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.