Accurate hydraulic fracturing engineering test in coal seam section with comb-shaped branch holes through layers

In order to solve the problems of large deviation of fracturing objects and low efficiency in the integral hydraulic fracturing and segmented hydraulic fracturing techniques of combing branch holes through layers, the precise hydraulic fracturing test of coal seam section with comb branch holes through the layer was carried out, and the drilling of 1 main hole and 4 branch holes and the precise fracturing of coal seam section were completed by adopting the technological process of “backward construction and fracturing at the same time”. The total fracturing length was 248.3 m, the total fluid injection volume was 5 730 m3, and the initiation pressure of coal seam section with each hole ranged from 10.6 MPa to 11.8 MPa. The maximum pumping pressure is 14.9-16.8 MPa. The test results show that during the fracturing process, the coal seam rupture process appears as “local structure weak surface opening stage - initial crack initiation stage - periodic sawtooth micro-fracture accumulation stage - obvious fracture stage”, as the fracturing continues, the water pressure when obvious fracture occurs is increasing, and the cumulative period of sawtooth micro-fracture is also lengthening. Compared with non-fracturing, the average permeability coefficient of coal seam after fracturing increases to 14.08 times of the original, and the average attenuation coefficient of drilling flow decreases to 0.56 times of the original. Compared with the previous overall hydraulic fracturing test of combing borehole in the same horizontal control range of this tunnel, the volume fraction of gas extraction from single hole in the precision hydraulic fracturing test of coal seam section with combing branching hole in this tunnel has increased by 2.7 times and the pure gas extraction has increased by 11.8 times.