Research on the Optimized Layout of Boreholes in Low-Permeability Coal Seams to Eliminate Gas Drainage Blank Zones

Abstract

With the increasing depth and complexity of coal mining in China, the gas control in coal seam has become increasingly important. Gas predrainage through boreholes is an effective means of gas control, and the effective drainage radius of boreholes is a significant index for gas drainage. Taking Sijiazhuang Coal Mine as the research object, the gas drainage radius of seam 15 at different drainage times have been measured. A gas–solid coupling model for coalbed gas drainage has been established by governing equations between porosity, permeability, stress, and seepage, which was applied to optimize the layout of coalbed gas drainage boreholes. The results show that the drainage radius of conventional boreholes in Sijiazhuang Coal Mine increases with time, namely, 3.0, 4.0, 4.4, and 5.0 m on 35, 70, 100, and 120 days, respectively. The model validation results are consistent with the field measured values, confirming the feasibility of the established model in simulating gas drainage effectiveness. When the borehole spacing is less than or equal to 3 m, the effective drainage rate reaches 100% with no blind zones; when the spacing is greater than or equal to 3.5 m, the effective drainage rate drops below 82.7% and blind zones appear. Therefore, the optimal spacing should be controlled within 3 m. The comparison of layout patterns shows that under the same number of boreholes, the double-row staggered (three-flower) layout has a larger negative pressure acting area and more uniform pressure reduction in the roof and floor compared with the single-row layout, with the time to meet standards shortened from 58 to 55 days, significantly improving the drainage efficiency.