Position characteristics of horizontal-directional long boreholes in overlying strata and drainage mechanism of pressure-relief gas in longwall mining

The pressure-relief gas drainage in longwall mining goaf is an important part of the simultaneous extraction of coal and gas in the green mining technology system. The horizontal-directional long boreholes arranged in overlying strata for pressure-relief gas drainage have been widely used in recent years. The boreholes in mining overburden are different from the high drainage roadway and ordinary high-level boreholes. When the pressure-relief gas is extracted from the overburden of goaf, the boreholes response and sensitivity to the mining overburden damage and gas migration have some significant particularities. Through theoretical analysis, numerical simulation and engineering example verification, the position characteristics and pressure-relief gas drainage mechanism of horizontal-directional long boreholes in mining overburden were studied. The technical principle of extracting pressure-relief gas by horizontal-directional long boreholes was described. Combined with laboratory tests and numerical simulation, three position characteristics of hori-zontal-directional long boreholes were analyzed. The gas accumulation degree at the borehole area is high, which provides the concentration conditions for borehole gas drainage. The fractures at the borehole area are relatively developed, which provides gas source guarantee for borehole drainage. The stratum at the borehole area is less affected by mining, which provides stability conditions for the borehole. On this basis, the drainage mechanism of horizontal-directional long boreholes was revealed from three aspects: the degree of gas accumulation in mining fractures, the permeability of mining strata and the stability of mining boreholes. The theoretical criteria for determining the position of horizontal-directional long boreholes was proposed, and the method for determining the position of the boreholes drainage was given, which were verified by numerical simulation and engineering examples. The research results show that according to the borehole position criteria, the maximum pure volume of gas drainage of the borehole arranged is 2.59 m³/min, which is 2.56 times that of other comparative boreholes. Affected by horizontal-directional long boreholes drainage, the maximum gas volume fraction in return airway is 0.11%−0.72%, and the gas volume is 0.69−2.79 m³/min, which verifies the reasonability of pressure-relief gas drainage based on the position determination method of horizontal-directional long boreholes. The research results can provide a technical basis for the layout of horizontal-directional long boreholes to extract the pressure-relief gas in goaf, thereby improving the pres-sure-relief gas extraction rate and promoting the development of green mining technology of the mine.