High-strength coiled tubing (CT) was subjected to sulfide stress cracking (SSC) testing for the purpose of defining acceptable operating zones in sour environments, both with and without inhibition. In this case, 130 ksi specified minimum yield strength tubing was used. This paper presents a summary of the results with conclusions, lesson learned, and recommendations for sour zones of high-strength CT with and without inhibition.
Coupons of CT were tested in accordance with NACE TM0177 Method B (four-point bent beams) (NACE TM0177-2005) to determine if cracking occurred in different sour environments. Sour environments were tested ranging from in-situ pH of 2.8 to 5.5 and H2S partial pressure of 0.005 to 10 bar. Testing was performed both including and excluding chemical inhibition. Coupons tested were removed from as-milled tubing and tubing that had been subjected to low-cycle plastic fatigue under pressure on a laboratory CT fatigue testing machine. Plasma arc bias welds, high frequency induction seam welds, and parent material coupons were tested.
Without inhibition, coupons tested in mild, intermediate, and severe sour environments failed because of sulfide stress cracking. However, when chemical inhibition was mixed into the aqueous environment, coupons tested in severe sour environments produced acceptable results, meaning that no sulfide stress cracking occurred. The acceptable results indicate that, when properly managed, high-strength CT can be used successfully in a manner similar to lower-strength CT when operating in aqueous downhole environments with high sour gas partial pressures. The sour environments tested with and without inhibitor were plotted on a pH versus H2S partial pressure diagram. Based on the failure or no-failure results of the testing, recommended operating zones were then determined and are presented.
The development of a high-strength CT SSC testing program is also presented in this work. Guidelines for the test program include the preparation of bias weld, seam weld, and parent material specimens and also simulation of the cyclic plastic strain and internal pressure typical of CT operations.