Management of bacteria remains a concern for most Operators and the ability for micro-organisms to generate hydrogen sulphide (H2S) that results in reservoir souring and microbiologically influenced corrosion is well documented.
This paper describes two studies carried out by Clariant Oil Services, in conjunction with an oilfield microbiology service company, where molecular techniques were utilized. These studies provided a detailed assessment of system bacterial levels in two UK North Sea platforms which had historically encountered integrity failures related to microbiologically influenced corrosion.
In each system, two methods of microbiological survey were carried out. Quantitative Polymerase Chain Reaction (qPCR) was utilized to determine the overall bacterial contamination of the system and Next Generation Sequencing (NGS) was utilized to determine the location of bacterial species capable of causing microbiologically influenced corrosion.
In system 1, qPCR showed significant H2S producing bacteria in the oil process train, closed drains drum and oily water separator. NGS indicated that the source of bacterial contamination in the closed drains drum was from the oil process train. The oily water separator, historically identified as the source of bacterial contamination of the closed drains drum, was found to contain distinctly different bacterial species to the other process vessels on the platform.
In system 2, production fluids are processed from two fields; one with seawater breakthrough and one where seawater breakthrough has not been detected. Microbiologically influenced corrosion failures have been observed downstream of where the produced fluids commingle. NGS showed predominantly sulphate reducing bacteria in the field with seawater breakthrough. Low levels of sulphate reducing bacteria were observed in the field with no seawater breakthrough, indicative of a shift in water chemistry. A moderate level of bacterial species known to cause or enhance microbiologically influenced corrosion were also detected in these field fluids.
Next Generation Sequencing has great potential to be used as a tool to identify bacterial species capable of causing microbiologically influenced corrosion in oilfield systems. These molecular techniques should be considered as a screening tool, particularly in systems which have a high frequency of failures attributed to microbiologically influenced corrosion.