A Method and Application of Material Fracture Energy Inversion for Numerical Calculation of Hydraulic Fracture Propagation

To study the effect of the initial crack on hydraulic fracture propagation behavior, the energy release rate is calculated using the virtual crack closure technique based on the extended finite element method.Based on the idea of PFC parameter calibration, a method for inverting the fracture energy of materials based on the relationship between water pressure and energy release rate of the crack tip is proposed.The fracture energy calculated by this method is taken as the material parameter.The obtained hydraulic pressure simulation value is compared with the experimental data and related literature, errors not exceeding 7%,so the reliability of the method is proved in this way.Based on this simulation, the mechanical behavior of hydraulic fracture is studied when water pressure is injected into the pre-existing crack.Using the maximum energy release rate criterion, the effects of in situ stress and the pre-existing crack on hydraulic fracturing propagation behavior were analyzed.The results show that for rock-like internal oblique cracks, tensile-shear failure occurs under the action of water pressure.The hydraulic fracture propagates in one direction in which the tensile stress contour in front of the crack is closest to the crack tip, and the radial shear stress in this direction is 0.Compared with the in situ stress, the pre-existing crack inclination has a greater influence on the crack initiation water pressure.Therefore, fracturing should be performed mainly by changing the injection angle or selecting the natural weak surface of 45°~60° to reduce the engineering cost.