Progressive Initiation Phenomenon of Radial and Axial Cracks of the Borehole During Rock Hydraulic Fracturing

Hard roof control is a technical problem that restricts the safe and efficient coal mine production. Hydraulic fracturing is an effective technical approach for controlling hard roofs. The progressive initiation of radial and axial cracks in boreholes during rock hydraulic fracturing is observed using a physical model experiment of hydraulic fracturing under true triaxial stress loading in a laboratory. After the hydraulic fracturing experiment, radial and axial cracks form inside the sample, and the two T-shaped hydraulic fractures are observed. The axial crack intersects the radial crack, however few intersections are observed, and the axial crack does not pass through the radial crack. Therefore, the initiation of the radial crack occurs earlier than that of the axial crack. When a crack is preferentially initiated in the radial direction of the borehole during hydraulic fracturing, the local principal-stress field of the borehole changes spatiotemporally owing to the squeezing effect (water wedge effect) of water pressure in the radial crack. The direction of the minimum principal stress changes from parallel to the borehole axis in the initial state to perpendicular to the borehole axis. When the borehole water pressure satisfies the borehole-rupture condition, borehole fracture along the axial direction is observed, progressively initiating radial and axial cracks, which are not coincidental. The formation of a radial hydraulic fracture does not mean that imply the development of radial or axial. Three conditions must exist for the development of radial and axial hydraulic cracks during hydraulic fracturing. Highlights Progressive initiation phenomenon of radial and axial cracks is discovered in lab. Radial hydraulic fracture initiates before the axial hydraulic fracture. Spatial turn of local principal stress of borehole caused by radial crack is analysed. Progressive initiation mechanism of radial and axial hydraulic fracture is revealed.