Effect of the number of ethylene oxide units on the properties of synthesized tailor-made cationic gemini surfactants for oilfield applications

The solubility and heat stability of the surfactants are the most prominent features required for oilfield applications. Commercial surfactants such as petroleum sulfonate, polyoxyethylene ether, and alkyl benzyl sulfonate suffer because of stability issues under high salinity and high-temperature conditions. Herein, two new cationic amido-amine type poly(ethylene oxide) gemini surfactants containing lower and higher degree of ethoxylation were synthesized. It was observed that the addition of a sufficient number of ethylene oxide groups between the hydrophobic tail and the ionic head group of the cationic gemini surfactant significantly enhanced their solubility. The surfactant containing a higher degree of ethoxylation was found to be soluble in formation water and seawater. However, the surfactant with a lower degree of ethoxylation precipitated in formation water and seawater and hence was not studied further. Thermogravimetric analysis results of the surfactants containing a higher degree of ethoxylation exhibited degradation temperature above 300 °C that was greater than the typical reservoir temperature (90 °C). The structure characterization results of the aged sample of surfactant at 90 °C for 90 days demonstrated the survival of the original chemical structure, revealing a high long-time heat stability. The reduction in critical micelle concentration (CMC) was observed upon enhancing salt concentration, and the minimum CMC was noticed in seawater (3.91 × 10⁻⁵ mol/L). Rheological investigations revealed that the viscosity, as well as storage modulus, was reduced upon increasing the concentration of surfactant. The surfactant also exhibited superior foaming properties compared to the commercial surfactants. The cationic amido-amine type poly(ethylene oxide) gemini surfactants showed excellent surface and thermal properties and are potential candidates for oilfield applications such as enhanced oil recovery in harsh reservoir conditions.