The use of high strength American Petroleum Institute (API)(1) 5CT Q-125 Type 1 Cr-Mo steel tubing is planned for treated seawater injection wells for several deepwater projects. The API 5CT Q-125 Type 1 Cr-Mo steel tubing offers multimillion dollar savings per well, compared to the widely used Super Duplex 25 Cr (Unified Numbering system [UNS](2) S32760) tubing. Holding oxygen levels at an average of 10 ppb, with alarm and shut-in levels of 40 ppb, will maintain corrosion rates at an acceptable level for the life of the field.

Several field case histories will be presented from operations in deepwater West Africa, where seawater floods have been in service for 7 to 9 years (or more) without failure. Corrosion rate calculations of the expected wall loss of the Cr-Mo steel tubing and connection were also made to ensure that sufficient mechanical properties remain intact for the life of the projects. The results of the 40-pin field caliper surveys from deepwater West Africa water injection wells will be presented. Carbon steel piping from a subsea water injection jumper retrieved from the field showed minimal corrosion after over 8 years and over 53 million bbls of injected seawater.

The seawater treating process design consists of a series of course and fine filters, a 40-μ security filter, followed by the sulfate removal unit (SRU) membranes. A gas-stripping tower removes oxygen down to 40 ppb. Chemical treatment follows, removing oxygen to less than 10 ppb. The sulfate removal membrane is a nanofiltration system that rejects bacteria along with some chlorides and removes sulfate to less than 40 ppm. Multiple oxygen sensors prevent unplanned excursions and there is no bypass piping around the stripping tower or sulfate membrane equipment.

Low levels of oxygen in solution are not thermodynamically favored in the presence of dissolved iron. Ionic equilibrium software was used to predict the equilibrium oxygen concentration as a function of dissolved iron in the seawater.

Subject
Water, Corrosion rate, Sulfates, Walls, Calipers, Seawater injection, Seawater, Tubing, Penetration, Water injection, Towers, Oxygen, Surveys
Keywords:
Cr-Mo steel, low alloy steel, seawater injection, tubing selection, oxygen corrosion
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2016
Association for Materials Protection and Performance (AMPP)
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