The excessive friction pressure loss due to the small tubing diameter and the curvature (which believed to cause secondary flow) of coiled tubing (CT) often limits the maximum obtainable flow rate in CT operations. This paper presents an experimental study of drag reduction performance of several polymer solutions in coiled tubing using a sophisticated full-scale coiled tubing test facility for an industry-university joint research project. The facility includes of seven reels of coiled tubing with diameters of 1", 1–1/2", and 2–3/8", fluid mixing and pumping equipment, and data acquisition system. Fluids investigated include water and solutions of polymers currently used in the well drilling and completion industry - Xanthan gum, partially hydrolyzed polyacrylamide (PHPA), guar gum, and hydroxyethylcellulose (HEC). Experimental results showed that the amount of friction drag reduction differs significantly among the different types of polymers at various concentrations. Data interpretation and analysis revealed that the CT diameter and the CT-to-reel drum diameter ratio are important geometrical parameters affecting drag reduction. The flow data indicate that the onset of turbulent flow in CT is suppressed in the polymer solutions. This paper introduces the concept of the modified drag reduction envelope for flow of polymer solutions in coiled tubing.


Coiled tubing has found many applications in the petroleum industry which include drilling (CTD), cementing, wellbore cleanout, acidizing and hydraulic fracturing, etc. 1 But, the excessive friction pressure loss due to the relatively small tubing diameter and the tubing curvature (which is believed to cause secondary flow) of the coiled tubing often limits the maximum obtainable fluid injection rates. A recent experimental investigation2 indicates that the frictional loss in coiled tubing is significantly higher than in straight tubing.

One way to increase the injection rate and reduce pumping cost is to use drag reducing additives (or drag reducers). It has been found that typical drilling and completion fluids, usually polymer solutions, exhibit some drag reducing property. Therefore, it is of practical importance to investigate the drag reduction properties of these solutions in coiled tubing.

There has been extensive research on drag reduction phenomena with dilute polymer solutions, soap solutions, or suspensions in straight pipe flow. But no information on drag reduction of polymer solutions in CT is available.

In this study, extensive experiments using a full-scale coiled tubing test flow loop have been conducted for the joint industry research project - the Coiled Tubing Consortium at the University of Oklahoma. This paper presents the experimental results and the characteristics of drag reduction performance of typical drilling and completion fluids tested in the coiled tubing. Major factors affecting drag reduction performances are also discussed.

Literature Review

Frictional pressure in turbulent flow can be drastically reduced by adding small quantities of certain long chain polymers to the solvent such as water. This phenomenon is called drag reduction. Generally, credit is given to Toms3 for being the first to observe the phenomenon, therefore, drag reduction is also called Toms phenomenon. Toms was investigating the mechanical degradation of high polymer solutions in pipe flow. It was found that a solution of polymethyl methacrylate in monochlorobenzene required a lower pressure gradient than the solvent alone to produce the same flow rate.

Several references4–9 that appeared in the petroleum literature indicate the importance and potential applications of drag reduction to this industry. Savins4 reported pipe flow tests using a number of synthetic and natural polymeric materials and three tubing sizes. Factors affecting the drag ratios were studied. He also compared the test data with Dodge-Metzner10 friction factor correlation and observed the "diameter effect".

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