Sunday, May 9, 2010
Water injection is common applied in maintaining crude reservoir to increase crude productivity. For offshore facilities, seawater is commonly lifted and treated for water injection purpose. The water injection rate is subject to reservoir condition and crude production. Particle present in seawater potentially results formation blockage and reduce injectivity. Oxygen present in seawater potentially results severe corrosion of transfer pipeline and injection tubing. Alga and bacteria present in seawater may potentially growth result corrosion and formation blockage. Therefore seawater use for injection shall be treated prior transfer and injection.
Seawater TreatmentSeawater used for water injection will goes through a series of treatments :
- Filtration - remove particles
- Deaeration - remove oxygen
- Chemical injection - prevent foaming, corrosion, alga / bacteria growth
Seawater lifted will pass through filtration package. The filtration package commonly consist of two levels of filtration. First level filtration is also known as Coarse Filtration where the filter is provided to remove particle with size larger than 80-100 micron. The common required removal efficiency is 98%. The filter is normally equipped with auto-backwash facilities e.g. rotational backwash motor. Second level filtration is also known as Fine Filtration where the filter is provided to remove particle with size large than 2 micron with removal efficiency of 98%. Similarly this filter is equipped with auto-backwash facilities with the assistance of blower.
Seawater is aerated in ambient contains high oxygen contents. Typically the seawater is considered saturated with oxygen and this quantity is sufficient to results significant corrosion in transfer pipeline and injection tubing. Corrosion is increased with increased in quantity of oxygen in seawater. The oxygen level in the seawater is commonly deaearated down to 20-40 ppb in the deaeration column. Oxygen scavenger is injected downstream of deaeration column to further bring the oxygen level down to 10-20 ppb. Two main methods are used for deaeration :
- Vacuum deaeration
- Gas stripping
Present of large quantity of small particle may results particle passing filtration, accumulates, agglomerate and finally results plugging of formation. Coagulant may be required to be injected upstream of filtration package to promote particle coagulation and filtration.
Seawater may be contaminated with hydrocarbon when it is lifted. Seawater used for processing cooling, any leakage in the seawater heat exchanger also result Hydrocarbon present in the seawater. Presented of hydrocarbon in seawater may results foaming in deaeration column. Therefore, anti-foam may be required to suppress foaming.
Bacteria presents in seawater may results corrosion and alga growth which release solid waste to promote formation plugging. Hypo-Chloride is injected to prevent bacteria and alga growth. One shall take note present Chlorine is seawater may also results corrosion. Concentration subject to type of Biocide, however 5 ppm level could be good guess.
Present of Chlorine, residue bacteria and residue oxygen promote corrosion. Therefore corrosion inhibitor (CI) is injected to prevent / minimize corrosion. Concentration subject to type of CI, however 5 ppm level could be good guess.
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