X-ray MCT-Based Quantitative Characterization of the Microscopic Remaining Oil Occurrence State under Different Displacement Systems

To reveal the variation law of the microscopic remaining oil occurrence state under different displacement systems, this study carried out displacement simulation experiments using a micrometer CT scanning equipment. The experiments included continuous water flooding, water flooding converted to CO2 flooding, and water flooding converted to polymer flooding. The occurrence state of microscopic remaining oil was quantitatively characterized at different displacement stages under different displacement systems. The results indicated that water flooding converted to CO2 flooding and water flooding converted to polymer flooding could significantly improve the sweep efficiency, and the recovery rate increased by 13.6 and 5%, respectively, compared with continuous water flooding. Under the three displacement systems, with the progress of displacement, the average volume of remaining oil decreases continuously, the total number of remaining oil blocks increases continuously, the distribution of remaining oil becomes more and more dispersed, and the contact area ratio of remaining oil decreases continuously. Compared with continuous water flooding, the decrease degree of average volume of remaining oil, the increase degree of total number of remaining oil, and the contact area ratio reduction in water flooding converted to CO2 flooding and water flooding converted to polymer flooding change obviously, but the change in the degree of water flooding converted to CO2 flooding is the largest. During the displacement process, the cluster remaining oil is gradually converted to other types of remaining oil. The proportion of cluster remaining oil decreases, and the proportion of other types of remaining oil increases. The remaining oil in the residual oil state is mainly cluster, and other types of remaining oil coexist. The remaining oil occurrence types are highly dispersed in water flooding converted to CO2 flooding and water flooding converted to polymer flooding.