Long-term souring treatment using nitrate and biocides in high-temperature oil reservoirs

The biogenic production of hydrogen sulfide (H2S) by sulfate-reducing bacteria (SRB) and consequently the souring of reservoirs are still major problems in oil industry. Biocides and/or nitrate are used to control SRB activity in oil reservoirs, but long-term studies are still needed to prove their efficacy. In this study, two high-temperature (80-84 degrees C) oil reservoirs were analyzed over three years. Nitrate and tetrakishydroxymethyl phosphonium sulfate (THPS) were added to the water injection system (WI) at the beginning of secondary oil recovery in oil reservoir 1, while nitrate was only added 19 months after the beginning of secondary oil recovery in oil reservoir 2. The H2S concentration was quantified monthly in production wells, and the total bacterial community (based on the gene coding for 16S rRNA) and SRB (based on dsrA and apsAB genes) were determined using fluorescence in situ hybridization (FISH) and PCR-DGGE analyses. Nitrate plus THPS controlled H2S production for 34 months in oil reservoir 1. THPS injection in oil reservoir 2 controlled H2S levels for 17 months, and the further addition of nitrate in water injection did not control H2S production. PCR-DGGE analyses and the molecular identification of the dominant groups showed a predominance of thermophilic bacteria, including different SRB (such as Desulfocaldus and Desulfonauticus) and nitrate-reducing bacteria (NRB - Marinobacter) in oil reservoir 1 and mesophilic SRB (Desulfovibrio) and NRB (Halomonas and Acinetobacter) in oil reservoir 2. The strategy chosen during secondary oil recovery modulated the microbial community and, consequently, changed the dynamics of H2S production.