Effect of Shear Slippage on the Interaction of Hydraulic Fractures with Natural Fractures
Authors: Agrawal Shivam , Shrivastava Kaustubh , Sharma Mukul M. 
Published: January 2019

Micro-seismic data suggest that complex fracture networks are formed frequently in unconventional reservoirs due to the interaction of hydraulic fractures (HF) with natural fractures (NF). Understanding this interaction is critical for optimizing fracturing design. It is generally accepted that under certain conditions, a propagating HF can cause remote shear failure of a NF before intersecting with it. This fact is not accounted for in the development of the existing fracture interaction criteria. The goal of this study is to account for these dynamic interactions and present new criteria that define the conditions under which a HF will cross, kink, branch, or turn along a NF.

We have used our peridynamics-based poroelastic fracturing simulator in this study, which solves for rock displacements and fluid pressure in a fully coupled and implicit manner. Shear failure of the NF is modeled using a Mohr-Coulomb failure criterion. The frictional force on the NF surface is modeled implicitly. The stress distribution around the HF is monitored as the NF approaches it. Considering the effects of shear failure, different propagation behavior such as turning, and crossing are characterized as a function of in-situ stress ratio, angle of approach, NF characteristics, and matrix permeability. It should be noted that the peridynamics model used in this study does not require a crossing criterion as an input, rather it can predict the interaction behavior based on local poroelastic stresses.

The model is validated against the analytical crossing criteria derived using Linear Elastic Fracture Mechanics (LEFM) by ignoring remote shear slippage prior to intersection and poroelasticity in our model. Recent experimental observations that show an increase in approach angle before intersection of a HF with a NF are also used to test the model. Remote shear failure of the NF before intersection results in relaxation of the stresses locally. This in turn leads to the HF bending towards the NF. Though these effects are found to be important in low permeability rocks (100 nD), they are more pronounced in high permeability rocks (10 mD). In high permeability rocks, poroelastic effects are much more significant, leading to greater stress relaxation and thus a near-orthogonal modified approach angle. When stress relaxation due to remote shear slippage of the NF is considered, the HF is more likely to turn along the NF. For low angles of approach and low stress ratios (1.0-1.1 for low permeability rocks and 1.0-1.2 for high permeability rocks), the poroelastic crossing criteria derived in this study are considerably different from those derived using LEFM. However, for near-orthogonal angles of approach or high stress ratios, the crossing criteria do not change much.

The poroelastic crossing criteria derived here can serve as direct inputs for discrete fracture network models simulating the growth of complex fracture networks (Shrivastava and Sharma, 2018). The results and insights presented in this paper improve the understanding of the formation of such complex fracture networks in unconventional reservoirs.