Directional hydraulic fracturing (DHF) is popular with hydraulic fracturing operations in coal mining to create cave-hard roofs, in which radial initial notches are created around open borehole walls before injecting high-pressurized fluid. Despite extensive field application of DHF, the three-dimensional irregular hydraulic fracture (HF) geometry in DHF remains unclear, and the HF re-orientation mechanism requires comprehensive understanding. Here, we experimentally examined factors affecting HF re-orientation in DHF in transparent gelatin samples with a self-developed experimental device. We found that it is the ratio between the differential stress and gelatin elastic moduls that determines HF re-orientation rather than the absolute magnitudes of these two factors. Both shear failure and tensile failure occur during HF re-orientation. The HF tends to propagate asymmetrically, and the step-like HF geometry is likely to form in gelatin samples with low elastic moduli and under high differential stresses. HF re-orientation is not necessarily a near-borehole effect, and HFs can propagate along the notch direction for longer distances in stiffer gelatin samples under relatively low or moderate differential stresses. Finally, recommendations are provided for the effective utilization of DHF at coal mine sites.

During advancing the working face at Guojiawan Coal Mine, hard rock faults are encountered, which hinder the normal cutting of the shearer. Hydraulic fracturing is applied to pretreat the hard rock materials. Un-directional hydraulic fracturing experiments in transparent gelatin samples are carried out. The influence of the differential stress, the borehole dip angle on the initiation and propagation of un-directional hydraulic fractures (HFs) are investigated. Three field test schemes of hydraulic fracturing are proposed and implemented at the 51207 working face of Guojiawan Coal Mine. Compared with the results of different test schemes, the cutting current ratio of the shearer is reduced by 51.70% and the cutting efficiency is increased by 89.93%. Therefore, the feasibility of hydraulic fracturing to improve the cuttability of the hard rock materials is verified, which provides a theoretical basis and field guidance for hydraulic fracturing pretreatment of the hard rock materials at a coal mining working face.