Abstract
Souring is the undesirable production of hydrogen sulfide (H2S) in oil reservoirs by sulfate-reducing bacteria (SRB). Souring is a common problem during secondary oil recovery via water flooding, especially when seawater with its high sulfate concentration is introduced. Nitrate injection into these oil reservoirs can prevent and remediate souring by stimulating nitrate-reducing bacteria (NRB). Two conceptually different mechanisms for NRB-facilitated souring control have been proposed: nitrate-sulfate competition for electron donors (oil-derived organics or H2) and nitrate driven sulfide oxidation. Thermodynamics can facilitate predictions about which nitrate-driven mechanism is most likely to occur in different scenarios. From a thermodynamic perspective the question “Which reaction yields more energy, nitrate driven oxidation of sulfide or nitrate driven oxidation of organic compounds?” can be rephrased as: “Is acetate driven sulfate reduction to sulfide exergonic or endergonic?” Our analysis indicates that under conditions encountered in oil fields, sulfate driven oxidation of acetate (or other SRB organic electron donors) is always more favorable than sulfide oxidation to sulfate. That predicts that organotrophic NRB that oxidize acetate would outcompete lithotrophic NRB that oxidize sulfide. However, sulfide oxidation to elemental sulfur is different. At low acetate HS− oxidation is more favorable than acetate oxidation. Incomplete oxidation of sulfide to S0 is likely to occur when nitrate levels are low, and is favored by low temperatures; conditions that can be encountered at oil field above-ground facilities where intermediate sulfur compounds like S0 may cause corrosion. These findings have implications for reservoir management strategies and for assessing the success and progress of nitrate-based souring control strategies and the attendant risks of corrosion associated with souring and nitrate injection.
Highlights
Souring, the undesirable production of hydrogen sulfide (H2S) in oil reservoirs by sulfate-reducing bacteria (SRB), is a common problem during secondary oil recovery when sea water is injected into the reservoir to maintain the high pressures required for oil extraction (Vigneron et al, 2017)
The aim of this work is to provide a thermodynamic framework to evaluate the energetics of the various pathways potentially involved in nitrate-facilitated oil reservoir souring control
This does not necessarily imply that nitrate-facilitated souring control works via direct competition between nitrate-reducing bacteria (NRB) and SRB for organic electrons donors
Summary
The undesirable production of hydrogen sulfide (H2S) in oil reservoirs by sulfate-reducing bacteria (SRB), is a common problem during secondary oil recovery when sea water is injected into the reservoir to maintain the high pressures required for oil extraction (Vigneron et al, 2017). (ii) Nitrate driven sulfide oxidation: The sulfide produced by SRB during souring is re-oxidized with nitrate as electron acceptor (Table 1, R8) The latter mechanism potentially results in a cryptic sulfur cycle (i.e., regenerated sulfate can be re-used by SRB if electron donors are available; Hubert et al, 2003) but as long as nitrate is available sulfide will be essentially absent from the system. As such the two mechanisms are potentially stoichiometrically identical (nitrate-driven oxidation of electron donors either directly or via intermediate S cycling), with the essential difference between them being (Figure 2) that in pathway (i) organotrophic nitrate reducers dominate, while pathway (ii) hinges on the activity of sulfide oxidizing nitrate reducers
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