Injection water on all Gullfaks platforms have been treated with nitrate to reduce H2S production caused by Sulfate Reducing Bacteria (SRB). Results from Gullfaks B (GFB) and Gullfaks C (GFC), which have had the most stable injection and production during the treatment period, show a decrease in BS production, when the treated injection water reaches the producers. In the water injection systems the change from biocide treatment to nitrate treatment resulted in more than 1000-fold reduction in SRB number and 10-20-fold reduction in sulphate respiration activity. Following nitrate injection, the SRB population was replaced with an equally large population of NRB in the biofilm and corrosion level in the water injection system dropped by more than 50%. This result corresponds well with the observations from a similar water injection treatment at Veslefrikk.
The process of reservoir souring, caused by the growth of Sulfate Reducing Bacteria (SRB) in oil reservoirs, represents a major problem for the oil industry.
Introduction of nitrate as electron acceptor for anaerobic respiration is an alternative to biocide treatment. The idea has been that stimulation of nitrate reducing bacteria (NRB) could inhibit growth of SRB1, 2, 3. Nitrate has been used successfully to inhibit microbial sulfide production in oil-contaminated wastewater 4, 5. Laboratory experiments with oil reservoir model columns have shown that injection of nitrate inhibits sulfide production 6,7. Telang et al. 8 showed that nitrate injection in water injection wells led to enrichment of sulfide-oxidizing, nitrate-reducing bacteria in the reservoir. Jenneman et al. 9 showed reduced HzS production from injection and production wells after treatment of injection water with 400 mg/L of NH4NO3.
Myhr et al. 7 showed in laboratory experiments that microbial HzS-production is inhibited by nitrate, in concentrations that is applicable to offshore oil fields. As a result of this the injection water at the Veslefrikk field (North Sea) was treated with nitrate. Microbiological monitoring of the water injection system showed reduced growth and activity of SRB in the water injection system, and a concomitant reduction in microbial induced corrosion (MIC) in C-steel topside seawater injection systems ~0.
On the basis of results obtained from the model experiments 7 and successful treatment of the water injection system at Veslefrikk ~0, several oil fields on the Norwegian Continental Shelf have been treated with nitrate since 1999, among them all the Gullfaks platforms. In this paper results from nitrate treatment of the water injection system and the effect on HzS production from the reservoir (reservoir souring) at Gullfaks B and C are presented.
MATERIALS AND METHODS
The Gullfaks platforms GFB and GFC
The Gullfaks field is located on block 34/10 in the northern part of the North Sea, west of Bergen city in Norway. The production start up was in 1986, 1988 and 1989 for the Gullfaks A, B, and C platforms respectively. The field uses gas and seawater injection (WAG) as pressure support. The water injection volumes have, from each platform, been varying between approximately 30.000 m 3 and 70.000 m 3. During the latest years, approximately 50.000 m 3 and 40.000 m 3 have been injected on GFB and C respectively. Injection pressure downstream of the water injection pumps are close to 200 bars. The seawater intake depth is 70 m on GFA/B and 120 m at GFC. Oxygen is removed by vacuum deaeration including bisulfite scavenger, to less than 20 ppb. Temperature downstream of the deaerator is approximately 25°C. On GF the water was originally filter
