Effects of physicochemical properties of injected seawater on microbial processes, corrosion, and souring.

Abstract

Physicochemical parameters such as salinity, temperature, electron donor, and electron acceptor availability play a crucial role in microbial activity within petroleum reservoirs. These parameters are significantly influenced by the injection of water for secondary oil production, resulting in the establishment of temperature, salinity, and chemical gradients that have a profound impact on microbial communities and their activity within petroleum systems. Seawater or produced water are often used for water injection, with the latter practice referred to as Produced Water for Re-injection (PWRI). The re-injection of produced water helps minimize the transportation and usage of water from external sources. This study utilized anoxic microcosms to investigate the impact of temperature and salinity on microbial processes, specifically sulfate-reduction, using sediments from River Tyne Estuary as a source of microbial inoculum. Various experiments were conducted using synthetic injected seawater-produced water (ISW: PW) mixtures resembling North Sea and Arabian Gulf PWRI operational scenarios. Salinities ranged from 42 g/L TDS to 212 g/L TDS, and four temperatures were tested, varying between 15°C and 60°C. All incubations contained sulfate as an electron acceptor and a mix of volatile fatty acids (VFAs) as electron donors, with some experiments including steel coupons to assess the impact of microbial activity on corrosion. Aqueous sulfide, sulfate, other anions, VFAs, and microbial community composition were periodically evaluated in each microcosm. Moreover, surface analysis of steel coupons was conducted in specific experiments to assess the potential for microbially influenced corrosion using gravimetric measures. The results of this Ph.D. study indicate that temperature and salinity significantly impacted microbial community composition. Low salinity (42 g/L & 64 g/L) at 30°C promoted the enrichment of a sulfate-reducing consortium dominated by Desulfobulbus sp., Desulfobacter sp., and Desulfotignum sp., culminating in complete sulfate-reduction and the metabolic removal of VFAs (acetate, propionate, and butyrate). In contrast, high salinity (107 g/L, 127 g/L, and 150 g/L) at 30°C selected for Halanaerobium sp. and resulted in substantially lower levels of sulfatereduction/sulfide production and VFAs metabolism. At low salinity (42 g/L & 64 g/L) and 600C, spore-forming sulfate-reducing prokaryotes (SRP) primarily Desulfomatoculum sp. and the spore-forming, non-sulfate reducing bacterium Pelotomaculum sp. were selected. In contrast, microbial communities were effectively suppressed at higher salinities (107 g/L, 127 g/L, and 150 g/L) and 60°C, with no microbial activity observed, supporting the notion of palaeopickling in high-temperature, high-salinity petroleum reservoirs. Furthermore, in low-temperature, lowsalinity conditions, there was a succession from completely oxidizing sulfate-reducing acetoclastic SRP to acetoclastic methanogenesis, characterized by the enrichment of acetoclastic methanogens related to Methansarcina sp. at the end of incubation. As salinity increased, acetate oxidizing SRPs were inhibited, leading to the accumulation of acetate in microcosms. When steel iii coupons were included in certain microcosms, variable but overall low levels of mass loss were recorded (less than 0.2 mm/yr.), indicating a low degree of microbially influenced corrosion. Nonetheless, surface morphology analysis of corrosion coupons in North Sea incubations at 30°C, revealed multiple pitting nucleations morphologies, indicating potentially different pitting corrosion mechanisms at play. The findings from this study imply that microbially influenced corrosion would indeed be less significant at higher salinities and temperatures than at lower salinities and temperatures, with practical consequences for oil field management practices.