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裂缝韧性和破坏模式的影响


水力裂缝的发生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到层理的控制。然而，层理对裂缝韧性和压裂行为各向异性的控制机制尚不清楚。本研究采用离散元法 （DEM） 进行了一系列数值缺口半圆弯曲 （NSCB） 试验，研究了层理特性对断裂韧性和断裂模式各向异性的影响。基于 DEM 框架，提出了一种新的仿真方法，以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呈负相关β与层理间距和层理强度呈正相关;层理强度和间距对 β = 0°–60° 的试样的断裂模式有显著影响，而 β = 90° 的试样几乎不受层理面的影响。页岩中 CTOD 和 FPZ 的演化表现出明显的阶段性特征。由于低角度层理面对峰前裂纹挠度的强烈抑制作用，CTOD 和 FPZ 在峰载荷前后的层理角方面表现出相反的趋势。本研究有助于理解各向异性页岩的裂缝扩展过程，可为页岩储层的水力压裂设计提供指导。