Analysis of Shear Slip Ahead of Hydraulic Fracture Along a Frictional Interface Not Aligned with Principal Stresses and Conditions for Secondary Branching

Stress field at the trailing edge of the shear-slip zone ahead of a hydraulic fracture (HF) traveling along a natural fracture (NF) not aligned with maximum horizontal stress is studied in the context of its potential for inducing secondary fracture branching from the NF. The size and amount of shear-slip and the stress parallel to the NF wall are computed. Localized tensile stress occurs at the trailing edge of the shear-slip zone, with a logarithmic singularity which is similar to the elastic solution for a half-space subjected to a surface shear traction. The induced tensile stress may lead to a small surface crack being nucleated and HF branching away from NF. From the stress solution, maximum stress intensity factor K1max at the tip of a prospective crack and its corresponding length are estimated. If K1max is greater than fracture toughness, a crack can be nucleated. Example cases show that crack initiation strongly depends on HF length, NF angle, coefficient of friction and stress anisotropy. Crack initiation occurs only when HF reaches a sufficient length since the shear-slip zone length is proportional to HF length, which in turn affects K1 at the crack tip. 1. INTRODUCTION Interaction of a hydraulic fracture (HF) and a weak interface, which can be an open or poorly cemented natural fracture (NF) or a bedding plane and is hereafter referred to as a NF in general, plays an important role in fracture stimulation of unconventional formations. When a HF intersects a NF, the interaction may result in HF crossing NF directly or with an offset (also referred to as a step-over), or being arrested by the NF. In the latter case, HF can dilate and propagate along the NF if fluid pressure exceeds the normal stress on the NF. The crossing/arrest behavior at the moment of fracture intersection has been extensively studied. Various empirical/theoretical criteria had been investigated and proposed (Warpinski and Teufel (1987), Blanton (1982, 1986), Renshaw and Pollard (1995), Gu and Weng (2010), Gu et al. (2011), Chuprakov et al. (2013)) and been incorporated in complex fracture network models (Weng et al. (2011), Weng et al. (2016)). The understanding of a HF crossing a weak interface with an offset is somewhat limited. Cooke and Underwood (2001) studied numerically the stress parallel to the interface on the opposite wall ahead of the tip of a vertical HF approaching a weak bedding plane. They attributed the initiation of offsetting fracture to tensile peaks in the stress on both sides of the intersection point due to local interfacial opening and slippage near the intersection point for "moderate-strength" interfaces. These peaks move closer together and their magnitude increases as the distance between the fracture tip and the interface becomes small, and they occur in the shear-slip region ahead of the opening segment. Fracture offset has also been studied numerically by Thiercelin and Makkhyu (2007), and Chuprakov et al. (2010).