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 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 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).
The requirement to deform coiled tubing beyond its yield strain during spooling operations is common knowledge in the coiled tubing industry. The flexure strains imposed are typically in the range of 2% to 3% plastic strain. Plastic deformation, even to a seemingly minor extent, is know to significantly reduce the resistance of carbon steels to sulphide stress cracking (SSC) and is therefore prohibited in conventional oilfield tubular goods. For coiled tubing, such repetitive cold working, has not impeded well work-over operations in sour environments that have been performed successfully for many years. For under-balanced drilling using coiled tubing (DUCT) in sour wells, however, the Alberta Energy and Utilities Board (AEUB) recognized a need to establish an industry recommended practice (IRP) that included the use of CT drilling in critical sour reservoirs.
A preliminary IRP 6.3 has been published1 based on available information, publications2,3 and on limited static test data commissioned by the AEUB Drilling And Completions Committee (D.A.A.C). The material and operating constraints contained in this draft of the IRP was tantamount to eliminating the use of DUCT in sour reservoirs. It was subsequently recognized that for a more realistic and reliable fitness-for-purpose evaluation of material performance in sour conditions, additional test data, based on dynamic testing procedures and full body CT test specimens, was required. For this purpose, a joint industry project (JIP) was initiated by BJ Services. The complete results of this JIP, including the research performed independently by Shell Canada and BJ Services, were published recently in complete detail4 with the hope of improving the IRP guidelines and for the general benefit of the coiled tubing and oil and gas industries. The present paper summarizes the major results pertaining to the CT70 and CT80 carbon steel tubing and presents the complete test results for the CRA CT that were not published previously.