Challenging the Industry's Understanding of the Mechanical Friction Reduction for Coiled Tubing Operations
Authors: Livescu Silviu , Craig Steven , Watkins Tom 
Published: October 2014

The requirement for intervention operations in long laterals continues to grow. For instance, in North America and North Sea, it is required to run 2 and 2 3/8-in coiled tubing (CT) in 12,000 and 25,000 ft laterals, respectively. One of the most critical well intervention problems nowadays is that many extended reach wells that need to be serviced are more than 30–40% unreachable and contribute to lost productivity for energy companies around the world. 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.

To reach the remaining 30–40% un-reachable length, lubricants are required to work in conjunction with other systems. Obviously, the use of lubricants for well interventions is not new. Typical field results of current systems show a 17–25% reduction in the coefficient of friction (CoF), from a generic 0.24 to 0.18 – 0.20. However, these results compare poorly to the industry-wide lab rotational friction tests that do not take into account any downhole parameters. CoF values between 0.03 and 0.08 are usually reported in such lab tests, but never achieved in the field.

In this paper, the results of an extensive set of laboratory measurements using the first in industry linear friction apparatus are reported. This instrument was designed to take into account the downhole effects of temperature, pressure, CT sliding speed, surface roughness, and fluid composition. The lab results show that the pressure and the sliding speed have weak effects of friction, while temperature, surface roughness, and fluid type and composition have strong effects. More than 6,000 measurements were performed with many combinations of CT and casing samples and lubricants currently used in field operations. Based on these laboratory results, a new lubricant was designated that reduces the linear CoF by approximately 42–58% (from the default CoF of 0.24 to 0.10–0.14) under downhole conditions. Friction of this magnitude is expected to make it feasible to run CT in all previously un-reachable laterals and decrease the operational time when fluid hammer or tractor tools are used.

Several North America field trials with the new lubricant are also reported. The field CoF values obtained are in the 0.12 to 0.14 range and validate the new laboratory methodology. Friction reduction of this magnitude is expected to double the reach in long lateral wells without lubricant. Previous field CoF calculation studies for determining predictable lateral reach are also reviewed. The laboratory and field results with the new lubricant challenge the current industry understanding of the CT friction and show great benefits for our industry in extending the reach of lateral wells.