The Effects of Drillstring Eccentricity, Rotation and Buckling Configurations on Annular Frictional Pressure Losses While Circulating Yield Power Law Fluids
Authors: Erge Oney , Ozbayoglu Evren M. , Miska Stefan Z. , Yu Mengjiao , Takach Nicholas , Saasen Arild , Oljeselskap Det Norske , May Roland 
Published: June 2014

Abstract
An experimental study followed by comprehensive flow modeling is presented. The experiments were conducted on a horizontal well setup with drillstring under compression, considering the influence of rotation on frictional pressure losses of Yield Power Law (YPL) fluids. Flow through various buckling configurations with and without drillstring rotation was investigated. A new correlation is presented for the transition from laminar to turbulent regions in concentric and eccentric annuli. A broad model of flow of YPL fluids is proposed for concentric, eccentric and buckled configurations. The model includes the effects of rotation in laminar, transitional and turbulent flow.
A 91 ft. inner pipe was rotated while applying axial compression during flow. At the no-compression case, eccentricity of the inner pipe is varied as the drillstring rotated. The aim for such a design was to simulate actual drilling operations. The test matrix involves flow through sinusoidal, transitional and helically-buckled drillstring. The effect of pitch length is investigated. Helical modes with two different pitch lengths were tested. Eight distinct YPL fluids were used to examine the dependence of pressure losses on fluid parameters. In the theoretical part, a stability criterion is modified to determine the onset of transitional flow of YPL fluids and a correlation is proposed for practical purposes. In addition, pressure loss prediction models are presented for the flow of YPL fluids through concentric, eccentric, free and buckled configurations of the drillstring, with and without rotation. The proposed models are compared with data from the literature and the experiments.
It has been observed that increasing eccentricity and rotation causes an earlier transition from laminar to turbulent flow. Increasing eccentricity decreases pressure losses. In addition, the buckled configurations show a further decrease in frictional pressure losses as the compression increases. In the helical mode, decreasing the pitch length results in a decrease in pressure losses. Rotation tests with free drillstring show an increase in pressure losses as the rotary speed of the drillstring increases. Also, rotating the drillstring while it is compressed suggests an elevated increase in pressure losses due to amplified vigorous motion of the drillstring. Distinct differences in the effects of buckling and rotation are observed in laminar, transitional and turbulent flow. The greatest differences are found in the transition region.
Flow of YPL fluids is one of the greatest challenges in the modern drilling industry. Studies that correspond to actual drilling conditions are substantially important in reducing these challenges. The information obtained from this study can be used to improve the control of bottomhole pressure during extended reach (ERD), horizontal, managed pressure (MPD), offshore and slim hole drilling applications. Consequently, this theoretical and experimental research has the potential to lead to safer, deeper and more precisely controlled oil/gas well drilling operations.