Technology Update: Biocide Treatment Program Reduces Premature Coiled Tubing Failures
- Lemuel Edillon (STEP Energy Services) | Matthew Henderson (Fusion Technologies) | Stan Leong (OSP)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- June 2016
- Document Type
- Journal Paper
- 20 - 21
- 2016. Copyright is retained by the author. This document is distributed by SPE with the permission of the author. Contact the author for permission to use material from this document.
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Premature coiled tubing (CT) failures have occurred in the United States, in areas such as the Eagle Ford and Haynesville formations, and the Permian Basin. In Canada, these failures once were not considered as prevalent. However, a presentation at the 2015 SPE/ICoTA conference in The Woodlands, Texas, changed that perception. A paper presented there detailed how a major Canadian CT service provider experienced a series of CT string failures while performing bridge plug millout operations in the Montney formation in northeastern British Columbia (Edillon et al., 2015).
With each trip in or out of the wellbore, CT strings incur fatigue that can be estimated using simulation software based on the CT outside diameter (OD), material grade, and operating conditions. CT strings can fail for a variety of reasons, including external mechanical damage, corrosion, and excessive diametrical growth. Premature failure results not only in operational delays and the associated costs, but critical safety risks for onsite personnel.
For these reasons, a study was undertaken to determine the root cause of the CT string failures in the Montney. The results showed that microbial influenced corrosion (MIC) was a contributing factor. This led the CT service provider to instigate laboratory and field studies to create a biocide treatment program. The program has successfully mitigated further CT string failures in the Montney and has now been applied to CT programs in the Eagle Ford formation.
CT String Analysis
Seven failures of 60.3-mm (2.375-in.) CT strings supplied by multiple manufacturers were experienced over 2½ months. The strings failed in the range of 18% to 49% fatigue—well below their predicted 80% fatigue life. Preliminary analysis of the failure points revealed pinholes and internal corrosion in the base material and at the bias weld. The filler material used in bias welds has different mechanical properties than the base pipe material, making it weaker and more fatigue-prone.
To determine the failure mechanism, CT strings from four of the seven units were further analyzed by the manufacturers. In all four cases, internal corrosion pitting was observed. Comprehensive analysis on one string, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, and metallography, was performed by the manufacturer and the University of Calgary. The analysis revealed that failure occurred at the bias weld and indicated microcorrosion pitting and transverse cracking in the weld filler material (Fig. 1). The corrosion morphology (rounded shape with light green corrosion debris) and the presence of sulfur in the corrosion product provided indirect evidence of MIC.
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