Research on gas pre-drainage in coal roadway strips by combined ordinary drilling and directional drilling

When single layered ordinary drilling or directional drilling are used in gaspre-drainage in coal roadway strips, the long boreholes easily deviate from the trajectory and the cost of directional drilling is high. With the case study of 21601 excavation working face of Qinglong coal mine, the method of gaspre-drainage in coal roadway strips by combined ordinary drilling and directional drilling is proposed to solve the above problems. Numerical simulation results show thatwhen extracting the gas by single drilling, the axial gas pressure contour of drill hole at the end position of drill hole in the early stage is "V" shaped distribution. As the extraction time increases, the "V" shaped distribution of gas pressure gradually smooths. The gas pressure in the radial direction of the borehole gradually increases outwards in a ring shape with the borehole as the center. The effective extraction radius reaches 3.80 m at 93 d of pre-drainage. The ordinarydrilling and directional drilling can effectively control the gas in the range of 15 m in front of the two sides of the coal roadway and 200 m in front of the coal working face respectively. Field application results show that when ordinary drilling and directional drilling are combined for pre-drainage, the average value of total gas extractionis 19.86×103 m3, the average gas extraction volume fraction is 53.5%, the average pure gas extraction flow is 1.97 m3/min, the average mixed gasextraction is 3.68 m3/min, and the residual gas content is less than 8 m3/t.The gas extraction effect is proved to be good.