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ABSTRACT: Hydraulic fracture branching has been observed in the laboratory and in the field. Our previous studies demonstrate that hydraulic fractures can branch into multiple directions for weak anisotropic in-situ stresses when intermedium injection rate and fluid viscosity are used. However, it is quantitatively unclear how the in-situ stress anisotropy affects hydraulic fracture branching. Understanding this question is important for the optimization of hydraulic fracture design for real-world applications because reservoirs are subject to varying degrees of in-situ stress anisotropy. To address this problem, we designed a transparent true triaxial (TTT) cell that allows us to investigate hydraulic fracture branching under different anisotropic stress conditions with distinct injection parameters and sample heterogeneity. This self-designed TTT cell has a transparent bottom window for us to record hydraulic fracture propagation in real-time. We completed a few benchmark experiments to verify the capability of the TTT cell for hydraulic fracturing tests. The most striking experimental observation is that hydraulic fractures can grow and curve in the direction inclined to the minimum principal stress. This crack growth can stop at some critical points, after which new hydraulic fractures are initiated in the direction normal to the minimum principal stress. The curvature-stop-reinitiation cycle repeats during hydraulic fracturing and generates crack strands. Our experimental finding agrees with the observations at the hydraulic fracturing test site. Therefore, the TTT cell offers unique capabilities for us to experimentally understand complex hydraulic fracture branching and stranding behavior in subsurface conditions. 1. INTRODUCTION Hydraulic fracturing is a key technology that enables economic hydrocarbon production from low-permeability reservoirs (Smith and Montgomery, 2015). The same technology is also used to develop the Enhanced Geothermal System (EGS) for carbon-free geothermal energy extraction from the subsurface (Frash et al., 2014; Fu et al., 2021). Significant efforts have been made to advance our understanding of hydraulic fracture initiation and propagation in rock formations with anisotropic in-situ stresses (Haimson and Fairhurst, 1967; Detournay 2004; Fisher and Warpinski, 2012; Frash et al., 2015 & 2019). However, it remains unclear how do we effectively produce complex hydraulic fracture patterns in the subsurface which is very often subject to anisotropic in-situ stresses. |
