Investigation into Halfdan Water Injection Biofilm Using Chemical Microsensors

Abstract

Investigation into Halfdan Water Injection Biofilm Using Chemical Microsensors

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.