1. Introduction
The Guojiahe Coal Mine of Xuzhou Mining Group is located in the north of Linyou County, Shaanxi Province. In particular, the upper corner of the gas accumulation, a direct impact on the safety of coal mining face production. In the mining area, some methods such as pre-drilling, setting up air barrier and hanging water jet fan have not been adopted to solve the problem of gas exceeding the limit. In order to solve the problem of gas control in the upper corner, gas drainage project with large diameter roof borehole is introduced by CCTEG Xi’an Research Institute.
2. Solution
The directional roofing drilling is designed to be about 5m above the roof of the coal seam, about 15m away from the tailentry, and the size of the drilling field is 6m*5m*3m. At 200m from the working face, the No.1 drilling site is arranged, and the No.2 and No.3 drilling sites are arranged in turn, with a distance of 130m. The length of drilling hole is 105~163m and the distance of stubble pressing is 30m, which ensures that the gas in goaf is drained continuously during the mining process. The designed borehole is located in the upper part of the caving zone, the middle and lower part of the fracture zone. The vertical distance between the final hole and the roof of the coal seam is 24~49m, and the horizontal distance between the hole and the tailentry is 0 ~ 50m. Six boreholes are arranged in sector shape in turn in each drill site. The diameter of No.1 drilling site is 113mm, while No.2 and No.3 drilling sites adopt large diameter drilling site with 200mm diameter.
3. Construction situation
80 boreholes with 200mm diameter have been completed. Maximum borehole depth reached 280m, and total footage is 17400m. In order to ensure the position of the final borehole in the designed layer, rotary drilling and multiple reaming techniques are adopted for directional roof drilling.
4. Achieved accomplishment
Through the statistical analysis of the drainage effect, the curves of gas drainage concentration in different types of boreholes during the 30-day drainage period are obtained. Through the test of continuous drainage, the cumulative drainage of gas from large diameter roof boreholes is 715915m3, the highest concentration of gas is 0.71% in the tailentry and 0.27% in the upper corner. Obviously the project effectively solves the problem of gas exceeding the limitation in upper corner and tailentry. Thus, mining safety of the working face is ensured.
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