NEAR SURFACE EXTERNAL CASING CORROSION IN ALASKA: CAUSE AND MITIGATION

Recently, a number of external casing failures on a group of relatively new wells prompted an investigation into the cause. The investigation determined that external corrosion failures had occurred above the top of the casing cement within the conductor.

The surface casing corrosion was found to be caused by repetitive events of surface water entering into the top of the annulus between the surface casing and the conductor above the primary cement top. Testing of water and cement samples taken from the field indicate that the addition of oxygenated water and chemical salts that leach from the cement creates a low resistance electrolyte resulting in an extremely corrosive environment. The yearly replenishment of oxygenated water in this environment sets up an electro chemical cell that corrodes the surface casing. Elevated casing temperatures (~125 deg. F) accelerate the corrosion rates as well as the temperature gradient between the casing and the conductor creating a thermogalvanic corrosion cell.

Although damaged surface casing has been mechanically repaired on numerous wells by excavation and installation of welded sleeve patches, accessing the damaged surface casing can be difficult. Inhibiting the corrosion mechanism is considered a more tenable solution.

This paper will: 1. Provide an outline of extent of shallow external casing corrosion seen in the field; 2. Detail the mechanisms for the external corrosion; 3. Detail the mechanical repair procedures used to return the wells to service;

4. Discuss possible mitigation methods to inhibit corrosion on existing wells;

5. Discuss the treatment chosen to stop the corrosion; 6. Provide details of the first treatments to inhibit further shallow casing corrosion.

Understanding the mechanism, repairing existing damage, and mitigating future problems will have a significant rate benefit for operators on the North Slope of Alaska, and may be applied to other fields with similar problems worldwide.