通过黔北地区牛蹄塘组页岩的岩心观察显示,方解石脉作为天然裂缝的充填物,并在很大程度上影响着页岩的破坏模式,研究含方解石脉页岩的破坏特性对水力压裂过程中裂缝的起裂、扩展行为的预测与工程设计有重要意义。为揭示方解石脉对页岩力学特性和破裂特征的影响,进行0°、15°、30°、45°、60°、75°和90°7种倾角的单轴压缩和声发射试验,并结合CT扫描技术、有限元计算,构建了三维细观数值模型,讨论不同角度方解石脉对页岩的细观破坏过程和力学性能的影响,分析页岩微裂纹的时空演化规律。结果表明:(1)不同方解石脉角度页岩的声发射和应力-应变曲线形态变化基本相同,均经历压密阶段、弹性阶段、屈服阶段和峰后破坏阶段等4个阶段,各阶段区分明显,特征强度的变化曲线整体呈“U”形,变化程度不断加快,θ为75°时为最低值;(2)方解石脉显著影响页岩的破坏模式,随着角度的提高,由劈裂破坏转变为劈裂型剪切破坏再到剪切滑移破坏,最终变为劈裂张拉破坏;(3)重构的三维模型和物理试验的表现较为一致,并且能够观察到页岩内部和表面裂纹扩展与贯通过程,声发射的空间分布反映了不同阶段下的压缩、张拉和剪切破坏的单元类型,从微观角度揭示了含方解石页岩的破裂机制;(4)页岩的宏观力学特征同时受方解石和基质的影响,体现了明显的各向异性特征,方解石角度越高,对滑移导向作用越强,试件的力学性能越弱。

Core observations of shales from the Niutitang Formation in the northern Qianbei region show that calcite veins often act as natural fracture fillers and largely influence the shale damage patterns. The damage characteristics of calcite vein-bearing shales is important for the prediction of fracture initiation and extension behavior during hydraulic fracturing and for the engineering design. In order to reveal the influence of calcite veins on the mechanical properties and fracture characteristics of shale, uniaxial compression and acoustic emission tests are conducted at seven inclination angles of 0° to 90° in 15° increment. Combined with CT scanning technology and finite element calculations, a three-dimensional (3D) microscopic numerical model is constructed. The effects of calcite vein angle on the fine-scale shale damage process as well as shale mechanical properties are discussed, and the spatiotemporal evolution of shale microcracks are analyzed. The results show that (1) under different calcite vein angles the shale acoustic emission and stress-strain curves show similar curve shape changes, in four stages, namely compression-density, elasticity, yielding, and post-peak damage, with obvious distinctions between stages. The change curve of the characteristic intensity is “U”-shaped with a local minimum at θ of 75°. (2) Calcite veins significantly affect the damage mode: As the vein angle decreases, the damage mode changes from cleavage to cleavage-type shear, to shear-slip, and finally to cleavage-tension. (3) The reconstructed 3D model is largely consistent with physical testing data, providing insights into the process of crack expansion and penetration at shale’s interior and surface. The spatial distribution of acoustic emission reflects the compression, tension, and shear damage types at different stages, revealing the fracture mechanism of calcite-containing shale from the microscopic point of view. (4) Calcite and matrix simulatineously and anisotropically affect the macroscopic mechanical properties of shale: The higher the calcite vein angle, the stronger its slip guidance effect, and the weaker the mechanical properties of the specimens.