Numerical Study on Mode I Fracture Characteristics of Rock Under Confining Pressure

Hydraulic fracturing is a key technology for the efficient exploitation of deep oil and gas resources. Mode I fracture is the main fracture type of hydraulic fracturing. However, it is difficult to consider the process of mode I fracture under confining pressure in laboratory experiments. Based on numerical methods, this study aims to reveal the influence of confining pressure on mode I fracture of rock. A three-point bending numerical model was established by discrete particle method to monitor the tensile stress concentration zone at the fracture tip and analyze the fracture morphology and fracture parameters under different confining pressures. The results show that the tensile stress concentration zone changes from a horizontal narrow band to a semi-ellipse as confining pressure increases, and its distribution range becomes smaller. Confining pressure enhances the actual tensile strength and actual fracture toughness of the rock. The mode I fracture of rock under confining pressure needs to overcome the confining pressure and rock cohesion resistance. The compaction effect of confining pressure on the rock will strengthen the fracture performance of the rock itself. This strengthening mechanism is affected by the critical confining pressure. This work provides a theoretical basis for studying the mode I fracture of hydraulic fracturing. 1. INTRODUCTION Hydraulic fracturing is a widely stimulation method to increase oil and gas production (Detournay, 2016). Fracture mechanics has proven to be a useful tool for the investigation of brittle failure of rocks during hydraulic fracturing (Janiszewski et al., 2019; Yue et al., 2020). Generally, the fracture can be divided into three fundamental modes, tensile (mode I), in-plane shear (mode II), anti-plane shear (mode III), and mixed mode. The fracture is constrained by the in-situ stress state in the process of hydraulic fracturing and mode I fracture is the main fracture mode. However, it is difficult to conduct traditional fracture mechanics experiments under confining pressure, such as three-point bending, four-point bending and semi-circular three-point bending (Kuruppu et al., 2014; Wang and Hu, 2017; Zhu and Deng, 2023). It is of great significance to reveal mode I fracture rule of rock under confining pressure for further understanding of hydraulic fracturing mechanism.