Coal Geology & Exploration


Pre-drainage of coal seam gas with directional long boreholes is an effective technical means to achieve advanced control of coal mine gas area. Affected by mining deployment, negative-angle boreholes(downward holes) are widely used in production. Medium-hard coal seams have good porosity and usually do not require hole protection. However, the existing conventional methods can not effectively solve the problem of water accumulation in negative angle long boreholes. This paper takes the downward long boreholes in the No. 9 medium-hard coal seam of Longfeng Coal Mine in Guizhou as the research object. Two sets of directional long boreholes are constructed in the same area. The average inclination of the boreholes is –8°, and the borehole depth is 240-363 m, with vertical depth of –40.0 m. This paper compares and analyzes the difference in gas drainage effect between protected and unprotected long directional boreholes with long-distance and large vertical depth. The results show that in the early stage of drainage, with the negative angle orientation of the protection hole technology adopted, the average gas drainage volume of long boreholes is 2.09 m3/min, and 1.87 m3/min without hole protection technology. There is little difference between the two technologies. But the attenuation coefficient of gas drainage volume in directional long boreholes with protection technology is 61.54% of the unprotected holes. Taking 400 days of drainage as an example, the total amount of gas drainage from the directional long borehole with the hole protection technology is 1.40 times that of the unprotected hole. Theoretical calculations show that the time required for the drilling site to meet the standards by adopting the screen pipe protection technology is 157 days shorter than that of the unprotected drilling site. From the perspective of long-term drainage effect, in the absence of effective water drainage measures, the adoption of hole protection technology can effectively improve the gas drainage effect of negative-angle directional long boreholes.


medium-hard coal seam, down hole, screen pipe protection, borehole water accumulation, gas drainage, Guizhou Longfeng Coal Mine




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