Pilot Study On Seawater Nanofiltration Softening Technology Based On Integrated Membrane System For Offshore Oilfield Polymer Solution Preparation

With the unique selective separation characteristic of divalent ions as well asmultivalent ions, seawater nanofiltration （NF） could provide permeate with excellentquality for offshore oilfield water injection, on which there have been relevantresearch and application reports. However, the industrialized application of aqueouspolymer solution prepared by NF softened seawater for polymer flooding has notbeen reported yet.In this work, a pilot scale Ultrafiltration （UF）–NF integrated membrane system（IMS） has been investigated as a method of removing divalent ions from JiaozhouBay seawater and providing softened seawater to prepare polymer solution foroffshore oilfield. Pilot tests are conducted to investigated the Quality and steady ofUF pretreatment product water; the performance of three types of NF membranemodules （4.0inch） under different operation conditions, such as operation pressure,recovery and influent quality; the performance of the NF3membrane module hasbeen investigated at high system recovery; the effect of magnetic field on NF2membrane module; the removal efficiency of NF membrane for organic matter andnutrients; the viscosity of polymer solution prepared with different NF permeate atdifferent temperature.The results indicate that seawater nanofiltration softening technology foroffshore oilfield polymer solution preparation is viable for industrial application. UFpretreatment system could continuously and steadily provide qualified filtrate for NFsystem, despite of the original seawater quality. With the properly selected NFmembrane, the operation factors and the order of NF membrane modules had littleeffect on its permeate quality, which could keeps more than90%removal rate for Ca2+and Mg2+.（1） Operation factors affected the flux and recovery of all the membranesgreatly. However, they had little effect on the permeate quality of the properlyselected NF3membrane, which could keeps more than90%removal rate for Ca²⁺and Mg2+.（2） The selected NF3membrane module could provide good quality permeationwater with very low concentration of Ca²⁺and Mg²⁺even at very high systemrecovery rate. Therefore, seawater nanofiltration softening technology for oilfieldpolymer solution preparation is viable for oilfield application.（3） Magnetic field had positive effect on NF2membrane flux and negativeeffect on its rejection of Ca2+and Mg2+, which was beneficial for NF membraneanti-scaling and the long running.（4） Excellent removal efficiency for organics of all the three NF membranes hasbeen obtained. Also, NF membranes had certain effect on the removal of nutrientsand total P, total N.（5） Viscosity of the polymer solution prepared by different NF3permeationwater kept high and met the polymer flooding. NF3membrane module is applicableand viable for application of seawater softening to prepare polymer solution foroffshore oilfield.