Evaluating the Fitness-for-Purpose of Coiled Tubing for Underbalanced Drilling of Sour Wells


A multi-year joint industry program (JIP) has been completed involving an experimental testing program to evaluate the fitness-for-purpose of high-strength low-alloy steel coiled tubing (CT) in underbalanced drilling of sour oil and gas wells. The primary focus is the cold-worked material response to sour well conditions when subjecting the tubing to multiple cycles of plastic tension and compression strains in the range of 2% to 3%. The material performance and/or degradation parameters measured include low-cycle corrosion-fatigue (LCCF) life, critical stress corrosion cracking (SCC) stress intensity factor, KISCC, total plastic strain to failure, normalized fracture strain, sulfide stress cracking (SSC) and hydrogen induced cracking (HIC). Laboratory tests performed under the auspices of the JIP, consisted of low-cycle, axial-straining corrosion-fatigue with full body CT specimens submerged in aqueous H2S, H2S/CO2 varying pH, SSC corrosion inhibitors and NACE standard solution specified for determining the relative propensity for HIC and SSC. Various other testing was performed unilaterally by individual JIP sponsors, including double cantilever beam (DCB) and slow strain rate testing (SSRT).

Coiled tubing samples comprised the 70 ksi and 80 ksi yield strength grades, pre-fatigued with varying exposure duration to the corrosive environment. The assessment for suitability of CT in underbalanced drilling of sour wells, was made on basis of the various performance results and failure modes obtained in the laboratory testing. These results are also intended to further the development of industry operational guidelines and coiled tubing material selection for under-balanced drilling of sour wells.


Oilfield operations with continuous or coiled tubing necessitate the tubing string to be spooled on a working reel and be conveyed into the well bore with the aid of a guide arch or gooseneck. Both components of a coiled tubing unit (CTU), require the tubing to be bent beyond their yield point such that the maximum flexure strains imposed attain values in the range of 2% to 3% plastic strain. Conventional coiled tubing is manufactured from ASTM A606/607 Type A modified fine-grained steel strip continuously rolled into tube form and seam welded by electric resistance welding (ERW) followed by a full body stress relief heat treatment. The ranges in sizes, defined by outer diameter (OD) and wall thickness (WT), commonly produced are from 1 OD to 3-1/2 OD and 0.08 to 0.203 respectively. Yield strengths and ductility range from 65 ksi to 120 ksi specified minimum yield strength (SMYS) and 30% to 22% elongation, respectively. Maximum hardness values are HRC = 22 for the 70 ksi to 90 ksi grades and approximately HRC = 28 for the 100 ksi to 120 ksi grades.

Author: Luft, B., Szklarz, K. E., Nowinka, J., & Skrzypek, H.

Publisher: CORROSION 2002, 7-11 April, Denver, Colorado

Year: 2002

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