Title: Mechanistic Evaluation of Drillstring Integrity in Extended Reach Wells using Stiff String Torque and Drag Simulations under Variable Frictional Regimes
Authors: Eniye Oguta, Isaac Eze Ihua-Maduenyi
Volume: 9
Issue: 5
Pages: 83-98
Publication Date: 2025/05/28
Abstract:
Extended Reach Drilling (ERD) operations present complex mechanical challenges due to increased lateral wellbore contact, high frictional resistance, and intricate wellbore trajectories. This study employs a fully-coupled, finite-element-based stiff string torque and drag (T&D) model, implemented in WellPlan(tm), to investigate the downhole mechanical behavior of drillstrings in Well AB005-an extended reach well (ERW) located in the ABX Field, Niger Delta. The model rigorously quantifies axial, torsional, and lateral forces under multiple operational scenarios-tripping in, tripping out, rotating-on-bottom (ROB), and sliding-across a spectrum of openhole friction factors (OHFF: 0.20, 0.25, 0.30, 0.35), while maintaining a constant cased-hole friction factor (CHFF: 0.15). Key outputs include effective tension, Hookeload, axial drag, torque distributions, side forces, buckling profiles, and deformation-induced stretch components. Results demonstrate a strong correlation between OHFF and surface torque, which increased from 70,140 ft-lb to 87,504.8 ft-lb for a fixed bit torque of 2,000 ft-lb. Sinusoidal and helical buckling were prevalent during tripping in and sliding, particularly within the 5 7/8-inch drill pipe, where compressive effective tensions approached zero or became negative. Stress analyses revealed progressive mechanical degradation, with yield stress exceedances reaching 100% and fatigue onset observed in high-contact regions, especially at OHFF ? 0.30. ROB and tripping out operations, driven by sustained axial tension and dynamic load redistribution, consistently suppressed buckling tendencies across all friction scenarios. Axial stretch analysis decomposed total elongation into mechanical, ballooning, and thermal components, with mechanical stretch dominating and varying significantly with operation type and frictional load. The model further identified critical stress accumulations at trajectory curvature inflection points and casing shoe interfaces, highlighting zones vulnerable to casing wear, tool joint overload, and structural fatigue. This study affirms the necessity of advanced stiff string T&D models in accurately capturing real-world downhole mechanics in ERWs. The findings provide a robust engineering framework for optimizing drillstring and BHA design, managing frictional energy dissipation, and mitigating buckling and fatigue risks, thereby enhancing the mechanical reliability and operational efficiency of high-angle, long-reach drilling systems.