Mobilization of Oil through In Situ Emulsification in a Porous Medium: A Comparative Analysis of Oil Solubilization and Interfacial Tension Reduction

The addition of surfactants into the aqueous phase is acknowledged as a primary mechanism for the reduction of the interfacial tension (IFT) between oil and water, providing oil mobilization within confined conditions. Certain surfactants exhibiting ultralow interfacial tension can solubilize oil, thereby generating microemulsions, whereas other surfactants are limited to the formation of thermodynamically unstable emulsions. There is currently no evidence to determine which factor holds greater significance for oil mobilization within porous media where oil and water are present simultaneously. Two surfactants, each exhibiting ultralow interfacial tension, were systematically screened and assessed through a series of experiments, resulting in the formation of middle phase microemulsions and emulsions. The investigation focused on their performance in oil mobilization prior to and following water flooding, utilizing micromodel and core displacement experiments. It was found that cetyltrimethylammonium bromide (CTAB) with <i>n</i>-butanol featured good oil solubilization capability and could form microemulsions with a wide carbon number distribution of alkanes. Nonylphenol polyoxyethylene ether carboxylate (SS-231) featured the ultralow IFT with simulated oil and could form dense emulsions. The pore-scale and core-scale oil displacement experiments both showed that the CTAB with <i>n</i>-butanol could get a higher ultimate oil recovery than that of SS-231 without prewater flooding, while the results became contrary if the surfactants were injected after water flooding. Our work found that the mechanism of oil solubilization worked well when there was only an oil phase in the porous medium, and the emulsion formation became more important for oil mobilization at the high-water cut stage. The results indicated that the surfactant featured with good emulsification capability was preferred at the high-water cut stage while surfactants featured with high oil-water solubilization capacity could be good candidates if the remaining oil saturation was high in the reservoir.