Document Type : Original Research Paper

Authors

• Elyar Kavosian ¹
• Alireza Yarahmadi Bafghi ²
• Mohammad Fatehi Marji ³
• Manouchehr Sanei ⁴
• Hasan Shojaei ⁵

¹ Ph.D. Candidate of Rock Mechanics, Mining Engineering, Yazd University, Yazd, Iran

² Associate Professor of Mining and Metallurgical Engineering Department, Yazd University, Yazd, Iran

³ Professor of Mining and Metallurgical Engineering Deptartment Yazd University, Yazd, Iran

⁴ Assistant Professor of Mining and Metallurgical Engineering Department, Yazd University, Yazd, Iran

⁵ Senior Expert, Geology Department, National Iranian South Oil Company (NISOC)

Abstract

Conventional wellbore stability analyses commonly apply a single failure criterion and a constant Eaton exponent throughout the wellbore. Such simplified assumptions may overlook lithological and mechanical heterogeneity and reduce prediction accuracy. Although numerical methods may improve accuracy, their complexity often limits their use in rig-site operations, where rapid decision-making is essential. This study presents a field-calibrated analytical workflow for wellbore stability analysis that remains practical for operational use while maintaining geomechanical reliability.
The workflow evaluates the limitations of the single-criterion approach and identifies the most suitable failure criterion among Mohr-Coulomb, Mogi-Coulomb, Modified Lade, and Drucker-Prager for different lithological units. It also applies lithology-dependent stress coefficient multipliers to improve prediction of the safe mud weight window and refines pore pressure estimation through formation-specific calibration of the Eaton exponent. Tight-hole intervals were further analyzed analytically and interpreted using the stress polygon framework and Anderson stress regime concepts to better constrain in-situ stress conditions. In addition, the safe mud weight window for the Drucker-Prager criterion was determined using the analytical solutions proposed by Al-Ajmi and Zimmerman (2005) and Bahrami et al. (2020).
The workflow was applied to the K-Oil Field in the Dezful Embayment, southwest Iran, and further evaluated using additional wells from the same field. The results indicate that lithology-dependent calibration improves safe mud weight window prediction and supports development of a more reliable mechanical earth model for drilling design. Within the studied field, the calibrated parameters provide a practical basis for future well planning and geomechanical decision-making.

Keywords

• Wellbore Stability
• Failure Criteria
• Eaton Exponent
• Stress Coefficient Multiplier

Main Subjects

• Rock Mechanics