Sour Serviceability of Higher-Strength Coiled Tubing: Final Results

Abstract

Summary As early as 1999, the need for industry guidelines regarding the use of coiled tubing (CT) in sour wells was recognized (Luft and Wilde 1999). Small-scale standard National Assocation of Corrosion Engineers (NACE) tests (i.e., proof-ring, C-ring, double-cantilever-beam, 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 low-cycle fatigue performance of CT after sour exposure that could be used for CT-string management in real operating conditions. By 2003, uniaxial low-cycle corrosion fatigue tests of short, full-body CT samples were used to evaluate the sour performance of Grade-70 and -80 materials (Luft 2003). These tests, although they considered fatigue performance, were based on uniaxial reversing loads rather than reversing bending strains, which is what occurs in CT operations. In addition to this, the need for high-strength CT materials (i.e., 90-grade and higher) in sour wells required additional knowledge regarding their sour performance. By the end of 2004, a joint-industry project (JIP) among ExxonMobil Development Company, Shell Canada Limited, and BJ Services Company was initiated to investigate the serviceability of high-strength CT in sour environments, with the main emphasis on the measurement of low-cycle bending-fatigue life after sour exposure using full-body CT samples. Some of the early results of this project were included in a previous paper (Luft et al. 2007). The present paper summarizes more than 4 years of laboratory testing aimed at establishing the effect of sour exposure on the low-cycle fatigue life of CT materials. Other aspects related to the possible CT degradation from sour exposure are also included, such as the effect of varying sour-environment severities, performance of anticracking (H2S) inhibitors, effect of prefatigue, butt- and bias- weld performance, and tensile-strength integrity. The results have led to changes in the current operating procedures regarding CT-strings management and CT-material selection criteria for sour service.