Flexible Treatment Program for Controlling H2S in FPSO Produced-Water Tanks

Abstract

Summary Tanks used to store produced water on floating production, storage, and offloading units (FPSOs) are extremely susceptible to generation of high hydrogen sulfide (H2S) levels because of the activity of sulfate-reducing bacteria (SRB). The FPSOs operated by Petrobras in the Campos basin, offshore Brazil, all contain slop-water tanks, while some also have upstream oil/waterseparation tankage. Slop water, including produced water, ballast water from oil cargo ships, and deck water, contains SRB and their nutrients required for generating H2S. Additionally, solids accumulations at the tank bottoms provide an excellent environment for microbial growth. A 2002 field trial on an FPSO confirmed the viability of combined batch treatments using anthraquinone (AQ) and a THPS blend and their effectiveness in controlling H2S biogeneration better than previous treatment programs. AQ, a nontoxic SRB inhibitor, and tetrakishydroxymethyl phosphonium sulfate proprietary blend (THPS), an oilfield biocide, act synergistically to provide effective control of H2S biogeneration in this environment. The combined-chemical treatment strategy has now been implemented successfully on six Petrobras FPSOs. Flexibility has been important in developing the treatment programs because operating parameters are different for each FPSO and change with increased water-production rates. Options include the ability to inject the chemicals continuously or batchwise at different locations and to alter the volumes and ratios of chemicals for optimizing control over H2S and corrosion. This paper describes the individual FPSO water-flow and water-storage systems and discusses the customized chemical treatment programs. Included are field H2S data showing the evolution of the programs as they are being continually adjusted to optimize control of H2S generation and cost-effectiveness. Also included are results of laboratory microbial studies showing the synergy of anthraquinone and THPS and of corrosion studies that have impacted the direction of usage of these chemicals. With more than 100 FPSOs operating worldwide, the treatment program described can significantly affect the safety and environmental aspects of processing water containing SRB.