张拉作用下页岩裂缝扩展演化机制研究

Study on the fracture propagation mechanisms of shale under tension

为深入认识张拉作用下页岩裂缝的扩展演化机制及其层理方向效应,通过三点弯曲和巴西劈裂试验,系统研究张拉作用下不同层理方位页岩裂缝的扩展演化形态,探讨裂缝的扩展演化机制及其层理方向效应。结果表明:(1)三点弯曲时,裂缝沿Arrester方位扩展时,会出现弱层理开裂和断裂路径偏移,易分叉、转向形成复杂的扩展形态;裂缝沿Divider方位扩展时,会出现断裂路径偏移,易剪切转向;裂缝沿层理扩展时,断裂路径为层理,较平直,无转向现象。(2)巴西劈裂时,层理方位对裂缝的起裂点、起裂机制、扩展路径和扩展机制等影响较大;垂直或斜交层理加载时,裂缝多自加载鄂起裂,扩展路径一般呈弧形,多伴随沿层理和贯穿层理的剪切破裂,且扩展中张拉和剪切断裂此消彼长,共同控制裂缝的扩展形态;仅Short-Transverse方位裂缝自圆盘中心起裂,扩展路径为层理,无转向现象,为纯张拉裂缝。(3)张拉裂缝沿层理扩展时,应力能促使其转向;而当张拉裂缝垂直或斜交层理扩展时,易出现分叉、转向和弱层理开裂等复杂扩展行为,形成相对较复杂的扩展形态;裂缝的复杂扩展行为与受力条件、裂缝与层理的相对方位直接相关,层理和受力条件对裂缝的扩展起主控作用。

In order to deeply understand the fracture propagation mechanisms and bedding directivity effect of shale under tension,three-point bending and Brazilian tests were performed on Longmaxi shale specimens in various bedding orientations with respect to the loading direction to study the fracture propagation patterns and then to explore the fracture propagation mechanisms and bedding directivity effect under tension.For three-point bending condition,It is shown that weak bedding cracks and fracture path deflects in the case of a fracture propagating in the Arrester orientation,producing a complex fracture pattern with fracture branching and deflection,that fracture path usually deflects for a fracture propagating in Divider orientation,which is resulted from shear fracture across bedding planes,and that the fracture path is the bedding plane without deflection for the case of a fracture propagating along a bedding plane.For Brazilian condition,the bedding orientation has a great impact on the initiation point,initiation mechanism,propagation path and propagation mechanism of fractures.When the compressive load is loaded in the direction normal or oblique to the bedding,the fracture is usually initiated from the loading jaws with an arc-shaped fracture path and frequently accompanied by shear fractures across or along bedding planes,and the fracture pathway is determined by tensile and shear fractures which shift during propagation.Only in the Short-Transverse configuration,fracture starts from the disc centre and extends along the bedding plane without deflection.When a tensile fracture propagates along a bedding plane,it can be deviated away from the bedding plane due to the local stress.Complex fracture behaviours such as fracture branching,deflection and weak bedding cracking easily occurs while a tensile fracture propagates in a direction normal or oblique to the bedding.These complicated propagation behaviours are directly related to the stress condition and the relative orientation between the fracture and the bedding plane,and both the bedding and stress state play a dominant role in fracture propagation of shale under tension.