Identification and In-Depth Investigation of External Casing Corrosion in an Exploration Well Prior to Field Development

Abstract

Abstract External corrosion in new casings on the Norwegian Continental Shelf (NCS) is rare. An exception to this was discovered when reentering an exploration well that had been temporarily plugged and abandoned for 2 years. A pressure test showed the 9 5/8-in. liner was leaking. A multidisciplinary investigation was done to find the precise depth of the leak and its root cause. The proper logging technologies were carefully selected for this purpose. Due to the relatively low leak rate, it was suspected that holes in the liner were small. To locate these holes, noise, temperature, ultrasonic high-resolution corrosion, and multifinger caliper logs were recorded. One of the hypotheses for the root cause of the leak was corrosion resulting from the corrosivity of the formation fluid combined with crossflow between the depleted formations. To test this, a water-flow log was run to find any potential crossflow behind the liner. Modular dynamic tester sampling was performed to extract formation water samples from the interval for chemical composition analysis. The high-resolution corrosion data obtained from azimuthal and ultrasonic measurements showed the presence of multiple depths of external casing corrosion. These depths coincided with sections of the liner that were uncemented and therefore directly exposed to surrounding formation water. Analysis of the temperature data and the water-flow log during static well conditions enabled finding the precise location of the leak along with the detection of upwards water (or oxygen) movement behind the casing. Additionally, a detailed examination of water samples taken from the 12 ¼-in. section of the sidetrack well revealed that the formation water had a low pH and a high CO2 content, which were considered significant for the context of the NCS. Based on these observations, the corrosion rate was estimated at about 4 mm/year. This conclusion relied on the confirmation of the formation water flow from the depleted formations. This work helped to find the true root cause, which was found to be related to the selected well design, where the "overburden" was in communication with the reservoir section of a nearby field. As a preventive measure to avoid the recurrence of such issues in future similar wells in the NCS, the well design was changed during early field development.