For an offshore oil production field, a reservoir pressure maintenance scheme by the injection of minimally treated high-pressure seawater is being developed. The corrosion assessment focuses on the evaluation of degradation mechanisms of materials used to fabricate the major hardware items for transporting and injecting the raw seawater. The consequence of reservoir souring of the production system is also discussed.
Non-metallic materials are the preferred materials selection for the corrosive seawater service. However, these materials cannot always be used, because pressure limitations apply and mechanical damage cannot be accepted.
Where metallic materials have to be applied, the major corrosion threat is crevice corrosion due to the combined action of dissolved oxygen and chlorine. An assessment of the suitability of a range of alloys was carried out based on literature data and focused crevice corrosion tests for three candidate alloy types. Under shut-in conditions with temperatures up to 55 °C, it is concluded that alloys 925 and 2550 are not suitable for chlorinated seawater service even with less than 0.2 ppm of free chlorine. Alloy 725 is found resistant against crevice corrosion at this temperature with up to about 0.3 ppm of free chlorine. Alloy 625 and higher nickel alloys, as well as titanium alloys, have better corrosion resistance and are also suitable under these conditions.
Reservoir souring is controlled by biocide and nitrate injection. With surface equipment and production pipelines not being designed for sour service, up to 200 ppm H2S is acceptable in the produced gas under normal operating conditions. If more severe souring would occur, or under highpressure shut-in conditions, H2S scavenger injection will need to be applied.
In a mature offshore oil field, incremental reserves are to be developed and production sustained for an additional 15 years. This is to be achieved by a Minimally Filtered Seawater Injection (MFSI) project, which involves the implementation of a reservoir pressure maintenance and sweep improvement scheme by the injection of high-pressure raw seawater. The initial phase will consist of some injector wells, which are existing oil producers with high water cut and optimally placed for water injection. The producers will be converted to water injectors by replacing the existing carbon steel completion with corrosion resistant materials.
Originally, a conventional water injection development had been proposed which would include the installation of a large de-aerator tower and oxygen scavenger injection to control oxygen corrosion. This would result in the need to construct a new jacket to deliver the fully treated de-oxygenated water at the necessary rate. The raw seawater concept chosen has resulted in a reduction of the surface development cost by two-thirds. However, suitable construction materials must be selected to mitigate the corrosion risks associated with raw seawater injection. An optimal materials selection aims at controlling the corrosion risk in a cost effective manner.
