Technology and application of high efficiency gas extraction by directional long borehole hydraulic fracturing in coal seams of medium hardness and low permeability

Aiming at the problems such as poor permeability of coal seams and fast attenuation of gas extraction concentration and flow rate in Jurassic coalfield of Huanglong, the complete set of independently developed hydraulic fracturing equipment was used, the high-efficiency gas extraction technology of directional long borehole hydraulic fracturing of coal seams was put forward, the engineering application test was carried out in Huangling No.2 coal mine in Huanglong coalfield. Five directional long boreholes were drilled. The depth of single borehole was 240~285 m and the total footage was of 1 320 m. The integrated fracturing process was used to fracture coal seams in five boreholes. The cumulative amount of fracturing fluid was 1 557.5 m³, and the maximum pumping pressure for the single borehole was 19MPa. After fracturing, the gas extraction concentration and the drainage quantity per 100 m were increased by 0.7-20.5 times and 1.7-9.8 times respectively. Compared with ordinary boreholes, the initial gas-gushing strength of the fractured borehole was increased by 2.1 times, and the attenuation coefficient of gas flow of the boreholes was reduced by 39.6%. The test results show that the gas extraction effect can be significantly improved and the coal seam drainage ability can be increased after adopting the permeability improvement measures of hydraulic fracturing, providing technical support for the high-efficiency gas extraction of low-permeability coal seam in similar mining areas.