Nitrate has been added to the injection seawater in the Halfdan field since January 2001 in order to prevent reservoir souring. The Halfdan water injection pipeline contains a significant biofilm, made up of nitrate utilizing bacteria (NUB) and a small, but persistent population of sulfate reducing bacteria (SRB). In this paper, we demonstrate that various chemical microsensors can be used for assessing the souring potential and corrosivity of the biofilm formed in the Halfdan seawater injection system treated with nitrate. The results obtained were compared with a similar microsensor investigation on a biofilm formed using Halfdan injection seawater containing no nitrate. The biofilms were obtained by installing Sessile Bacteria Monitoring Tubes (SBMTs) both upstream and downstream of the nitrate injection point. Both biofilms were allowed to grow for four months.
The results showed that in the nitrate treated sidestream, nitrate was quickly reduced by NUB. However, no nitrate limitation occurred through the biofilm towards the metal surface. The redox potential of the nitrate treated biofilm was typically in the region of 0 mV (compared to a hydrogen reference electrode) through the entire biofilm. No free H2S was detected and no iron sulfide deposits were visually observed, as there was a complete absence of blackening (by FexSy) in the NUB biofilm. For comparison, the redox potential of the non-nitrate treated biofilm varied from -382 mV at the surface of the biofilm to -460 mV deep in the biofilm. No free H2S was detected but the biofilm was rich in Fe3S4 (Greigite) with no FeS (Mackinawite) or FeS2 (Pyrite) detected. The results show that corrosion due to the formation of H2S by SRB is prevented by nitrate injection in the Halfdan seawater injection system.
Souring of oil field reservoirs and microbiologically influenced corrosion (MIC) of pipelines and process equipment as a result of SRB activity are well established in the oil and gas industry. Nitrate injection into the Halfdan seawater injection system is designed to modify the bacterial populations in the water injection pipeline and the reservoir in order to prevent reservoir souring and minimize MIC in the pipeline. However, with the demonstrated persistent population of SRB in the Halfdan seawater injection system1, it is important to ascertain whether or not these microorganisms are producing H2S, which could occur in small microniches deep within the biofilm. No H2S has been detected in the Halfdan injection seawate2. However, small traces of sulfide produced could react with iron in the system thereby remaining undetected. The result is that SRB could continue to sour the Halfdan reservoir or be responsible for localized corrosion and pitting in the water injection pipeline and in well tubulars. Equally, the localized production of nitrite by the NUB biofilm could cause pitting corrosion.
The objective of the present study was to determine the chemical gradients occurring within a biofilm, both with and without nitrate. In particular, it was important to identify the chemical processes occurring close to a pipe wall as these processes have a direct impact on pitting corrosion.