Study on Sidewall Collapse Failure Mechanism of Hard and Brittle Shale of Longmaxi Formation in Western Sichuan

In view of the problem of easy collapse of shale shaft wall in Longmaxi Formation in Sichuan, the study of triaxial mechanical loading and well wall collapse failure law under the in-situ stress field and wellbore mechanics environment was carried out by using true triaxial test equipment. The direction and instability mode of shale shaft wall collapse were studied by changing the inclination angle of the layered joints; The dynamic trajectory of fractures during rock mechanical loading was recorded by endoscope, and the type of collapse instability of the well wall was studied according to the shape of the collapsed rock block; The dynamic process of shale wall failure and instability was simulated by using RFPA software. The results show that: When the inclination angle of the layered joint is 0°, the failure of the well wall is splitting and shear failure through the layer; With the continuous increase of the inclination angle of the layered joint, the rock on the well wall tends to shear and slip along the fracture surface; When the inclination angle of the layered joint is 90°, the rock in the well wall tends to split and tension along the layered joint. The results reveal the mechanism and main influencing factors of shale wall instability, which can provide guidance for subsequent study of well wall stability.