ABSTRACT: For an efficient design in a hydraulic fracturing operation, reliable predictions of the applied pressure, and fracture initiation and propagation are important. In this paper, a hybrid bonded particle system and finite element domain is used to study the crack propagation around a pressurized borehole. Different in situ stresses and borehole sizes are considered. In addition, the effect of material ductility and size effect are studied and discussed. The results show that borehole size has an important effect on the crack extension regime. It appears that due to the existence of an internal length in a bonded particle system, the observed size effect can be at least partially explained within the frame work of a nonlocal or gradient continuum theory.
Cavity expansion theory is a useful theory that has found some utilizations in geotechnical engineering. Specifically, it has been used widely to analyze problems related to deep foundations (Vésic, 1972), stability analysis of underground excavation (Ladanyi, 1967), in-situ testing (Mayne, 2001), and penetration tests (Salgado and Prezzi, 2007). Due to the usefulness and applications of this theory, there are several reports in the literature about the numerical analyses of the cavity expansion tests (e.g. Vu et al. 2005; Vrakas 2016; Tarokh et al. 2016; Molaei et al. 2016; Menendez et al. 2017).
Borehole breakout is a direct consequence of the in-situ stresses and stress concentration in rock resulting in preferential rock failure and has been found in nearly all rock types. Breakouts were observed for the first time in the quartzite and conglomerates of the Witwatersrand gold mine in South Africa (Leeman, 1964).
Throughout the study of crack propagation around a borehole, the peak pressure that is achieved during the pressurizing of a cavity has been debatable. This argument has resulted from the classical models which were presented by Hubbert and Willis (1957), and Haimson and Fairhurst (1967) which did not count for the existence of pre-existing fractures and size and rate effects in predicting the breakdown pressure.
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