Historically in our industry there have been many anecdotal claims of premature coiled tubing pipe failures during plug milling operations with recirculated fluid. Many of these failures shared related failure characteristics but no root cause was believed to have been identified by the service companies involved. In 2013, for one particular client, a series of failures on the coiled tubing string bias welds was observed, with a root cause due to the presence of sulfate-reducing bacteria (SRB) in the recirculated water. Two different mechanisms were identified as causing the premature failures on the bias welds: high-pressure corrosion fatigue that started at severe internal pitting caused by the presence of bacteria, and sulfide stress cracking caused by the SRB producing H2S.

A detailed analysis was also completed of time periods where fluid was spent stagnant in coiled tubing during rig-down/rig-up operations. Procedures were adjusted accordingly to prevent failure from further exposure to similar conditions by modifying recirculated fluid-handling volumes, improved biocide use, and rigorous multicomponent inhibition and purging during and between jobs.

Several specialist colleagues, familiar with SRB corrosion, were surprised at the speed SRB formed and created problems in coiled tubing in these environments. During the investigation process a rigorous fluid-testing protocol was developed. A prevention procedure was implemented during coiled tubing plug milling operations in this field. Afterward, no further coiled tubing failures were observed during similar operations in this area.

These failures have an industrywide impact in terms of personnel safety, operational disruption, and the cost and inconvenience of replacing coiled tubing strings. Information regarding coiled tubing failures related to bacteria exposure is only recently being published in our industry (Sherman 2014, Sherman 2015, Edillon 2015). Establishing the causes of these pipe failures and developing a procedure to minimize and/or eliminate their recurrence will allow safer operations for everyone involved in these operations. This paper presents the characteristics of these failures and the corrective actions taken to prevent further failures in coiled tubing exposed to similar conditions.

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