The Study On Gelation, Rheological Behavior And Dynamic Properties Of HPAM/Resole Cross-linking System

Partially hydrolyzed polyacrylamide (HPAM) is one of the most widely usedwater soluble polymers, which are now finding widespread use for near well-borewater and gas shut-off in the oil industry. The biggest difficulties for HPAM appliedin oil field are the poor heat stability and salt tolerance. The organic cross-linkingsystem of HPAM/resole becomes one of the most having used potential polymerflooding, for the good aging property due to the introduction of benzene ring. Thestudies on gelation, rheological behavior and dynamic properties of HPAM/Resolecross-linking system are very important to understand and improve the in situapplication of the gels technology and the development of cross-linked polymersolution theories.The cross-linked polymer solutions can be prepared by two methods. One typeis synthesized from polymer monomers by adding cross-linking agent. The othertype is prepared by a polymer solution with cross-linker. Because of the hugemacromolecular weight and so on factors, there are few resources to study the lasttype cross-linked polymer solution.In order to choose the best cross-linker a few organic cross-linkers werecompared by the microgel properties of HPAM/cross-linker. Then the chosencross-linker was reacted with HPAM. The recipe of cross-linking system was determined by the viscosity enhanced ability of cross-linker and the aging propertyunder hard environment of microgel. The industrial product of the chosencross-linker was successfully put in practice. The industrial and experimentalproducts were studied through experiments and tests. Results show that there is littledifference between them, and the industrial product totally satisfied the userequirement.The rheological behaviors of HPAM microgel with or without salt were studiedby a Haake rheometer RheoStress RS150 in the linear and non-linear rheologicalregime. And the gelation mechanism was deeply discussed. The gelation process ofa dilute solution, lasting a long time, can be divided in three steps: induction period,acceleration period and termination period. In the first stage the intra-molecularcross-links are formed which causes a small decrease of viscosity. However, there isa large rise in viscosity due to the formation of intermolecular cross-links in theacceleration period and the cross-linked three-dimensional network is established inthis stage. The third period extends over the longest time. The chemicallycross-linked network is strengthened in this stage. The heterogeneities caused by thedifference in connection between the intra and intermolecular cross-links, results ina shear thickening effect of dilute samples during the gelation process. After thegelation process the HPAM microgel become aging no mater with or without saltunder the reacting temperature. This is due to the hydrolysis of HPAM and thecontraction of cross-linked network.The viscosity of the obtained HPAM microgel will increase with theconcentration increase of either HPAM or the cross-linker. For the HPAM microgelwithout salt, the shear thickening effect will disappear with the increase ofconcentration. The HPAM microgel has larger viscosity at higher temperature by theanalysis of the flow curves of obtained at different temperatures.Dynamic rheological experiments illustrate the cross-linked hydrogel exhibits abehavior approaching the ideally chemically cross-linked network. For the HPAMsolution, G′becomes larger than G″when the frequency exceeds a certain value, which demonstrates the importance of the elastic part. However, for the formed gelin the accessible test frequency range, G′is always larger than G″and the crosspoint G_c where G″=G′can not be observed.The dynamic properties of the HPAM solution and the microgel were studiedby dynamic light scattering, and the influences of temperature and the light vectorwere also discussed, The intensity-time autocorrelation function illustrated thatboth the HPAM solution and the microgel have polydispersity and structuralinhomogeneity. The hydrodynamic diameter of the HPAM increase with theincrease of reacting time, and the diameter size has scaling behavior. The networkproportion of structure increases as the cross-linking reaction proceeds.