Smaller Coiled Tubing Diameter Achievable by the Use of Lubricants
Authors: Livescu Silviu , Craig Steven , Watkins Thomas 
Published: March 2015

The requirement for well intervention operations in laterals longer than 10,000 ft continues to grow. The current industry trend for servicing long laterals is to increase the coiled tubing (CT) diameter and use fluid hammer or tractor tools. While increasing the CT diameter remains a theoretical option to improve reach, practically, it creates logistical challenges with onshore road transport and offshore crane lifting/deck loading limitations. Other options such as using fluid hammer and tractor tools may have reasonable operational range, but they have significant limitations by increasing circulating pressures and operational complexity and reducing CT speeds before lock-up. Using lubricants is the simplest, most predictable, and, sometimes, most cost-effective method. However, most current field results show only a 15 to 20% reduction in the coefficient of friction (CoF), from a generic value of 0.24 to 0.20. This CT friction reduction is not sufficient to run CT smaller than 2-in. in long laterals.

In this paper, the current status of lubricant usage in well intervention operations is reviewed. Several results of an extensive laboratory program using the first in industry linear friction instrument especially targeting CT operations are also reported. More than 6,600 measurements have been performed to date with many combinations of CT and casing samples and lubricants currently used in field operations. Based on these laboratory results, a new lubricant was developed that results in significantly lower CoFs than previously seen under downhole conditions. The field CoFs with the new lubricant are in the 0.09 to 0.13 range (i.e., field CoF reduction of approximately 46 to 63% from the default value of 0.24) and validate the laboratory results. Numerical simulation results show that CT friction reduction of this magnitude facilitates the CT diameter size decrease in long laterals and brings great benefits to the industry by lowering the operational costs and carbon footprint. Decreasing the CT size is particularly beneficial in Southeast Asia where offshore smaller platforms and smaller CT sizes are vital for intervention operations in small diameter wells.

Introduction

CT has been used as a well intervention technology since the early 1960's. Since then, the CT diameter has steadily increased, driven by such operational requirements as higher flow rates, larger push and pull forces, and the need for entry into longer laterals (Portman, 1999). That is, larger CT are used because they can handle more flow, are stronger and stiffer. However, increasing the CT diameter requires larger and heavier equipment and creates logistical problems both onshore, with road transportation, and offshore, with crane lifting and deck loading limitations. In addition, larger CT diameter means lower annular flow rates, lower pressure snubbing, shorter fatigue life, lower burst and collapse pressures, and higher operational costs and carbon footprint.

The logistical challenges with larger and heavier equipment and higher operational costs could be overcome by using smaller CT diameter and additional technologies. There are several well documented case histories in literature that show how smaller CT sizes, such as 1-in., have been successfully utilized for scale removal, solids cleanouts, milling, perforating, and gas lifting operations (Terry et al., 2013). Some of the factors enumerated above favoring larger CT have already been addressed by the industry for using smaller pipes.