Mechanism of wellbore instability considering tubular-string contact with the wellbore wall.

Exploration and development of deep and extended-reach wells faces extreme challenges including an extremely narrow mud weight window and complex well trajectories, with frequent occurrences of wellbore instability during drilling and completion operations. Tubular strings in the wellbore are highly prone to wellbore wall contact. However, most current studies on wellbore instability neglect the contact interaction between tubular strings and the wellbore during drilling, wiper tripping, and casing running operations. Therefore, this paper quantitatively investigates how tubular-string contact pressures destabilize wellbore integrity, a novel model integrating soft-string model, beam-column model, and contact mechanics was developed to compute the contact pressure between tubular string and wellbore walls under point-contact and wrap-contact cases. An enhanced model coupling tubular string, wellbore trajectory, and rock mechanical properties was established to predict stress distribution around the wellbore. The wellbore instability coefficient was then determined using the Mohr-Coulomb criterion. Key findings demonstrate that tubular string contact induces stress perturbations-radial stress increases while circumferential stress decreases, creating asymmetric wellbore instability patterns. The instability coefficient increases with larger contact force, larger wellbore diameter, higher rock elastic modulus, higher Poisson's ratio and lower wellbore curvature radius. The instability coefficient exhibits symmetrical variation with azimuth angle, peaking at 0° and 180° while maintaining stability between 75° and 105°, with low sensitivity to inclination angles. Notably, drilling fluid density effects create a narrowed safe window, as higher densities damage contact areas while lower densities collapse non-contact regions. This study's novelty lies in its systematic incorporation of mechanical contact effects-a factor previously oversimplified in stability analyses. Practical solutions proposed include trajectory optimization and operational strategies. These findings advance wellbore stability theory and provide actionable guidelines for high-risk formations and extended-reach wells.