Contact Dynamics of a Rotary Drillstring System in Elliptical Wellbores

As drilling depths continue to increase in oil and gas operations, the associated downhole environment has grown more complex. Data from recent field measurements reveals significant differences between the major and minor axes of the wellbore cross sections, resulting in an elliptical rather than a circular geometry. Such a wellbore may affect the drilling trajectory and wellbore quality. In this work, a dynamic model of a downhole drillstring system incorporating elliptical wellbore constraints is developed using an Absolute Nodal Coordinate Formulation (ANCF)-based beam element, specifically designed to handle contact within both circular and elliptical wellbore cross sections. Numerical simulations are performed based on an actual well trajectory. The results indicate that the elliptical shape of the open-hole section affects the vibration of the drillstring and bit, thereby further influencing the shape of the actual drilling trajectory. The proposed approach can be used to predict the drilling process with an irregular wellbore. Simulation results can inform the selection and adjustment of drilling tools and control parameters for the upcoming drilling operation.