直剪条件下页岩裂缝扩展演化机制研究

Study on the propagation mechanisms of shale fractures under direct shear conditions

为深入认识页岩剪切裂缝的扩展演化机制及其层理方向效应,通过直剪试验,系统研究不同法向应力下不同层理方位页岩剪切裂缝的扩展演化形态,探讨直剪过程中雁列状裂缝成核、扩展、连接及贯通后形成宏观剪切裂缝的力学机制及其层理方向效应。结果表明:(1)直剪时,剪切力诱导的张拉作用在剪切面附近产生雁列状微裂缝,而雁列状微裂缝的进一步扩展、连接及贯通等形成宏观剪切破裂带,剪切破裂带的宽度不均一,表现出明显的非均质性。(2)对层状页岩,直剪时一般沿层理形成雁列状裂缝,而层理的开裂程度与开裂方向与层理方位直接相关,表现出明显的层理方向效应;沿层理剪切时极难形成斜穿层理的雁列状裂缝,剪切面为层理面,较平直、光滑;与层理成30°和60°角剪切时,均形成了沿层理的雁列状裂缝,且30°时更明显;垂直层理剪切时,仍能观察到一定程度的层理开裂,但已较微弱。(3)雁列状裂缝相互沟通后形成的剪切破裂面多呈锯齿状,摩擦滑动时易出现擦痕和磨损现象,而法向压力能明显抑制雁列状裂缝的产生,使剪切面趋于光滑。(4)当剪切力与层理成一定夹角时,剪切力能诱导大量的层理开裂,形成复杂的裂缝形态,而该裂缝形态多局限于剪切破裂带,表现出明显的变形局部化。

In order to deeply understand the propagation mechanisms of shear fractures and directivity effect of bedding,direct shear tests were performed on Longmaxi shale specimens with various bedding orientations with respect to the loading direction under different normal stresses to study the fracture propagation patterns. The evolution process of nucleation,propagation and coalescence of en-echelon fractures and directivity effect of bedding were investigated. The results show that,due to the shear-induced tensile stress,en-echelon micro-cracks are generated around the shear plane,further develop,connect and finally form a macroscopic shear fracture zone with a uniform width and obvious heterogeneity. For layered shale,the shear-induced en-echelon cracks typically develop along the bedding plane. The cracking degree and orientation of the en-echelon cracks are closely related to the bedding orientation,showing significant bedding directivity effect. It is very difficult to induce en-echelon cracks in the direction oblique to the bedding plane along which the shear force is exerted. When the angle between the shear direction and the bedding plane is 30° or 60°,the en-echelon cracks are generated along the bedding plane,especially for 30°. When the shear force is exerted normal to the bedding plane,weak bedding cracking could still be observed to some extent. The shear fracture surface formed by the coalescence of the en-echelon fractures often presents a serrated shape and occurs striation and abrasion in post-peak frictional sliding stage. However,the normal stress could obviously inhibit the formation of en-echelon cracks and make the shear plane smooth. When the shear force is exerted at a certain angle to the bedding plane,complex fracture morphology is formed by the shear induced bedding-parallel en-echelon fractures. The complex fracture morphology is mostly confined to the shear fracture zone,showing obvious localized deformation.