Subsea Water Intake and Treatment - The Missing Link?
Eirik Dirdal, Senior Engineer, Seabox

Water injection is the oil industry's most commonly used method to increase production and recovery rates. In fact, water injection - or so-called waterflooding - began accidentally in Pithole, Pennsylvania, USA, in 1865, and became common in Pennsylvania during the 1880s. In short, water is injected into the reservoir to increase or maintain pressure and for displacing the oil and pushing it towards a producing well.

As the oil industry moved from onshore to offshore, the concept of water flooding followed in its path. Since the 1970s, water treatment systems and injection pumps have been installed on processing decks of offshore platforms to send filtered and disinfected seawater into the reservoirs to increase production and oil recovery rates. The topsidetreated seawater, which is typically sourced from about 30 m below the sea sur face, can either be injected into the reser voir through dry injection wells located at a platform, or transported through flowlines to satellite subsea fields where it is injected through subsea injection trees. In recent years, subsea pumps have also become a reliable alternative, but traditional topside pumping is still the commonly used option.

The Topside Conundrum
There are also 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. In total, this can present significant weight problems – particularly for floating production units. In addition to being large and heavy, the traditional solutions for water treatment on platform topsides are quite energy consuming and use significant quantities of chemicals harmful to health and the environment in order to obtain proper treatment on the physical limitations that follows with a topside treatment system.

Another issue with topside treatment systems is that such equipment often has to be designed, engineered and constructed a long time before anyone knows exactly how the reservoir behaves. A common challenge for reservoir engineers is to predict exactly the dynamics of a reservoir over time and identify the optimal placement of injection wells based solely on appraisal well results. Assumptions rarely prove to be correct and the strategy for recovery normally changes during the lifetime of a field.

When Seabox star ted 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 sea bed are of less importance, an optimal treatment process for high quality water could be the driving design parameter. This has since resulted in the SWIT- technology(Subsea Water Intake and Treatment).

Enter SWIT
Moving technology from the surface to the seabed has traditionally been associated with numerous disadvantages. Higher risk and cost per well combined with lower recovery rates, less access for maintenance and upgrade , less robust technology and a lower level of predictability has been the norm. Although the subsea oil and gas industry has identified the right ways to manage these issues, a subsea solution is still widely considered the 'second best' alternative to its more experienced topside cousin.

Seabox aimed to use Well Processing as the vehicle to develop the technology that was later denoted as SWIT. A guiding principle behind SWIT 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 completely new way of thinking, as there are fewer restrictions to weight/space
consumption and number of available well slots. This new 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. From a financial point of view, this will also enable operators to defer some of the capital costs in a project, as long as it does not create any operational difficulties at a later stage.

There are also HSE benefits with placing equipment on the seabed, as different chemicals often required for operation of a topside plant are no longer necessary.

With all these factors in mind, the Seabox team is convinced that a subsea water intake and treatment technology is interesting from both a technology and financial perspective. However, the one thing that would really prove the SWIT technology is if it enabled oil companies to recover more oil than what they would have traditionally done with a similar topside system.

Seabox's subsea treatment unit is comprised of several pieces of kit assembled with the aim of maximising solids removal and disinfection of the seawater in a robust and reliable way suited for subsea use. The ultimate objective is to disinfect and filter seawater that will help prevent blocking of reservoirs or turning them sour when water is injected into them .

Whereas topside water treatment systems utilises water from near the surface , SWIT extracts water from close to the seabed, which represents an advantage as there are more stable temperatures on the seabed, a lower degree of bacterial concentration, and it is far away from potential platform discharges such as produced water, ballast water and drainage (rain ) water.

The treatment process takes place in a square-shaped box that is positioned on the seabed, hence the company name ‘Seabox’. The SWIT unit is also utilised to isolate the water to be treated from external conditions.

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 th SWIT unit. The water continues through a set of electro chlorination cells that produce sodium hypochlorite, which initiates the disinfection process .

Seabox has developed a purpose built sensor that can detect change in total residual oxidant level in the treated water compared to the surrounding raw seawater. Tests by Seabox shows that the chlorine level can be adjusted with an accuracy of 0.01 mg/l, which enables tight control of the disinfection process. The objective is to produce chlorine in the treated water that is only marginally higher concentration than the level in the untreated seawater.

