Sour Serviceability of Higher-Strength Coiled Tubing: Final Results

Abstract As earlier as 1999, the need for industry guidelines regarding the use of coiled tubing (CT) in sour wells was recognized2. Small scale standard NACE tests (i.e. proof ring, C-ring, double cantilever beam, and slow strain rate tests) have been performed to evaluate the behaviour of CT materials in sour environments1,3,4,5. 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 strings 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 70 and 80 grade materials5. 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 of high-strength CT materials (i.e. 90 grade and up) 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 paper6. The present paper summarizes more than 4 years of laboratory testing aimed to establish the effect of sour exposure on the low-cycle fatigue life of CT materials. Other aspects related with the possible CT degradation from sour exposure are also included, such as the effect of different sour environments severities, performance of anti-cracking (H2S) inhibitors, effect of pre-fatigue, butt-welds and bias welds 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.