As oil fields mature, oil production rates decline. Water injection, to either help maintain reservoir pressure or to help sweep more oil from the reservoir, is an established method to maintain production rates and the requirements for water injection are expected to grow year-over-year.

But it’s not as simple as just pumping available water, such as seawater, down into specifically drilled injection wells. The water must be treated to be disinfected, remove particles and sometimes adjust the chemistry of the water, which can mean having to remove sulfate and reducing salinity levels.

For mature offshore facilities, adding or even increasing water treatment and injection facilities can prove difficult, due to space or weight constraints, and may also be prohibitively expensive. While it’s possible to inject water into subsea wells, subsea water treatment technology has not been available until now, which means topsides facilities have been needed. This makes water injection expensive for long-distance tiebacks, due to the high-pressure pipelines and pumping equipment that would be needed.

By introducing subsea seawater treatment, topsides water treatment and pumping equipment, as well as long, high-pressure pipelines, would no longer be needed.

NOV, through its Seabox business in Norway, is paving the way. Early this year, the company deployed its first industrial full-scale Seabox subsea water treatment module in a fjord in Norway for an extended verification test. A second Seabox treatment unit has been built at CSUB in Arendal, Norway, and NOV’s Fiber Glass Systems facility in Plymouth, U.K. The unit has been shipped to Stavanger, Norway, for final outfitting.

In August it is due to be taken offshore and installed at ConocoPhillips’ Ekofisk facilities in the Norwegian North Sea. A third unit, called a SWIT, which will treat the water even further, is also due to be deployed in September.

Torbjørn Hegdal, NOV’s manager of business development for Seabox, said that while taking products subsea often makes them more complicated, the Seabox solution is fairly straightforward, and he said it also produces better quality water than you get using topside equipment while reducing topside footprints and lowering the environmental footprint of oil and gas production. “There are no moving parts. We have simplified it,” he told the Underwater Technology Conference (UTC) in Bergen, Norway, in June.

Hegdal called Seabox a modular distributed product. It gives operators water injection capability when and where they want it, he said. This can enable more proactive reservoir management.

“As you get more information about the reservoir, you can act on that and optimize it,” he said. “You have got to appreciate that there’s huge uncertainty about the subsurface. So flexibility is needed, and this lends itself to a distributed solution.”

The first Seabox unit was placed in 220-m (722 ft) water depth in February and by mid-June it had “100% uptime and extremely good results,” Hegdal said.

The Seabox system settles out particles and then disinfects the water so that no bacteria get put into the reservoir that could cause “souring.” It removes 99% of all particles above 8.5 micron, which is close to a typical matrix flooding specification. Matrix flooding provides better control of the waterflooding, aiding increased oil recovery. By combining the Seabox with the ultrafiltration module, practically all suspended particles are removed down to an absolute filtration of 0.1 micron, according to NOV.

The SWIT, which contains membranes, can then also be used to remove sulfates from the water, preventing scaling, and to reduce salinity, which alters the “wettability,” a mechanism that helps make the oil more mobile in some reservoirs.

A standard Seabox module, measuring 8 m by 8 m by 8 m (26 ft by 26 ft by 26 ft) and qualified to 3,000-m (9,843-ft) water depth, would handle 40,000 bbl/d of water and has a treatment unit called a “still room” where particles settle out, and a tray, which it sits in. The module is built in glass-reinforced polymer. The treatment unit needs to be serviced about every four years and is designed to be changed out easily, Hegdal told UTC.

When the water enters the treatment unit it passes through nine electrochlorinator cells at the inlet and two histidine-rich glycoprotein (HRG) cells at the outlet. In these cells hypochlorite and hydroxyl radicals are created and, using electrolysis and the long residence time provided by the still room, provide a thorough chlorine soak killing off any bacteria. The HRG cells at the outlet have a decomposing effect on any neutrally buoyant organic material and will also kill any remaining bacteria. The system is instrumented and monitored continuously.

The reverse osmosis membranes in SWIT are fed with water that has been pretreated by the Seabox module and a downstream ultrafiltration module using a feed pump. This drives the water across the membranes. The maintenance interval for the ultrafiltration module’s hollow fibers and the membranes of the SWIT are 2.5 years.

The complete system can be scaled to any size, Hegdal said. “It’s about distribution, putting it where and when it’s needed.”

The SWIT can, in principle, produce water as good as drinking water at the seabed, according to Hegdal. It is a matter of type of membranes and pressure applied across the membranes.

“It’s a disruptive technology and opens up innovative ways to develop new fields and optimize existing ones,” he said.