Improvement of stability of nano-SiO₂/HPAM/SDS dispersion systems and its effect on oil displacement performances

The turbidity and Zeta potential experiments of nano-SiO₂/HPAM/SDS dispersion systems at 60 ℃, 1.0×10⁴ mg/L NaCl brine and simulated formation water were first investigated and the results indicated that Ca²⁺ and Mg²⁺ ions were responsible for the instability of dispersion system. Then, the methods of reducing pH value and adding chelating agents were used to improve the stability of nano-SiO₂/HPAM/SDS dispersion system in simulated formation water, and the improvement effects and mechanisms were discussed based on sedimentation experiments and Zetasizer. The influences of these two methods on the oil displacement performances of dispersion system were analyzed by rheometer and interfacial tensiometer. The experimental results suggested that with the decrease of pH value, the stability of dispersion system was enhanced by the protection of H⁺ in the SiO₂ double electric layer and the enhancement of hydration forces between particles although the absolute value of Zeta potential(|ζ|) of dispersion system decreased. All of the chelating agents including Na₂EDTA, ATMP and Na₄EDTA can improve the stability of dispersion system. Ca²⁺ and Mg²⁺ ions were complexed and pH value of dispersion system was decreased with the addition of Na₂EDTA and ATMP, however, the decrease of pH value resulted in a sharp deterioration in the viscosity of dispersion systems. With the addition of Na₄EDTA, the pH value of dispersion systems increased. The|ζ|, viscosity, storage modulus and loss modulus of dispersion systems all increased, which were benefited from the formation of stable complexes between Na₄EDTA, and Ca²⁺ and Mg²⁺. Meanwhile, the ability of dispersion system to reduce oil and water interfacial tension was enhanced. Thus, after addition of Na₄EDTA with mass ratio of 0.4% (optimal mass ratio)in dispersion system with nano-SiO₂ mass ratio of 0.5%, the oil recovery was increased by 3.1%.