Simulating Cyclic Hydraulic Injection-Induced Fatigue with an Improved Discrete Element Method Model

Cyclic hydraulic fracturing (CHF) is currently used for subsurface energy extraction. However, a numerical model that can reproduce the hydraulic fatigue damage is not yet available. In this study, we have developed a fully coupled hydro-mechanical damage model using the Discrete Element Method (DEM). This model incorporates a novel time-dependent failure model to simulate the hydraulic fatigue response. Additionally, we employed the Grain-Based Model (GBM) to simulate heterogeneous incipient cracks, which permit fluid flow after breakage. We numerically calculated the hydraulic injection induced fatigue under varying levels of fluid pressure. These calculations have been validated by laboratory results, demonstrating a reasonable consistency in breakdown pressures. We have thoroughly investigated the effects of CHF on fracture network propagation, providing potential guidance for practical subsurface energy exploitation. 1. INTRODUCTION Hydraulic stimulation has been widely employed to create a permeable reservoir for subsurface energy extraction since the 1940s (Lecampion et al. 2018; Zimmermann et al. 2019). This process involves injecting pressurized fluids into reservoirs, leading to the formation of complex fracture networks composed of newly developed fractures and reactivated existing ones (Li et al. 2022). However, this stimulation treatment can trigger seismic events, potentially causing earthquakes and leading to public disapproval (Grigoli et al. 2017). Breakdown pressure, a crucial parameter in hydraulic stimulation, is influenced by rock properties, field stress, and fluid pressure (Xi et al. 2021). The breakdown pressure is generally high due to large field stress and fracture toughness. Consequently, the large pressures and injection fluid volumes have resulted in significant safety concerns (Ellsworth 2013). The goal of stimulation is to devise an advanced injection scheme that can control hydraulic fracturing propagation while simultaneously reducing unwanted seismic events (Majer et al. 2007). In this regard, numerous efforts have been made to develop effective stimulation strategies. Cyclic Hydraulic Fracturing (CHF) is one such strategy aimed at optimizing the development of fracture networks while minimizing environmental impact (Hofmann et al. 2018; Yoon et al. 2015; Zang et al. 2013). In contrast to conventional hydraulic fracturing, which adopts continuous injection, CHF employs periodic high- and low-injection pressures (Zhuang et al. 2019). The fluid-rock interaction within the incipient fracture tips can cause hydraulic fatigue, thus making it possible to adopt a lower breakdown pressure to reduce seismic risks (Rao and Ramana 1992; Zhuang et al. 2019). In addition, the permeability of rock is enhanced during CHF compared to conventional hydraulic fracturing (Zimmermann et al. 2019).