Study on Hydraulic Fracture Propagation in Hard Roof Under Abutment Pressure

Roof disaster has always been a major hazard in the working face with overlying hard roof. As an effective technology, directional hydraulic fracturing can alleviate the safety risk caused by hard roof. The fracturing region is located around working face or roadway, where the construction of mining and excavation is frequent. Thus, the hydraulic fracture (HF) propagation will be influenced by abutment pressure, which is to be studied by numerical simulation. Different simulation methods for hydraulic fracturing are compared and the extended finite element method (XFEM) is selected for its advantages in solving stress problems. The effect of stress concentration coefficient, lateral coefficient, in-situ stress and perforation angle on HF propagation is analyzed. The results show that the larger vertical stress gradient can shorten the re-orientated distance of HF, but the magnitude of vertical stress has little effect on HF propagation. The comparison of vertical extension range (VER) of HF shows that the larger perforation angle and smaller lateral coefficient is suitable for creating vertical HF. The principles to create vertical HF under abutment pressure are concluded. Finally, a typical case of directional hydraulic fracturing in hard roof is introduced, and the fracturing schemes are further designed based on the simulation results. This research provides a theoretical basis for roof fracturing and bridge the gaps of hydraulic fracturing under abutment pressure.