Conceptual Design of a Large Water Treatment System for Offshore Smart Water Injection

Water injection is a traditional method of secondary recovery used in the petroleum industry. Typically, even with the injection of water supplementing the initial energy of the reservoirs (primary recovery), less than 50% of the original volume of oil is produced. By altering the chemical composition of the injected water, tests performed in laboratory and in pilot units have shown significant oil recovery (5 to 40%), when compared with a field with traditional water injection. The oil industry has shown great interest in smart water (SMW). Basically, the SMW comprises a seawater stream with altered ionic composition and low total dissolved solids (TDS). Membrane separation is the most suitable method for producing SMW offshore, though, it has some limitations for water ion tuning. Reverse Osmosis (RO) membrane technology plays an important role in SMW generation process due to its hability to promote a high rejection for all ionics species. This paper describes the results of a topside conceptual design of a 39,500 m3/d Water Treatment System to produce SMW (WTSMW), that generates water with a TDS of 1,884 mg/l, from seawater with 40,000 mg/l of salt. The treatment scheme comprises RO membrane permeate plus seawater, with a device to recover energy from the RO reject stream. The product water is also used for oil desalting and may be used for membrane cleaning. Electrical consumption, weight and footprint were compared with a sulphate removal unit (SRU) of 35,400 m3/d, of an existing 150 Kbpd floating production storage and offloading unit (FPSO) used as reference design (RDF). It was also evaluated the suitability of the RDF to incorporate the WTSMW, concerning electrical consumption, weight and footprint. The WTSMW is heavier (571 t), requires one more module level, which resulted in a module 9 m higher, and consumes more energy (3.3 MW), despite the use of the energy recovery device (ERD). To receive the WTSMW, the generation nominal capacity (4 × 23 MW) of the RDF should be changed to 4 × 28 MW. The increase in height and weight do not represent any restriction for design or construction of a new production unit to include the WTSMW.