层状页岩巴西劈裂破坏特征颗粒流模拟研究

Particle flow simulation of Brazilian splitting failure characteristics of layered shale

页岩储层是非常规油气资源开发研究的热点之一,层理面性质对页岩的力学行为有重要影响。针对层状页岩巴西劈裂载荷下的破裂特征,采用颗粒流软件研究层理面倾角、黏结强度比值及天然裂缝等因素对层状页岩巴西劈裂试验抗拉强度及破裂模式的影响,结合颗粒细观力链及裂纹动态变化过程分析页岩细-宏观破裂演化机制。结果表明:层状页岩巴西劈裂试样呈现3种破坏模式:页岩基质劈裂拉伸破坏、沿层理的劈裂拉伸破坏和沿层理和页岩基质的拉伸、剪切复合型破坏;微裂纹萌生于圆盘顶部与底部加载处,峰值荷载后,微裂纹扩展贯通并出现较为密集的力链直至岩石形成宏观破坏;层理面黏结强度比值的大小影响着试样的破坏形态与峰值强度;随着层理角度的增加,抗拉强度逐渐减小;考虑天然裂缝的影响后页岩的抗拉强度更低,裂纹扩展模式更为复杂。

Shale reservoir is one of the research hotspots in the development of unconventional oil and gas resources. The properties of bedding plane have an important influence on the mechanical behavior of shale. According to the fracture characteristics of layered shale under Brazilian splitting load, the effects of bedding angle, bond strength ratio and natural fractures on the tensile strength and fracture pattern of layered shale in Brazilian splitting test were studied by adopting the particle flow code. The evolution mechanism of meso-macro fracture of shale was analyzed by combining the dynamic change process of particle microscopic force chain and crack. The results show that three types of fracture patterns are observed: Splitting tensile failure of shale matrix, splitting tensile failure along bedding, and tensile and shear composite failure along shale matrix and bedding. Microcracks initiate at the top and bottom loading points of the disk. After the peak load, microcracks propagate through and dense force chains appear until macroscopic failure occurs. The bond strength ratio of bedding plane affects the failure pattern and peak strength of specimen. With the increase of the bedding angle, the tensile strength decreases gradually. After the influence of natural fractures is taken into consideration, the tensile strength of shale becomes lower and the crack propagation becomes more complex.