Phase Field Modelling of Interactions Between Hydraulic Fractures and Natural Fractures

Hydraulic fracturing is a widely used technique applied in unconventional reservoirs to generate large fracture networks. Interactions between hydraulic fracture (HF) and natural fracture (NF) can impact the fracture topology and thus the subsequent productivity. Despite a large number of studies on HF–NF interactions, the HF propagation path is normally judged based on ad-hoc criteria to decide whether crossing or deflection occurs and the mechanism behind has not yet reached a unified understanding. Here, we use a phase-field model (PFM), which is based on a unified fracture propagation criterion, to investigate the influence of in-situ stress, fracturing operational parameters and NF orientation and strength. We analyze the mechanism behind different propagation patterns resulting from different kinds of NFs—non-cemented and cemented ones under different conditions. In particular, we compare the total energies between the symmetric propagation and asymmetric propagation to verify the minimum energy propagation path. Our results indicate that a higher stress anisotropy more likely leads to HF–NF crossing and a less fracture complexity. Injection rate influences propagation speed and fracture complexity. Within a certain range (30°, 45°, 60° in this study), the larger the approaching angle is, the more complex the fractures become. With the increasing strength contrast between NF and rock matrix, the material heterogeneity increases, encouraging HF to form complex fractures. Opening more strongly cemented NFs, which act as a barrier for propagation, consumes more energy than HF propagation outside the interface. Lower stress anisotropy and higher injection rate lead to higher initiation pressure, requiring more energy for propagation.