Sulfate-reducing bacteria are generally considered to have a profound impact on the petroleum industry
as their sulfide production activity contributes to reservoir souring and pipeline corrosion. SRB can be
controlled by injecting biocides into pipelines and above-ground facilities. A recent “green” alternative
for controlling reservoir souring is to inject nitrate, as nitrate is relatively harmless and is ultimately
reduced to nitrogen gas. Resident nitrate-reducing bacteria (NRB) reduce nitrate to nitrite, which is a
strong inhibitor of SRB, thereby inhibiting sulfide production. However, NRB-mediated oxidation of
sulfide with nitrate and/or chemical reaction between nitrite and sulfide can generate sulfur-polysulfide
(S-PS), which can expedite corrosion. S-PS is also rapidly formed by chemical reactions, when sour
produced waters, containing substantial sulfide concentrations, are exposed to air. Once formed, S-PS
can be removed by either of two alternative routes, which may thus reduce corrosion risk. In the
presence of (i) excess electron acceptor (e.g. nitrate), NRB may further oxidize the S-PS to sulfate,
whereas in the presence of (ii) excess electron donor (oil organics, e.g. acetate) the S-PS may be
reduced back to sulfide. A specialized group of sulfur-reducing bacteria catalyzes this reaction. A
representative of this group, Desulfuromonas acetoxidans, derives energy for growth from the reaction:
4 sulfur + acetate = 4 sulfide + 2 CO2. Because oil field waters tend to be electron donor rich and
electron acceptor poor, one would expect S-PS to be removed by the second route. A survey of the
microbial community in produced waters from an oil field with low bottomhole temperatures indicated
Desulfuromonas species to be common. Hence, once S-PS is formed by reaction of excess sulfide with
a limiting concentration of nitrate or oxygen, it may subsequently be effectively removed through the
activity of sulfur-reducing bacteria such as Desulfuromonas.
KEYWORDS: Sulfur-reducing bacteria, sulfate-reducing bacteria, sulfide, sulfur, polysulfide, microbial corrosion, nitrate injection