Pre-existing crack angle optimization of thick sandstone roof during directional hydraulic fracturing and its application to anti-rockburst in Shannxi and Inner Mongolia areas

Pre-existing crack angle is one of the important technical parameters for deep hole roof directional hydraulic fracturing. Through the fracture mechanics test of semi-circular bending and the analysis of hydraulic fracturing mechanics model,the fracture characteristics of the hard roof under the influence of pre-existing crack angle on the hydraulic fracturing induced by the directional grooving were studied. The research shows that the pre-existing crack inclination angle affects the crack initiation direction of the hydraulic crack,but the maximum principal stress plays a controlling role in the crack propagation direction. Under the certain condition of crustal stress,the critical water injection pressure decreases with the increase of the pre-existing inclination angle,the type I stress intensity factor K_Ⅰincreases gradually,and the type II stress intensity factor K_Ⅱincreases first and then decreases. In order to ensure that the hydraulic crack is subjected to a large tensile stress effect,and the shear expansion can be performed by the action of ground stress,the pre-existing crack inclination angle when the K_Ⅱreaches the maximum value should be valued according to the actual construction parameter. The fracture mechanics test and theoretical results were applied to the actual hydraulic conditions of the thick sandstone roof of the Bayangole coal mine. The strength and frequency of the coal cannon in the roof during the fracturing process and the microseisms and the change of stress near the auxiliary retracement channel before and after fracturing were compared,which proves that the range of hydraulic fracturing crack under the designed parameters effectively promotes anti-rock burst.