Emulsification Dynamics and Unlocking Methods for Residual Oil in Porous Media

Water injection has been widely used and studied worldwide as one of the most popular methods for oil recovery. However, the injected water is basically invalid once an oil reservoir enters the middle- and high-water-cut period, resulting in a remarkable waste of water resources and increasing the costs of produced water treatment. In this study, the flow characteristics of oil and water and the pore-scale distribution characteristics of remaining oil in the high-water-cut stage of a reservoir are studied. The characteristics of unlocking remaining oil through adjusting the injection–production strategy and the injection fluid are clarified. During this process, the enhanced oil recovery (EOR) mechanisms of chemical agents were demonstrated in detail by combining them with the characteristics of emulsifications and wettability. The remaining oil is present in two states: adhering to the pore surface and remaining in unswept pores. Increasing the water injection rate can expand the main flow channels, cyclic injection can disrupt the main flow channels, and injection–production conversion can reconstruct the main flow channels. For chemical flooding, there are three main ways to unlock the remaining oil: (1) stripping and emulsifying the adhered oil film; (2) emulsifying the oil layer by layer on both sides of the main flow channels; and (3) cutting and emulsifying the unswept oil. The results of this study can provide theoretical guidance for the rational utilization of water resources during oil reservoir development and the design of efficient utilization schemes for the remaining oil in oilfields.