The effect of CaCO3 saturation on sour corrosion was tested in a series of autoclave experiments. The main objective was to investigate a proposed mechanism where an inner layer of CaCO3 would form and reduce the protectiveness of iron sulfide layers. CaCO3 saturation ratios in the range of 0-4 were studied with two different gas compositions; 2.5 bar H2S + 2.5 bar CO2, and 0.05 bar H2S + 5 bar CO2. The temperature was 90°C and the salinity 100 g/L in all experiments. Weight loss corrsion rates were obtained, and the corrosion surfaces and corrosion products were analyzed with 3D profilometry and SEM/EDS. The presence of continuous calcic layers could not be observed in this study, but it was nevertheless found that the corrosion rates in solutions with dissolved calcium can be significantly higher than in calcium-free solution. The findings are relevant for the oil&gas industry as production rates of formation water typically will increase during mid/late life production, and several new field will produce formation water shortly after start-up.
A large part of the remaining natural gas reserves worldwide is sour, i.e. H2S-containing. Many oil and gas fields, especially in North America, Central Asia and the Middle East, are highly sour with an H2S fraction of several percent. Internal corrosion in sour environments is associated with a high risk of severe localized corrosion attacks. This has consequences for design and materials selection for sour oil and gas production facilities, where special requirements apply for use of carbon steels vs. CRAs as well as corrosion inhibition of carbon steels. However, the presence of H2S can also be an advantage for corrosion protection compared to purely sweet systems. Formation of more or less protective corrosion product layers will take place instantly on carbon steels under most conditions, which in some cases has removed the necessity of further anti-corrosion treatment. Being able to identify these conditions will provide an economical benefit from the aspects of material selection and corrosion control.
