Mine-back experiments, lab experiments, and cores taken through fractured rock show that hydraulic fracturing treatments can create swarms of fractures. Subcritical crack growth, microcracking ahead of the fracture tip and failure of weak planes in the rock are mechanisms that have helped explain fracture swarming observed in both natural and hydraulic fractures. In this work, we show that fracture swarms can also form in homogeneous rocks by complex stress fields induced in the reservoir by propagating hydraulic fractures or by other geologic events.

A 3-D fracturing simulator based on the Displacement Discontinuity Method (DDM) and a 3-D fracturing-reservoir simulator based on the Finite Volume Method (FVM) were used to simulate hydraulic fracture propagation. These models can accurately account for stress shadow effects and simulate fracture turning during propagation with the direction of propagation calculated using the maximum tangential stress criterion. Simultaneous propagation of multiple fractures is simulated to exhibit multi-stranded fracture propagation behavior in the absence of natural fractures in the reservoir. Additionally, we assess the impact of natural fractures in the reservoir on the created fracture swarms.

The stress shadow around a single fracture is spatially variable. Simulation of the propagation of multiple fractures shows that parts of the fractures propagating in lower stress contrast regions turn more readily than the parts of the fractures propagating in higher stress contrast regions. The smaller width of the fracture in the high-stress layers induces a smaller stress shadow while the larger width of the fracture surface in the low-stress layers induces a larger stress shadow. Higher stress interference in certain layers causes fracture turning in those layers and induces splitting of the fracture tip resulting in swarms of fractures. The FVM simulations clearly show this effect. Fracture branching and tip splitting are only evident when these simulations are run in 3-D. The fracture branching results obtained from the simulators show good agreement with field core-through observations from the Eagle Ford and the Permian basins. This work shows that the stress field created by propagating fractures can result in the creation of fracture swarms. It is shown that this branching can happen in the absence of material flaws and weaknesses. This new mechanism of fracture branching to create fracture swarms is quantified through both DDM and finite volume simulations.

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