Breakdown Pressure and Propagation Surface of a Hydraulically Pressurized Circular Notch Within a Rock Material
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ORIGINAL PAPER
Breakdown Pressure and Propagation Surface of a Hydraulically Pressurized Circular Notch Within a Rock Material Adam K. Schwartzkopff1,2 · Nouné S. Melkoumian1 · Chaoshui Xu1,3 Received: 8 November 2018 / Accepted: 26 September 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract Rock masses contain pre-existing cracks. These cracks are not usually considered when predicting the maximum injection pressure, i.e. the breakdown pressure, in hydraulic fracture stimulations. The conventional approach to predict breakdown pressure is to use the maximum tensile stress failure criterion to calculate the pressure when a point on the borehole wall reaches the tensile strength of the rock. In addition, a pre-existing crack intersecting a hydraulically pressurized section of a borehole may produce a non-planar crack propagation surface. It is important to predict these non-planar crack propagation surfaces to design productive hydraulic fracturing stimulations and to mitigate risks associated with uncertainties of the resultant crack propagation. To gain a better understanding of this problem, a series of hydraulic fracturing experiments were conducted to investigate the breakdown pressures and crack propagation surfaces of a pressurized circular crack represented by a thin notch, subjected to different external triaxial stresses. The results show that the breakdown pressures under the shear stress conditions studied can be estimated using only the resultant normal stress on the plane of the crack. As the material properties of the experimental specimens are well defined and the crack propagation surfaces were mapped, the experimental results presented in this study provide a very useful measured dataset for the validation of various modelling approaches. The propagation surfaces from experiments were found to align closely to the computational predictions based on the maximum tangential stress criterion. Finally, this study gives evidence in three-dimensions that via the hydraulic fracturing process, the propagation of an initially arbitrarily oriented crack will eventually realign to be perpendicular to the minor principal stress direction. Keywords Hydraulic fracturing · Fracture mechanics · Three-dimensional crack propagation · Breakdown pressure · Preexisting cracks Abbreviations CCNBD Cracked chevron notched Brazilian disc LEFM Linear elastic fracture mechanics * Adam K. Schwartzkopff aschwartzkopff@kumamoto‑u.ac.jp Nouné S. Melkoumian [email protected] Chaoshui Xu [email protected] 1
School of Civil, Environmental and Mining Engineering, Faculty of Engineering, Computer and Mathematical Sciences, The University of Adelaide, Adelaide, SA 5005, Australia
2
International Research Organization for Advanced Science and Technology, Kumamoto University, Kumamoto, Kumamoto Prefecture 860‑8555, Japan
3
School of Resource and Environmental Engineering, Wuhan University of Science and Technology, Wuhan, China
List of symbols a Radius or major axis
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