Nitrate has been added to the injected seawater in the Halfdan field since the start of injection in January 2001 in an attempt to prevent reservoir souring.
Indications are that the treatment is successful based on analyses of fluids from backflowed injectors and from the few producers with partial seawater breakthrough. No H2S has been observed in any of the backflowed fluids from the Halfdan water injectors. The small amounts of H2S produced by the production wells reduced after seawater breakthrough to a level typically less than half the level measured prior to seawater breakthrough.
Nitrate aims to encourage the growth of nitrate utilizing bacteria (NUB) at the expense of sulfate reducing bacteria (SRB). The mechanism applicable is a combination of out-competition of SRB by NUB and inhibition / suppression of SRB activity by NUB due to the formation of nitrite. Furthermore, one strain of SRB was found to be capable of switching to nitrate reduction (thereby acting as NUB) in an environment with high sulfate concentrations.
The Halfdan oil field is located in the Danish sector of the North Sea and is operated by Maersk Oil on behalf of the Danish Underground Consortium (DUC). The Halfdan oil and gas accumulation was discovered in December 1998 by a 30,000 ft (9,144 m) long horizontal well drilled from the Dan field. Production of oil and gas commenced in February 1999. Two wellhead platforms are currently installed in the Halfdan field. Gas and liquid separation is carried out on Halfdan and the partially stabilized oil is further processed on the Gorm-C Platform. The gas is exported to the Dan-FF Platform.
The Halfdan field 1 is laterally extensive and comprises a high porosity (25-30%) low permeability (0.5-2 mD) chalk reservoir located outside structural closure. The producing horizon is the Maastrichtian Age Chalk with a reservoir temperature and pressure of typically 80°C and 4,200 psi (29 Mpa). The field is developed with long horizontal wells of 10,000-15,000 ft (3,048-4,572 m) reservoir sections drilled in a dense parallel pattern of alternate producers and injectors with well spacings of 600 ft (183 m). At present, 26 producers and 12 water injectors have been drilled.
Water injection for reservoir pressure maintenance and line sweep commenced in January 2001 from one injector, HDA-04. Injection water is supplied from the Dan-FF platform, via a 9 km long sub-sea pipeline made of carbon steel. From 2001 to early 2003, the injection water was used as cooling water in the Dan-FF production process prior to being pumped to Halfdan. The Halfdan water injection rate has currently reached a level of 100,000 bbls/day (15,900 m3/day).
Small amounts of H2S are often measured in the produced gas when new wells are brought on-stream. Levels typically vary between 0.2 to 20 ppm in the gas phase. The origin of the H2S is microbiological as Sulfur isotope ratio (~345) determinations have shown these to fall within the known boundaries of biologically derived ~i345 signatures. The H2S is believed to originate from SRB introduced by the use of seawater based fluids during drilling, completion and stimulation operations. Addition of biocides to these fluids is being introduced.
By the end of 2002, seawater breakthrough had been observed in three producing wells on Halfdan (HDA-03XA, HDA-07 and HDA-34C).
HALFDAN INJECTION WATER TREATMENT
The risk of reservoir souring in the Halfdan field is high due to the high nutrient content of both formation and injection water. The Halfdan formation water contains 350 mg acetate/1 and 50 mg propionate/1 (referred to as VFA, Volati