Treatment Of Oily Wastewater Produced In Polymer Flooding Using Compound Agents And Combined Process

With polymer flooding applied to enhance oil recovery and increase oil production in many oilfields,large amount of wastewater,which is much more difficult to treat than that from water flooding,has been produced.The produced water from polymer flooding contains a quantity of residual hydrolyzed polyacrylamide(HPAM),which seriously impedes the oil-water separation.The present wastewater treatment process in the oilfield fails to treat the wastewater from polymer flooding properly within usual settling time.As a result,the effluent can not reach the reinjection standards of oilfield, and serious environment pollution would also be created if the wastewater was discharged to land surface.The wastewater treatment has become a bottleneck for the application of polymer flooding;therefore it has high application value and environment benefits to develop effective treat and recycle technologies for the wastewater produced in polymer flooding.In this thesis,the stability mechanism of produced water from polymer flooding was studied systematically.A series of hydrophobically modified flocculants were synthesized and then mixed with two demulsiflers.The compound agents could improve the oil-water separation efficiently.A combined chemical-biological process was evaluated in order to make the effluent meet classâ… National Wastewater Discharge Standard.The main contents and results are as follows:1.Simulative samples of wastewater and oil were prepared based on the molecular weight distribution of HPAM in produced water and the formation conditions of oil/water emulsion.The effect of HPAM on oil/water emulsion stability and its mechanism was studied through the measurement of oil concentration in water phase,water content in oil phase,oil-water interfacial force and viscoelasticity, Zeta potential and particle size distribution ofoil droplets in the sewage.Moreover, the synergism of HPAM and suspended solid(SS)on oil/water emulsion stability was determined when they exist in oil-water interface together.It is shown that, because HPAM has a long molecular chain and good viscoelasticity,it is able to enhance the strength of oil-water interface by absorbing on it.As a result,it is more difficult to break the oil-water interface when oil droplets collide and distort. Furthermore,HPAM can increase the electrostatic repulsion force among oil droplets;and the collision and coalescence probability will be reduced consequently.In the early stage of the oil/water emulsion formation,the coalescence of oil droplets is restricted by the two factors,leading to lesser droplet diameter and lower floating velocity.As a result,the separation of oil and water becomes more difficult.Solid particulate matter brought out during the oil recovery can form a network structure with HPAM on oil-water interface,which will enhance the strength of interfacial film and prevent the film from breaking. The synergism of HPAM and SS is too strong to be neglected.2.A series of polyacrylamide-dimethyldiallyammonium chloride-butylacrylate terpolymers[P(DMDAAC-AM-BA)]with different molecular weight,cationic percentage and hydrophobic percentage were synthesized by the micellar free radical copolymerization technique.The effect of synthesis condition on polymer characteristics was discussed,and the products were also analyzed.Fourier transform infrared spectroscope,nuclear magnetic resonance, thermogravimetry-differential scanning calorimeter,transmission electron microscope and dynamic light scattering analysis were used for the structural characterization.The multi-initiation system was composed of redox initiator (K₂S₂O₈-Na₂S₂O₃)and thermal decomposition initiator AIBN,and the optimum mass ratio of Na₂S₂O₃/K₂S₂O₈/AIBN is 1:2:1.The results showed that polymerization conversion rate and polymer molecular weight increased when the multi-initiating system was used.The optimum conditions of polymeric reaction were presented as follows:initiators content is 0.15%of monomers mass,initial temperature is 30℃,reaction time is 6h,monomers concentration is 30%,and urea content is 2.0%of monomers mass.Infrared spectrogram and nuclear magnetic resonance of products showed that characteristic absorption peaks of all the three monomers did exist in the polymer,which confirmed the terpolymerization had occured.Thermo gravimetric analysis showed the polymer had good thermo-stability.Transmission electron micrograph studies showed that the polymer appeared a branch likeness structure,and the volume was broadened when the concentration of hydrophobie monomer increased.Dynamic light scattering analysis showed that P(DMDAAC-AM-BA)exhibit obvious intermolecular hydrophobic association in aqueous solutions when the concentration of P(DMDAAC-AM-BA)was 100 mg/L.The number of molecular aggregate was 16-29.3.Flocculants and demulsifiers were selected separately and then combined to accelerate the oil-water separation.In order to find the active mechanism of the compound agents,the flocculation process,interfacial characteristics,Zeta potential and diameter distribution of floes were examined.Because of the hydrophobic group,flocculant BES-32 performed better than normal cationic organic flocculants on the treatment of wasterwater from polymer flooding.SD-46,a mixture of two nonionic demulsifiers,could dehydrate water from oil rapidly and reduce the oil content in the sewage by enchanting the coalescence of oil droplets.BES-32 and SD-46 were therefore selected to compound CHP-01 and CHP-02.It indicated that the compound agents could improve the oil-water separation efficiently,by enhancing floes formation velocity and strength.They possess the advantages of both flocculants and demulsifiers.CHP-01 has the same effect as SD-46 on interracial tension and viscoelasticity and the same effect as BES-32 on Zeta potential.At the initial stage after dosage added,BES-32 helps oil droplets congregate rapidly,and then SD-46 breaks the interface film,which makes the fine droplets coalesce and become large ones.The compound agent has good stability and is appropriate for pilot test.4.Based on the results of bench tests,the compound agents CHP-01 and CHP-02 were brought into pilot plant production,cooperating with Dongying Yuanda chemical company.And a three months field test was carried out in Tuo-1 Association Plant of Shengli Oilfield in order to evaluate the actual efficiency of the compound agents.It indicated that the technological process of the pilot production was feasible,and the quality of product reached the laboratory level.The product is applicable to treat produced wastewater from polymer flooding,because it could dehydrate water from oil rapidly and reduce the mediate layer effectively.The new product could improve the dehydration and decreases oil and SS content in the sewage with less dosage.As a result,the frequency to wash and renew the filtration materials was reduced.The application of the compound agents make it is possible for the wastewater produced in polymer flooding to reach the discharging standard after advanced treatment.5.In order to improve the water quality of effluent,a combined process was set up, including "Flocculation","Hydrolysis Acidification" and "dynamic membrane bioreactor(DMBR)".The results showed that the combined process was stable after optimizing the operation parameters.The application of compound agent CHP-01 in flocculation could reduce the COD efficiently.After flocculation,COD could decrease from 612.2 mg/L to 178.9 mg/L with a removal efficiency of 70.77%.After hydrolysis acidification,the COD fell to 147.1 mg/L,and the removal rate was 17.8%.The hydrolysis acidification also could control the undulation of the effluent quality.After aerobic biological process,the COD reduced to 89.7 mg/L,with 49.86%removed.The COD of effluent could steadily reach the classâ… National Wastewater Discharge Standard before 1998,and the classâ…¡National Wastewater Discharge Standard after 1998.Other pollutants of effluent could reach the classâ… National Wastewater Discharge Standard after 1998.