Criteria for the progressive initiation and propagation of radial and axial fractures of borehole during rock hydraulic fracturing

ABSTRACT

The behavior of hydraulic fractures initiated along the radial and axial directions of borehole has been investigated by the author in previous hydraulic fracturing experimental and engineering applications. Based on this, and considering the stress fields at the tips of fractures, a mechanical model of the progressive initiation and propagation of radial and axial hydraulic fractures in the borehole is established in this paper. Subsequently, based on the mechanical model, criteria for the progressive initiation and propagation of radial and axial hydraulic fractures are postulated. According to the criteria, the axial fracture begins at the midpoint of the radial fracture. When the second and third principal stresses near the axial fracture tip are equal, the axial fracture begins to turn. Finally, through an actual triaxial fracturing experiment, the injection pressure curve and internal fracture shape of a test block were obtained, which are the discrimination factors of the criteria. The fracture initiation pressure of the radial and axial fractures of the test block satisfied the fracture initiation condition of the criteria. The accuracy of the criteria in predicting the turning position of the axial fracture was 89.6% in comparison to the experimental data. Radial and axial fractures can propagate horizontally and longitudinally on the hard roof to fully break it, which provides a new idea and essential theoretical basis for the control of the end hanging roof of coal mining face.

KEYWORDS:

• Hydraulic fracturing
• radial fracture
• axial fracture
• disturbance stress
• initiation and propagation criterion

Nomenclature

Table

Acknowledgments

Financial support for this work, provided by the National Natural Science Foundation of China (No. 52004269, 51774272), is gratefully acknowledged.

Disclosure statement

The authors declare no conflict of interest.

Additional information

Funding

This work was supported by The National Natural Science Foundation of China [52004269, 51774272].

Notes on contributors

Luying Shao

Luying Shao is a graduate student of China University of mining and technology. His research interest is hydraulic fracturing for coal-rock mass.

Bingxiang Huang

Bingxiang Huang is a Professor at State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology. He is a supervisor of Master and Ph.D. students. His research interest is hydraulic fracturing for coal-rock mass, engineering application includes hard roof control, hard top-coal weakening, directional stress transferring, relief of local stress concentration, permeability improvement of coal seam, coal bed methane exploitation, and natural gas hydrate exploitation.

Xinglong Zhao

Xinglong Zhao is an Assistant Researcher at State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology. His research interest is the mechanism of pore pressure gradient effect of hydraulic fracturing

Yuekun Xing

Yuekun Xing, Ph. D, Research Associate, works at China University of Mining and Technology. Research interests revolve around hydraulic fracturing in the unconventional reservoir. Recent efforts on the hydraulic fracturing have focused on (1) modelling of high-temperature rock plasticity and fracture behaviours in hydraulic fracturing, and (2) Comprehensive rock fracturing characterization with acoustic emission (AE) and digital image correlation (DIC). The selected publications are listed below:(1) Xing Yuekun, Zhang G, Li S. Thermoplastic constitutive modelling of shale based on temperature-dependent Drucker-Prager plasticity. International Journal of Rock Mechanics and Mining Sciences, 2020, 130: 104305.(2) Xing Yuekun, Huang B, Ning E, et al. Quasi-Static Loading Rate Effects on Fracture Process Zone Development of Mixed-Mode (I-II) Fractures in Rock-Like Materials. Engineering Fracture Mechanics, 2020: 107365.(3) Xing Yuekun, Zhang G, Wan B, et al. Subcritical fracturing of sandstone characterized by the acoustic emission energy. Rock Mechanics and Rock Engineering, 2019, 52(7): 2459-2469.(4) Xing Yuekun, Zhang G, Luo T, et al. Hydraulic fracturing in high-temperature granite characterized by acoustic emission. Journal of Petroleum Science and Engineering, 2019, 178: 475-484.(5) Xing Yuekun, Huang B, et al. Recent Advances in Nonlinear Fracturing Characteristics of the Hydraulic Fracture in the Deep Reservoir. China Rock 2020. (outstanding paper award)