Corrosion in the presence of CO2 was studied under supercritical conditions (high pressure and moderate temperature) using different carbon steels and various corrosion resistant alloys (CRA's). An objective of this work was, among others, to put the results in perspective relative to various CO2 corrosion models that have been developed and published over the years. In particular, the NORSOK, FREECORP, and OLI models were used for comparison, while others were discussed as well. The systems investigated were (a) supercritical CO2 (SC CO2) saturated with water (no separate water phase), (b) a water mist phase in equilibrium with SC CO2, (c) a water/brine phase in equilibrium with SC CO2. It was found that the OLI model best simulates the experimental corrosion rates observed in system (c) where coupons were corroded in the aqueous phase at high velocities in the rotating cage. The varying susceptibility of different carbon steels to corrosion under these conditions is highlighted. CRA steels are comparatively more resistant.

This paper will share some of the collaborative efforts completed within DeepStar® Global Deepwater Technology Development Program as part of Phase XII projects in X500 Drilling, Completions & Intervention committee leading to gap identification, industry standards, and guideline deployment. Selecting safe economical materials for production from the increasingly more important HPHT horizons is a serious challenge because of the required combination of ‘safe and economical’. This challenge is being discussed in the literature as well as the standard writing community at great length. The effort aims at economical materials remaining within safe limits without being overly conservative. It is well known that qualification for sour service has occurred experimentally within a matrix of pH and H2S partial pressure ever since NACE MR0175 was first established some 40 years ago. It has now been recognized within the past 5+ years that it is the H2S concentration in solution which is responsible for the corrosion damage. As the total pressure increases the solubility is no longer proportional to the partial pressure, rather a fugacity correction needs to be applied. As the total pressure still increases further into the realm of 15,000 to 20,000 psi a heretofore not fully appreciated effect occurs in that the high methane pressure reduces the solubility even more than would have been predicted by the fugacity correction alone. Its consequence regarding corrosion, however, is still not well established because testing at these high pressures is, and will be for some time, problematic. This makes it clear that acquiring representative and relevant environmental data for the MSBOD is only part of the task at hand. The realistic translation of field environment to the test environment is the other part. Hence, appropriate physicochemical models for extreme conditions must become an integral part of the development of the BOD. In this environment there are two aspects that need to be vigorously pursued. The qualification procedures for the proposed metallurgy need to be clearly delineated and their relevance demonstrated. Test protocols need to be strengthened and results need to be interpreted statistically for relevance. In this context methods for identifying (system) in situ conditions based on external analytical data will also be critically discussed. A methodology will be proposed whereby the H2S partial pressure on x axis of the familiar pH-pH2S diagram can be displaced by the fugacity based H2S concentration. This will result in a reduction of excess conservatism while maintaining an adequate safety margin. This work was initiated and funded by DeepStar, Global Deepwater Technology Development Program as part of Phase XII projects in the (X500) Drilling, Completions & Intervention Committee.

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The linear polarization technique in principle is a convenient and rapid way for determining corrosion rates. In practice, one finds that it is often applied under conditions alien to the assumptions on the basis of which it was derived. This paper will present a series of comparative observations between linear polarization, resistance probe, and weight loss measurements in different systems. The systems studied were: (1) Hydrochloric acid containing acetylenic inhibitors, (2) a two phase system containing hydrocarbon and water saturated with H2S, and (3) tap water containing various inhibitors. An equation will be derived showing the effect of ohmic resistance due to corrosion product buildup. The effect of polarization on the properties of a corrosion product layer will be discussed qualitatively.