Phase-Field Modeling of Hydraulic Fracturing Under Heterogeneities And/Or Layering of Rock Properties and In-Situ Stress

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ABSTRACT Hydraulic fracturing is a process of creating artificial fractures to enhance the hydraulic conductivity of nearly impermeable rock masses, enabling an economic development of many subsurface energy systems. Due to heterogeneities and layering of rock properties and in-situ stress, hydraulic fracture propagation commonly involves highly complex patterns. To investigate the effects of material heterogeneities and layered rock structure on hydraulic fracturing, numerical modeling usually plays a central role. Most existing numerical methods, however, face a formidable challenge in handling fracture propagation with complex geometries, which can potentially occur in heterogeneous and layered rock formations. A remedy for this challenge is to use the phase-field method. Basically, the phase-field method models the fracture propagation as evolution of a diffusely distributed damage variable, so it allows ones to model hydraulic fracture propagation without explicitly tracking the crack geometries. In this study, we adopt a recently developed phase-field approach to hydraulic fracturing to numerically investigate the effects of heterogeneities and layered structure on near-wellbore hydraulic fracture propagation. Compared with existing phase-field models, this new phase-field model features its capability to capture a rock strength that is independent of the phase-field regularization length. This capability thus allows ones to simulate hydraulic fracture nucleation and propagation in a heterogeneous and layered domain with a uniform regularization length. We then perform a series of hydraulic fracturing simulations with layering of in-situ stress and heterogeneous critical fracture energy. We believe the outcome of this numerical study can improve our understanding of the mechanisms behind the hydraulic fracture propagation in heterogeneous and layered rock formations. INTRODUCTION For many subsurface applications, such as oil and gas production from tight reservoir and enhanced geothermal systems (EGS), it is necessary to hydraulically fracture the rock to enhance its permeability and favor fluid circulation. The fractures are generated by injecting a high-pressure fluid (e.g., water). The characteristics of the generated fractures determine the efficiency of the stimulation treatment and possible hazards associated with the fracturing process. The propagation direction, the geometry, and the extent of the hydraulically generated fractures are greatly influenced by the in-situ stress conditions and the heterogeneity of rock properties (Yang et al., 2004; Xu et al., 2019; Zoback et al., 2022). Therefore, it is crucial to develop numerical tools that can reliably and efficiently model hydraulic fracture nucleation and propagation in heterogeneous porous media.