Experimental evaluation of non-ionic mixed surfactant formulations at high-temperature and high-salinity conditions

Non-ionic alkyl polyglucoside (APG) surfactants have been considered as eco-friendly, nontoxic and biodegradable surfactants. This study investigated the feasibility and advantages of new formulations for mixed surfactants consisting of a commerical APG product and one of the following alkyl amines (AAs) – dodecylamine (C12A), tetradecylamine (C14A), hexadecylamine (C16A), and octadecylamine (C18A) for improved oil production applications in carbonate reservoirs under high-temperature and high-salinity (HTHS) conditions. The physicochemical properties including compatibility, surface tension, interfacial tension (IFT), static adsorption, wettability alteration of the solutions comprising APG and various AA under HTHS conditions were evaluated. The self-assemble behavior of molecules between APG and C12A was characterized based on the Scanning Electron Microscopy (SEM) analysis. The formulations with an ultra-low IFT were introduced and the mechanisms resulting in such ultra-low IFT were further studied and discussed. The yielded APG/AA formulation was injected as the chemical flooding agent into a micromodel to investigate the efficiency of oil displacement. Commercial APG products exhibited excellent compatibility with the simulated injection water at reservoir temperature. After mixed with AA, the APG solution became hazy or phase separated above a critical concentration (Ccrit) of AA. Ccrit was found to decrease with an increase of the hydrocarbon chain, suggesting that more APG surfactants are required to solubilize the AA with longer alkyl hydrocarbon chain. In the presence of AA, the critical micelle concentration (CMC) of APG surfactant significantly reduced due to the formation of a compact micellar structure. APG yielded a low IFT at 90 °C and a concentration of 2 g/L in the order of 10−2 mN/m, while an ultra-low IFT in the order of 10−3 mN/m was obtained upon addition of a small amount (0.3–0.5 g/L) of C12A. SEM images and proposed IFT reduction mechanisms indicated that APG and C12A synergistically generated a more compact structure via hydrophobic interaction between the hydrophobic moieties and hydrogen bonding between the hydrophilic parts of the chemicals comparing the aggregated structure formed by APG alone, such effect could eventually lead to a decrease in IFT between oil and water. The amounts of chemical adsorption and contact angle of an oil droplet on carbonate cores were not affected upon addition of AA. The micromodel test showed t