Study Of Salt Tolerance Mechanism And Synthesis Of Amphoteric Polyacrylamides

In recent years,more and more produced water has been produced in oil fields.Using the produced water economically and environmentally is an important target in the oilfield production.As an important reservoir stimulate technology,hydraulic fracturing has been widely used in the oilfield which will cost a large amount of water.The produced water can be effectively utilized as a work water for fracturing fluids.However,the composition of the produced water is complex and the salinity of it is high.which puts high requirements on the salt tolerance of the fracturing fluid thickeners.It is necessary to make the mechanism of the interaction between ions and thickeners in high-salinity water clear,for showing guidance for the development of thickeners which are suitable for oilfield applications.The paper designs and synthesizes an amphoteric polyacrylamide with certain degree of salt tolerance named PAAD through research.The properties of the sample and its interaction rules to calcium and sodium ions are studied by the solubility test,viscosity test,rheological test,particle size test,zeta potential test,SEM test,AFM test,etc.Results show that the addition of salt will first increase the viscosity of the thickener solution and then reduce it.But when the salt concentration is high enough,the viscosity of the solution will tend to be stable and stay a fixed value.There are four main stages of the interaction between salts and PAAD:The viscosity rise phase,the viscosity rapid decline phase,the viscosity transient stabilization phase,and the viscosity slow decline phase.The critical points appear at the cationic charge molar concentration of salt about 0.2×10-2 mol/L,0.9×10-2 mol/L and 1.8×10-2 mol/L,respectively.The increase of monomer-2 content can help to improve the salt tolerance of the sample.PAAD is more tolerant to sodium salts than to calcium salts.It is proved that the salt tolerance of the sample is limited by the molecular chain electrical properties and the dispersibility of the polymer molecules.To get polymer which can tolerant salt higher,this paper replaces the cationic monomer with monomer-4 which has similar properties to the monomer-3 and armed with a long alkyl chain.The long alkyl chain can separate the polymer molecular chains,hinder the agglomeration of molecules and enhance the dispersibility of samples which expose more sulfonic acid groups,extending the second stage of the sample and salt action.Besides,PAAM,the polymerization product of monomer-4 can exhibit hydrophobic association properties in aqueous solution which further improves salt tolerance.The polymer is characterized and tested.In comparison with the performance of PAAD,it is found that the dispersibility of the sample is optimized,salt resistance and other performances are also improved.The salt-tolerant mechanism of amphoteric polyacrylamide is analyzed by analyzing the previous research data.The results show that anti-polyelectrolyte effect and electrostatic shielding effect appear when the copolymer is mixed with calcium chloride or sodium chloride in the solution.the former effect will increase the viscosity of the solution,while the latter will greatly reduce the viscosity when it exceeds a certain critical value.Calcium ions act as a cross-linking agent to the polymer,which make PAAD and PAAM more tolerant to sodium chloride than to calcium chloride.AM15'-0.5,the performances of the optimal amphoteric polyacrylamide thickener sample is determined according to the water-based fracturing fluid performance evaluation standard.The results show that AM15'-0.5 in pure water and high concentration brine have wonderful rheology property.sand carrying property.compatibility and gel breaking performance.