INTRODUCTION
Sulfate-reducing bacteria (SRB) pose a threat to oilfield produced water facilities due to microbiologically influenced corrosion and biogenic sulfides. Produced water systems are susceptible to the establishment of oil-coated SRB-enriched biofilms that are typically inaccessible to conventional biocide regimes. To address this, a strategy has been designed using a conventional biocide in combination with anthraquinone (AQ), a nontoxic compound that specifically inhibits the SRB sulfate-reduction pathway. [1,2] AQ is sulfides in water injection systems due to a synergistic action on sessile SRB. [3,4] In the present study, AQ and a tetrakishydroxymethyl phosphonium sulfate blend (THPSBLEND) were used to treat slop water tanks on a Floating, Production, Storage and Offloading Unit (FPSO) operated by Petrobras in the Campos Basin, offshore from Brazil. High levels of hydrogen sulfide (>30 ppm in the water) were being generated by aggressive SRB present in the tanks, resulting in safety, economic, and discharge issues. Conventional treatment with glutaraldehyde was ineffective. The AQ-THPSBLEND cotreatment immediately reduced and maintained hydrogen sulfide levels at <1 ppm. During the course of treatment, the amount of AQ applied was increased and THPSBLEND decreased, which resulted in a stable, cost-effective scenario for bacterial control that enabled safe discharge of the water into the marine environment due to low ecotoxicity. This finding is significant in that it provides an economically favorable protocol to mitigate biogenic sulfides in oilfield water tanks and vessels and minimizes personnel and environmental risk during treatment and discharge.
Oilfield waters are rich in nutrients that support the growth of microbiological organisms. A diverse community of bacteria, including general aerobic and facultative anaerobic bacteria (GAB) and sulfate reducing bacteria (SRB), are commonly found in produced waters in oil and gas production systems. Uncontrolled growth and activity of bacteria can create severe operational, environmental, and human safety problems, including corrosion, solids production, and hydrogen sulfide (H2S) generation. [3]
Safety and environmental issues become more acute at offshore locations such as Floating, Production, Storage and Offloading units (FPSOs). The increased hazard at a FPSO is linked to higher risk of personnel exposure in the event of a leak due to limited options for evacuation. For effective control of microbial activity in offshore locations, a biocide treatment program must provide onboard safety; penetration of oil, sediment, and biofilm; rapid kill and persistent control of planktonic and sessile bacteria; and compliance for discharge.
A Petrobras FPSO offshore from Brazil encountered severe H2S problems due to persistent SRB populations in the produced water slop tanks. The flow dynamics at this location made it very difficult to implement an effective batch biocide treatment to control the bacterial populations present in the slop tanks. The previous treatment program with glutaraldehyde proved to be ineffective.
A dual treatment program was designed to target rapid and effective planktonic SRB kill with sustained control of sessile SRB. The program utilized a tetrakishydroxymethyl phosphonium sulfate blend (THPSBLEND) as the primary biocide, supplemented by batch treatments with anthraquinone (AQ), a biostat.
THPS also offered high effectiveness against SRB and GAB, sustained control of
bacterial growth, performance independent of H2S, relatively low toxicity and ease of
handling with respect to safety due to its low volatility.
AQ
