Using Thermodynamics to Predict the Outcomes of Nitrate-Based Oil Reservoir Souring Control Interventions

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.

Metabolites of an Oil Field Sulfide-Oxidizing, Nitrate-Reducing Sulfurimonas sp. Cause Severe Corrosion

“…However, the activities of organotrophic NRB (11, 14, 15) and other nitrate-driven processes, such as FeS or Fe 2+ oxidation (61, 62), likely proceed in parallel. Furthermore, it is possible that organotrophic NRB may even outcompete soNRB under certain conditions (63), so the prediction of proper nitrate dosing strategies in the field remains challenging.…”

Nitrate respiration and diel migration patterns of diatoms are linked in sediments underneath a microbial mat

“…The controlling factors that determine whether DNRA or denitrification is dominant in a habitat are a subject of debate. In the past studies have focussed on the impact of kinetics (King and Nedwell, 1985;Tiedje, 1988;Behrendt et al, 2014;Kraft et al, 2014;McTigue et al, 2016;Murphy et al, 2020), inhibition and/or thermodynamics (Dong et al, 2011;Kraft et al, 2014;Bonaglia et al, 2017;Dolfing and Hubert, 2017 (Consalvey et al, 2004). Craticula cuspidata perform photosynthesises (PS) in the afternoon and accumulate NO 3 − (turquoise ellipses = stored NO 3…”

Reservoir souring: sulfur chemistry in offshore oil and gas reservoir fluids

“…Biocides are potentially hazardous to oilfield personnel and the environment and difficult to inject deep into reservoir making treatment of SRB distant from injection well challenging. Nitrate injection at the injection well or at the production well in the produced water treatment is an alternative to biocide treatment (Sunde et al 2004;Sturman and Goeres 1999;Dolfing and Hubert 2017;Myhr et al 2002). Unlike biocides, nitrate flows readily into an oil-bearing formation and shifts the microbial activity from SRB to nitrate reducing bacteria (NRB).…”

“…It is also an effective H 2 S scavenger as it reacts with sulfide resulting in the generation of elemental sulfur and nitrogen (Sturman and Goeres 1999). Microbial competition between NRB and SRB for electron donors (oil-derived organics or H 2 ) and nitrate-driven sulfide oxidation are the two mechanisms proposed for the NRB-facilitated souring control (Sunde et al 2004;Dolfing and Hubert 2017). There are concerns that nitrate treatment shifts the corrosion risk from production to injection wells due to the oxidizing potentials of nitrate and nitrite (Martin 2008;Hubert et al 2005).…”

“…Depending on the ratio of nitrate to sulfide, fully oxidized sulfate or partially oxidized sulfur-polysulfides may be generated; sulfide is oxidized to sulfate at high nitrate to sulfide ratios with the reduction of nitrate to nitrite, while, at low ratios, sulfur-polysulfide formation is favored (Sturman and Goeres 1999). The latter conditions can be encountered at oilfield topside at low temperatures (4-85 °C) (Dolfing and Hubert 2017). These intermediate sulfur species could also be formed rapidly by chemical reactions, when soured produced water containing substantial sulfide concentrations is exposed to air (Johnston et al 2010).…”