Experimental Study and Modeling of Yield Power-Law Fluid Flow in Annuli with Drillpipe Rotation

Abstract

Abstract Helical flow of non-Newtonian fluids in concentric and eccentric annuli is of great interest in oil/gas well drilling. Experimental results and field measurements demonstrated that frictional pressure loss in the annulus can be affected substantially by the rotation of the drillpipe. Previous studies indicated that the influence of pipe rotation on annular friction pressure loss is affected by fluid properties, flow regime, annular diameter ratio and eccentricity. This article presents results of in-depth experimental and theoretical investigations conducted on the flow of yield power law (Herschel Buckley) fluids in concentric and eccentric annuli with pipe rotation. The aim of this study is to develop a reliable hydraulic model that accounts for the effect of pipe rotation in annular pressure loss calculations. Extensive flow experiments with polymer-based fluids were carried out using a newly developed testing facility. Four different annular geometries have been considered for the investigation. Five different formulations of test fluid were prepared by varying concentrations of Xanthan Gum (XG) and Polyanionic Cellulose (PAC) for each annular geometry. Accurate fluid characterization was made using the horizontal pipe sections and a rotational viscometer. In addition to the experimental work, theoretical analysis of helical flows in concentric annuli was carried out. Experimental results indicate the strong presence of shear thinning and inertial effects when inner pipe rotates. In highly eccentric annuli, inertial effects dominate the phenomenon of shear thinning and result in increased pressure loss as the speed increases. Inertial effects can be generated due eccentricity and/or geometric irregularities of the annulus that substantially influence the velocity field as the pipe rotates. The experimental measurements and theoretical analysis presented in this investigation are very useful for hydraulic program optimization and well control, particularly for managed pressure drilling (MPD) operations. Introduction Helical flow of non-Newtonian fluids in concentric and eccentric annuli is of great interest in oil well drilling. Laboratory test results 1–7 and field measurements 8–16 demonstrated that frictional pressure loss in the annulus can be affected substantially by the rotation of the drill pipe. The effect of pipe rotation on friction pressure loss is mainly influenced by fluid properties (rheology and density), flow regime, diameter ratio and eccentricity. The change in friction pressure loss due pipe rotation is attributed to different flow phenomena such as:shear thinning in non-Newtonian fluid;inertial effects resulting from pipe eccentricity and/or geometric irregularities such as drillpipe wobbling or eccentricity variation and;formation of secondary flows. Shear Thinning: Shear thinning in non-Newtonian flows tends to reduce the friction pressure loss due to the coupling of axial and rotational flow through shear rate dependent apparent viscosity function. Inertial Effect: Streamlines in concentric annular flow form perfectly helical pattern. This means that the magnitude of velocity of a fluid element at a given radius from the center is constant. However, a rotating eccentric pipe and/or wobbling drillpipe generates very complicated flow patterns in the annulus, resulting in substantial variation of the velocity of a fluid element along the streamline. Consequently, the inertial effects that positively influence the friction pressure loss counteract the effect of shear thinning in eccentric annuli.