The initiation and propagation of hydraulic fractures are closely related to the fracture ability of rocks. Such processes in shale reservoirs are, to a certain extent, controlled by bedding. However, the control mechanism of bedding on the anisotropy of fracture toughness and fracturing behavior remains unclear. In this study, a series of numerical notched semi-circular bend (NSCB) tests are conducted using the discrete element method (DEM) to investigate the influence of bedding properties on the anisotropy of fracture toughness and fracture patterns. Based on the DEM framework, a novel simulation method is proposed to accurately identify two key fracture indicators, the fracture process zone (FPZ) and crack tip opening displacement (CTOD), to reveal the fracture driving mechanism. The results show that the fracture toughness of shale is negatively correlated with bedding angles β but positively correlated with bedding spacing and bedding strength. Both the bedding strength and spacing significantly influence the fracture pattern of the specimens with β = 0°–60°, whereas the specimen with β = 90° is scarcely affected by the bedding planes. The evolution of the CTOD and FPZ in shale exhibits distinct phased characteristics. Due to the strong suppression effect of low-angle bedding planes on pre-peak crack deflection, the CTOD and FPZ exhibit opposite trends with respect to bedding angles before and after the peak load. This study facilitates the understanding of the fracture propagation process of anisotropic shale and could provide guidance for hydraulic fracturing design in shale reservoirs.

中文翻译：

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层理对各向异性页岩断裂韧性及破坏模式的影响


水力裂缝的起裂和扩展与岩石的破裂能力密切相关。页岩储层中的此类过程在一定程度上受层理控制。然而，层理对断裂韧性各向异性和破裂行为的控制机制尚不清楚。在本研究中，采用离散元法（DEM）进行了一系列数值缺口半圆弯曲（NSCB）试验，以研究层理性能对断裂韧性各向异性和断裂模式的影响。基于DEM框架，提出了一种新颖的模拟方法来准确识别断裂过程区（FPZ）和裂纹尖端张开位移（CTOD）两个关键断裂指标，以揭示断裂驱动机制。结果表明，页岩断裂韧性与层理角度β呈负相关，与层理间距、层理强度呈正相关。层理强度和间距对β=0°~60°试件的断裂形态影响显着，而β=90°试件几乎不受层理面影响。页岩中 CTOD 和 FPZ 的演化表现出明显的阶段性特征。由于小角度层理面对峰前裂纹挠度的强烈抑制作用，峰值载荷前后层理角度的CTOD和FPZ表现出相反的趋势。该研究有助于理解各向异性页岩裂缝扩展过程，可为页岩油藏水力压裂设计提供指导。