Summary
Recently, a number of external-casing failures on a group of relatively new
wells in the Kuparuk field of Alaska prompted an investigation into the cause.
The investigation determined that external surface-casing-corrosion failures
had occurred near 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 indicates 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
electrochemical cell that corrodes the surface casing. Elevated casing
temperatures (≈125°F) accelerate the corrosion rates and increase the
temperature gradient between the casing and the conductor, creating a
thermogalvanic-corrosion cell.
Although damaged surface casing has been repaired mechanically 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
- Provide an outline of the extent of shallow external-casing corrosion seen
in the field.
- Detail the mechanisms for the external corrosion.
- Detail the mechanical-repair procedures used to return the wells to
service.
- Discuss possible mitigation methods to inhibit corrosion on existing
wells.
- Discuss the treatment chosen to stop the corrosion.
- Provide details of treatments used to inhibit further shallow-casing
corrosion.
Introduction
The Kuparuk field is located on the North Slope of Alaska, USA,
approximately 30 miles west of Prudhoe Bay (Fig. 1). The Greater Kuparuk Area
(GKA) includes Kuparuk and a number of smaller oil pools in the operating unit.
The wells are divided almost equally between producers and injectors.
The majority of GKA wells are configured with a conductor casing (CC),
surface casing (SC), production casing (PC), and tubing string. A smaller
number of wells have a single-casing design. Within specified limits, annular
pressure is allowed on most wells, and each casing string is expected to handle
the pressure within its allowable range (CO 494, Proposed Rules
2003).
The SC functions as an element of a secondary or tertiary layer of
protection between the reservoir and atmosphere. For single-casing wells
without a packer, the SC is the primary layer of protection. Therefore,
degradation of the SC caused by corrosion processes is a threat to the
integrity of a given well. SC corrosion can result, and has resulted, in wells
being removed from service because of the loss of the layer of protection.
Prevention of SC-corrosion problems is considered very important to maintain
the mechanical integrity of the field, ensuring personnel safety and
environmental protection and maximizing the service life from the wells.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
24 February 2006
- Meeting paper published:
8 May 2006
- Revised manuscript received:
17 April 2008
- Manuscript approved:
28 April 2008
- Version of record:
10 December 2008
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