Characterisation of mechanics and flow fields around in-seam methane gas drainage borehole for preventing ventilation air leakage: A case study

Coal mine methane (CMM) drainage via adopting in-seam horizontal boreholes is an effective way to decrease ventilation cost and production delay, ensure mining safety, reduce greenhouse gas emissions and increase the supply of energy resource. Ventilation air leakage into borehole, however, occurs frequently, which decreases the methane-drainage efficiency. Despite extensive anti-leakage method investigations, a very few studies have been focused on spraying air-proof materials on the roadway rib and optimizing borehole sealing along the borehole, along with taking the effects of sequential excavations of roadway and borehole into account. In this paper, based on data of a real coal mine, a fully coupled mathematical model was developed by incorporating coal permeability with coal mechanical properties and gas adsorption/desorption and was then implemented into a Finite Element software. This numerical simulation was used to analyse transient stress and dynamic air-leakage flow fields around the drainage borehole. Simulation results indicate that: (1) four stress areas (I to IV) exist around the roadway based on different relief status of the tri-axial stress, and stress distributions around the borehole in those four areas are different from each other; (2) based on different air-leakage degrees, on the roadway rib, there are four air-leakage levels around borehole. Meanwhile, there are four leakage areas along the borehole: FAA, SAA, HAA and VAA, and the optimal sealing length should be over 17 m (exceeding HAA) to prevent air leakage. Air-leakage results in (2) are verified mutually with the mechanics outcomes in (1). Meanwhile, real data obtained from field experiments is introduced to validate the simulation results. All those outcomes can allow for the optimal design of the spraying area and order and borehole sealing, thus provide scientific basis for an integrated anti-air-leakage method including spraying air-proof materials on the roadway wall and sealing borehole effectively, to prevent ventilation air leakage and maximize methane drainage performance.