川西硬脆性页岩力学特征及井壁稳定性研究

在硬脆性页岩地层钻井过程中井壁失稳现象频发，严重阻碍了现场钻井作业的进程。针对小塘子组页岩钻进过程中发生的井下垮塌现象，通过开展系统室内实验测试，并结合实验数据建立的理论模型，评价分析小塘子组页岩地层的井壁稳定情况。结果显示，小塘子组页岩属于典型的硬脆性页岩，层理缝发育，在外力或高应力环境下可以产生张性裂缝；钻井液浸泡效应可降低小塘子组页岩地层力学强度，层理走向及力学弱面效应对小塘子组页岩力学性能影响明显；裂缝走向、井眼轨迹及井壁渗流场影响小塘子组页岩地层井壁稳定性，当裂缝面与井眼轴线之间夹角满足一定关系，井壁岩石容易沿裂缝面发生滑移垮塌。

Study on Mechanical Characteristics and Wellbore Stability of Hard Brittle Shale in Western Sichuan

In the drilling process of hard brittle shale formation, the phenomenon of wellbore instability occurs frequently, which seriously hinders the process of field drilling. In view of the phenomenon of downhole collapse in the process of shale drilling in Xiaotangzi Formation, we carryied out systematic laboratory tests of the shale cores of Xiaotangzi Formation established the theoretical model with experimental data, and evaluated and analyzed the wellbore stability of Xiaotangzi Formation. The results showed that Xiaotangzi Formation is the typical hard brittle shale with developed bedding cracks, which can produce tensile fracture under external force or high stress. Drilling fluid immersion effect can reduce the mechanical strength of shale formation in Xiaotangzi Formation. The influence of bedding strike and weak plane effect on the mechanical properties of shale in Xiaotangzi Formation is obvious. Fracture strike, wellbore trajectory and wellbore seepage field affect wellbore stability of shale in Xiaotangzi Formation. When the angle between fracture surface and borehole axis satisfies a certain relationship, the borehole wall rock is prone to slip and collapse along the fracture surface.