The interaction between hydraulic fractures (HF) and natural fractures (NF) will lead to complex fracture networks due to the branching and merging of natural and hydraulic fractures in unconventional reservoirs. In this paper, a newly developed hydraulic fracturing simulator based on discrete element method is used to predict the generation of complex fracture network in the presence of pre-existing natural fractures. By coupling geomechanics and reservoir flow within a dual lattice system, this simulator can effectively capture the poro-elastic effects and fluid leakoff into the formation. When HFs are intercepting single or multiple NFs, complex mechanisms such as direct crossing, arresting, dilating and branching can be simulated. Based on the model, the effects of injected fluid rate, viscosity, NF orientation, NF permeability, NF cohesion and stress anisotropy on the HF-NF interaction process are investigated. Combined impacts from multiple parameters are also examined in the paper. The numerical results show that large values of stress anisotropy, intercepting angle, injection rate and viscosity will impede the opening of NFs.
According to the microseismic observations, hydraulic induced fractures in shale reservoir often deviate from the conventional bi-wing fracture geometry and lead to complex fracture network, which may be caused by stress shadow effect from multiple hydraulic fractures (HFs) or interaction with pre-existing natural fractures (NFs). It has been demonstrated that the widespread presence of natural fractures in most shale formation will significantly impact the induced fracture pattern. Understanding the interaction between HF and NF plays a crucial role in quantifying the stimulated reservoir volume (SRV) and optimizing the well completion strategy.
As shown in Figure 1, there are several possibilities that might occur when hydraulic fractures intersect with natural fractures[1,2]:
(1) The hydraulic fracture directly crosses the natural fracture and keeps propagating with its original direction;
(2) The hydraulic fracture crosses the natural fracture with an offset, where hydraulic fracture will deflect into the natural fracture for a certain distance and resume its propagation along the maximum stress direction at some weak points of natural fracture;