Biogenic production of hydrogen sulphide (H(2)S) is a problem for the oil industry as it leads to corrosion and reservoir souring. Continuous injection of a low nitrate concentration (0.25-0.33 mM) replaced glutaraldehyde as corrosion and souring control at the Veslefrikk and Gullfaks oil field (North Sea) in 1999. The response to nitrate treatment was a rapid reduction in number and activity of sulphate-reducing bacteria (SRB) in the water injection system biofilm at both fields. The present long-term study shows that SRB activity has remained low at < or =0.3 and < or =0.9 microg H(2)S/cm(2)/day at Veslefrikk and Gullfaks respectively, during the 7-8 years with continuous nitrate injection. At Veslefrikk, 16S rRNA gene based community analysis by PCR-DGGE showed that bacteria affiliated to nitrate-reducing sulphide-oxidizing Sulfurimonas (NR-SOB) formed major populations at the injection well head throughout the treatment period. Downstream of deaerator the presence of Sulfurimonas like bacteria was less pronounced, and were no longer observed 40 months into the treatment period. The biofilm community during nitrate treatment was highly diverse and relative stable for long periods of time. At the Gullfaks field, a reduction in corrosion of up to 40% was observed after switch to nitrate treatment. The present study show that nitrate injection may provide a stable long-term inhibition of SRB in sea water injection systems, and that corrosion may be significantly reduced when compared to traditional biocide treatment.

The distribution of thermophilic marine sulfate reducers in produced oil reservoir waters from the Gullfaks oil field in the Norwegian sector of the North Sea was investigated by using enrichment cultures and genusspecific fluorescent antibodies produced against the genera Archaeoglobus, Desulfotomaculum, and Thermodesulforhabdus. The thermophilic marine sulfate reducers in this environment could mainly be classified as species belonging to the genera Archaeoglobus and Thermodesulforhabdus. In addition, some unidentified sulfate reducers were present. Culturable thermophilic Desulfotomaculum strains were not detected. Specific strains of thermophilic sulfate reducers inhabited different parts of the oil reservoir. No correlation between the duration of seawater injection and the numbers of thermophilic sulfate reducers in the produced waters was observed. Neither was there any correlation between the concentration of hydrogen sulfide and the numbers of thermophilic sulfate reducers. The results indicate that thermophilic and hyperthermophilic sulfate reducers are indigenous to North Sea oil field reservoirs and that they belong to a deep subterranean biosphere.

A mathematical model for reservoir souring caused by growth of sulfate reducing bacteria (SRFJ) in the reservoir has been developed. The model is a 1D numerical transport model based on conservation equations and includes bacterial growth rates, the effect of nutrients, water mixing, transport and adsorption of H2S in the reservoir formation.Two basic concepts fm microbial H2S production were tested using the model: H2S production in the mixing zone between formation water and injection water (mixing zone model), and H2S production due to SRB growth in a biofilm in the reservoir rock close to the injection well (biofilm model). In both cases &S adsorption by reservoir rock was considered Field data obtained from three oil producing wells on the Gullfaks field correlated with H,S production profiles obtained using the biofilm model, but could not be explained by the mixing model.The biofilm model implies that &S production is correlated to the quality of the injection water, especially nitrogen (N) and phosphorous (P). The use of chemicals (containing N and P) in injection water treatment should therefore be reduced to a minimum.

A mathematical model for reservoir souring caused by growth of sulfate reducing bacteria (SRFJ) in the reservoir has been developed. The model is a 1D numerical transport model based on conservation equations and includes bacterial growth rates, the effect of nutrients, water mixing, transport and adsorption of H2S in the reservoir formation.Two basic concepts fm microbial H2S production were tested using the model: H2S production in the mixing zone between formation water and injection water (mixing zone model), and H2S production due to SRB growth in a biofilm in the reservoir rock close to the injection well (biofilm model). In both cases &S adsorption by reservoir rock was considered Field data obtained from three oil producing wells on the Gullfaks field correlated with H,S production profiles obtained using the biofilm model, but could not be explained by the mixing model.The biofilm model implies that &S production is correlated to the quality of the injection water, especially nitrogen (N) and phosphorous (P). The use of chemicals (containing N and P) in injection water treatment should therefore be reduced to a minimum.

In this study, bacteria from oil installations were used in growth experiments with 15 different chemicals normally used in injection water treatment. Growth experiments were performed using a mixture of bacteria grown under aerobic and anaerobic conditions, and SRB pure cultures grown anaerobically. pure cultures grown anaerobically. The results showed that chemicals would be utilized as N-sources, as P-source and as C-sources for bacteria. These chemicals included oxygen scavenger, scale inhibitors, polyelectrolytc, surfactant, antifoaming agents and chelating agents. The remaining chemicals did not support growth under the test conditions of these including biocides, contained nutrients that may be utilized under field conditions. From this study it is concluded that the growth potential of water treatment additives may be substantial and should therefore be investigated prior to chemical selection. Introduction lnjection water is routinely treated with chemicals to improve water quality. These chemicals include corrosion inhibitor, defoamers, oxygen scavengers, surfactants, biocides and scale inhibitors. Many of these chemicals contain carbon (C), nitrogen (N) and phosphorus (P), and may serve as substrates and/or mineral nutrients for bacteria. Added to sea water, which contains low amounts of nutrients, the result may well be to fertilize the injection water, giving reduced water quality, extra growth of bacteria and increased corrosion rates. North Sea water typically contains 1mg/1 total organic carbon. Of this, only a minor part (approximately 1%) is readily available to the bacteria. The concentration of utilizable carbon in sea water may be below 0.01 mg/1. Nitrogen is found in sea water primarily as nitrate, typical values for North Sea water is 0.6 mg/l, and the content of phosphate is 0.06 mg/1. phosphate is 0.06 mg/1. Chemical addition varies considerably between operators and fields. Calculated back to continous addition, common concentrations of additives can be 1 mg/l for each of the additives; antifoam, polyelectrolyte and scale inhibitor. Oxygen scavenger, polyelectrolyte and scale inhibitor. Oxygen scavenger, biocide and corrosion inhibitor is normally added in concentrations of 5-10 mg/1. The amount of available carbon and mineral nutrients added to the sea water may therefore increase substantially during water treatment (table 1). In water treatment one important step is to remove oxygen from the water. This is done in the decration tower, either by gas stripping (with natural gas) or by evacuation. The result is a steep oxygen gradient over the tower. After deacration the treated water should contain less than 0.05–0.2 mg/l oxygen, which is removed with oxygen scavengers. Despite this treatment, traces of oxygen may be found, at least periodically, throughout the water injection system. periodically, throughout the water injection system. Chemical additives may therefore be attached by, aerobic as well as anaerobic bacteria. P. 727