High-Strength Coiled Tubing Expands Service Capabilities
Authors: Chitwood G.B. , Lewis P.C. , Fowler S.H. , Zernick W.M. 
Published: October 2008

This paper discusses the development of a new grade of coiled tubing which has 100,000 psi minimum yield strength. Enhanced properties are imparted through new heat treatment techniques. Raising the yield strength from the 70,000 psi minimum currently being used to 100,000 psi increases the acceptable tensile/pressure envelope by almost 50 percent, yet resistance to sulfide stress cracking is improved. Cycle testing reveals that bending fatigue, a common cause of failure with coiled tubing, is reduced by approximately 50 percent with the new grade. By increasing the strength of the coiled tubing, the variety of fast and economical services that can be performed is expanded.

INTRODUCTION

Well servicing with coiled tubing has been in existence for nearly 30 years, providing advantages unrealized from conventional jointed tubing.1 Beginning with coiled tubing of about 50 ksi yield strength made from as-rolled steel, performance was adequate for the sizes being used and the pressures encountered. However, as demand increased for larger sizes, deeper depths and higher pressures, new higher strength steels in the 70 ksi yield strength range were employed. These materials were generally hot rolled as-rolled steel products and the tubing was subsequently cold worked, seam annealed and tempered. Also, better methods of splicing the tubing strip together greater operating parameters.

Yet as the oil and gas industry finds more applications for coiled tubing in servicing wells, it becomes desirable to further increase working pressure and depth capabilities. Substantial gains in pressure and depth cannot be realized with existing materials; the plateau in yield strength of as-rolled steel products is only about 80 ksi. Rather, it is necessary to move on to more sophisticated heat treating operations to raise the properties significantly. This is especially true since coiled tubing requires operation in the plastic state and in an H2S environment that can cause sulfide stress cracking (SSC).

INITIAL APPROACH

Several factors were considered during the initial development of high-strength coiled tubing. First was the necessity to maintain adequate SSC resistance. Second was the need for a manufacturing technique that yielded consistent quality and also allowed the tubing to be repaired when necessary. The selection of a chromium-molybdenum steel appeared to satisfy all of these needs. Numerous sources cite the excellent SSC resistance of a chromium-mo1ybdenum steel, strengthened by a quench and temper (Q&T) heat treatment.2,3

The material chosen, a low-carbon 4100 series steel, has sufficient hardenability to achieve the uniform, martensitic microstructure from heat treatment required for optimum SSC resistance, and yet has with low enough carbon to be field repair welded. To take full benefit of the heat treatment, though, the tubing would need to be heat treated after it was spliced together at the factory.

Therefore, the decision to provide fully heat treated tubing required an entirely new manufacturing process. Since the sheet coil used to make the tubing only comes in 3000 5000 ft. 1engths, sect ions must be sp1iced together to make the necessary 15,000 25,000 ft. length of tubing, either when the strip is still flat or after rolling and seam welding into tubing.