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 , May Roland 
Published: September 2015

Summary
An experimental study followed by comprehensive flow modeling is presented. The experiments were conducted on a horizontal annulus 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. Correlations of critical Reynolds numbers are presented that predict the onset and offset of transition from laminar- to turbulent-flow 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 rotary speed causes an earlier transition from laminar to turbulent flow. Results suggest reduced pressure losses with an eccentric pipe. In addition, buckled configurations showed a further decrease of frictional pressure losses as the compression increases. In the helical mode, two pitch lengths are compared, and decreasing the pitch length resulted in a decrease in pressure losses. Rotation tests are conducted with free and buckled configurations. Rotation in the free-drillstring mode showed an increase in pressure losses as the rotary speed of the drillstring increases.
Amplified vigorous motion of the drillstring is visually observed as the drillstring is buckled while rotating. Rotating the drillstring while buckled showed a further increase in pressure losses compared to rotating in free mode. This additional increase in pressure losses is attributed to the more-dynamic motion of the drillstring. Distinct differences of pressure losses in the effects of buckling and rotation are observed in laminar, transitional, and turbulent flow. Significant differences are measured in the transition region.