Wellbore Stability Prediction Model for Complex Reservoirs: Application to the Bozi Gas Field in the Tarim Basin

The primary objective of this paper is to propose a method for predicting the safe mud weight window within the Bozi gas field situated in the Tarim Basin and establish a 3D wellbore stability model. The approach combines Mogi–Coulomb criterion using intermediate stress for wellbore shear failure. It considers rock shear breakdown in fracture pressure calculations and predicts reservoir shear slip with Jaeger criterion and drilling fluid invasion on the weak surface in calculating wellbore tensile failure. The study finds that instability in the well wall of this block is not solely due to the intermediate stress criterion or shear fracture of intact rock. Instead, during drilling with high-density fluid, the weak surface (fracture) undergoes shear-slip damage before intact rock. The collapse is a result of the interplay between bit replacement, dynamic drilling loads, and drilling fluid intrusion, causing tensile rupture on the weak surface and repeated fluid leakage. Analysis of wellbore instability in 16 wells within ultra-deep complex formations shows that the collapse pressure prediction accuracy is 87.5%, and the loss pressure prediction accuracy is 93.7%. Compared to traditional methods, this approach is more accurate and practical, providing essential guidance for resolving wellbore instability issues in drilling operations. In conclusion, this paper introduces a novel and more precise method for predicting the safe mud weight window in the Bozi gas field.