Analyzing the Optimization of Unloading Gas Extraction Drilling Arrangement Based on Stress Distribution in the Protected Layer

Abstract

In the process of protected seam mining, the reduction in stress and the enhancement of the gas mobility that affects the protected seam are crucial manifestations of the protection effect. Taking the working face of E8-32010 and the upper D5-6 coal seam of the Six Mines of Ping Coal Company Limited as the study object, the research method combining theoretical analysis, numerical simulation, and a field test was adopted. In combination with the actual production, we adopted the stress distribution law pertaining to the coal body of the protected seam under the condition of 2.2 m mining height. When the length of the tendency of the working face mining is under the condition of 2.2 m mining height and when the working face mining inclination lengths are 120 m, 160 m, 200 m, 220 m, 240 m, and 280 m, the stress distribution law that regulates the coal body of the protected seam is analyzed, and, based on the stress distribution law, the unpressurized extraction drilling holes are designed, and the effect of the coal body stress at the final hole position of the unpressurized extraction drilling holes on the efficiency of unpressurized extraction is determined through on−site extracting data. The research results indicate the following: (1) as the tendency length of the working face increases, the degree and range of pressure unloading that affects the protected layer are increasing, the stress increase in the deep D5-6 seam is larger than that of the shallow D5-6 seam, and the tendency direction can be divided into the stress elevation area, pressure unloading area, and stress elevation area from the shallow zone to the deep zone. Moreover, the minimum stresses in the pressure unloading area are 7.80 MPa, 6.42 MPa, 5.59 MPa, 5.59 MPa, 5.42 MPa, 5.30 MPa, and 5.21 MPa, and the minimum stress is less than 60% of the original stress; (2) the vertical stresses at the final locations of the No. 1, No. 2, and No. 3 drill holes after the protective layer is mined are 16.42 MPa, 10.74 MPa, and 6.72 MPa, respectively, and the pure amount of gas extracted from the unpressurized extracting drill holes has increased immensely; the higher the rate of unloading, the greater the increase, and, the more the unpressurized extracting drill holes are extracted, the greater the increase. The higher the unloading rate, the greater the increase: 19.77–21.31 times, 41.62–41.68 times, and 68.68–74.66 times the pure amount of gas extracted from the corresponding pre−pumping holes; (3) the No. 3 depressurized extraction borehole is 261.02–281.04 times, 191.77–205.55 times, and 138.43–148.18 times higher than the No. 1, No. 2, and No. 3 pre−pressurized extraction boreholes, respectively, and 6.09–7.14 times and 2.28–2.49 times higher than the No. 1 and No. 2 depressurized extraction boreholes, respectively. The research results can not only provide a theoretical basis for verifying the protection effect of the protected layer but also a scientific rationale for the layout of the unpressurized extraction drill holes.