Subsea Water Treatment Comes of Age
- Karen Bybee (JPT Assistant Technology Editor)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- August 2011
- Document Type
- Journal Paper
- 69 - 70
- 2011. Offshore Technology Conference
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- 86 since 2007
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This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 21578, "Subsea Water Treatment Comes of Age," by David Pinchin, SPE, Well Processing A/S, originally prepared for the 2011 Offshore Technology Conference, Houston, 2-5 May. The paper has not been peer reviewed.
This subsea seawater-treatment system has undergone several phases of development since its inception in 2002. It works by extracting water from the seabed area (where there are inherent advantages such as space, stable temperatures, lower degree of bacterial concentration, and distance from platform discharges). A combination of residence time within a “still room,” two steps of electrochemical disinfection, and chemical dosing are implemented to achieve disinfection and solids reduction.
Water injection (waterflooding) is by far the most used method of increasing oil recovery from an oil reservoir. Water quality is an important factor for maximizing sweep efficiency (displacement of oil) during waterflooding and also preventing reservoir souring. Key objectives of seawater-treatment plants are therefore to minimize the blocking of reservoir pores (e.g., with solids, biomass, and scale) and to control reservoir souring.
The full-length paper describes a subsea seawater-treatment system that is seen as filling the “missing link” to the already proven technology of “raw” seawater injection on the seabed by subsea pumps. To be accepted as a viable water-treatment process, it must be capable of providing at least the same level of reservoir protection as afforded by a topside seawater-treatment plant.
Subsea Treatment System and Testing
The subsea treatment unit comprises several equipment items assembled in such a way as to maximize solids removal and disinfection of the seawater. The process uses the seabed environment and makes use of its beneficial effects. A still room is used to isolate the water to be treated from external conditions (e.g., storms and tidal movements). The inlet compartment is designed to allow for a good degree of solids settling with sufficient density to drop out. The water then is chlorinated by electrolysis as it transfers to a second compartment where it is afforded a long residence time to allow the chlorine reaction to take place and to allow further solids settling. The water exiting the still room then flows through a hydroxyl-radical generator where a final oxidation process takes place. The treatment unit therefore generates its own treatment chemicals by electrolysis and requires only a combined power/control signal interface cable. For the testing, a submerged lift pump was used to induce flow through the subsea treatment unit and up to an onshore sampling/control container.
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