Abstract
Less than one year after recompletion, a producing well was shut in due to penetrating wall thickness loss in the API 5CT L80 Type 1 top completion. The short service life included a history of numerous coil tubing jobs, several shut-in periods and only limited production periods. The root cause of a longitudinal local wall thickness loss of approximately 7 mm in 10 months was examined. No indications of wear were detected on the examined tubular and on the basis of the geometries of the longitudinal track and the coil, respectively, the coil itself cannot have induced the penetrating tracks by metal-metal contact. The API 5CT L80-13Cr pup joints were also corroded, indicating a possible effect of oxygenated seawater.
A plausible explanation for the failure mode is that the numerous coil tubing operations caused local mechanical damage to protective scale and iron sulfides exposing the bare metal in a track, thereby facilitating localization of corrosion on the inside surface of the tubular. The controlling degradation mechanism is corrosion due to aggressive fluids, including untreated, oxygen-containing seawater used during the interventions. It is essential that the prescribed treatment of seawater with corrosion inhibitor, oxygen scavenger and biocide is followed in all cases and during all interventions.
