Study On Factors Influencing Viscosity Of Polymer Solution Prepared By IMS Softened Seawater And The Pilot Research

Polymer flooding is one of important tertiary oil recovery technologies, and theapplication of this technology can effectively improve oil recovery. Polymer floodingtechnology requires plenty of fresh water; however, fresh water is scarce in offshoreoilfield. Seawater has high salinity and hardness, so it can not be used for preparingpolymer solution. Therefore, it is critical to find suitable softened water resource forpolymer preparation and to improve efficiency of oil displacement in offshoreoilfield.In this paper, in order to meet the requirements of polymer flooding in offshoreoilfield, three kinds of polyacrylamide with different molecular weight termed asHPAM-A, HPAM–B and HPAM–C, respectively, are used to study the mainfactors influencing viscosity of the polymer solution, including polymer solutionconcentration, temperature, stirring speed, shearing rate, salinity, cationic, pH valueand dissolved oxygen content. In order to lessen the shortage of fresh water inoffshore oilfield, a pilot scale Ultrafiltration (UF)–Nanofiltration (NF) integratedmembrane system (IMS) are used to treat Jiaozhou Bay seawater, and the softenedseawater is used for preparation of polymer solution.The most optimum softenedseawater is selected for pilot polymer flooding preparation.The results show that the viscosity of HPAM solution increases with the HPAMconcentration, so the appropriate HPAM is selected in consideration of the oilrecovery efficiency and economic benefit. The viscosity of the polymer solution isinfluenced by dissolving mixing speed and mechanical shearing, because HPAM hasa structure of polymer chain. And the viscosity of HPAM solution reduces30%whenthe temperature rises from20℃to60℃.HPAM is anionic, so the cationic ions in water could have great influence on the viscosity of HPAM solution. The viscosity of the HPAM solution reduces with theincrease of TDS and divalent cation concentration. When the concentration oftrivalent cation increases to a certain value, the polymer solution could formflocculation. Because Fe2+has reducibility, so it has serious influence on HPAM withthe existence of oxygen. The viscosity of HPAM solution is also influenced by pHvalue, when pH is between7~9, HPAM solution has the maximum viscosity.Dissolved oxygen has oxidative degradation effect on HPAM, so the deoxygenationby nitrogen could removal86%dissolved oxygen and the content of dissolvedoxygen could be reduced to about1mg·L-1, meanwhile, the initial viscosity of HPAMsolution increases by50%and the viscosity retention rate can keep80%through tendays aging at65℃.Four kinds of softened seawater (terms as NF1, NF2, NF3and NF4, respectively)produced by an integrated membrane system (IMS) are used to prepare HPAMsolution. The results show that: the NF1and NF4have the shortest polymerdissolving time, both of which are about sixty minutes and the viscosity of NF1andNF4polymer solution are also higher than those of NF3and NF2; when the polymerconcentration of NF1and NF4solution are1500mg·L-1, the viscosity is greater than30mPa·s, so it can satisfy the viscosity requirements of polymer flooding; thepolymer solution prepared by softened seawater has good viscosity-temperatureproperty, shear thinning property, anti-shear property and thermal stability; thepolymer solution prepared by NF1, NF4have good compatibility with core, and theresistance coefficient of NF1and NF4are7.46and8.0, the residual resistancecoefficient of NF1and NF4are2.32and2.58.The pilot research of polymer solution preparation by softened seawater showthat: the pilot dissolution time of HPAM-C in NF4softened seawater is about90min;the viscosity of polymer solution is30.4mPa·s at1500mg·L-1, which could meet theviscosity requirement of polymer flooding; the polymer solution prepared by the pilotsoftened seawater has good injection performance, and the resistance coefficient andresidual resistance coefficient evaluated by core test are6.98and2.67, respectively.The above results provide a certain qualified reference for increasing the polymer solution viscosity for oilfield flooding and the large scale application ofintegrated membrane seawater softening system in offshore oilfield.