Optimizing borehole spacing for coal seam gas pre-drainage

In-seam boreholes are widely used in underground coal mines to improve gas pre-drainage. An important consideration in this gas extraction method is selecting the appropriate borehole spacing. In this study, appropriate boreholes spacing was determined using FLAC3D software and control equations for seepage and deformation analysis and coupling equations for porosity and permeability analysis. A range of borehole spacing was considered that took into account the influence of drainage time and the permeability of the coal. A series of borehole spacing tests were conducted at an underground coal mine in Henan Province, China. The results showed that coal permeability, drainage time and negative gas pressures were the main factors to consider when determining an optimum borehole layout. The negative drainage pressure and the drainage time are very important factors when determining an optimal borehole layout. The optimal drainage radius measured during the field test was 3-4 m, which confirmed the results of the FLAC3D numerical simulations.