Research on the Microscopic Residual Oil Activation Mechanism of Heavy Oil in Different Water Content Stages

This paper investigates the occurrence characteristics, mobilization behavior, and controlling mechanisms of microscopic remaining oil in heavy-oil systems at different water-cut stages by using a visual microfluidic platform. A series of displacement and pressure-ramping experiments were conducted at 65 °C on homogeneous and heterogeneous chips with different permeabilities and oil viscosities. Image-based saturation processing and oil-phase area recognition were further employed to quantify remaining-oil morphology, area fractions, and unit threshold pressure. Results show that, with increasing water saturation, the continuity of the oil phase deteriorates progressively, and the remaining oil evolves from cluster oil to elongated cluster oil and spot and corner-trapped oil. Accordingly, the threshold pressure increases nonlinearly, especially at high-water-cut stages. For the same water saturation, higher permeability leads to lower threshold pressure because of larger pore-throat radii and better connectivity, whereas higher viscosity raises the threshold pressure due to stronger viscous resistance. Compared with homogeneous chips, heterogeneous chips exhibit higher threshold pressure because local pore-throat bottlenecks dominate capillary resistance. The image-recognition analysis indicates that the areal fraction of cluster oil decreases continuously, while that of the discontinuous oil phase, comprising elongated cluster oil and spot and corner-trapped oil, generally increases. The unit threshold pressure of cluster oil rises monotonically, whereas that of the discontinuous oil phase first decreases and then increases, reflecting the combined effects of oil fragmentation, migration, and trapping during water flooding.