| The oilfield of Yanchang is low pressure,low production,low permeability(extremely low permeability)and micro-fracture development.The main exploited reservoirs are Yan'an Formation and Yanchang Formation(including Chang 2 and Chang 6).The main methods to enhance oil recovery include CO2 foam flooding,Hydroxypropyl hydrazine-based fracturing,and water injection in the same layer.With the continuous implementation of waterflooding development measures,the components of produced water are becoming more and more complex,and the difficulty of treatment is increased.Treatment processes of the produced water existing are difficult to meet the reinjecting water quality requirements of the low permeability(Very low permeability)oilfield.Based on the water quality characteristics and requirements of the oilfield of Yanchang,the difficulties in the treatment of produced water and the instability of water quality after treatment were analyzed.A "eddy multiphase synergistic ozone oxidation+ozone dissolved gas coagulation air flotation" reaction system was constructed.which was simplified into eddy multiphase synergistic dissolved ozone coagulation flotation(DOCF system).The mechanisms of ozone oxidation,coagulation and ozone air flotation and synergism in the vortex state were explored.Multi-function pretreatment device with the functions of oil removal,impurity removal,sterilizing and iron removal,desulphurization and demulsification,and scale inhibition and corrosion prevention was developed,which was optimizing the control parameters of the DOCF system process,treating the inorganic carbon membrane filter material as the end treatment water,up to the requirements of the water quality for low-permeability and ultra-low-permeability oil fields in northern Shaanxi,and improving the recovery rate of oilfield water injection.The effects of additives such as fungicide 1227,isothiazolinone,corrosion inhibitor SW-639 and fracturing fluid on the emulsion stability of oilfield produced water were analyzed.The results showed that the interfacial tension of oil and water gradually decreased with the increase of additive concentration,Zeta potential When the particle size of the suspended particles is smaller,the dehydration rate of the demulsification decreases,the water transmittance decreases after the coagulation treatment,and the stability of the produced water system is enhanced.At the same time,The physical chemistry factors,corrosive factors and fouling factors have been studied in terms of the causes of water quality deterioration after treatment.The physical factors caused delayed flocculation were the emulsified oil content in the treated water and the excessive amount of coagulant added..The main influencing factors and scale composition of the produced water were analyzed,which showed that the order of the influencing corrosion rate was:pH>Fe3+>SRB>dissolved CO2>HCO3->dissolved oxygen>SO42->TGB>S2-.The fouling factor is mainly caused by fouling ions producing CaCO3,sulfide and other scale substances at different pressures.These factors will lead to unstable water quality after water treatment directly or indirectly,the suspended solid content of the water injection wellhead increases and causing formation damage.The"eddy current multiphase synergistic ozone flotation"(DOCF system)was constructed,The number of openings in the vortex reactor,the ratio of inner and outer cylinder diameters,and the optimal gas-liquid ratio were optimized.and a DOCF system laboratory experiment setup was established to examine the correlations between ozone oxidation,UV intensity,coagulation reaction,ozone floatation,and water quality stability of the produced water treatment standards.The interrelationships of the coagulant,ozone concentration,contact time and inlet pressure in the system were discussed.The optimum conditions of the process parameters were optimized by orthogonal experiments and response surface analysis:when the pH value is 8.0,the ozone concentration was 55 mg/L,the inlet pressure is 0.3 MPa,the dosage of PAC is 29.5mg/L,the reaction time is 5min,the UV lamp irradiation intensity was 110W,the dosage of APAM is 2.0mg/L,the temperature is 30-40?,The O2 and O3 tail gases produced by the system are further recycled to maximize the use of the exhaust gas.The effect of key factors on the apparent reaction kinetics of ozone oxidation was investigated by using the removal rate of COD before and after the reaction.Empirical kinetic equations were established by analysis of reaction rate constants and key factors:When the amount of PAC added ?30 mg/L,the PAC was positively correlated with the oxidation rate equation of simulated production fluid,and and the kinetic empirical equation is C=C0 exp)(-3.14×10-5Q0.7095G0.2048W0.0549).When PAC added>30mg/L,PAC is negatively correlated with the oxidation rate equation of simulated production fluid,and the kinetic empirical equation is C=C0 exp(-4.02×10-4Q0.7095G-0.5474W0.0549).where C is the simulated COD concentration of produced water at time t,C0 is the initial COD concentration of produced water,Q is the dosage of ozone,G is the dosage of PAC,and W is the UV intensity.This paper analyzes the reaction principle of DOCF system for deoiling,impurity removal,iron removal sterilization,demulsification and desulphurization,and scale inhibition and corrosion protection.The mechanism of demulsification and destabilization of DOCF system,mechanism of destabilization removal.The DOCF system multi-function high-efficiency pretreatment device is designed and developed.The field tests of impact resistance and anti-pollution were carried out,which showed that the device can supply 400-898mg/L oil and 74-368mg/L suspended solids to 20-30mg/L and 5-40mg/L respectively,the removal rates can be reached 95%and 89.2%respectively,and the operation was stable.The inorganic carbon membrane was selected as the end treatment through the contact angle and chemical stability analysis of the membrane material.Through the operation of the flux,backwash flow,backwash cycle,backwash time,transmembrane pressure difference,the oil content before and after processing changes in suspended solids and other operational parameters to evaluate the performance of inorganic carbon membrane anti-pollution impact resistance.The results showed that when the incoming oil content is 30mg/L,and the suspended matter is 60mg/L,when the inflow flow is 5.0m3/h,the initial concentrated water discharge flow is 1.5L/h,and the cycle of initial water production is 30 minutes,the backwash water flow is 4.5m3/h,the pulse interval is 20s,the circulating flow rate is 45m3/h,after treating,the oil content and the suspended matter are below 1.0 mg/L,and the transembrane pressure has small change,stable operation,strong anti-pollution and impact resistance.The DOCF system-inorganic carbon membrane process was used to treat the produced water in the Extended Oilfield ××Joint Station.The results showed that the produced water after treating meets the requirements for reclaimed water from the low-permeability oilfields in the extended oilfields:the oil content?1.0mg/L the suspended solids?1.0mg/L,the median particle size?1.0?m,the corrosion rate?0.076mm/a,SRB bacteria content?10 ge/mL,TGB bacteria content?100ge/mL,iron bacteria are not detected,iron content?0.5mg/L,sulfide?2.Omg/L,soluble CO2?1.0mg/L and well compatibility with the formation.The simulated wellhead water analysis of physical indicators(oil content,suspended solids,median particle size),ionic stability energy evaluation and corrosion scale stability shows that the water quality is stable after treatment and will not cause secondary pollution.From the aspect of technical comparison,process operation cost and social benefits,the technical and economic comparison of the conventional chemical oxidation-dissolved air flotation process and DOCF system-inorganic carbon membrane process in the extended oilfield was conducted..The results show that the process is in operation cost,effluent quality and impact load.The ability is superior to the chemical oxidation-dissolved air flotation treatment process,the treatment cost is as low as 1.345yuan/t,and it has strong impact resistance,high treatment efficiency,stable operation and low operating treatment cost.It is worthy of further application and application to extend the water quality requirements of low-permeability and ultra-low-permeability oilfields in oilfields and improve oilfield water recovery and development. |
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