Mutual interference of layer plane and natural fracture in the failure behavior of shale and the mechanism investigation
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ORIGINAL PAPER
Mutual interference of layer plane and natural fracture in the failure behavior of shale and the mechanism investigation Peng Zhao1 · Ling‑Zhi Xie1,2 · Zhi‑Chao Fan1 · Lei Deng1 · Jun Liu2 Received: 20 May 2020 / Accepted: 10 August 2020 © The Author(s) 2020
Abstract Shale contains a certain amount of natural fractures, which affects the mechanical properties of shale. In this paper, a bondedparticle model in particle flow code (PFC) is established to simulate the failure process of layered shale under Brazilian tests, under the complex relationship between layer plane and natural fracture. First, a shale model without natural fractures is verified against the experimental results. Then, a natural fracture is embedded in the shale model, where the outcomes indicate that the layer plane angle (marked as α) and the angle (marked as β) of embedded fracture prominently interfere the failure strength anisotropy and fracture pattern. Finally, sensitivity evaluations suggest that variable tensile/cohesion strength has a changeable influence on failure mechanism of shale, even for same α or/and β. To serve this work, the stimulated fractures are categorized into two patterns based on whether they relate to natural fracture or not. Meanwhile, four damage modes and the number of microcracks during the loading process are recognized quantitatively to study the mechanism of shale failure behavior. Considering the failure mechanism determines the outcome of hydraulic fracturing in shale, this work is supposed to provide a significant implication in theory for the engineering operation. Keywords PFC-based model · Damage modes · Tensile/cohesion strength · Brazilian tests · Hydraulic fracturing
1 Introduction Hydraulic fracturing is an effective technique to stimulate shale reservoirs, which enables the boom of shale gas/oil development over the world (Yang et al. 2014; Yin et al. 2017; Liu et al. 2019a, b, 2020; Zhou et al. 2019; Zhao et al. 2020a). Shale is usually characterized as layered and contains complex natural fractures (Liu et al. 2016, 2017). Previous achievements noted that these natural fractures make the mechanical properties of shale more complex, resulting in that the failure patterns of shale mainly depend on the mechanical characteristics of the layer planes and natural fractures as well as the relationship between them (e.g., Edited by Xiu-Qiu Peng * Jun Liu [email protected] 1
State Key Laboratory of Hydraulic and Mountain River Engineering, College of Architecture and Environment, Sichuan University, Chengdu 610065, China
Institute of New Energy and Low‑Carbon Technology, Sichuan University, Chengdu 610065, China
2
März et al. 2008; Vervoort et al. 2014; Tan et al. 2015; Li et al. 2019; Feng et al. 2020). Therefore, there is a strong need to better understand the mechanical characteristics and failure process of shale regarding fracturing engineering applications (Li et al. 2019, 2020; Zhao et al. 2020b). Natural fractures are variable to shale with a different geological backgr
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