Study On High-Efficiency Water Clarifiers For Polymer Flooding Produced Water

Compared with water-flooding produced water, polymer flooding produced water hasexhibits higher viscosity and tighter o/w emulsion, lower rising or sinking rates of oil dropletsand solid particles during gravitational setting. The industrial scale application of polymerflooding in Daqing Oilfield has brought about numerous troubles for produced watertreatment such as poor performances of skim tanks, excessive dosage of water clarifiers,overload of filters, and accumulation of slop oil at the top of water in skim tanks which tendsto upset crude oil treaters when it is recycled into the feed of FWKOs, resulting indeterioration of the qualities of both re-injection water and crude oil export. The poorperformances of the incumbent coagulants and flocculants on polymer flooding producedwater were found to be the major factor leading to inefficient treatment of polymer floodingproduced water due to formation of sticky flocs that do no efficiently sink but tend to suspendin the filter feed or rise to the top of water inside skim tanks where they enter the slop oil,leading to pollution of the slop oil and filter media, as well as frequent out specification ofre-injection water and crude oil export.The rising and sinking tendencies of flocs under different conditions were studied first inthis thesis. It was found that in addition to the factor of flocculant, the oil content of producedwater can significantly affect the bulk density of flocs. Flocs sink when the oil content ofuntreated produced water was below100mg/L. While the oil content of produced water wasbetween100and300mg/L, most of the flocs suspended in the water because their apparentdensity of flocs was close to that of water. When the oil content of produced water exceeded300mg/L, the flocs showed a strong tendency of rising. The higher the oil content was, themore difficult was it for the flocs to sick.Aimed to solve the numerous problem troubling produced water treatment by two-stagegravitational settling and one-stage deep bed filtration such as poor compatibility of the incumbent flocculants with the anionic polymer in produced water, poor performances of theincumbent flocculants on polymer flooding produced water, pollution of filter media by flocsremaining in the effluent of secondary skim tanks, a new water clarifying agent, named asQS7003, which was composed of cationic high molecular weight organic flocculants and aweighting agent acting as both centers of flocculation and density modulator, was developedfollowing the principles of higher bulk density and sinking rate of flocs. The new waterclarifier featured flocculating, weighting and reverse demulsifying functions. It showed goodperformances on produced water with polymer content below600mg/L and can be applied inproduced water treatment facilities adopting gravitational settling and deep bed filtrationprocess if bottom sludge in skim tanks can be easily drained.A limited field trial of the new water clarifier QS7003was conducted, in which it wasdiluted with water to form a suspension and injected simultaneously at two points of atwo-stage gravitational settling and one-stage filtration process handling polymer floodingproduced water. Water quality determination, determination of the distribution of suspendedsolid particles throughout skim tanks and sludge drainage of skim tanks were undertakenduring the field trail. Over85%of the flocs generated by the addition of QS7003sank to thebottoms of the skim tanks when50mg/L of the new water clarifier was injected into apolymer flooding produced water with an oil content of300mg/L and a polymer content of240mg/L, significantly improving the sinking performances of the flocs. A dosage of10mg/Lof QS7003reduced the residual contents of oil and suspended solids of the effluent water ofsecondary skim tanks respectively from181mg/L to47.8mg/L and from65.1mg/L to27.9mg/L, with residual contents of oil and suspended solids of filter effluent water meeting thequality specification for re-injection into high permeability reservoir, over70%of the flocsformed during the water treatment end up in the sludge at the bottoms of skim tanks.