Produced water from polymer flooding in Daqing oilfield was investigated and the typical characteristic includes: (1) high concentration of polymer (polyacrylamide, 120-190 mg/l), (2) very stable oil-colloid emulsion at presence of polymers (suspended colloid, 50-100 mg/l), and (3) increased concentration of dissolved hydrocarbon and silica (oil, 160-800 mg/l). To overcome the economic and environmental challenges of reusing the produced fluid from polymer flooding, a combined biological/microfiltrating process with capacity of 1500 m3/day was field tested for removing oil and suspended solids. The purified water met the quality criteria proposed by Daqing oilfield for reinjection: < 8 mg/l oil concentration, < 3 mg/l suspended solids, and < 2.0μm of median particle size of suspensions.
This paper investigates the bench scale study of biodegradation of polymer-containing produced water including bacterial selection and enrichment. The enriched bacteria from local produced water pond, including Bacillus, Halomonas, and Arhodomonas, were investigated by DNA sequencing. Kinetics of biodegradation of different combination was identified and influences of additives, such as nutrient and oil suspensions, were optimized for enhancing biodegradation efficiency. According to the bench scale study, a pilot test of biodegradation/filtration for produced water purification was carried on for one year. The fully operational field demonstration suggests that the combined biological/filtratiing process can significantly reduce the hydrocarbon content and break the polymer-oil-colloid emulsions for more efficient oil/water separation. The oil concentration was reduced from 689 mg/l to 3.51 mg/l. The suspended solid was reduced from 164 mg/l to 4.9 mg/l and the average median particle size of suspensions was reduced to 1.5 μm. The operating cost of the biological process is estimated at 26 cents/m3. Other economical factors including capital investment, land use, and operating cost were discussed.