Evaluating Sub-Optimal Nitrate Application Under Simulated Field Conditions Using High-Pressure Bioreactors

Abstract Microbiological oilfield souring, driven by sulfate-reducing microorganisms (SRM) producing hydrogen sulfide (H2S), presents significant economic and operational challenges for the oil and gas industry. This study investigates the efficacy of intentional sub-optimal nitrate treatment to mitigate souring under simulated subsurface conditions using pressurised bioreactors. Triplicate bioreactor systems were operated under high-pressure, low-temperature conditions for 13 weeks, mimicking near-wellbore environments with volatile fatty acids (VFAs) and low residual hydrocarbons. Test bioreactors received a calculated 50% nitrate underdose, while controls received no nitrate. Results demonstrated that sub-optimal nitrate dosing achieved a 73% reduction in sulfide production, exceeding theoretical predictions and reducing sulfide concentrations by 81.3% relative to controls. Enhanced VFA consumption and nitrate utilisation indicated additional metabolic pathways potentially involving hydrocarbon-derived electrons. Microbial community analysis revealed consistent suppression of SRM activity and stable nitrate-reducing microorganism (NRM) populations in the test group, aligning with observed chemical data. This work highlights the critical role of accurate nitrate dosing within the thermal viability shell (TVS) and underscores the complexity of microbial interactions in reservoir-like conditions. The findings provide valuable insights for optimising nitrate-based souring mitigation strategies, emphasising the need for tailored dosing approaches to address reservoir-specific dynamics.