Abstract

This paper presents experimental results of the effects of coiled tubing curvature on the drag reduction behavior of polymeric fluids in turbulent flow. The experimental setup consists of a 10-ft section of 1/2-in. OD straight tubing and four coils of coiled tubing of the same diameter. Fluids investigated included water, guar, hydroxypropyl guar (HPG), and Xanthan fluids at various polymer concentrations. It is found that the coiled tubing curvature could reduce the drag reduction by 10 to 30% as compared with straight tubing, depending on the flow conditions. The curvature of tubing also delays the onset of drag reduction. The percentage drag reduction in coiled tubing may be improved if fluid is pumped at higher rates. The results of the present study were also verified with the previous full-scale experiments and are found to be representative of the drag reduction behavior of similar fluids under the field flow conditions.

Introduction

Frictional pressure of fluids in tubing can be drastically reduced by adding drag-reducing additives such as certain long-chain polymers or surfactants to fluids pumped. This drag reduction property of fluids is more desirable in pumping operations through coiled tubing than through conventional straight tubing, since the flow rates through coiled tubing reels are often limited due to the small tubing diameter (coiled tubing should be small enough for the whole length of tubing string to be spooled on the reel drum). The tubing curvature will cause secondary flow, which further increases the frictional pressure drop in coiled tubing.

Since the discovery of the drag reduction phenomenon of polymer solutions in pipe flow by Toms1 in 1948, there have been numerous studies on this fascinating and certainly important subject of fluids engineering. The papers by Lumley2, Hoyt3,4, Virk5, and Berman6 provide some extensive reviews on this topic. Comparatively, information on drag reduction of fluids in coiled tubing is very scarce. It would be quite logical to believe that the drag reduction behavior of polymer solutions in coiled tubing would be significantly different than in straight tubing. The centrifugal forces during fluid flow in coiled tubing will result in secondary flow in vortical forms in the cross-section of tubing.7,8 This secondary flow, superimposed on the main axial flow, will change the flow field and, for drag-reducing fluids, affect the drag reduction mechanisms.

Most of the previous studies on fluid flow in coiled pipes have focused on the flow of Newtonian fluids.9 Only a few investigated the flow of non-Newtonian fluids in coiled pipes.10–13 In recent years, the rapid increase of coiled tubing applications in the oil and gas industry has driven research activities on coiled tubing hydraulics using full-scale experiments.14–16 Shah and Zhou16 investigated the drag reduction behavior using a full-scale coiled tubing test facility which was established for the Coiled Tubing Consortium supported by the industry. Useful observations on the effects of polymer concentration and coiled tubing curvature on drag reduction were provided. Note that the curvature ratios of the coiled tubing reels in the investigation were limited ? the combinations of tubing diameter and reel drum diameter resulted in a range of curvature ratio of 0.0113 to 0.0185. To include this important parameter into friction factor or drag reduction correlations, further investigation on fluid flow through coils with a wider range of curvature ratio is apparently desired. Based on this consideration, a lab-scale test loop was constructed using 1/2-in. OD stainless steel tubing. It consists of a 10-ft straight tubing section and four replaceable coils with curvature ratio of 0.01, 0.019, 0.031, and 0.076 respectively. This range of curvature ratio is believed to be able to cover the curvature ratios encountered in the field operations. Besides the saved costs of running the full-scale experiments, better quality controls can be achieved concerning the fluid preparation, fluid degradation as well as testing conditions with the present lab-scale tests.

The objective of this paper is to present the new experimental results of coiled tubing curvature on the drag reduction behavior of polymeric fluids in coiled tubing.

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