1. Introduction
Xiayukou Coal Mine is a coal and gas outburst prone mine. 2# coal seam is used as the upper protective layer adjacent to the main mining 3# coal seam. When the 2# coal seam is mined, the floor rock layer moves and the coal seam expands to relieve pressure, which causes gas in the lower 3# coal seam escapes to the working face and the goaf through the cracks in the rock bottom. The process may lead to gas overflow and threaten coal mining safety. Conventional floor penetration boreholes for gas drainage has problems such as large engineering volume, high labor intensity, poor hole sealing effect, low drainage concentration, low drilling utilization rate, and high comprehensive management cost.
2. Solution
CCTEG Xi’an Research Institute introduced underground directional drilling technology to control the pressure relief gas problem in the protective layer. Compared with conventional floor penetration drilling, directional drilling has a controllable trajectory, and has a long extension distance at the target horizon, a wide coverage area and fewer blind areas. It also greatly reduces the amount of drilling work and labor intensity and avoids problem of sealing and joint leakage caused by more drilling, which leads to greatly improved gas drainage effect. In addition, the effect of gas drainage by directional drilling was investigated and compared, and the best gas drainage layer was selected for the construction of directional drilling for different strata.
3. Construction situation
From 2017 to 2019, directional drilling for pressure relief gas drainage has been carried out on 4 working faces including 21218, 21220, 23208, 23210 in Xiayukou Coal Mine. 17 directional drilling boreholes have been completed with a total footage of 9335m. Directional drilling boreholes have much better gas drainage effect than the conventional drilling. The 3 working faces currently under construction have been safely mined. This method is continue to be applied to other working faces.
4. Achieved accomplishment
The gas drainage effect of directional boreholes in the 21218 working face was investigated and compared with conventional borehole gas drainage effects. It was shown that with the increase of the drainage time, the gas drainage effect of conventional boreholes tended to attenuate. However, drainage effect trend of directional drilling borehole is relatively stable. Gas drainage volume of directional drilling is 3.96 times that of the conventional drilling. Among directional drilling boreholes, borehole nearby the 3# coal seam has a better gas drainage effect.
After adopting directional drilling technology combined with pressure relief gas drainage, the total concentration of gas drainage in the mine is increased by 8.5-15%. Gas power generation in a single month reaches 1.1 million degrees, realizing gas comprehensive management and utilization model of pressure relief mining, gas extraction, and coal & gas combine mining.
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