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.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
28 December 2006
- Manuscript approved:
21 June 2007
- Version of record:
20 September 2007
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