In Situ Wettability and Pore/Fluid Occupancies during Low-Salinity Surfactant Flooding in Oil-Wet Carbonates

The combination of low-salinity brine injection and surfactant flooding was shown to significantly enhance oil production in oil-wet systems, yet the direct pore-scale transport mechanisms responsible for the additional recovery are still not well understood. In this study, we employ a micro-CT scanner to directly probe the pore-scale fluid configurations, local displacement patterns, and in situ contact angles under reservoir conditions. Specifically, we investigate the flow of different wetting fluid systems through a set of dynamically aged miniature limestone core samples after addition of the surfactant and alteration of salt concentrations in the formulated brine solutions. Characterization of local wettability reveals an accelerated reversal from oil-wet toward neutral-wet for the low-salinity surfactant injection compared to that of either low-salinity waterflooding or surfactant flooding alone. The in situ contact angles change significantly with the injection of even small pore volumes of low-salinity surfactant solution, and the effect is even more profound when the injection follows a low-salinity waterflood. The main mechanism responsible for the observed oil recovery enhancement appears to be the invasion of the brine front into smaller pores and corners of the pore elements, which is facilitated by the rapid wettability reversal induced by the low-salinity surfactant injection.