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Abstract
Shallow surface casing failures have occurred in some wells in the Kuparuk oil field located on the North Slope of Alaska. A previous investigation into these failures has shown that they are caused by shallow external corrosion of the surface casing within the conductor annulus. The corrosion is believed to be caused by the presence of oxygenated surface water in the annulus and exacerbated by the extremely corrosive environment created when this water mixes with the ion-rich cement.

To combat this issue a field wide corrosion mitigation program began in 2006. The program focuses on injecting a corrosion inhibiting compound into the outer most annulus. At the time of this paper, 870 out of 1270 wells have had the mitigation completed. The goal of this compound is to coat the pipe and stop the influx of water into the annulus; without the water influx the corrosion should be reduced.

Concurrent with the mitigation program, a memory logging tool was used to monitor corrosion rates. This tool surveys variations in pipe metal thickness by measuring variation is velocity and amplitude of alternating magnetic waves. The log results display total metal volume by calibrating the tool at a depth that is assumed to be undamaged. The accuracy of this tool was verified for specific casing configurations on 14 wells that were excavated and inspected after logging. Since then, this log has been run several months prior to, at the time of, and several months after injecting the corrosion inhibitor into the annulus on various wells.

Using the data available from the logs and surface casing failures, the effectiveness of the corrosion mitigation program was evaluated. The main focus of this paper will be the methodology and conclusions obtained from the evaluation.

This paper will:

1.     Review the cause of shallow surface casing corrosion.
2.     Discuss current mitigation methods for reducing this corrosion.
3.     Detail how these methods vary based on well head design.
4.     Discuss the logging tool, its limitations, and how it was used for evaluation.
5.     Summarize the results of the evaluation.
6.     Discuss recommendations for improving the effectiveness of corrosion mitigation project on remaining wells.
7.     Outline the future of the corrosion mitigation project.

Introduction
The Kuparuk oil field is located on the North Slope of Alaska, about 30 miles west of Prudhoe Bay (Fig. 1). The Greater Kuparuk Area (GKA) includes the Kuparuk reservoir along with several smaller oil pools in the operating unit.

The most common design for wells in this operating unit includes Conductor Casing (CC), Surface Casing (SC), Production Casing (PC), and tubing. With this design the SC functions as a tertiary layer of protection between the reservoir and atmosphere.

However, around 5% of the 1270 wells use a single-casing design. This design uses only CC, SC, and tubing, making the SC the secondary layer of protection between the reservoir and atmosphere. In this case, corrosion of the SC is a serious and dangerous breach in the integrity of the well. Mitigation of SC corrosion is considered vital in maintaining the integrity of the wells, safety of personnel, and protection of the environment. These single-casing designed wells were used as the focus for the mitigation evaluation study discussed in this paper.