pipe in Fractured Soft Coal Seam

Large diameter borehole drilling in fractured soft coal seam may lead to problems such as drilling blockage, perforation, and hole collapse. The protective screen pipe in the drilling hole can support the damaged coal, prevent the drilling blockage, and improve the coal permeability. The mathematical model of gas drainage by large diameter borehole in fractured soft coal seam was established. Taking the 3205 working face in Shanxi Fenxi Zhongxing Coal Mine as the background, the numerical simulation was carried out by importing the mathematical model into COMSOL Multi-physics software to study the gas migration law around large boreholes with protective screen pipe in fractured soft coal seam. The results show that the gas flow velocity in section A of the borehole and pipe is between 0 and 0.25 m/s, and the gas flow velocity in the borehole is smaller than that in the pipe and screen hole. The gas velocity in the borehole is lower than that in the protective screen pipe. In the large diameter borehole, the protective screen pipe can support the collapsed coal and prevent the borehole blockage. The free gas in the borehole can be quickly extracted out through the screen holes, which ensures the gas extraction efficiency. The results provide a guidance for promotion and application of the technology of large diameter borehole with protective screen pipe in field.