Practice of enhanced gas extraction by staged fracturing with comb-shaped long hole in coal mine roof

In order to solve the problems of drilling construction difficulty, low gas drainage concentration, poor drainage effect and inability to realize the predrainage of large area, based on the existing directional drilling technology and hydraulic fracturing technology in coal mines, combined with the previous research results, the staged hydraulic fracturing technology of comb-shaped long borehole in roof was proposed, and the field test was carried out in Sangshuping No.2 well in Hancheng mining area.The test results show that the main hole length of comb-shaped hole in roof was 588 m, including 8 branch holes, the total drilling footage was 1 188 m, the distance between main hole and coal seam was 0-3.28 m, and the plane coverage was about 12.5 m. The hole was divided into 4 stages with staged hydraulic fracturing technology of immovable pipe string. The cumulative injected water volume was 2 012 m³ and the maximum pumping pressure was 8.74 MPa. After fracturing, the maximum influence radius was more than 30m, and the fractures were mainly located under the borehole and extended to the coal seam. During the stable drainage stage of fracturing borehole, the pure gas drainage volume was 1.18 m³/min, and the average drainage concentration was 43.54%. The pure gas drainage quantity of segmented hydraulic fracturing borehole with comb-shaped long borehole in the roof was 1.2 times of that of hydraulic slotting borehole and 4.0 times of that of bedding borehole in this coal seam. The test results show that the staged hydraulic fracturing technology of comb-shaped long borehole in roof can effectively avoid the problems such as collapsed hole, stuck hole, spray hole, etc in the conventional drilling process of this coal seam, and realize the pre-drainage of gas in a large area of broken soft coal seams of low permeability. It provides a new idea and new method for gas pre-drainage in the area of broken soft coal seams of low permeability.