Failure and deformation characteristics of shale under true triaxial stress loading and unloading under water retention and seepage

A multifunctional true triaxial fluid-structure coupling system was used to conduct water retention and seepage tests of shale under true triaxial loading and unloading stress paths. The stress-strain evolution law of shale specimens under different experimental conditions was obtained, and the corresponding deformation and strength law was analyzed. The evolution law and failure characteristics of cracks in shale were obtained by CT scanning images before and after the experiment. The results show that under the condition of water retention, the volumetric strain of shale specimen increases first, then decreases and finally continues to increase with the increase of deviational stress, indicating that the volumetric change has experienced a process of compaction-expansion-compacting. The partial stress-maximum horizontal strain curve of the sample increases first and then decreases, while the deformation of the sample in the direction of intermediate principal stress shows the characteristics of repeated compression and expansion. In the seepage test, the permeability - maximum horizontal strain curve can be divided into two parts before and after fracture according to the deviant stress - maximum horizontal strain curve. Before fracture, the compression velocity of the specimen in the loading direction exceeds the expansion velocity in the unloading direction, resulting in a decrease in volume and a decrease in permeability. With the increase of deviatoric stress, cracks occur inside the particles and continue to spread from the tip until the cracks break through the shale specimen. In this process, the pore fissure area increases and the permeability of the sample increases rapidly. In terms of fracture evolution, for the water-retaining test, dense tensile and shear cracks appear on the failure plane perpendicular to the direction of maximum and minimum principal stress, and complex shear fracture network appears on the failure plane perpendicular to the direction of intermediate principal stress. For the seepage test, heavy shear failure occurs throughout the original fracture of the sample. With the increase of the penetration depth, the crack shape on the failure surface perpendicular to the direction of intermediate principal stress gradually changes from single type to complex type.

本研究采用多功能真三轴流固耦合（true triaxial fluid-structure coupling）系统，开展了真三轴（true triaxial）加卸载应力路径下页岩的保水与渗流试验。获取了不同试验条件下页岩试样的应力应变演化规律，并对其相应的变形与强度特性开展了分析。通过试验前后的CT（Computed Tomography）扫描图像，得到了页岩内部裂纹的演化规律与破坏特征。结果表明：在保水工况下，页岩试样的体积应变（volumetric strain）随偏应力（deviational stress）的增大呈现先增大、后减小、最终持续增大的变化趋势，说明体积变化经历了压密-扩容-再压密的过程。试样的偏应力-最大水平应变（maximum horizontal strain）曲线先升后降，而试样在中间主应力（intermediate principal stress）方向的变形表现出反复压缩与扩容的特征。在渗流试验中，依据偏应力-最大水平应变曲线，可将渗透率（permeability）-最大水平应变曲线划分为破裂前后两个阶段。破裂前，试样在加载方向的压缩速率大于卸载方向的扩容速率，导致体积减小、渗透率降低。随着偏应力的增大，颗粒内部萌生裂纹并从裂纹尖端持续扩展，直至裂纹贯通整个页岩试样；此过程中孔隙裂隙总面积不断增大，试样渗透率快速升高。在断裂演化方面，保水试验中，垂直于最大与最小主应力方向的破坏面上出现密集的张拉剪切裂纹（tensile and shear cracks），而垂直于中间主应力方向的破坏面上则形成复杂的剪切断裂网络（shear fracture network）。渗流试验中，试样原生裂隙整体发生剧烈剪切破坏（heavy shear failure）；随着裂纹贯通深度的增加，垂直于中间主应力方向的破坏面上的裂纹形态逐渐由单一型转变为复合型。