Many studies attributed wellbore instability problems to physical and chemical interactions between rocks and drilling fluids; however the impact of drillstring vibration was neglected. This study aims to investigate the effect of drillstring vibration on wellbore stability; and determine major factors lead to drillstring collision with wellbore.

Drillstring vibration mechanisms are reviewed and correlated with wellbore instability field case to identify type of drillstring vibration responsible for wellbore failure. Then, drillstring equations of motion are solved utilizing Runga-Kutta numerical scheme; and sensitivity analysis is done to assess the effect of each variable (inputs) on drillstring radial displacement. Moreover, new model is developed to predict critical ranges of drillstring vibration that collapse wellbore. Model bases on rock mechanics principles, whirl vibration mechanism and Newton second law.

Results show a direct relation between whirl vibration initiation and wellbore instability; however axial and torsional vibrations have negligible effect. Wellbore failures resulted from drillstring vibration are more likely to happen in small hole sizes (6 in) than bigger holes. Drill collar length and drillstring revolutions per minutes are the primary factors aggravate drillstring collision with wellbore, while wellbore inclination and mud weight have minor impact. When drillstring vibrates and hits wellbore wall, hole collapse happens due rock compressive failure, rock fatigue, and reduction in rock strength. The innovated model estimates values of drillstring accelerations that collapse wellbore, and predicts optimum mud weight needed for safe drilling before and after reduction in rock strength due to drillstring vibration.

Research findings reduce risks of wellbore collapse and drillstring failure resulted from drillstring vibration by: presenting recommendations in drillstring design and drilling optimization; and utilizing model outputs to compute critical limits of drillstring vibration and optimum mud weight to prevent drillstring collision with wellbore.

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