SUBSEA WATER TREATMENT
For injection of seawater, SWIT represents the next step in IOR by adding treatment of raw seawater to the toolbox of available subsea technologies. Please note that treatment is essential for long term protection of the reservoir from blocking and souring.

With SWIT we bring together a set of proven and some completely new technologies that is assembled into a single all-electric subsea unit. The idea of treatment on the seabed is not new, but it has been a general understanding that current and storms would stir up the seabed and result in raw seawater with poor quality. The SWIT technology meets these environmental and quality challenges, using the 3 basic steps described below. The treated high quality water is then pumped into the reservoir through a conventional injection pump and a Christmas tree.

1. SOLIDS REMOVAL
Solids removal is the first step where solids are removed to stop them from entering the reservoir and blocking it. Solids removal testing has been done to verify the efficiency of the SWIT technology given the conditions in the North Sea.  A model has been submitted to variations in tidal flowrate, seawater temperature, solids loading and water extraction rates. Representative seabed solids (silt) were used.

Particle size distributions were measured at the external inlet and in the SWIT extract flow during testing. The figure to the right shows a significant reduction in particle concentration. The testing not only confirmed our expectations, but also provided design-data for a variety of seabed conditions which can be used for designing systems for specific applications.

2. RESERVOIR SOURING CONTROL
Seawater sterilisation, essential for souring control, is conducted via a two stage electrochemical technique, that eliminates bacteria as well as other organic matter. The two stages consist of a Hydroxyl Radical Generator (HRG,) and an Electro Chlorinator (EC). The efficiency of the HRG unit and the EC unit on planktonic bacteria have been tested to find the most efficient levels of operation.

The result of this planktonic testing is shown in the figure to the right. It shows that the treatment is very efficient, by reducing the bacteria count from 100 000 to 13 CFU (Colony Forming Units).  The two treatment stages complement each other in that the HRG has a very strong instantaneous effect on bacteria, while the EC releases chlorine, that has a long retention time and will further reduce the bacteria that "slip through".

3. SYSTEM STERILISATION
The seawater at the injection point to the reservoir will contain low levels of dissolved oxygen. However, this and advantage as the anaerobic bacteria that leads to souring does not like oxygen. Still over a period of time it is a possibility that anaerobic slime will build up in the well bore on the way to the injection zone, Shock dosing of biocide is needed to destroy this film, but will be less frequent than topsides as we have a very efficient sterilsation system with the HRG and EC units assisted somewhat by the oxygen.

Shock dosing is done topsides using liquid chemicals with about 20% active ingredient. SWIT uses solids biocide chemicals, with nearly 100% active ingredient, which is obviously more weight and space effective. A simple and robust way of dissolving and injecting this chemical on a batch basis is employed. Refill of the biocides will be only be required once a year or perhaps every second year dependent on the application. The refill process is simple and can be done as a part of the yearly inspection of the subsea system.