Synthesis and performance evaluation of betaine type zwitterionic surfactants containing different degrees of ethoxylation

A large fraction of the world's remaining oil occurs in high salinity/high temperature reservoirs. The design of surfactants for such harsh reservoir is challenging owing to poor thermal stability and solubility in reservoir brine. The selection of the wrong surfactant can lead to low production and damage the reservoir because of rock dissolution which ultimately blocks the pore space. Previous studies indicate that sulfobetaine-based zwitterionic surfactants exhibited excellent surface and thermal properties; however, they become insoluble in high salinity brine particularly with a long hydrophobic tail. In this work, we tuned the chemical structure of the sulfobetaine-based zwitterionic surfactants by incorporating ethoxy units within the framework of the surfactant to improve its water solubility in presence of salts (213,734 ppm). The surfactants were synthesized by reacting 3-(dimethylamino)-1-propylamine with glycolic acid ethoxylate lauryl ether followed by treatment with 1,3-propanesultone. The chemical structures of the products were confirmed by FT-IR and NMR spectroscopy. The solubility tests revealed good solubility of the synthesized surfactants in formation water (213,734 ppm), seawater (57,643 ppm), and deionized water. Thermogravimetric analysis data exhibited excellent heat stability of the surfactants and the major degradation in the surfactants structure was observed after 280 °C which is higher than the real temperature of the reservoirs (≥90 °C). The critical micelle concentration values of both surfactants in formation water at 60 °C were 1.53 × 10⁻⁴ mmol/L and 1.10 × 10⁻⁴ mmol/L. The related surface tension (γ_cmc) readings were 27.65 mN/m and 29.26 mN/m respectively. Betaine type zwitterionic surfactants containing different degrees of ethoxylation showed excellent heat stability and surface properties which mark them a suitable material for high temperature and high salinity reservoirs.