Evaluation of 3D Fluid–Solid Coupling Wellbore Collapse Characteristics of Shale Under Fracture Influence: A Case Study from Weixinan, China

The fractures of Weixinan oil shale simultaneously influence drilling fluid invasion and solid strength and makes the collapse pressure difficult to predict. The indentation–NMR combined experiments were conducted to analyze the collapse characteristics, and the relationship between water porosity and shale strength was established. The experiment results show that water infiltration still occurs in oil-based drilling fluids in the short term and leads to a significant strength decrease. 3D numerical modeling was used to analyze water migration and shale strength weakening under fluid–solid coupling. It was found that fractures act as seepage channels and this aggravates the clay hydration and the shale instability. This causes high collapse pressure under specific well inclination angles and azimuth angles. The research results provide important references for shale wellbore stability analysis and engineering practice with complex geostress conditions and hydration.