Abstract Multi-stage fracturing of horizontal wells to recover shale gas has attracted substantial renewed interest in the physical and mechanical characteristics of shale. The mechanical characteristics, typically the strong anisotropy, significantly affect the nucleation and propagation of hydraulic fractures, as the nucleation mechanisms and propagation pathways primarily depend on the interaction between the actual in situ stress conditions and the anisotropic mechanical properties. However, there remains a lack of effective experimental data on the mechanical properties of the rock matrix and bedding planes. To investigate the mechanical properties, a series of tests, including Brazilian, direct shear and three-point-bending (TPB) tests, were performed on variously shaped Longmaxi shale samples in distinct bedding orientations relative to the loading directions. The results showed that the tensile strength, cohesion, internal friction angle and mode-I fracture toughness of the bedding planes are 4.713 MPa, 8.93 MPa, 31.216° and 0.566 MPa·m1/2, respectively, which are significantly lower than the rock matrix, corresponding to values of 13.164 MPa, 16.175 MPa, 36.222° and 0.957 MPa·m1/2, respectively. This finding demonstrated that the bedding layers are weakness planes on tensile strength, shear strength and fracture toughness in a quantitative manner. However, the values for the rock matrix and Arrester orientation are generally very similar; hence, the mechanical parameters of the rock matrix, especially the fracture toughness and tensile strength, can be approximated by the values determined in the Arrester orientation. For fractures propagating in the direction normal or oblique to bedding, a complex fracture geometry with tortuous propagation pathways is usually generated by bedding cracking and/or fracture deviation towards the bedding-parallel orientation. The mechanical characteristics of the bedding layers play a vitally important part in shale gas development, including the fracture-initiation pressure (FIP) prediction, borehole stability analysis, hydraulic fracture propagation pathways, and complex fracture network generation.

中文翻译：

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页岩层理面力学特性试验研究

摘要 水平井多级压裂回收页岩气已引起人们对页岩物理力学特征的重新关注。机械特性，通常是强各向异性，显着影响水力裂缝的成核和传播，因为成核机制和传播路径主要取决于实际原位应力条件和各向异性力学性能之间的相互作用。然而，仍然缺乏关于岩石基质和层理面力学特性的有效实验数据。为了研究机械性能，进行了一系列测试，包括巴西、直剪和三点弯曲 (TPB) 测试，对不同形状的龙马溪页岩样品进行了相对于加载方向不同的层理方向。结果表明，层理面的抗拉强度、内聚力、内摩擦角和I型断裂韧性分别为4.713 MPa、8.93 MPa、31.216°和0.566 MPa·m1/2，显着低于岩石基质, 分别对应于 13.164 MPa、16.175 MPa、36.222° 和 0.957 MPa·m1/2 的值。这一发现表明，层理层在抗拉强度、剪切强度和断裂韧性方面是定量的薄弱面。然而，岩石矩阵和避雷器方向的值通常非常相似；因此，岩石基质的力学参数，特别是断裂韧性和抗拉强度，可以通过在避雷器方向中确定的值来近似。对于沿与层理垂直或倾斜方向扩展的裂缝，具有曲折扩展路径的复杂裂缝几何形状通常是由层理开裂和/或裂缝向层理平行方向偏移而产生的。层理层的力学特性在页岩气开发中起着至关重要的作用，包括裂缝起始压力（FIP）预测、井眼稳定性分析、水力裂缝扩展路径和复杂的裂缝网络生成。具有曲折扩展路径的复杂裂缝几何形状通常是由层理开裂和/或裂缝向层理平行方向偏移而产生的。层理层的力学特性在页岩气开发中起着至关重要的作用，包括裂缝起始压力（FIP）预测、井眼稳定性分析、水力裂缝扩展路径和复杂的裂缝网络生成。具有曲折扩展路径的复杂裂缝几何形状通常是由层理开裂和/或裂缝向层理平行方向偏移而产生的。层理层的力学特性在页岩气开发中起着至关重要的作用，包括裂缝起始压力（FIP）预测、井眼稳定性分析、水力裂缝扩展路径和复杂的裂缝网络生成。