Subsea Water Treatment Comes of Age

Abstract. The development and implementation of innovative new technologies is becoming increasingly critical to the global oil industry as reserves continue to be discovered in more marginal fields. Existing reserves also need increased support to get the oil out of the ground as fields reach maturity, so any new technologies that can improve traditional methods of production whilst offering increased oil recovery are eagerly sought after by industry. This paper describes the development of just such a new technology that is seen as filling the ‘missing link’ in the already existing capability to inject ‘raw’ seawater on the seabed. A ‘total’ seabed waterflood system gives a number of advantages and freedom from topsides restrictions such as weight/space consumption and number of available well slots. Being able to inject unlimited amounts of good quality seawater (into specific areas of the reservoir that is most beneficial for oil displacement) enables a significant change in mindset in the field of offshore secondary recovery and reservoir drainage strategy. The new levels of flexibility in reservoir drainage strategy should lead to increased oil recovery from: existing oil reservoirs, new reservoirs and enable marginal (e.g. satellite) reservoirs to be economically developed. The new 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 away from platform discharges). A combination of residence time within a ‘Still Room’, two steps of electrochemical disinfection plus chemical dosing are put to good effect in disinfection and solids reduction. A full scale pilot plant has recently completed its testing on the seabed with the help of a global technology funding program from Shell, TOTAL, ConocoPhillips, GDF SUEZ and the Research Council of Norway. The new treatment system has produced some very good water quality results that compare favourably with water quality achievable from topsides seawater treatment plants in the key areas required for waterflooding. The technology is applicable for offshore oil fields at any stage of their production lifetime and is now ready for field applications. Total subsea waterflooding (including treatment) is now achievable on the seabed for the very first time. Introduction 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 maximising sweep efficiency (displacement of oil) during waterflooding and also preventing reservoir souring. Key objectives with seawater treatment plants are therefore to minimise the blocking of reservoir pores (e.g. with solids, biomass and scale) and controlling reservoir souring. This paper describes a new subsea seawater treatment system that is seen as filling the ‘missing link’ to the already proven technology of ‘raw’ seawater injection on the seabed via subsea pumps (Neil Rogerson et al., Sept 2007). In order to be accepted as viable water treatment process it must therefore, be capable of providing at least the same level reservoir protection as afforded by a topside seawater treatment plant.