Vibration Modeling and Analysis Under Backreaming Condition
Authors: Samuel Robello , Mirani Ankit 
Published: September 2015

Abstract
One of the major challenges recently encountered is vibration of the drillstring under backreaming condition (simultaneous pumping, rotating, and pulling out of the hole). Traditionally, much attention is focused on predicting vibration and measurements under normal drilling operations, and guidelines are provided to avoid the resonant frequencies to mitigate tool failures. Vibration during backreaming was never thought through, and the effects are not well understood during this scenario. The tools with complex profiles and designs become an additional excitation source of vibration when backreaming. Current methods lack backreaming rate modeling to predict drillstring vibration under this scenario as the representation of the system dynamics is different. The purpose of this study is to investigate the dynamic response of the string when vibrating under this backreaming scenario.
This paper presents an analytical methodology for modeling and predicting severe damaging vibrations, analysis techniques, and guidelines to successfully avoid the damages to downhole tools and their associated downhole assemblies when backreaming. The dynamic analysis model is based on forced frequency response (FFR) to solve for resonant frequencies, which can effectively identify the modes of vibration. The method provides easy transfer function from the bit or hole-opening tool to the drillstring components. In addition, a mathematical formulation includes viscous, axial, torsional, and structural damping mechanisms as damping treatment is important when the string is in backreaming state. The model also considers the effect of fluid flowrate. The model also uses the integral of bending and torsional energies for quantification of the severity and provides the range backreaming rotation, which needs to be avoided. It has also been found that strong harmonics can occur at low rotation because the tension present in the bottomhole assembly vis-à-vis compression during normal drilling and cause tool failures during backreaming. This will also result in instability in the system; and thus, high cyclical stress in the components and slow backreaming rate. The paper also evaluates the capabilities and limitation of the existing models in predicting resonant frequencies.
It has been found that the drillstring may not be undergoing severe chaotic or stochastic oscillation in the backreaming scenario, but phase reversal can happen when hole-opening tools are present. It has also been found that under this dysfunction, the torsional vibration is a predominant cause of tool failure. Numerical examples with different excitation sources are considered to benchmark the solutions. Also the analysis provides an estimation of relative bending stresses, shear forces, and lateral displacements for the assembly used. Using the study, severe vibrations causing potentially damaging operating conditions were avoided, which was a major problem in the nearby wells. Simple guidelines were provided to estimate the operating backreaming rate of the drilling parameter to mitigate and avoid the downhole tool failures. Extensive simulations were carried out to compare the data from the downhole vibration sensors. The paper includes three severe vibration incidence data; whereby, the model manages to estimate, predict, and avoid severe vibration.