Hydraulic fractures can either grow in planar structures or branch into multiple strands. However, the conditions that cause these fracture patterns to occur are unknown and the controlling parameters are yet not understood. To answer these questions, we conducted laboratory hydraulic fracturing experiments using analog-rock samples that were constructed with controlled heterogeneity for repeatable experiments. Fluids of different viscosities were injected at various volumetric rates to investigate their controls on hydraulic fracture patterns. We observed three distinct phenomena on heterogeneous samples when injection rates and fluid viscosities were varied. Specifically, slow injection of low-viscosity fluids results in diffusion-dominated injection. Injection of medium-viscosity fluids at a moderate rate leads to hydraulic fracture branching. Further increase of both factors causes planar hydraulic fracture pattern. For samples without natural fractures, hydraulic fracture branching disappeared. Our repeatable experiments suggest two key findings: (1) Hydraulic fracture branching is only possible when heterogeneity, natural fractures, or pre-existing weak planes exist. (2) Branching can be controlled by manipulating the injection rate and fluid viscosity for a given formation. Our findings can help design the injection parameters to optimize hydraulic fracture branching in the subsurface for increased hydrocarbon production and recovery.
Hydraulic fracturing has launched a new era in the oil and gas industry because it permits economic hydrocarbon production from unconventional reservoirs. Significant progress has been made to understand the initiation and propagation of a fluid-driven tensile crack in tight formations (Haimson and Fairhurst, 1967; Detournay 2004; Fisher and Warpinski, 2012). Numerous numerical models have also been developed to predict the hydraulic fracture growth in the subsurface (Dahi-Taleghani and Olson, 2011; Zhang et al., 2012). However, hydraulic fracture patterns in the subsurface remain poorly understood.
Hydraulic fractures in the field usually interact with geological discontinuities that include bedding planes and natural fractures. Although these geological discontinuities are most likely closed due to compression, rock creep deformation, and geological diagenesis over million years, they can affect the fracture propagation and result in a complex fracture network partially depending on the in-situ stress anisotropy (Warpinski and Teufel, 1987; Jeffrey and Settari, 1995). Researchers have investigated the crossing/arresting behavior of hydraulic fractures when they encounter geological discontinuities (Blanton 1982; Olson et al., 2012; Fu et al., 2018). These studies, however, predicted no branching of the hydraulic fractures. The branching behavior of hydraulic fractures has been observed (Tan et al., 2017; Frash et al., 2015 & 2019; Li et al., 2020), but no studies exist to explore the conditions that enable hydraulic fracture branching.