Study On Microstructure Control Of Viscoelastic Fluid Solution Based On C_nDEDAM Cationic Surfactant

To promote the application of viscoelastic surfactant (VES) in oil field processes as acidizing fracturing and enhanced oil recovery, a serious of quaternary ammonium cationic surfactants (Dimethyl-aminoethyl-long-chain alkyl lammonium chloride, CnDEDAM, n=14,16,18) were synthesized from2-Chloroethylamine hydrochloride and long-chain alkyl dimethyl amine (CnDAM) at a relatively high temperature for a few hours. Besides, the structure of the surfactant C18DEDAM was confirmed by FT-IR, MS and1HNMR tests. The synthesis was optimized and the best yield was obtained under the condition that the pH value was modulated to7, while raw material ratio (na/nb) was fixed at1.2, and temperature was fixed at70℃, and the reaction was carried on for7hours. The critical micelle concentration (CMC) values of these three surfactants (C14DEDAM, C16DEDAM, C18DEDAM) were obtained by conductivity measurements, with values fixed at2.69×10-3mol/L,7.53×10-4mol/L,1.82×10-4mol/L respectively.The experiments found out that (1) the cationic surfactant CnDEDAM (take C18DEDAM as an example) could respond to pH value change, the surfactant solution could achieve an obvious higher viscosity when pH value was relatively low, and the critical micelle concentration (CMC) decreased with the decrease of pH value;(2) the surfactant C18DEDAM could interact with counterion salts to form wormlike micelles, the mixed solutions of C18DEDAM with inorganic salts sodium chloride (NaCl) and organic salts sodium salicylate (NaSal) were studied respectively, viscosity and viscoelastic measurements revealed that the solution property of C18DEDAM-NaCl mixed system was much better than C18DEDAM-NaSal mixed system;(3) maleic acid (MA) could enhance the solution property of C18DEDAM-NaCl mixed system, conductivity measurements, rheology measurements, Dynamic Light Scattering (DLS) measurements and Scanning Electron Microscope (SEM) measurements showed that C18DEDAM-MA-NaCl mixed system is one kind of smart wormlike micelle system whose property could be switched by pH value: when the pH value was less than7, the mixed system behaved as a viscoelastic fluid (the viscoelasticity of the mixed system peaked when the pH value was fixed at5); when the pH value was above7, the viscoelasticity of the mixed system disappeared completely and behaved as a typical Newtonian fluid; The increase of NaCl concentration could induce the micromorphology of C18DEDAM-MA-NaCl mixed system to form rod-like micelles form sphere micelles, and the intersection point of G’-G" curves to appear in lower frequency region as well; C18DEDAM-MA-NaCl mixed system was milk-white solid (gel) at ambient temperature, while transparent viscoelastic fluid at higher temperature. The viscosity of the mixed system decreased with the increase of temperature, and a relatively higher one was retained even in high temperature;(4) as the carbon chain length of the quaternary ammonium cationic surfactant CnDEDAM decreased, both of the viscosity and viscoelasticity of CnDEDAM-MA-NaCl (n=14,16,18) mixed system declined significantly.