Characterization of Anisotropic Fracture Properties of Silurian Longmaxi Shale
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ORIGINAL PAPER
Characterization of Anisotropic Fracture Properties of Silurian Longmaxi Shale L. Ren1,2 · H. P. Xie1,3 · X. Sun1,2 · R. Zhang1,4 · C. B. Li3 · J. Xie1,4 · Z. T. Zhang1,4 Received: 19 May 2020 / Accepted: 21 October 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract Fracking is widely applied to enhance shale gas mining, and insight into the fracture behaviors of shale rocks is important. To characterize the fracture properties of Lower Silurian Longmaxi shale, a chevron-notched deep beam specimen, which inherits the advantages of notched deep beam and chevron-notched specimens, is introduced, and several three-point bend tests were conducted on the Longmaxi shale specimens in three principal fracture orientations: the divider, short-transverse, and arrester orientations; the anisotropy in the critical shape coefficient, fracture toughness and fracture energy were then examined and highlighted. The results demonstrate that the critical shape coefficient depends on the anisotropic elasticity of the investigated shale, and a maximum error in the calculated fracture toughness of 9.2% could occur if a critical shape coefficient for isotropic rock is adopted. For the tested shale, the mode I fracture toughness in the divider orientation is close to that in the arrester orientation, with both values being approximately 1.5 times that in the short-transverse orientation; in addition, a progressive decrease in fracture energy from the arrester orientation to the short-transverse orientation is apparent. The fracture surface morphology was observed by scanning electron microscopy, and obvious material deterioration was found near the fracture surfaces for the arrester and divider samples. Keywords Anisotropy · Shale · Crack propagation · Fracture toughness · Fracture energy · Shale gas
1 Introduction Benefitting from hydraulic fracturing techniques, gas mining of Lower Silurian Longmaxi marine shale has achieved a breakthrough in South China (Jin et al. 2018). Three typical propagation regimes exist in the hydraulic fracturing treatment of shale formations, i.e., the toughness-dominated * R. Zhang [email protected] * Z. T. Zhang [email protected] 1
Key Laboratory of Deep Earth Science and Engineering (MOE), Sichuan University, Chengdu 610065, China
2
College of Architecture and Environment, Sichuan University, Chengdu 610065, China
3
Guangdong Provincial Key Laboratory of Deep Earth Sciences and Geothermal Energy Exploitation and Utilization, Shenzhen University, Shenzhen 518060, China
4
College of Water Resources and Hydropower, Sichuan University, Chengdu 610065, China
regime, the leak-off-dominated regime and the viscous-dominated regime, which can be determined by the dominant energy dissipating process (Jin and Arson 2020). For the toughness-dominated regime, the mode I fracture toughness of shale, KIc, which characterizes the ability of a material to resist crack propagation caused by mode I loading, is a key mechanical parameter that controls hydrau
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