Strain and Stress Analyses Ahead of the Propagating Fracture Through Bedding Planes Using Digital Image Correlation (DIC)
Authors: Bhadariya Vishesh , Lee Hunjoo P. 
Published: August 2024

ABSTRACT:
Bedding planes in sedimentary rocks interfere during HF propagation and create complex networks. This investigation utilized semi-circular bend (SCB) test to analyze how opening mode fractures interact with bedding planes with bonded discontinuities of various spacings and orientations. To evaluate strain and stress fields ahead of the crack tip, SCB test results were analyzed using Digital Image Correlation (DIC). Results indicate that fracture diversion occurs for smaller approach angle and wider spacing. Increase in interface spacing and number resulted in longer fracture diversion distance. DIC results show a higher stress zone appears in the vicinity of the crack tip before the fracture initiation from the notch. Also, before fracture initiate and propagate to bonded interfaces, strain and stress concentrations were developed at the interfaces suggesting failure ahead of the fracture tip. As the fracture diverted into bonded interface, a high shear stress region observed during the propagation along the interface indicating mixed-mode I-II propagation. The results are significant for understanding the influence of bedding plane characteristics on fracture propagation and height containment. Additionally, analyzed strain fields from DIC provide a unique opportunity to better interpret fracture complexity in the field from Distributed Acoustic Sensing (DAS) technology.

1. INTRODUCTION
The most common features of sedimentary rocks are stratified bedding planes and bedding layers. It is well known that these layer boundaries and bedding planes act as mechanical discontinuities that inhibit the height growth of hydraulic fractures and induce a complex fracture network, which ultimately governs the stimulated rock volume (SRV) (Flewelling et al., 2013). Micro-seismic monitoring also suggests that these frictional interfaces play a significant role in fracture complexity and limit fracture height growth (Yushi et al., 2017). Furthermore, numerical investigation by Wei et al. (2018) shows that the vertical containment of hydraulic fracture is influenced by bedding planes and their orientation, rock stress state, Young's modulus, and fracture toughness. Lee et al. (2015) measured the fracture toughness of the arrester configuration, relevant to fracture height growth scenario, and reported that it is 150% greater than that of the other configurations due to frictional resistance along the bedding planes.
When hydraulic fractures interact with bedding planes, various types of interactions are observed in the field, including crossing, arrest, diversion, and offset at bedding interfaces (Thiercelin et al. 1987; Helgeson and Aydin, 1991; Cooke and Underwood, 2001; Zheng et al., 2022).
Cooke and Underwood (2001) demonstrated that in crossing/offset case, the opening of the interface near the approaching fracture tip is responsible for fracture arrest at the bedding plane. When the frictional interfaces are weakly bonded, the propagating fracture is more likely to divert into the interfaces (Gudmundsson, 2011; Lee et al., 2015; Bhadariya et al., 2023). In addition, other studies on rock specimens with pre-existing discontinuities show that the interaction results are affected by the approach angle and spacing (Damjanac and Cundall, 2016; Zhu et al., 2021). However, in a laboratory setting, it is difficult to analyze the fracture propagation and interaction process as it occurs within a short period of time, typically within seconds.