页岩岩相类型对裂缝形成及演化的影响研究——以黔西北地区龙马溪组为例

沉积环境的变迁导致岩相的递变,进而控制着裂缝的形成和演化,揭示不同页岩岩相中裂缝的发育规律及有关机制,对于“甜点”层的脆性精细评价具有重要的工程意义。以黔西北地区龙马溪组页岩为例,通过X‐射线衍射实验、偏光显微镜、岩石力学实验以及有限元数值模拟,对不同岩相类型进行了划分,深入探讨了不同页岩岩相类型岩石的力学性质以及破坏模式,最后再现了两种不同岩相叠置的地质体模型中裂缝的形成和演化的动态可视化过程。结果表明:龙马溪组富有机质页岩层段主要发育硅质页岩、钙质硅质混合页岩以及粘土质硅质混合页岩等3种岩相类型;硅质页岩具有较高的应力跌落速率和弹性能量转化效率,裂缝拓展能力较强,岩石整体具有较强的脆性,纵向和横向裂缝均有发育且相互之间发生交会和联结,而钙质硅质混合页岩裂缝拓展能力次之,最弱者为粘土质硅质混合页岩;粘土质硅质混合页岩夹层对孔隙水压力和主应力的传播有较强的应力屏障效应,裂缝跨层传播能力随应力的急剧下降而受到削弱,钙质硅质页岩应力屏障效应较弱,裂缝具有较强的跨层传播能力。这些研究成果对“甜点”层优选、水平井地质导向以及压裂施工参数的调整等具有较好的参考价值。

Study on the influence of lithofacies types on the formation and evolution of fracture: A case study of Longmaxi Formation in Northwest Guizhou

The change of sedimentary environment leads to the gradation of rock facies, which controls the formation and evolution of fracture. It is of great engineering value for the fine evaluation of the brittleness of the “sweet spot” layer to reveal the development rule and related mechanism of fractures in different shale facies. In this paper, by means of X-ray diffraction experiment, polarized light microscope, rock mechanics experiment and numerical simulation, the different shale lithofacies types are classified, and the mechanical properties and failure modes of different lithofacies types are discussed in depth. Finally, the dynamic visualization process of fracture formation and evolution in two different lithofacies superimposed geological body models is given. The results show that the organic-rich shale interval of Longmaxi Formation mainly develops three types of lithofacies, such as siliceous shale, calcareous-siliceous mixed shale, and clayey-siliceous mixed shale. Siliceous shale has high stress drop rate and elastic energy conversion efficiency when its failure, and has strong fracture development ability and brittleness as a whole, the longitudinal and transverse fractures are developed and intersect and connect with each other. Calcareous-siliceous mixed shale ranks second in fracture expansion ability, while clay siliceous mixed shale has the weakest fracture expansion ability. The clay-siliceous mixed shale interlayer has a strong stress barrier effect on the propagation of pore water pressure and principal stress. The ability to propagate fractures across layers is weakened with a sharp decrease in stress. The calcareous-siliceous shale has a weak stress barrier effect, and the fracture has a strong ability to propagate across layers. It is expected that the relevant research results will provide some reference and guidance for the research of "sweet spot" layer optimization, horizontal well geological guidance and fracturing construction parameter adjustment.