Investigation of Anionic-Nonionic Surfactants for EOR in Carbonate Reservoirs

In this work, four linear alkyl ether carboxylates (AECs) C₁₁E₅A, C₁₁E₁₁A, C₁₂E₄A, and C₁₂E₇A were examined as agents for surfactant EOR. Their main properties that have a key effect on surfactants' performance during flooding were investigated: stability, interfacial tension (IFT), wetting ability, and spontaneous imbibition in limestone samples. The thermal stability and salinity tolerance was correlated with the molecular structure of AECs. IFT on the boundary with n-decane and crude oil was evaluated with the spinning drop tensiometry method at five temperatures in the range between 25°C and 70°C and various salinities. The wettability was studied through contact angle measurements of deionized water on the limestone plate surface. The core plates were aged in oil for 14 days to obtain the hydrophobic carbonate surface and then treated with surfactant compositions for 48 hours at 70°C. The wettability alteration mechanism was assessed through the Rock-Eval pyrolysis method, which was not previously applied for this purpose. The experiments showed that ethylene oxide (EO) chain length influences the stability of AECs. It was found that the temperature rise substantially impacts a surfactant with a longer EO chain C₁₁E₁₁A. In contrast, brine salinity significantly affects the interfacial behavior of AEC with a shorter EO chain. C₁₂E₇A strongly hydrophilizes the surface, and the contact angle decreases from 110° to 15÷20°. The Rock-Eval pyrolysis analysis indicated that C₁₂E₇A has a better wetting ability due to a combination of "cleaning" (washing of hydrocarbons from the core sample surface) and "coating" (adsorption of surfactant molecules on cleaned spaces) mechanisms.