In May 1974, Dubai Petroleum Company started the injection of seawater in the Fateh Mishrif reservoir. Hereafter is described the water treatment, the corrosion control through the waterflood plant and the subsea water lines, the scale prevention in the injection wells and the prevention in the injection wells and the scale control in the producing wells.
The seawater is lifted from 100' below the surface through a lifting caisson common to three lifting pumps. Two electro-chlorinators provide a hypochlorite solution in order to kill the bacteria and marine organisms in the lifted water. The hypochlorite solution is injected through a distribution ring located at the bottom of the lifting caisson, therefore all the lifted water is treated. The electro-chlorinators can be operated either automatically or manually to insure a chlorine content of 1 to 1.2 mg/l in the lifted water.
The average total suspended solids (T.S.S.) in the raw water is 1 mg/l with a maximum of 4 to 5 mg/l. From the caisson, the water goes through strainers then to the upflow sand filters. Before entering the filters a coagulant is injected into the water. The injection of bentonite as a filter aid was found unnecessary and has been discontinued. After filtration the TSS is less than the specified 0.2 mg/l. The TSS is determined by filtration on a 0.45 millipore filter. To meet the TSS specification the filters are flushed every 48 or 72 hours according to the raw water TSS.
A defoamer is added to the water before entering the vacuum towers where the water is deaerated by vacuum. The remaining oxygen content is less than 0.02 mg/l. Since the water injection was started it has never been necessary to add an oxygen scavenger to meet the specified oxygen content of 0.02 mg/l. The filtered deaerated seawater is then injected in the Fateh Mishrif reservoir through 24 wells. Once a week a bactericide is injected as a batch treatment upstream of the sand filters to kill the remaining bacteria. Two different bactericides are used alternatively.
The oxygen content of the water is monitored continuously. The iron, hydrogen sulfide and sulfate reducing bacteria contents are regularly monitored on the waterflood plant and on the remote platforms to determine adjustments necessary in the water treatment.
The raw seawater contains an average of 6.5 mg/l of oxygen, 42 to 44,000 mg/l of dissolved salts and has a low resistivity of 17.60 ohm-cms; therefore it is very corrosive. The lifting caissons, the strainer bodies and the lifting pump discharge headers are made of epoxy lined carbon steel. The lifting pumps are composed of stainless steel and monel which are corrosion resistant. From the discharge header to the vacuum towers, vinyl ester resin fiberglass reinforced piping has been used successfully.
The vacuum towers are lined with rubber. Downstream of the vacuum towers the oxygen content of the water is less than or equal to 0.02 mg/l.
