Hydraulic-fracturing treatments are an essential part of well completions in unconventional reservoirs. Hydraulic fractures provide access to the low-permeability reservoir rock, but they also interact with natural fractures to create unpredictable complex fracture networks. Recently, the concept of hydraulic fracture branching, as opposed to simple planar fractures, has been predicted by numerical models to be a mechanism for the high production rates observed from unconventional oil and gas wells. Here, we investigated this concept by performing laboratory experiments. These used gypsum plaster in a heterogeneous 2D grid for analog-rock samples to quantitatively analyze hydraulic fracture branching. The grid included a low-permeability matrix and highpermeability ‘weak layers’ that represent closed natural fractures. Oils with different viscosities were used as the injection fluids to induce hydraulic fracture branching with differing intensity. The experimental results indicate that the intensity of fracture branching can be controlled by the injection parameters. This branching phenomenon was observed by simultaneous growth of the hydraulic fractures in orthogonal/oblique directions, eventually creating a dense network of fractures. Ongoing, we seek a criterion to optimize branching for a given rock type using measurable parameters and tailored stimulation fluids.

1. INTRODUCTION

Economic hydrocarbon production from shale reservoirs relies on multistage hydraulic fracturing treatments and horizontal wells. Therefore, numerous studies have been conducted to better understand initiation and propagation of a fluid-driven tensile fractures in low permeability rocks. These efforts include the study of viscous fluid flow in cracks (Detournay, 2004), characterization of created fracture surfaces in solids (Fisher and Warpinski, 2012; Ishida et al., 2004; Cuderman and Northrop, 1986), recognition of fluid lag behind the crack tip (Lecampion and Detournay, 2007), examination of elastic/plastic deformation of solids (Perkins and Kern 1961; Nordgren 1972; Geertsma and de Klerk 1969; Warpinski et al. 2013), and analysis of the leak-off of fluid from the fracture (Bunger et al., 2005). These studies paved the way to the analytical and numerical models of singlestrand planar hydraulic fracturing process.

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