Numerical analysis on the mechanism of hydraulic fracture behavior in heterogeneous reservoir under the stress perturbation

Hydraulic fracturing is the key technology employed to alter the permeability of tight reservoirs. In this article, the discrete element fluid-solid coupling method is improved, and sinusoidal stress waves are applied at the boundary of a heterogeneous reservoir to simulate perturbation. The influence of stress wave frequency on the mode of crack propagation in the hydraulic fracturing process and the internal stress changes in specimens under different in situ stress ratios are investigated. The simulation results suggest that (1) under a constant injection rate, an increased frequency is associated with a gradual increase in the breakdown pressures, an increase in the time required for cracking breakdown, and faster propagation of cracks propagating; and (2) the hydraulic fracture propagation direction obviously deflects towards the stress wave propagation direction, and the failure mode becomes more complicated under the combined action of the ground stress and the stress wave; and (3) the maximum principal stress inside the specimen is compressive stress, and the curves of the maximum principal stress and maximum shear stress are periodic vibration, with the characteristics of a large amplitude at a low frequency and a small amplitude at a high frequency. The initiation of cracks near the injection hole results from a reduction in the maximum principal stress and an increase in the maximum shear stress. The analysis indicates that the stress wave has a non-negligible influence on hydraulic fracturing and provides a better guideline for understanding the mechanical nature of hydraulic fracturing under the influence of stress perturbation.