Workovers of sour wells (H2S contaminated) including gas, oil and water injection wells, present special challenges. Fields in Russia, Canada and Saudi Arabia are some of the most significantly affected. Add higher temperatures and specialty equipment and sometimes failures can result. Coiled tubing has become one of the mainstays in treating these types of wells. Failures can be such that the tubing is recovered completely with leaks while catastrophic parting of the coiled tubing can leave several thousand feet including bottomhole assemblies to be fished.

Corrosion protection to the proper level of weight loss (< 0.02 to 0.05 lb/ft2) and zero localized surface defects (pitting and stress cracking) both in live and spent acid solutions is essential to protecting the production equipment. Doing this is a step towards avoiding coiled tubing failures during acid treatments down coiled tubing. However, more than this is essential to the safe usage of coiled tubing. Included in this is proper selection of metallurgy for the downhole environment, appropriate use of hydrogen sulfide inhibitors, non-destructive testing of the coil prior to usage, periodic sampling of the coil for physical examination and more are still in the development stages.

This paper reviews two case histories of coiled tubing failures during workovers, including the analyses that were performed and recommendations defined of practices to avoid reoccurrence. Further, a suggested set of guidelines are presented for the safe usage of coiled tubing for workovers in sour wells.

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