Root cause analysis of the corrosion-related coiled tubing failure

Coiled tubing (CT) is widely used in the oil and gas industry. However, corrosion-related failures are frequently reported. Research into the causes of failures leads to improvement in the design of components and processes. In this study, a new CT sample and a CT sample with perforated wall that had failed after a few acidizing operations were selected for analysis. Scanning electron microscope (SEM) images at the fracture site showed that CT damage was caused by the low cycle fatigue. In addition, light and scanning electron microscopy (SEM) showed that a corrosion pit acted as the initiator of the crack. Elemental analysis using energy dispersive X-ray spectroscopy (EDS) indicated the presence of an iron oxide layer and a layer associated with the Sb containing inhibitor. The corrosion damage investigation showed that the internal CT wall pits likely formed during storage due to the acidizing operations in the areas where the remaining liquid was still at the tube bottom.