Abstract
The redevelopment of complex fault-block oilfields in the Southern Ordos Basin faces dual challenges of slim-wellholes (Φ120 mm) and high curvature (build-up rate > 57°/30 m), where the passability of slim (Φ73 mm) dual-bend screw drilling tools (1.5° + 3°) is particularly critical. Moving beyond traditional geometric or static analysis, this study establishes an innovative "casing-open hole" passability evaluation model that explicitly couples the effects of Weight on Bit (WOB) and borehole curvature, based on the beam-column theory and contact mechanics, while considering the unique constraints of slim tools. In the casing section, geometric constraints verified the feasibility of the tool's passage with a maximum lower bend point displacement of 47.3 mm inside Φ124.3 mm casing. In the open-hole section, for curvature radii of 15-30 m, the model quantifies the synergistic mechanism between WOB (1-3 T) and curvature, revealing that passability is significantly more sensitive to curvature changes than to WOB variations. Results indicate the tool can pass through a 30 m curvature radius under the full WOB range (1-3 T), while the safe WOB threshold decreases to 1.8 T at a 15 m radius. Stress concentration factors reach 2.1-2.8, with a peak stress of 749.4 MPa at the lower bend. This study provides a critical bend angle formula and a drag-thrust balance criterion for optimizing tool running, offering a quantitative and more reliable design basis compared to conventional empirical methods for high-curvature sidetracking in the Southern Ordos Basin. Field applications demonstrated a substantial increase in successful running operations, validating the model's superior predictive capability and practical value. The findings provide not only technical support for well redevelopment in this region but also a theoretical reference for drilling engineering under similar complex wellhole conditions.