Nanofluids composed of liquid suspensions of nanoparticles may soon permit accelerated recovery of hydrocarbons from oil and gas reservoirs. Here, we present a series of flooding experiments at different capillary numbers to quantify the performance of a polymeric nanofluid compared to brine using the sintered glass-beads. A high resolution X-ray microtomography (micro-CT) was used to visualize oil and brine distribution in a sintered bead pack before and after nanofluid flooding. The results of flooding experiments showed that an additional oil recovery of approximately 15% is possible with nanofluids compared to brine at low capillary numbers and is as effective as high capillary number brine flooding. Nanofluid induced additional oil recovery decreases as we increase the capillary number, and the total oil recovered shows a marginal decrease. At first glance, these results are opposite of what one expects in the conventional EOR, where oil recovery is known to increase progressively with increasing capillary number. These results cannot be explained based on mobilization theories due to the reduced capillarity. Our results however are consistent with the mechanism of wettability alteration caused by structural disjoining pressure leading to the formation of the wetting nanofluid film between oil and substrate. Unlike brine displacement, X-ray micro-CT images show that mobilization of oil from the porous medium by a nanofluid is fairly uniform even at low capillary numbers and large-scale clusters of oil are absent. Our findings in this paper are expected to promote the understanding of EOR by nanofluids.