A New Damping Model for Nonlinear Drillstring Dynamics: Understanding the Effects of Rotation, Eccentricity, and Confined Fluid Flow and Their Impact on Unconventional Drillstring Design
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AbstractSince the pioneering work of Bailey and Finnie in 1960, drillstring dynamics continues to be an active area of research within the industry. While much effort is put into developing accurate descriptions of the complexities associated with the downhole environment, proper modeling of the various damping mechanisms acting on the drillstring remains underdeveloped. A typical approach to modeling the damping downhole consists of utilizing a generalized, or proportional, damping model based on measurements of the actual system response. This technique can be fairly useful when done properly but does not actually quantify the effects of various environmental or operational parameters, such as fluid characteristics or string rotation, on the overall behavior of the drilling assembly.This study presents a nonlinear, semi-analytical, fluid-force model specifically developed to account for the various downhole characteristics that contribute to energy dissipation such as pipe eccentricity, lateral velocity, rotation speed, fluid rheology, and flow rate. This new fluid-force model is combined into an already proven drillstring model which was developed to embody the fully coupled flexibility of the drillstring, arbitrary wellbore curvature, frictional contact, and complex tool geometry. Using the improved model, the paper analyzes the nonlinear behavior of drillstrings with a focus on lateral vibrations in modern unconventional wellbores. Specific attention is given to studying the damping effects on the dynamic response of the drillstring and BHA during rotation with a Rotary Steerable System (RSS).The results shown through this investigation help to quantify the dynamics associated with modern drilling operations. Effects of fluid properties, flow rate, and rotation speed on the nonlinear behavior of the drillstring are examined through numerical studies of rotating an RSS assembly in an unconventional horizontal wellbore. Through the results, it is shown that proper modeling of the fluid forces acting on the drillstring helps to explain how BHAs, under certain conditions, can be safely operated within a range of resonant frequencies. Advanced visualizations of these time-domain simulations also reveal a unique observation that could have a significant influence on expanding the drilling envelope in automated operations.
