Investigation of Natural Fracture Orientation and Clustering Impact on Fracture Propagation Using the Semi-Circular Bend (SCB) Test
Authors: Bhadariya Vishesh , Somishetty Varun K. , Lee Hunjoo P. 
Published: September 2023

ABSTRACT
Sedimentary rocks contain abundant pre-existing discontinuities that directly impact the trajectory of the fracture propagation. In this study, the Semi-Circular Bend (SCB) test was employed to investigate the interaction of opening mode fracture with multiple pre-existing discontinuities. In the first set of SCB specimens embedded with homogenous parallel inclusions, mechanical properties analogous to the host rock, demonstrated that fracture diversion is more likely to occur for smaller approach angle and wider spacing. The second set of samples were designed to contain heterogeneous inclusions to simulate clusters of mineral-filled natural fractures, which have different mechanical properties compared to the host rock. The stiffness, strength, and thickness of the inclusions were varied by adjusting the mixing ratio of gypsum powder and water. The results indicated a similar trend for approach angles as in the first set, while the samples with weaker and softer inclusions showed longer fracture diversion distance than the samples with stiffer and stronger inclusions. As the thickness increased, the fracture diversion distance became longer. The findings of this study will help to evaluate the Stimulated Rock Volume (SRV) in naturally fractured formations, which is directly related to the hydrocarbon production volume.

INTRODUCTION
Pre-existing discontinuities in tight and low permeability formations act as weak planes that influence the propagation of hydraulic fractures and the ultimate fracture network, which is directly related to the overall hydrocarbon recovery. Micro-seismic interpretations (Fisher et al., 2002), core observations (Anders et al., 2014) and logging activities (Khoshbakht et al., 2009) provide an insight into the orientation and distribution of these discontinuities in fine-grained layered structures (Lai et al., 2022). Previous studies have experimentally and numerically investigated the impact of orientation, stress anisotropy and mechanical properties of natural fractures on fracture propagation (Fatahi et al., 2017). The interaction of an opening mode fracture with pre-existing discontinuities could lead to fracture arresting, crossing or diversion depending on the approach angle, stress anisotropy as well as strength of the natural fracture (Warpinski and Teufel, 1987; Dahi-Taleghani and Olson, 2009; Lee at al., 2015; Wang et al., 2018). Bahorich et al. (2012) performed an experimental study on hydro-stone block samples containing inclusions representing cemented natural fractures to characterize the interaction behavior between the hydraulic fracture and the inclusions. To conduct systematic tests for sensitivity analysis, other studies utilized the Semi-Circular Bend (SCB) test to investigate the impact of approach angle, thickness, stiffness, and strength of the natural fracture on interaction with the opening mode fracture (Lee at al., 2015; Wang et al., 2018; Huang et al., 2020).