Research On The Treatment Efficiency Of Extra-low Permeability Oilfield Produced Water By Demulsification-recirculated MBR

Petroleum is facing with declining reserves as a result of increasing demand.Extra-low permeability oilfield exploitation has been a primary developingdirection. Because of low porosity and permeability, extra-low permeability oilfieldis prone to form rock stratum locking damage during water flooding. Therefore,higher quality injection water is demanded. The existing produced water disposalprocesses are difficult to meet the standard of extra-low permeability oilfieldreinjection water quality. Based on Daqing Oilfield exploitation correlated technicalproblems, the article aims to develop a highly efficient and stable produced waterdisposal process for extra-low permeability oilfield and investigate thecorresponding mechanism.The demulsification–recirculated MBR (Membrane Bioreactor) process isdeveloped by analyzing the produced water quality of extra-low permeabilityoilfield. The influencing factors of demulsification have been investigated and themain mechanism of demulsification is confirmed. According to the results, newflocculant is developed and its operating conditions are optimized. Operatingcondition parameters of recirculated MBR are obtained and relevant kineticsresearch is carried out. The impacts of surfactant and flocculant on quality ofproduced water and membrane fouling mechanism are investigated. Finally, thispaper conducts the technical and economical analysis on the demulsification–recirculated MBR. The main results are as follows:Inorganic salts are believed to be the principal influence factor ondemulsification. Inorganic salts demulsification process is simulated based onexperimental results and the main mechanism of demulsification is confirmed:inorganic salts interact with oil bead surface charges as well as surfactants inproduced water after ionization, thus Zeta electric potential changes. When chargeneutralization plays a leading role, Zeta electric potential decreases and electricdouble layer is compressed, thus emulsion is destroyed. Moreover, demulsification-flocculation kinetics equation of produced water in extra-low permeability oilfieldis deduced.Flocculant HXN, with the main components of poly-silicate aluminum sulphate (PSAS), Ca2+and a trace of polyacrylamide, has been developed and itsoperating condition has been optimized without regulating temperature or pH value:reagent concentration is4~6mg/L, stirring intensity is60~100r/min, stirring timeis12~25min, settling time is35min. Field test shows good treatment efficiencies:the average oil content is7.8mg/L, average SS is51.4mg/L.Through investigating the variations of oil content, COD and suspended solidcontent and average diameter, the system optimum operating condition is obtained:the hydraulic retention time is6h, sludge retention time is25d, sludgeconcentration is3800~4300mg/L, aeration strength (measured by DO) is3mg/L;feed water pressure of membrane module is about0.07~0.12MPa, outlet pressure ofconcentrated liquid is about0.06~0.08MPa. Furthermore, relevant kinetics equationhas been established by studying the organic matter degradation and biomassaccumulation in recirculated MBR.Surfactant and flocculant have impacts on treatment efficiency of producedwater and membrane fouling in recirculated MBR. With regard to the low surfactantcontent, the removal of oil and COD is influenced slightly while it has both positiveand negative impacts on membrane fouling, thus surfactant can be neglected duringrecirculated MBR process. Flocculant contributes to the removal of COD, oil andsuspended solid. Its principal contribution may be the removal of colloidalsubstance and emulsified oil which are non-biodegradable. This improves thestability of process removal efficiencies greatly and reduces the load of membranemodule simultaneously. After the addition of flocculant, the sludge floc sizeincreases and SMP (soluble microbial products) decreases, which reduce themembrane pore blocking and membrane fouling to some extent. The pollutantsremoval efficiency is increased after the addition of PAC with20~30mg/L in thebioreactor.The extra-low permeability oilfield produced water has been treated bydemulsification–recirculated MBR process and the shock load test has been carriedout. The result shows that effluent water quality meets the produced waterreinjection index of extra-low permeability oilfield: oil content is2.04~4.11mg/L,SS is0.36~0.97mg/L, average particle size is0.7μm, no sulfate-reducing bacteriaare detected, iron bacteria and saprophytic bacteria contents are below30cfu/mL.Operation cost evaluation illuminates that treating produced water will cost 5.97RMB/m3. Thus it can be seen that demulsification–recirculated MBR process isan efficient and economical treatment method for extra-low permeability oilfieldproduced water.Finally, this paper compares the traditional physicochemical process in DaqingOilfield to demulsification–recirculated MBR process on the basis of processoperation, investment cost, operating cost and maintenance both technically andeconomically. In general, demulsification–recirculated MBR process is superior tophysicochemical process on operating cost, removal efficiency and shock loadcapacity.