The crack initiation and propagation process of hydraulic fracturing were reproduced using a coupled particle flow code (PFC) computational fluid dynamics (CFD) method in mixed ground. The breakdown pressure of mixed ground was investigated based on orthogonal test principles and five influencing factors were analysed. The crack propagation process in mixed ground was investigated under different normal bond strength ratios between hard and soft grounds and stress ratio. The minimum principal stress has the most influence on breakdown pressure, followed by stress ratio. Stress ratio was the determining factor for crack initiation direction. The result can be used in the engineering design of hydraulic fracturing projects.
Hydraulic fracturing is a process of initiation and propagation of cracks, which is used widely in the petroleum industry to stimulate oil and gas wells to increase their productivity (Adachi et al., 2007). Various numerical methods have been developed to investigate hydraulic fracturing. The finite element method (FEM) and the boundary element method (BEM) have been introduced to simulate hydraulic fracturing in complex structures (Papanastasioou, 1997; Vychytil and Horii, 1998; Hossain and Rahman, 2008; Aghighi and Rahman, 2010). The discrete element method (DEM) has also been used to investigate hydraulic fracturing from the microscopic perspective (Al-Busaidi, Hazzard, and Young, 2005; Shimizu, 2010; Shimizu, Murata, and Ishida, 2011; Han, Damjanac, and Nagel, 2012; Wang et al., 2014), and many significant conclusions have been obtained. Particle Flow Code (PFC), which is a DEM, has become an effective tool for simulating crack propagation, though it is not perfect (Potyondy and Cundall, 2004). However, there is little information in the literature concerning the hydraulic fracturing process in mixed ground based on DEM. In the current study, PFC2D (Itasca, 2010) was used to simulate the hydraulic fracturing process in mixed ground. The factors influencing breakdown for mixed ground were analysed based on orthogonal test principles. Crack propagation in mixed ground was investigated under different normal bond strength ratios between hard and soft ground (Rnb) and stress ratios (Rσ).