Effects of Confining Stresses, Pre-crack Inclination Angles and Injection Rates: Observations from Large-Scale True Triaxial and Hydraulic Fracturing Tests in Laboratory

Abstract

Hydraulic fracturing techniques have been used in a wide range of engineering applications, such as petroleum production, geothermal resource exploration, nuclear waste storage, nonconventional natural gas extraction, and cave mining. As a complicated coupled multi-physics process, hydraulic fracturing poses great challenges for modeling (Khristianovic and Zheltov 1955; Geertsma and De Klerk 1969; Settari and Cleary 1986) and numerical simulation methods (Carrier and Granet 2012; Nguyen et al. 2017). Therefore, laboratory experiments are crucial for improving our understanding of hydraulic fractures and verifying the theoretical results. Abundant experimental work has been reported involving various aspects of hydraulic fracturing. Abass et al. (1994) experimentally studied the hydraulic fractures reorientation process from perforations. Zoback et al. (1977) investigated the effect of in situ stresses on the propagation direction of hydraulic fractures. Zhou et al. (2010) further found that in situ stresses also determine the number of hydraulic fractures induced. Lamont and Jessen (1963) studied the interaction between natural fractures and hydraulic fractures. Teufel and Clark (1981) reported that the interfaces in layered rocks have a considerable influence on forming complex crack patterns, and Zhao et al. (2016) studied the cracking behavior of rock-like materials containing multiple flaws. Ishida et al. (2004) conducted experiments on crack propagation in granite driven by fracturing fluids with different viscosities. Zhao et al. (2017a, b) and Chen et al. (2017, 2019) studied the hydro-mechanical behavior of single rock fractures due to the morphological changes of the fractures. AlTammar et al. (2018) revealed that a region of high pore pressure can attract hydraulic fractures. The previous experiments mainly studied hydraulic fracture initiation from cased well bores with oriented perforation that is a dominant technique in oil industry for controlling crack initiation location and propagation direction. In contrary, in cave mining engineering open boreholes combined with prefabricated cracks are more frequently used to achieve directional fracturing, since we expect a single macro fracture to cut the hard roof instead of dense crack networks as in oil industry. In implementation of such directional hydraulic fracturing methods, prescribed cracks are conventionally created by cutting a penny-shaped notch perpendicular to the borehole orientation around the walls and consequently form a transverse hydraulic fracture (Chernov 1982; Chernov and Kyu 1996; Chernov et al. 1997; Jeffrey * Haojie Lian [email protected]