Raw seawater injection is an increasingly used technology in oil production. The injection of water allows more oil to be retrieved from a well. A large advantage of using raw seawater compared to treated seawater is that no surveillance is needed, and hence, the manning can be reduced. However, raw seawater requires the use of more corrosion resistant alloys than treated seawater. UNS S33207 is a newly developed hyperduplex stainless steel for umbilicals. It has a minimum PREN of 50. Recently, large diameter tubing has been developed with minimum yield strength of 101 ksi in the solution annealed condition, which results in a high strength material with a low level of anisotropy for the mechanical properties. The crevice corrosion resistance was evaluated by laboratory testing. In addition, crevice corrosion testing of UNS S33207 in natural seawater with severe crevices was performed, demonstrating very good crevice corrosion resistance. Furthermore, no indications of stress corrosion cracking in SSRT-testing in synthetic seawater at 80°C were found. The mechanical properties of the large diameter tubing and the results from the corrosion testing show that UNS S33207 is a good candidate tubing material for raw seawater injection.
Water injection into a well is a method wherein water is injected into a reservoir to increase the pressure and thus the production of the reservoir. Another effect is that it pushes the oil to the well. Water injection makes production over a longer time possible and hence increases the volume that can be retrieved from the well. Several kinds of water can be used, such as produced water, seawater, fresh water and aquifer water. In offshore production, it is often most practical to use seawater, and filtering, deoxygenating and biociding are often performed.1 However, it may be a difficult task to control the quality of the treated seawater because it is a complex issue, and thus the injection of raw seawater may be simpler.2,3 For this application, tubing with threaded connections similar to that in OCTG applications is used. Pit et al. have further studied several Ni-base alloys, UNS N09925, UNS N06975 and UNS N07725 for raw seawater injection. They used a potentiostatic set method in which the potential was increased every 3 hours with 50 mV and a disc spring controlled crevice setup was used3. UNS N07725 was found to be acceptable in 0.2 to 0.3 ppm chlorinated seawater between 50 and 60°C, but UNS N09925 and UNS N06975 were not. It is not always possible to use a nonmetallic material, due to pressure limitations, because mechanical damage is not acceptable.3 The corrosion resistance of carbon steel is not high enough for a design life of 15 years.3 In a study by Mack and Carminati one superduplex grade was included, and they suggested that it is resistant in seawater with 100 ppb oxygen where the temperature altered between 38°C and 78°C
