Microbially Induced Organic Acid Attack in a Sweet Natural Gas-Gathering System

Abstract

Abstract Microbially produced organic acids in contact with mild steel pipelines can cause an accelerated form of corrosion similar in appearance to a CO2 type of attack. While the two types of attack may appear to be similar in nature their corrosion mitigation methods are different. Chemical corrosion inhibitors which are effective in controlling CO2 attack could be rendered ineffective when treating microbially induced corrosion. This case study outlines the procedure taken in identifying the internal corrosion mechanisms responsible for the microbially induced corrosion (MIC) failures, selection of the treatment chemicals, monitoring techniques, health and safety aspects of chemical programs and the potential environmental impact of such programs. Background Alberta Energy Company Ltd. owns and operates approximately 3100 shallow gas wells and 3200 kms of flowlines, operating at an average pressure of 1000 kPa in the Suffield Field. The field is situated on a Canadian Forces Military Range located in Southeastern Alberta, Canada. The production, gathering and treatment of sweet gas commenced during 1976 from the Milk River, Medicine Hat and Second White Specs formations. These gases contain less than 0.2% CO2, and no H2S. Line failures were first discovered during 1983, and have increased in frequency as the system aged. By 1991 the total cumulative failures reached 96. Extrapolation of a semi-log plot of cumulative failures versus time (Figure 1) indicated up to 1000 failures could occur by 1998 if the corrosion process was allowed to continue unchecked. Previous third party investigations found that the corrosion failures were attributed to the activity of Sulfate Reducing Bacteria (SRB) or CO2 corrosion attack. The SRB corrosion was justified based upon culturing of deposits and the presence of iron sulphides. CO2 corrosion was justified by the presence of iron carbonate in the corrosion deposit. The presence of the iron carbonate was attributed to an acid attack resulting from CO2 dissolution in the water under the system conditions. Evaluation of the gas and liquid chemistry and recognition of the low system pressure (initially 2800 kPa), however, make CO2 induced corrosion an unlikely cause of these failures attributed to CO2 attack. An initial bioassay identified a chemical which had a high probability of success in controlling microbially induced corrosion (MIC). P. 997^