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/water-separation 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. Introduction The use of FPSOs and floating storage and offloading units (FSOs) to produce oil or to process oil and water associated with offshore production has increased to approximately 106 units currently in operation worldwide (Offshore 2006). These FPSOs and FSOs are ships containing multiple tanks for separating oil and water, storing oil before offloading into tankers, and processing waters. Produced water typically flows into slop tanks, where it may also combine with drainage water from decks or ballast water from cargo ships. Slop tanks are in many cases the final separation stage in which residual entrained oil is removed from the water before its discharge to the sea, offloading, or reinjection. Environmental concerns dictate that total oil and grease (TOG) is a crucial water-quality criterion before discharge, but the level of H2S is also critical because of its high toxicity and corrosivity to carbon steel. Oil/water separators and slop-water storage tanks are prime locations for activity of SRB and the subsequent generation of high levels of H2S. SRB are particularly abundant in most oilfield waters, including seawater. The slop waters also typically contain all the nutrients required by the SRB for their growth and dissimilatory respiration, reducing sulfate to sulfide. Environmental conditions in the slop-water tanks, especially the presence of sludge and solids deposits at the bottom of the tanks, are quite favorable for these anaerobic bacteria to form biofilms. These solids are also protective to the SRB and impede the action of chemical biocide treatments for controlling bioactivity. Health, safety, and environmental aspects associated with the presence of the toxic gas on offshore structures make it necessary to implement effective SRB- and H2S-control procedures while still maintaining compliant water for discharge. Petrobras currently operates 10 FPSOs and FSOs in the Campos basin, approximately 180 km northeast of Rio de Janeiro. The FPSOs are located in water depths from 160 to 1240 m. Some units have production facilities (FPSOs), while others (FSOs) only receive produced oil and water from other platforms. All have slop-water tanks with varying degrees of SRB activity, depending on the producing-fluids composition and reservoir characteristics. The fields subjected to seawater injection are prone to biogenic H2S generation, and those with low-salinity formation water and low reservoir temperature tend to be the most susceptible. As described previously, the dual treatment program of a biocide, THPS, and a biostat, AQ, was quite successful at controlling H2S biogeneration on one of the Petrobras FPSOs (Penkala et al. 2004). This dual treatment program has been implemented subsequently on five additional FPSOs, as its effectiveness has continued to be validated under the varying water and SRB conditions on each unit. This paper discusses the progression of the program as it has been customized and continually adjusted by each FPSO to optimize the control of H2S generation and cost-effectiveness. Also presented are laboratory data showing the synergy of THPS and AQ in dual treatments, as well as results from corrosion studies with the two chemicals.