Field investigation of multi-stage pulse hydraulic fracturing for improving permeability of coal seam in directional long borehole in underground coal mines

Directional long borehole has been promoted in underground coal mines. However, the hydraulic fracturing technology based on directional long borehole in underground coal mines is still in the initial stage. The uneven fracture distribution and small number of fractures in the conventional hydraulic fracturing with constant injecting rate is the technical bottleneck for directional long borehole fracturing. Combined the advantages of multi-stage hydraulic fracturing and pulse hydraulic fracturing, multi-stage pulse hydraulic fracturing technology is proposed, which can effectively overcome the disadvantages of few and uneven distribution of hydraulic fractures for conventional hydraulic fracturing with constant injection rate in whole section of long borehole. The complete set of equipment for multi-stage pulse hydraulic fracturing in directional long borehole has been developed. Field test of this technology for permeability improvement has been carried out. After multi-stage pulse hydraulic fracturing, the average extraction concentration of boreholes increased by 33%, the pure flow rate of boreholes increased from 0.07 m3/min to 0.35 m3/min, the cumulative daily pure extraction flow increased from 100 m3 to 500 m3, and the daily pure extraction flow of boreholes increased by about 400 m3. The technology improves the fracturing effect of directional long borehole more significantly. Meanwhile, the construction with the help of directional drilling rig is simple, convenient, easy to operate, safe, and reliable. Compared with conventional hydraulic fracturing with constant injection rate in underground coal mines, it improves the fracturing effect and increases the mechanization of hydraulic fracturing in underground coal mines and reduces the labor intensity.