Carbon steel pipelines are widely used for injection of sea and other waters into oil and gas wells so as to increase the rate of recovery, particularly from mature fields. Internal corrosion usually is mild. However, cases of very aggressive channelling corrosion along the bottom of the pipeline have been observed. Practical experience and anecdotal observations have attributed this to microbiologically influenced corrosion even though extensive use is made of preventative measures including biocides, oxygen scavengers, corrosion and scale inhibitors, and pipeline pigging. Interpretation of data and observations for five water injection pipelines, made available by industry, indicate that microbiologically influenced corrosion may play a part in causing channelling corrosion but that the most likely cause is under-deposit corrosion under pipe debris that settles during periods of pipeline shut-downs and low water velocity.
The present study compares corrosion mass loss and pit depth measurements on carbon steel corrosion coupons exposed under similar operating parameters, but with different biological consortia. One set of data were obtained from standard flush disc corrosion coupons used to monitor corrosion rates in a water injection pipeline on the North Sea continental shelf. The coupons were exposed on average for 6 months over 6 years operational time. These data are compared with published corrosion data of coupons exposed in abiotic district hot water systems from several power plants situated in Europe. The exposure time for these coupons was 9 months. Both systems were anoxic and in the same temperature range and are comparable. Observations regarding relationship between MIC and bacterial consortia, bacterial numbers and type, water quality and corrosion products are also made. The corrosion rate of the water injection pipeline is approximately 10 times higher compared with the corrosion rate in the abiotic district hot water system. It is concluded that the increased corrosion on the carbon steel coupons in the early stage is caused by MIC. This is also supported by the chemical and biological information available for the pipelines. The results reported here constitute the first step of an overall study to improve the level of understanding of the bacterial contribution to the total corrosion rates of carbon steel in water injection flowlines. Such understanding is expected to improve management and operational decision-making for practical control of corrosion in the field, by providing predictions of expected life time as a function of control of biotic consortia (e.g. through pigging, and biocide treatments). Further, it will facilitate decisions concerning choice of pipeline construction materials for future design.
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