Subsea Water Intake and Treatment

The SWIT Unit Has Been Designed to Provide Low Velocities and Laminar Flow to Have an Effective Settlement of Solids

Water injection is the oil industry's most commonly used method to increase production and recovery rates, and since the 1970s water treatment systems and injection pumps have been installed on processing decks of offshore platforms.

However, there are a number of challenges with a topside water treatment system. The treatment equipment and injection pumps on a platform are large, heavy and occupy scarce deck space. They are energy-consuming and use significant quantities of chemicals harmful to health and the environment.

Another issue with topside treatment systems is that such equipment often has to be designed, engineered and constructed long before anyone knows exactly how the reservoir behaves.

When Seabox started toying with the idea of moving the process equipment to the seabed, it was based on an assumption that it would free up deck space and offer added flexibility in the field development phase. In addition, as the space limitations at the seabed are of less importance, an optimal treatment process for high-quality water could be the driving design parameter. This has since resulted in the subsea water intake and treatment (SWIT) technology.

Enter SWIT

A guiding principle behind SWIT technology was that it would only be considered by the oil companies if it could compete with, and preferably outperform, a topside system when it comes to the quality of injection water, capex and opex. Locating the technology at the seabed provides a new way of thinking as there are fewer restrictions to weight/space and number of available well slots. This approach gives rise to another important mechanism, which is the inherent flexibility that allows the operator to adjust reservoir drainage strategy during the life of the field based on the dynamics of the reservoir.

Three technology steps

Chlorine has been used for many years to treat water to control microorganisms because of its capacity to inactivate pathogenic microorganisms quickly. The effect of the process is dependent on the chlorine concentration, time of exposure and pH of the water. This method is widely used for topside treatment plants and is used as the first step in the SWIT treatment process.

The water enters the SWIT through grids at the top of the structure from a suction pressure that is created by a pump located downstream of the SWIT unit. The water continues through a set of electrochlorination cells that produce sodium hypochlorite, which initiates the disinfection process.

Unlike a typical topside treatment plant, where the processes are adjusted to platform weight/space limitations, the SWIT unit allows a chlorine exposure time of 90 to 120 minutes compared to the typical 60 to 90 seconds for topside facilities. The net result is an effective kill rate and collapse of the cell structure for organic species.

Another important key element is the solid settling. The SWIT unit has been designed to provide low velocities and laminar flow to have an effective settlement of solids. In the pilot test with high suspended solids levels, the particle removal achieved was 99% of all particles greater than 15 microns. The typical requirement for fracked waterflooding conditions is 99% removal of all particles above 24 microns. A 40,000-bbl/d capacity industry version of SWIT has been designed for this specification.

The last step of the treatment process happens as the water passes through a hydroxyl radical generator. This module will generate highly reactive hydroxyl radicals that will clean the water further in addition to breaking the dead organics down into even smaller particles.

Game-changing pilot results

The SWIT unit's biggest challenge has so far been a recent 15-month full-scale pilot trial in the Oslo Fjord in Norway.

Despite variable quality of the feed water, the pilot's results showed a water quality with an average particle size below 10 microns, a typical solids concentration of 0.5 mg/l and a silt density index measurement of 5. The pilot showed no biofilm for the complete test duration in two SWIT-treated sample lines compared to similar control lines with raw seawater.

All methods used for bacterial analysis indicated a significant reduction of both general aerobic bacteria and sulfate-reducing bacteria (SRB) in the two SWIT-treated sample lines compared with the control line.

The pilot test also showed that the combination of electrochlorination and the hydroxyl radical generator is highly efficient in removal of SRB and delaying the onset of biofilms. Another finding is that even with no biocide used in one of the test lines during the 15-month SWIT test period, no biofilm developed for the duration of the test.

Seabox also managed to prove the system's operational reliability during the trial. Through the 15 months of testing, a 99.8% uptime was achieved with only minor debugging of the software at the start of the test. In comparison, typical topside treatment systems have an uptime of about 85%.

The SWIT treatment process provides high-quality water in areas that are essential for increasing the sweep efficiency and avoiding reservoir souring. As such, it fills a technology gap by enabling a total subsea waterflood system, thus increasing IOR beyond what is possible with traditional topside water injection systems. This can unlock oil reserves that would otherwise have been unrecoverable and at a much lower cost.

Source: E&P

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Taxonomy

• Treatment
• Water Treatment & Control
• Technology