Abstract
Oil reservoir souring and associated material integrity challenges are of great concern to the petroleum industry. The bioengineering strategy of nitrate injection has proven successful for controlling souring in some cases, but recent reports indicate increased corrosion in nitrate-treated produced water reinjection facilities. Sulfide-oxidizing, nitrate-reducing bacteria (soNRB) have been suggested to be the cause of such corrosion. Using the model soNRB Sulfurimonas sp. strain CVO obtained from an oil field, we conducted a detailed analysis of soNRB-induced corrosion at initial nitrate-to-sulfide (N/S) ratios relevant to oil field operations. The activity of strain CVO caused severe corrosion rates of up to 0.27 millimeters per year (mm y-1) and up to 60-μm-deep pitting within only 9 days. The highest corrosion during the growth of strain CVO was associated with the production of zero-valent sulfur during sulfide oxidation and the accumulation of nitrite, when initial N/S ratios were high. Abiotic corrosion tests with individual metabolites confirmed biogenic zero-valent sulfur and nitrite as the main causes of corrosion under the experimental conditions. Mackinawite (FeS) deposited on carbon steel surfaces accelerated abiotic reduction of both sulfur and nitrite, exacerbating corrosion. Based on these results, a conceptual model for nitrate-mediated corrosion by soNRB is proposed.IMPORTANCE Ambiguous reports of corrosion problems associated with the injection of nitrate for souring control necessitate a deeper understanding of this frequently applied bioengineering strategy. Sulfide-oxidizing, nitrate-reducing bacteria have been proposed as key culprits, despite the underlying microbial corrosion mechanisms remaining insufficiently understood. This study provides a comprehensive characterization of how individual metabolic intermediates of the microbial nitrogen and sulfur cycles can impact the integrity of carbon steel infrastructure. The results help explain the dramatic increases seen at times in corrosion rates observed during nitrate injection in field and laboratory trials and point to strategies for reducing adverse integrity-related side effects of nitrate-based souring mitigation.
Highlights
Oil reservoir souring and associated material integrity challenges are of great concern to the petroleum industry
To test the effect of the accumulation of different S and N metabolites on corrosion, carbon steel coupons were exposed to growing cultures of strain CVO at different initial N/S ratios (Fig. 1; see Table S1 in the supplemental material)
Iron supplied to CVO cultures with initial N/S ratios between 0.4 and 5.7 corroded at rates between 0.09 and 0.27 millimeters per year, whereas sterile sulfidic or sulfide-free incubations resulted in corrosion rates of Յ0.04 mm yϪ1 (Fig. 1A)
Summary
Oil reservoir souring and associated material integrity challenges are of great concern to the petroleum industry. Mackinawite (FeS) deposited on carbon steel surfaces accelerated abiotic reduction of both sulfur and nitrite, exacerbating corrosion Based on these results, a conceptual model for nitrate-mediated corrosion by soNRB is proposed. The results help explain the dramatic increases seen at times in corrosion rates observed during nitrate injection in field and laboratory trials and point to strategies for reducing adverse integrity-related side effects of nitrate-based souring mitigation Petroleum reservoirs and their associated facilities for processing and transporting oil, gas, and water represent vast engineered ecosystems that harbor a large diversity of microorganisms. Sulfate-reducing, fermentative, acetogenic, methanogenic, metal-reducing, February 2019 Volume 85 Issue 3 e01891-18 Another microbiological phenomenon that affects oil field operations is reservoir souring, i.e., the production of hydrogen sulfide (H2S) by sulfidogenic microorganisms in water-flooded reservoirs. Under the much-less-controlled conditions in oil field-wide nitrate applications, mixed accounts have been given regarding the efficacy of nitrate injection [16, 17], highlighting the need for a better understanding of the variables governing its success
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