Dynamics of Surfactant Imbibition in Unconventional Reservoir Cores

Despite the promising results observed from the utilization of interfacial-active additives in enhancing imbibition-based oil recovery from tight reservoirs, the predominant mechanisms governing this process remain inadequately understood. A meticulously designed workflow was implemented to conduct experimental and modeling studies focusing on imbibition tests performed on tight cores utilizing surfactant and microemulsion. The primary objective of this research was to investigate the response of oil recovery to these additives and to develop a robust and reliable model that incorporates the intricate interactions, thereby elucidating the underlying mechanisms. We systematically designed and prepared two imbibition fluids, namely surfactant (AES) and microemulsion (mE), while utilizing brine as a reference fluid. A comprehensive investigation was conducted to analyze the physicochemical properties of these fluids, encompassing phase behavior, density, viscosity, and wettability alteration, with the aim of establishing fundamental knowledge in the field. Imbibition tests were carried out on oil-wet cores to observe the response of oil production and optimize the experimental methodology. Subsequently, we proposed a numerical model that fully coupled the evolution of relative permeability and capillary pressure with the dynamic processes of emulsification, solubilization, and molecular diffusion. All tested fluids exhibited favorable density (1.05-1.07 g/cm³) and viscosity (1.0 cp) at the reservoir temperature of 44 °C. AES effectively reduced the oil-water interfacial tension (IFT) to 10^-1 mN/m, while mE achieved an ultralow IFT of 10^-3 mN/m, accompanied by strong emulsification capability and a high solubilization ratio. Both solutions demonstrated the ability to alter the wettability of the rock surface from oil-wet to water-wet, albeit through different mechanisms (adsorption for AES and solubilization for mE). In line with the IFT and phase behavior experiments, imbibition tests on cores revealed that aqueous solutions with interfacial-active additives resulted in significantly higher oil recovery compared to pure water. Notably, the core treated with mE exhibited the highest oil recovery, reaching 36.5% of the original oil in place (OOIP). To further elucidate the observed effects, a modeling study was conducted, considering the aforementioned mechanisms. The results demonstrated the crucial role of emulsification/solubilization in the imbibition process.