Sulfur-Reducing Bacteria May Lower Corrosion Risk in Oil Fields by Coupling Oxidation of Oil Organics to Reduction of Sulfur-Polysulfide to Sulfide

Sulfate-reducing bacteria (SRB) are generally considered to have a profound impact on the petroleum industry as their sulfide production activity contributes to oil 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 (SPS), which can expedite corrosion.  SPS is also rapidly formed by chemical reactions, when sour produced waters, containing substantial sulfide concentrations, are exposed to air. Once formed, SPS 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 SPS to sulfate, whereas in the presence of (ii) excess electron donor (oil organics, e.g. acetate) the SPS 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 SPS to removed by the second route. A survey of the microbial community in produced waters from an oil field with low bottomhole  temperature indicated Desulfuromonas species to be common. Hence, once SPS is formed by reaction of excess ulfide with a limiting concentration of nitrate or oxygen, it may be subsequently be effectively removed through the activity of sulfur-reducing bacteria such as Desulfuromonas.