Unlike a typical topside treatment plant, where the processes are adjusted to platform weight/space limitations, the SWIT unit allows for a chlorine exposure time of 60-120 minutes compared to typically 60-90 seconds for topside facilities. This is the second step of the process, where the water enters a chamber inside the SWIT that is called the ‘stillroom’. The net result is an effective kill rate and collapse of the cell structure for organic species.

Another important key to the second stage is the solid settling. The SWIT unit has been designed to give rise for low velocities and laminar flow in order to have an effective settlement of inorganic solids. In the pilot test with high-suspended solids levels, the particle removal achieved was 99 percent removal of all particles greater than 15 micron. Typical requirements for fracked water flooding conditions is 99 per cent removal of all particles above 24 micron. A 40,000 bpd capacity industry version of SWIT has been designed for this specification.

Step three of the treatment process happens as the disinfected water leaves the stillroom and passes through a hydroxyl radical generator. This module will generate highly reactive radicals that will clean the water further in addition to breaking the dead bacteria down into even smaller particles. Seabox holds the patent and exclusive rights for subsea-use of this technology.

All equipment items that are deemed to have an operational lifetime (cells, valves etc) are housed in a Retrievable Treatment Unit. The all-electric cartridge is typically powered by electricity from a nearby platform, or from a subsea pump located nearby. The power, control and monitoring systems enable the operator to perform remote operation and control of the treatment system.

Oslo fjord Pilot
After the SWIT concept's inception in 2002, the technology has gone through tens of thousands of hours of engineering, research and development, plus a series of technical qualification programmes. The SWIT unit’s biggest challenge so far, however, has been a recent full-scale pilot trial in the Oslo fjord in Norway.

In a joint industry project backed by ConocoPhillips, Shell, Total, GDF Suez and the Norwegian Research Council, the SWIT technology was put through its paces during a 15-month pilot.

The scope of the joint industry project covered the design of a full-scale SWIT pilot plant including a stillroom enclosure, electro chlorinator cells, hydroxyl radical generator cells, power, control and monitoring system. The project also covered fabrication, transportation, installation and hook-up of the SWIT system in 65 m water depth. The pilot plant was capable of treating 15,000 barrels per day.

The Oslo fjord location was chosen because of challenging water quality and proximity to the NIVA marine research centre at Solbergstrand, Norway.

Game-changing Results
Despite seasonal changes that resulted in variable quality of the feed water , the pilot’s results showed a water quality with an average particle size below 10 micron, a typical solids concentration of 0.5 mg/l, and a silt density index (SDI) measurement of 5. The pilot showed a significant delay in the onset of biofilm 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 (GAB) and sulphate reducing bacteria (SRB) in the two SWIT treated sample lines compared with the control line.

The pilot test also showed that the combination of electrochloriation and the hydroxyl radical generator is highly efficient in removal of SRBs and delaying the onset of biofilms.

Another interesting finding is that no biocide was used during the oneyear SWIT test period. This opens up possibilities for alternative dosing regimes and types of chemicals, such as lower doses of nitrate chemical, to control SRB in the reser voir itself.

With risk-adverse oil companies’ understandable scepticism towards new subsea technologies, it was also important that Seabox managed to prove the system's operational reliability during the trial.

Through the 15 months of testing, an impressing 99.8 per cent 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 approximately 85 percent.

SWIT's Potential
On a global basis, injection of seawater into an oil reser voir is the most effective and commonly used IOR technique. Currently global water injection is 240 MMbpd, which is three times the global oil production. By 2020 analysts project that global water injection will be seven times the global oil production.

Water injec tion has t ypically shown to double recoverable reser ves compared to a traditional pressure depletion (from 18-22% to 35-45%).

Optimised water flooding may further increase recoverable reserves by up to 10 percentage points (i.e. up to 45-55%). Additional IOR/EOR techniques combined with water injection (e.g. designer water, surfactants) will further increase recoverable reserves. The SWIT technology can also be utilised for onshore injection purposes by pumping the treated water from the coast to the targeted onshore wells. This is a more responsible alternative than using groundwater, which instead could be used as drinking water.

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 by traditional topside water injection systems. This can unlock oil reserves that would otherwise have been unrecoverable - at a much lower cost.

Putting a fully-fledged factory on the seabed is not the fanciful notion it may once have seemed and with the SWIT technology available, it seems like the missing link in the already capability to inject ‘raw’ seawater on the seabed has finally been filled.