A water-injection experiment was performed on a water-wet reservoir core plug that was filled with brine first and then displaced by synthetic oil. A X-ray Computed Tomography (CT) was used to take snapshots of the process of oil-water displacement at predefined time intervals to characterize the distribution of remaining oil. The quasi-real time images were used to understand the pore-scale phase displacement mechanisms and the distributional pattern of the remaining oil. Four forms of the distributional patterns, i.e. network, porous, isolated and film shape, were observed and analyzed with respect to the injected pore volumes (PV). The results show that with the increased level of water injection, the volume of the oil phase continuously decreases, and the morphology of the oil phase changes from initial continuous network-like to film shape forms. At 15 pore volumes (PV), the network-like remaining oil disappears and transforms into isolated and film-like forms. The statistics of the volume for each form of the remaining oil show that the isolated blobs increase with increasing water injection, by contrast, the average volume of the remaining oil decreases with increasing water injection. The rate of volumetric changes is fast before 5 PV but slow in the later period.