Abstract

This paper presents an overview of optimization of water treatment cycle for a seawater injection plant by focusing on corrosion and chemical management. To approach this purpose, a step-by-step review has been implemented on the plant consisting of 40km pipelines, 3-stage filtrations, a deaeration tower, and treatment chemicals.

Solid invasion, mineral scale deposition and corrosion, along with chemical treatment associated problems stemming from aged water injection plant, cause injectivity loss and increase in operational cost. The plant treats seawater for injection into two of Iranian offshore oilfields. It has now been in operation for 26 years. During these years, damages had been occurred in the system and relatively in the reservoir due to the above-mentioned problems. Besides, it suffers from lack of corrosion monitoring system and has never been inspected for corrosion potential, even though pig cleaning has been run frequently.

Field data and laboratory tests indicated that one of the major problems associated with seawater injection in this oilfield results from inadequate filtration and presence of sulfate salts such as CaSO4, as well as Sulfate Reducing Bacteria (SRB). The finest filter size is determined to be lower than 2? according to core injectivity tests while it is 10? on the site. Use of oxygen scavengers beside the deaeration tower to reduce oxygen concentration to the appropriate level (<0.5ppm) raises the concentration of sulfate and consequently shows up as CaSO4 precipitate.

Application of scale and corrosion inhibitor and bactericide, as well as their efficiency, are discussed in details. Having NACE TM0299-99 Standard in mind, a corrosion monitoring system has been proposed for the plant. It was proved that with few replacements in chemical injection points the system efficiency increased by 30% .The outcome of the study was, more effective treatment, less formation damage, as well as reduced cost and quantity of chemicals used per volume of oil produced and treated injected water.

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