Numerical test and application of gas pre-drainage in an extra-thick seam by using ground vertical boreholes

This study is to solve the problems of the low content and high gush of gas under high-intensity mining conditions in Tashan coal mine, and to make up the disadvantages of high cost and difficult operation of large-scale physical experiments and field tests. According to the geological and gas occurrence conditions of regional coal seam which covers 8101 working face in Tashan coal mine, numerical testing schemes were designed to optimise and analyse the gas pre-drainage effect in the extremely thick seam by using ground vertical boreholes. Based on the pore characteristics of coal-rock (body), we developed mathematical constitutive models of gas-solid coupling and permeability-damage coupling in the gas-bearing coal-rock (body) rupture process. A numerical model of gas drainage was established by using RFPA2D-Gasflow software, and the relevant physical parameters, simplified initial conditions and boundary conditions were established. The following results can be achieved through the above numerical tests. First, the reasonable borehole bottom location of ground vertical boreholes is at the bottom of coal seam. Then, on the basis of considering the drilling cost and the drainage effect, the reasonable borehole spacing between adjacent ground vertical boreholes is from 50 to 60 m. Meanwhile, the actual application effect of gas extraction by using ground vertical boreholes in 8101 working face demonstrates a good gas drainage effect when the borehole bottom location is at the bottom of coal seam and the borehole spacing is 50 m. To a certain extent, the rationality and feasibility of numerical tests are further verified.