Subsea Seawater Injection: A New "Raw" Solution?

Conventionally water injection facilities have been located topsides and sea (or aquifer) water filtered, treated with chemicals and de-aerated prior to injection. A new approach being trialed is to treat and inject raw sea water directly from facilities located on the seabed. Will this transform the provision of water injection to small accumulations where the cost of supplying topsides-based facilities is too great? Willie Wilson (William.Wilson@dti.gsi.gov.uk), Senior Development Engineer with the DTI Licensing and Consents Unit in Aberdeen , investigates.

Certain types of reservoirs require pressure support in order to maximise the economic recovery of hydrocarbons. Injecting treated water into a reservoir is a production support mechanism that has been widely utilised to provide pressure support and voidage replacement. In assessing the specification requirements for a water injection (WI) system, the following issues need to be addressed:

• Injection pressure.
• Scaling potential, as a result of injection and formation water interaction.
• Filtration specification, in order to mitigate any potential impact on reservoir permeability.
• Potential for reservoir souring and the impact on downstream production facilities.

Conventional practice has seen the WI generation facilities being located topsides on the host platform facility. Seawater or aquifer water has typically been used as the source water for injection purposes. The WI treatment process usually involves the injection water being filtered (to a defined micron level), chemically treated (biocides, oxygen scavengers, scale inhibitors) and de-aerated to defined standards prior to its pressure being raised (typically 2-300 bar) for injection purposes. A standard WI facility would typically consist of the following key components:

• Water Lift Pumps
• Filtration skids
• De-aeration vessel
• Chemical injection
• WI Injection pumps

For a subsea tieback, a dedicated WI riser and pipeline system would also be required to deliver the injection water from the platform to the subsea wells. The development of small fields, long distance tiebacks or a requirement for higher injection pressures, may render "conventional" approaches such as topside WI facility upgrades and/or provision of new pipelines impractical due to economic considerations. For these cases and for potential host platforms which do not have existing facilities or are faced by space and/or weight constraints, a Subsea Raw Seawater Injection (SRSI) system may offer a potential solution. As the name suggests, the treatment and injection facilities are essentially moved down onto the seabed.

Subsea Water Injection Pump Unit (photo courtesy of Framo Engineering) (Click for larger view)

Seabed pumping is an ever evolving technology, with pumps having been used on a number of subsea projects and, in addition, the Norsk Hydro Troll Pilot project [1] has demonstrated the application of seabed pumping for produced water re-injection. In its simplistic form, an SRSI system would typically comprise the following building blocks:

• A control console and variable speed drive (VSD) located topsides on the host platform.
• A subsea umbilical(s) between the host platform and the subsea pump unit, in order to provide electrical power, hydraulic lube oil and injection chemicals.
• A support base structure for the WI pump unit.
• A WI pump(s) and motor.
• A filtration unit on the WI pump suction inlet.
• Chemical injection facilities on the pump inlet.
• Injection jumpers or in-field flowlines from the pump outlet to each WI well.
• A protection structure for the pump unit assembly.

The filter unit design is a key element. It must be capable of removing solids and neutrally buoyant particles from the water stream to the required filtration level. This will be key in reducing the potential for solids ingress into the pump unit. The pump unit design will need to address this factor. The filter unit should incorporate some "cleaning" mechanism to ensure it functions to the required standard. The discharge back into the water column from this cleaning process must be deposited well clear of the filter suction inlet in order to avoid a "closed loop" situation.

Injection of aerated seawater may require the use of high grade steels for injection spools and WI well tubing. Various chemicals can be delivered through the umbilical from the platform, to enable chemical dosing of the injection water at the pump inlet for bacteria control and scale prevention, in a similar manner to topside applications. A detailed assessment will be required to confirm if raw seawater injection is suitable for each specific reservoir.

Injecting low temperature seawater may lead to the generation of subsurface thermal fracing, which in certain cases could provide beneficial effect for the injection process. The other potential benefit associated with a SRSI system is that it removes the Capital Expenditure (Capex) associated with upgrades to existing topside WI facilities or provision of a new WI riser/pipeline system. This needs to be balanced against the costs associated with the procurement and installation of the complete SRSI system in order to assess if it can offer a viable commercial alternative to "conventional" WI solutions on a case by case basis.

Considering the Operational Expenditure (Opex) aspects, the SRSI system depends on the platform facility to provide power (a few Megawatts) to the subsea pump and injection chemicals. High component reliability and system uptime availability must be the operational performance target. To deliver this, a systematic approach to the engineering, manufacturing and integration testing process is key. A uncomplicated pump system, utilising reliable components, designed to allow efficient component change out, in conjunction with a clear maintenance policy, are key points in delivering an efficient Opex strategy.

A number of companies are active in developing this technology. For the right application, SRSI technology may offer an attractive solution and it will be interesting to monitor if this technology breaks through in future field developments.

References

1. Framo Engineering Limited, World's First Subsea Water Injection Pump , Technical Bulletin, May 1999 ( http://www.framoeng.no/publications/Troll%20Pilot%20Apr%2099.pdf )

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