Laboratory Evaluation of Natural Fracture and Hydraulic Fracture Interactions Using Digital Image Correlation

Hydraulic fracture (HF) refers to inducing fractures in a solid material by injecting a fluid at a high rate into it. Understanding the patterns of induced hydraulic fractures within a rock mass is of high importance to many energy geosystems. In this experimental study, the pattern of induced HFs is evaluated by utilizing digital image correlation (DIC). Specifically, we investigate the interactions between a hydraulic fracture (HF) and a pre-existing natural fracture (NF). Using DIC, we examine the evolution in the local strain field in an intact and a pre-cut PMMA specimens as water is injected into the loaded specimens. From theory, it is anticipated that an HF initiates at the blunted notch fabricated at the top of the model wellbore in the PMMA samples and propagates upward as the applied boundary conditions require. This is verified for the intact PMMA specimen in this experimental study. The presence of an NF however forces the HF to curve and deviate into the NF. The interactions between induced HF and pre-existing natural fractures depend on the stress anisotropy, the relative orientations of natural and hydraulic fractures, and the bonding strength of natural fractures. 1 INTRODUCTION In many energy geosystems, such as enhanced geothermal and unconventional fossil energy systems, enhancing reservoir permeability relies on strategically creating and expanding fracture networks through hydraulic stimulation. This technique centers on the deliberate interaction between artificially induced hydraulic fractures (HFs) and the pre-existing network of natural fractures (NFs) within the subsurface geological formations. Understanding this interaction is crucial, as it directly influences the effectiveness of production and the extent of induced seismicity (Warpinski & Teufel, 1987; Zhang et al., 2020; Chang & Yoon, 2022). This approach aims to initiate new fractures and extend and interconnect to existing fractures, thereby optimizing the reservoir's permeability and fluid flow pathways. Interactions between HFs and NFs are influenced by various factors such as the angle of intersection between the fracture planes, the prevailing stress field within the reservoir, NF properties, rock matrix permeability, and fluid properties, among others (Warpinski & Teufel, 1987). An intricate convolution among these factors determines the fracture interaction scenarios, like their propagation, arrest, slippage, dilation, etc. (Jeffrey et al., 2009; Chang et al., 2015).