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

Abstract

Abstract The 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 H2S, H2S/CO2, 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 H2S 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 non-inhibited sour environment was reduced to between 15% to 20% of the life in air depending upon the pH level. With the application of an H2S 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).