摘要：

纹层作为页岩的关键组构特征，对岩石破裂行为与裂缝扩展具有显著控制作用，直接影响页岩油气的高效开发。然而，传统宏观力学实验方法受岩石纹层、基质和天然裂缝的耦合影响，难以单独识别纹层结构对页岩宏观力学性质和宏观破裂机制的影响模式。为此，本文基于有限元—离散元耦合方法(FDEM)，建立了考虑不同岩相页岩纹层分布的细观力学模型，定量表征了纹层方向、厚度和数量等参数对页岩单轴抗压强度、弹性模量、断裂模式和裂缝形态的影响规律。结果表明：(1)长英质和灰云质等硬质纹层是主导岩石力学性质的关键因素。页岩模型中引入硬质纹层后，单轴抗压强度提高17.6%~35.2%，弹性模量提高28.0%~39.8%，裂缝复杂度增加。(2)水平和垂直硬质纹层厚度从600 μm增大至2000 μm，页岩弹性模量和单轴抗压强度分别提高19%和4.8%，Ⅰ型拉张断裂降幅达70%。纹层厚度≤800 μm时，形成宏观剪切缝的同时伴随复杂的拉张分支裂缝，纹层厚度≥1500 μm时，分支缝逐渐减少。(3)硬质纹层数量由5条增至25条，页岩弹性模量和抗压强度分别提升37.8%和8.0%，Ⅰ型拉张断裂降幅达47.1%，分支缝数量增加，裂缝复杂度提高。(4)硬质纹层力学性质越高，对页岩强度的增强作用越显著。岩石基质与硬质纹层间力学性质差异越大，对宏观强度的影响也越明显。此外，随着纹层强度增大，Ⅰ型拉张断裂逐渐向Ⅰ—Ⅱ型混合断裂转变，产生分支缝更少。研究结果有望为深入理解页岩纹层结构对其宏观力学性质的影响机制提供理论支撑。

关键词: 页岩, 纹层, 有限元—离散元耦合方法, 数值模拟, 岩相, 裂缝扩展

Abstract:

Laminations, as a key fabric feature of shale, exert a significant control on rock failure behavior and fracture propagation, thereby directly influencing the efficient development of shale oil and gas resources. However, conventional macroscopic mechanical experimental methods are strongly affected by the coupled effects of laminations, rock matrix, and natural fractures, making it difficult to isolate and identify the specific influence of lamination structures on the macroscopic mechanical properties and failure mechanisms of shale. To address this issue, this study employs a finite-discrete element method (FDEM) to establish mesoscale mechanical models that incorporate lamination distributions in shale with different lithofacies. These models are used to quantitatively characterize the effects of lamination orientation, thickness, and number on the uniaxial compressive strength, elastic modulus, fracture modes, and fracture morphology of shale. The results indicate that: (1) Hard laminations are the key factor governing shale mechanical properties. After introducing hard laminations into the shale models, the uniaxial compressive strength increases by 17.6%~35.2%, and the elastic modulus increases by 28.0%~39.8%, accompanied by an increase in fracture complexity. (2) As the thickness of horizontal and vertical hard laminations increases from 600 μm to 2000 μm, the elastic modulus and uniaxial compressive strength of shale increase by 19% and 4.8%, respectively, while the proportion of Mode I tensile fractures decreases by up to 70%. When the lamination thickness is ≤ 800 μm, macroscopic shear fractures are formed together with complex tensile branching fractures, whereas when the lamination thickness is ≥ 1500 μm, the number of branching fractures gradually decreases. (3) As the number of hard laminations increases from 5 to 25, the elastic modulus and compressive strength increase by 37.8% and 8.0%, respectively, while the proportion of Mode I tensile fractures decreases by 47.1%, and the number of branching fractures increases, resulting in enhanced fracture complexity. (4) Higher mechanical properties of hard laminations lead to a more pronounced strengthening effect on the shale. The greater the mechanical contrast between the rock matrix and hard laminations, the more significant the impact on macroscopic strength. In addition, with increasing lamination strength, the dominant failure mode gradually transitions from mode I tensile fracture to a mixed mode I-II fracture mode, accompanied by fewer branching fractures. The findings are expected to provide a theoretical basis for an in-depth understanding of the influence mechanism of shale lamination fabric on its macroscopic mechanical properties.

Key words: shale, lamination, FDEM, numerical simulation, lithofacies, crack propagation

中图分类号: 

• P618.13

• TE122