21 september 2015
Seawater sulfate removal goes deep
Underwater membrane solution for sulfate removal
Among the major challenges in the increasingly complex environment of offshore oil systems are the space and weight limitations on what the industry calls “topsides:” oil platforms and Floating Production, Storage and Offloading (FPSO) units. With the need to find room for items such as oil and gas processing facilities, power generation utilities, crew living quarters, safety and drilling equipment, the trend in the industry is to look to subsea production systems in which the equipment can be installed under water, on the seabed.
One such process that has been shown to be suitable to make the underwater dive is the technology for removing sulfates from seawater, prior to injection into the oil reservoir. The seawater is injected to maintain the pressure in the reservoir, a process known as “waterflooding,” to increase the oil-production rate and, ultimately, the oil recovery.
Sulfate removal is crucial as a scale-control measure in reservoirs containing high levels of barium or calcium to prevent any precipitation or mineral deposition as well as to prevent well souring caused by sulfate-reducing bacteria. Seawater sulfate removal units using specialized nanofiltration (NF) membrane systems are widely deployed today on topsides.
SPRINGS® (Subsea Process and Injection Gear for Seawater) is the underwater membrane solution for sulfate removal, the first time the treatment has been applied in subsea processing. The technology, developed through a joint research program between Total, Saipem and Veolia, has completed a successful subsea deep-water test program and is being readied for full-scale deployment.
In addition to reducing topsides space and weight demands, major advantages offered by SPRINGS® include simplification of the pretreatment and greater flexibility in the water injection pattern for improved sweeping of the reservoir. Pretreatment is simplified as the higher-quality water from the ocean depths reduces the filtration requirement upstream of the membranes. Placing a dedicated SPRINGS® module at each injection well allows the injection water quality and capacity to be matched to the needs of the reservoir for optimal sweeping.
The standard SPRINGS® module has a capacity to treat up to 60,000 barrels of seawater per day. Initial applications are expected to occur either on existing production facilities where a subsea solution may be the only technically or financially viable option for water injection, or on new developments that require long tie-backs to small, remote oil fields for which a subsea solution may be the best economic option.