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Abstract Coiled tubing (CT) strings are costly and long lead items which are prone to sulfide stress cracking (SSC) and stress corrosion cracking (SCC) when exposed to sour conditions. This paper provides the real-time observations where CT exposure to harsh sour well conditions led to the occurrence of premature failure in CT strings resulting from SSC. The paper will also describe how the application of improvised protection practices can lead to quantifiable enhancement of CT string life. The paper will also explain the optimum grade selection for a CT string which remains the foundational factor influencing the life expectancy of the string and also outline best practices for CT string maintenance and management on and off jobs. Analyzing mass data of CT string utilization spreading over 200 plus well intervention CT runs shows the dominating control factor is the application of SSC inhibitors with unique scavengers to inhibit the corrosive environment the string is exposed to. Chemical protection in combination with appropriate fatigue derating was primarily evaluated in this study. The results were exceptional having observed zero sulfide stress related failures resulting from exposure to hydrogen sulfide (H2S) concentrations as high as 70,000 ppm (7%) and high-pressure high-temperature (HPHT) conditions over a period of four years. When it comes to the observation of coiled tubing life expectancy, the application of improved practices and procedures and the inference resulting from these practices, are time-bound and dynamic. The pre-job evaluation of the scope and wellbore conditions was a key turning point as it affected the job design. Specific jobs may have sour conditions, but with a lower partial pressure, the design of chemical protection can remain mild. Practices in this paper proved effective for a range of H2S and downhole conditions where H2S partial pressure was always above 0.05 psi. Concurrently, the study concluded that a scope that calls for prolonged CT string exposure to downhole conditions and prohibits access to pumping chemical, will not be the right candidate to implement the recommended practices in this study. In conclusion, using effective scavengers and inhibitors along with implementing the recommended practices such as coiled tubing fatigue derating remained the basis for establishing the best protection practice for such unforgiving environments. The study this paper presents dispenses an operating range for a coiled tubing to safely operate in specific high H2S environments. Implementation of synonymous design of chemical protection and coiled tubing derating routine establishes a moderate precedent for operating low carbon allow pipes in harsh sour environment while maximizing pipe life.
Kumar et al. (Mon,) studied this question.