Quantitative study of 3D numerical simulation on optimizing borehole layout spacing of gas drainage

In order to optimize the spacing of boreholes and realize efficient drainage, based on the fluid solid coupling model, a three-dimensional geometric model was established to make it closer to the field reality, the gas drainage processes of a coal mine with different borehole spacing were simulated using COMSOL software. The results show that: the effective drainage area was visualized by the three-dimensional diagram of the isobaric surface with gas pressure of 0.74 MPa, and the influence of borehole spacing on the extraction efficiency was discussed. The results show that: the effective extraction radius is about 1.5 m during gas extraction with single borehole at 120 days; when the borehole spacing d was 5 m during gas extraction with multiple boreholes at 120 days, the isobaric surface with gas pressure of 0.74 MPa was approximately cylindrical around all the boreholes, but it was sunken to the interior(there is a blanking zone); when the borehole spacing were 2.1 m, 3 m, 4 m, 5 m and 6 m respectively, the order of the volumes of the effective drainage area V changed with the growth of time; at 120 days, the order was V_{d=5 m}>V_{d=4 m}>V_{d=3 m}>V_{d=2.1 m}>V_{d=6 m}. According to the three-dimensional diagram of gas pressure isobaric surface and the order of effective drainage area volumes, the optimal borehole spacing is determined to be 4 m. In this paper, a numerical investigation method for borehole spacing based on effective extraction radius, superposition effect, shape of isobaric surface of three-dimension of gas pressure and volume of effective extraction area was presented, providing reference for optimizing layout of borehole spacing in underground coal mines.