The in situ saponification of crude oil lowers the interfacial tensions of oil–water systems in chemical-enhanced oil recovery (cEOR). When the interfacial tension is sufficiently low, emulsion phases occur, and the fluid phase distribution in the pore space changes, affecting oil displacement efficiency. This change in capillarity allows the injection water to penetrate different classes of pores, which must be reflected in the distribution of fluids in the pore space. This issue still needs to be adequately researched and is not reflected in the classic analyses of oil cluster size. We have performed μCT-based coreflood experiments to study the displacement efficiency of crude oil by alkaline solutions acting as cEOR agents for in situ saponification. The measured fluid distributions were analyzed using oil-cluster size statistics. In addition, the pore classes from which the oil was preferentially displaced and the degree of displacement as a function of alkali concentration were determined. Despite ultralow interfacial tension and emulsion formation, a clear separation of the oil and aqueous phases was observed over a wide range of alkali concentrations, allowing a two-phase analysis of the microscopic fluid distribution. It was found that (a) a Lorentz analysis of the cluster volume distribution is useful to characterize the cEOR displacement efficiency. It shows the tendency of oil clusters toward a more uniform size distribution toward optimal alkali concentration. (b) the additional oil recovery toward optimal displacement conditions comes from the fraction of smaller pores; at optimal conditions, all pore classes are flooded almost equally and to a high degree, equivalent to miscible displacement, i.e., extremely low interfacial tension.
