Effects of Produced Water Re- Injection (PWRI) on Reservoir Souring and Microbial Community Dynamics

Enhanced oil recovery (EOR) utilises injection of pressurised, disinfected, and deoxygenated seawater to maintain the reservoir’s pressure. When the injected seawater (ISW) mixes with produced water (PW), temperature and salinity gradients are formed, producing an open loop for electron donors (e.g., volatile fatty acids or VFAs), electron acceptors (mainly sulphate), and nutrients, thereby potentially, and selectively enriching for sulphate-reducing prokaryotes (SRPs) across a range of salinities and temperatures encompassing conditions for their optimal growth. We conducted anoxic microcosms experiments with ISW:PW mixes mimicking conditions for North Sea PWRI across a salinity gradient (42, 64, 107, 127, and 150 g/L total dissolved solids (TDSs)), and temperatures of 30°C and 60°C. Incubations were conducted over a 250- day period. We discuss the effect of PWRI temperature and salinity gradients on sulphidogenesis, sulphate-reduction, VFA metabolism (consumption/production), and microbial community dynamics. The temperature and salinity gradients are the key influencers of microbial community dynamics. Low-salinity (42-64 g/L TDS) PWRI at 30oC enriched an SRP consortium (Desulfobulbus sp., Desulfobacter sp., and Desulfotignum sp.). This was accompanied by complete sulphate-reduction and removal of VFAs (such as acetate, propionate, and butyrate). High-salinity (107, 127, and 150 g/L TDS) PWRI at 30oC shifted the microbial communities from SRP to thiosulphate-reducing and/or fermentative Halanaerobium sp. These incubations exhibited low/negligible sulphate-reduction, sulphide production, and VFA consumption. Low-salinity (42-64 g/L TDS) PWRI at 60oC shifted the microbial communities towards spore-forming SRP (Desulfomatoculum sp., Pelatomaculum sp.), whereas high-salinity (107, 127, and 150 g/L TDS) PWRI at 60oC markedly exhibited no clear microbial growth (Palaeopickling reservoirs phenomenon) (Head, I.M., Gray, N.D. and Larter, S.R., (2014). Life in the slow lane; biogeochemistry of biodegraded petroleum containing reservoirs and implications for energy recovery and carbon management. Frontiers in Microbiology, 5, p. 108946). Microbial succession from acetoclastic SRP (Desulfobacter sp.) to acetoclastic methanogenic archaea (Methanosarcina sp.) under low salinity (42 g/L TDS) at the lower temperature (30°C) revealed a shift in electron accepting processes from sulphate-reduction to methanogenesis. Additionally, the pitfalls of analytical procedures when analysing high-salinity waters, and therefore barriers in decision-making from the data, will be explored.