Hydraulic fracturing technology with directional boreholes for strata control in underground coal mines and its application

The application of hydraulic fracturing technology on rock fracturing for hard and complete roof, and destressing for roadways with high stresses is increasingly widespread. Based on a top coal caving working face with extra-thick coal seam and complete roof layers in the Caojiatan coal mine in Shaanxi Province, the roof rock geomechanical measurements, compressibility tests, theory of hydraulic fracture propagation, and 3D numerical simulation were carried out, and the determination approaches for fracturing rock horizon above a working face, fracturing borehole layout and parameters, and fracturing technology were put forward. The underground industrial trial and a comprehensive surface microseismic monitoring with actual time were conducted, and the spatial distribution characteristics of hydraulic fractures in roof were obtained. Meanwhile, shield resistance, periodic weighting step distance and duration distance, weighting dynamic load coefficient, and roof breaking energy were monitored and analyzed, and the hydraulic fracturing effect was synthetically evaluated. A complete set of technology for hydraulic fracturing with directional boreholes in underground coal mines were preliminarily established, which includes determination of fracturing rock horizon and parameter design, fracturing technology and equipment with directional boreholes, monitoring of hydraulic fracture spatial distribution, and comprehensive evaluation of hydraulic fracturing effect. The underground trial pointed out that: on the in-situ stress regime in the Caojiatan coal mine, that is the minimum principal stress is vertical stress, the hydraulic fractures were mainly horizontal fractures with a propagation distance of about 80 m on both sides of boreholes, the hard and complete roofs above the working face were effectively weakened, and regional roof reconstruction was achieved. The intensive ground pressure appearance of the working face was obviously reduced in the fracturing area, and production safety was ensured. Finally, the existing issues associated with hydraulic fracturing were discussed, and technical developments were envisaged.