The Low Cycle Fatigue and Plastic Strain Response of Coiled Tubing in a Sour Environment With and Without Corrosion Inhibitor Protection

Abstract

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractThe post-yield material performance of 70 and 80 ksi yield strength carbon steel and high-strength corrosion resistant alloy (CRA) coiled tubing (CT) has been investigated experimentally to evaluate its response to a sour environment following plastic bending straining induced during coiled tubing operations. Although this investigation was specifically targeted for under-balanced drilling of sour wells, the results obtained can also be interpreted with respect to CT work-over operations. The four different types of laboratory testing performed were low cycle corrosion fatigue (LCCF), NACE proof ring tests, double cantilever beam tests (DCB) and slow strain rate tests (SSRT). Full-body CT specimens were used for the SSRT and LCCF testing procedures and only SSRT test results are presented for a specific CRA material. The test environments consisted of aqueous H 2 S, H 2 S/CO 2 , varying pH and the standard solution specified by NACE for determining the relative propensity for HIC and SSC. Tests were also performed using the NACE solution treated with an H 2 S corrosion inhibitor. This paper presents the key findings from the carbon steel CT testing, the preliminary data for the experimental CRA CT and the basis on which these materials were evaluated for under-balanced drilling applications. Both 70 ksi and 80 ksi carbon steel grades were found to be fit-for-purpose for sour wells in accordance with certain qualifications discussed. A preference for the lower strength, 70 ksi CT has been identified when CRA materials are not being considered. For carbon steel, the low cycle fatigue (LCF) life in a noninhibited sour environment was reduced to between 15% to 20% of the life in air depending upon the pH level. With the application of an H 2 S corrosion inhibitor, the LCF can be increased significantly to about 2/3 of the life obtained in air. The results of this research are currently being implemented for a revised issue of Industry Recommended Practises for Critical Sour Under-balanced Drilling-Drill String (IRP 6.3).

Sour Serviceability of Higher-Strength Coiled Tubing: Final Results

“…It is interesting that these sour-fatigue-life values from reversebending-fatigue tests are on the same order of magnitude as those reported by Luft (2003) from uniaxial corrosion fatigue tests performed on 70-and 80-grade CT full-body samples; the sour fatigue life of samples with inhibitor (probably no internal cracking, see Effect of H 2 S Anticracking Inhibitor section) was approximately 67% of the fatigue life measured in air, while for those tests without inhibitor (probably damaged by internal cracking) the sour fatigue life was between 15 and 20% of the fatigue life measured in air.…”

“…In addition, these tests do not give data that could be related with the low-cycle fatigue performance after sour exposure; such data could be used for strings management in real operating conditions, as corresponding data are used for CT operations under sweet conditions. By 2003, uniaxial, low-cycle corrosion fatigue tests of short full-body CT samples were used to evaluate the sour fatigue performance of 70-and 80-grade CT materials (Luft 2003). Lowcycle fatigue life was measured in noninhibited and inhibited sour environments.…”

“…Small-scale standard National Assocation of Corrosion Engineers (NACE) tests (i.e., proof-ring, C-ring, double-cantileverbeam, and slow-strain-rate tests) have been performed to evaluate the behavior of CT materials in sour environments (Cayard and Kane 1997;McCoy 2005;McCoy and Thomas 2006;Luft 2003). Although these tests evaluate the crack-resistance properties of the material in sour conditions, they do not give data regarding the lowcycle fatigue performance of CT after sour exposure that could be used for CT-string management in real operating conditions.…”

Welding Thermal Accumulation During Butt-welding of Coiled Tubing

“…Homogenous microstructure is recommended to be achieved through modify chemistries and manufacturing process [5]. Coiled tubing should be welded together with butt welding in field to get enough length, but unfortunately, its outstanding 1 Xiao LI, Kai Shi, Yanming Liu, Hongduo Wang.…”

Workovers in Sour Environments: How Do We Avoid Coiled Tubing (CT) Failures?

“…For underbalanced-drilling operations in sour wells, performance testing of corrosion inhibitors demonstrated that there should always be an H 2 S inhibitor incorporated where CT is used (Luft 2003;McCoy and Thomas 2006).…”