Wellbore Stability Assessment of the Luzhou Block Shale Gas Reservoir Based on Mogi–Coulomb–Jaeger Failure Assessment Model

Abstract

During deep shale gas development, complex in situ stress conditions, abnormal formation pressure, and well-developed bedding planes result in complicated wellbore instability mechanisms. Conventional homogeneous rock models are unable to accurately characterize the different failure behaviors between matrix failure and bedding-plane slip. To address this issue, a Mogi–Coulomb and Jaeger-based multi-mechanism failure assessment model was developed for the Longmaxi shale gas reservoir in the Luzhou Block by integrating logging data, core experiments, and field drilling information. The results show that bedding structures significantly control rock strength and wellbore failure modes. The proposed model effectively identifies different failure mechanisms, including matrix shear failure, bedding slip, and tensile failure. Furthermore, the effects of well inclination, azimuth, and bedding angle on collapse pressure were analyzed, and well azimuth was found to have the greatest influence on the safe drilling window. This study reveals the controlling role of matrix–weak-plane strength contrast in shale wellbore instability and provides theoretical support for well trajectory optimization and drilling parameter design in deep shale gas development.