Pore-Scale Characterization of Remaining Oil Evolution During Waterflooding in Offshore Sandstone Reservoirs Using Micro-CT and U-Net Semantic Segmentation

After offshore sandstone reservoirs enter the high-water-cut development stage, remaining oil gradually changes from a continuously movable state to a dispersed, isolated, and locally trapped state. Therefore, pore-scale dynamic characterization of remaining oil is important for clarifying its occurrence state and identifying potential targets for further recovery. In this study, natural cores from a typical offshore continental sandstone reservoir were used to conduct micro-computed tomography (micro-CT) scanning experiments at different waterflooding stages. Fine identification of the rock skeleton, water phase, and remaining oil, as well as segmentation of remaining-oil units, was achieved using U-Net semantic segmentation and three-dimensional digital image reconstruction. Multiple parameters were extracted, including volume, surface area, equivalent diameter, major-axis length, intermediate-axis width, minor-axis thickness, shape factor, and Euler number. Combined with manually labeled results of typical remaining-oil unit, a decision-tree method was used to establish classification criteria for remaining-oil occurrence states. The results showed that, with increasing pore volumes injected (PV), remaining oil gradually evolved from an initially large-scale and continuously connected state to a small-scale dispersed trapping. The surface area and volume of remaining oil continuously decreased. Meanwhile, the shape factor decreased, and the Euler number increased, indicating that the continuous oil phase was continuously cut and stripped into remaining-oil units with lower topological connectivity during waterflooding. At the early stage of waterflooding, clustered flow dominated in cores with different permeabilities, accounting for 96–99% of the remaining-oil volume percentage. Under high pore volumes injected (PV), the remaining oil volume percentage decreased to 30–65%, whereas the remaining oil volume percentage of columnar flow and droplet flow increased to 16–37% and 5–13%, respectively. These results indicated that waterflooding promoted the transformation of remaining oil from large-scale connected occurrence to locally trapped states under pore–throat constraint. Further comparison of cores with similar permeability but different pore structures showed that differences in pore–throat geometry, local connectivity, and pore–throat matching still significantly affected the fragmentation degree and spatial trapping mode of remaining oil. This study established a pore-scale method for dynamic identification, quantitative characterization, and type classification of microscopic remaining oil during long-term waterflooding in offshore sandstone reservoirs. The results provide a pore-scale basis for fine remaining-oil potential tapping in the high-water-cut stage.