Failure characterization of Longmaxi shale under direct shear mode loadings
Failure characterization of Longmaxi shale under direct shear mode loadings
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DOI：
10.1016/j.ijrmms.2021.104936
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发表时间：
2021-12
影响因子：
7.2
通讯作者：
H. Lu;H. Xie;Y. Luo;L. Ren;R. Zhang;C.B. Li;J. Wang;M. Yang
中科院分区：
文献类型：
--
作者：
H. Lu;H. Xie;Y. Luo;L. Ren;R. Zhang;C.B. Li;J. Wang;M. Yang
研究页岩的剪切破坏对页岩气藏水力压裂及井壁稳定性分析具有重要意义。本文对龙马溪页岩立方试样进行了一系列直剪试验，α= 0°、30°、60°和90°，其中层理方向α定义为层理方向与剪切方向之间的夹角。分析了剪应力-位移关系、破坏形态和抗剪强度的各向异性。对于α= 30°和60°的试样，在峰值剪应力附近存在较大的应力波动，峰值剪应力处的剪切位移分别在α= 60°和α= 0°时达到最大值和最小值。当α= 0°时，破坏面为沿沿着软弱层面剪切滑动形成，破坏面较光滑，分维值较低;当α= 30°、60°和90°时，剪切过程中，破坏面两侧形成斜裂缝，破坏面较粗糙。抗剪强度和粘聚力在α= 0°时达到最小值，内摩擦角在α= 90°时最小。最后，考虑材料细观结构和应力状态引起的各向异性，提出了一种新的各向异性剪切破坏准则，并与试验结果进行了比较。在新准则下，最大抗剪强度所对应的层面角随外加正应力的变化而变化。
Insight into the shear failure of shale is significant for the analysis of hydraulic fracturing and wellbore stability in a shale gas reservoir. In this work, a series of direct shear tests is conducted on cubic Longmaxi shale samples with various bedding orientations, i.e.,α= 0°, 30°, 60° and 90°, where the bedding orientationαis defined as the angle between the bedding and shearing direction. The anisotropies in the shear stress-displacement relation, failure pattern and shear strength are then analyzed and highlighted. Stress fluctuation is widely observed near the peak shear stress for most samples withα= 30° and 60°, and the shear displacement at the peak shear stress reaches its maximum and minimum values atα= 60° andα= 0°, respectively, for a given normal stress. The failure surface is formed by shear sliding along a weak bedding plane ifα= 0°, resulting in a smooth failure surface with a much lower fractal dimension value; whenα= 30°, 60° and 90°, some inclined cracks form across the failure surface during shearing, resulting in much rougher fracture surfaces. The shear strength and cohesion reach their minimum values atα= 0°, while the smallest internal friction angle is observed atα= 90°. Finally, by considering the anisotropies induced by the material microstructure and stress state, a new anisotropic shear failure criterion is presented and found to achieve good agreement with the test results. With the new criterion, the bedding angle corresponding to the maximum shear strength is found to vary with the applied normal stress.