Produced water is the largest volume waste stream in oil and gas production and is a particular problem for late-life operation of oil fields. The huge waste volumes result in two obvious consequences and cost drivers; fast declining reservoir pressure and tremendous power used to handle it. Produced water reinjection has been used as a main strategy for both managing produced water volumes and supporting reservoir pressure. Treating produced water subsea as close as possible to the production wells has several advantages such as smaller topside footprint, minimizing the energy demand and associated emissions, reducing the well back pressure and increasing production rates. While some work has been done in the past on subsea use of deoiling hydrocyclones, robust high performance subsea produced water treatment equipment and processes have not yet been made available. The Compact Flotation Unit (CFU) is a well proven technology for topside use and has been considered promising for subsea use, but never before been tested at subsea relevant pressures. Aker Solutions with partners Aker BP, Equinor and Total and support from Research Council of Norway have successfully run a joint industry project to qualify this technology for subsea application.

Pilot testing of a subsea CFU was carried out in Equinor's large scale test facilities in Porsgrunn, using crude oil, natural gas and synthetic produced water. The pilot CFU was tested with pressures significantly above that used for conventional topside CFUs, and at various temperatures. Gas bubbles were injected by use of ejectors designed to provide the desired bubble size. Oil droplet size, flow rate and oil in water concentration were the major control variables in addition to pressure and temperature. The effect of injection of flocculant was also investigated. The main topic of interest was the oil removal efficiency of the CFU under high pressure.

The analyses of the results showed that the CFU's efficiency improved with increase of the operational pressure. This was likely to be related to the changes of physical properties of gas, oil and water as pressure increases, and the changing balance between physical phenomena in the process. The CFU also demonstrated robust performance with a large turndown ratio.

Test of flocculant at high pressure showed instant and substantial improvement in the CFU efficiency, and the effect of flocculant was not compromised by the high pressure.

The promising results suggest that CFU performs well at elevated pressure and is a viable solution for subsea produced water treatment.

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