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 ½-in. outside diameter (OD) straight tubing and four reels of coiled tubing of the same diameter. Fluids investigated included water, guar, hydroxypropyl guar (HPG), and xanthan fluids at various polymer concentrations. It was found that the coiled-tubing curvature could reduce the drag reduction by 10 to 30% as compared to straight tubing, depending on the flow conditions. The tubing curvature also delays the onset of drag reduction. The percentage of 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 full-scale experiments and were found to be representative of the drag-reduction behavior of similar fluids under field flow conditions.
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 because the flow rates through coiled-tubing reels are often limited owing to the small tubing diameter (coiled tubing must be small enough for the whole length of tubing string to be spooled on the reel drum). The tubing curvature causes secondary flow, which further increases the frictional pressure loss in coiled tubing.
Since the discovery of the drag-reduction phenomenon of polymer solutions in pipe flow by Toms (1948), there have been numerous studies on the important subject of fluids engineering. The papers by Lumley (1969), Hoyt (1972, 1990), Virk (1975), and Berman (1978) provide 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 from that 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 (Dean 1927, 1928). This secondary flow, superimposed on the main axial flow, will change the flow field and, for drag-reducing fluids, will affect the drag-reduction mechanisms.
Most of the previous studies on fluid flow in coiled pipes have focused on the flow of Newtonian fluids (Zhou and Shah 2004). Only a few studies have investigated the flow of non-Newtonian fluids in coiled pipes (Mashelkar and Devarajan 1976a, 1976b, 1977; Mishra and Gupta 1979). 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 (Azouz et al. 1998; McCann and Islas 1996; Shah and Zhou 2003). Shah and Zhou (2003) investigated drag-reduction behavior using a full-scale coiled-tubing test facility that was established for the joint-industry Coiled Tubing Consortium. Useful observations on the effects of polymer concentration and coiled-tubing curvature on drag reduction were provided. It should be noted that the curvature ratios of the coiled-tubing reels in the investigation were limited?the combinations of tubing diameter and drum diameter resulted in a range of curvature ratio of 0.0113 to 0.0185. To include this important parameter in friction-factor or drag-reduction correlations, further investigation on fluid flow through coils with a wider range of curvature ratios is apparently desired. On the basis of this consideration, a lab-scale test loop was constructed with ½-in. OD stainless-steel tubing. It consisted of a 10-ft straight tubing section and four replaceable coils with curvature ratios of 0.01, 0.019, 0.031, and 0.076. This range of curvature ratios is believed sufficient to cover the range normally encountered in field operations. In addition to the lower costs, better quality control during fluid preparation and testing and less fluid degradation can be achieved with the lab-scale tests, rather than conducting tests in a full-scale flow loop.
The objective of this paper is to present the experimental results of the effects of coiled-tubing curvature on the drag-reduction behavior of polymeric fluids.