Experimental Investigation of the Enrichment Characteristics and Governing Patterns of Microscopic Residual Oil During Chemical Flooding

Understanding the enrichment mechanisms of residual oil in the high water-cut stage is essential to achieve highly efficient regulation of chemical flooding. By establishing a one-dimensional (1D) real-digital-core model, this study systematically investigates the regulatory mechanisms of various chemical systems on residual oil mobilization, migration, and enrichment post-waterflooding. The results show that the length of microscopic residual oil is inversely linear with the displacement capillary number. The key requirement for the spatial enrichment of residual oil is the synergistic matching between high initial residual oil content and strong displacement driving force. Single polymer flooding exhibits a syringe-like pattern, in which the pressure jump induced by the high viscosity at the displacement front overcomes capillary resistance and forms a banded enrichment zone. In contrast, single surfactant flooding can greatly reduce the interfacial tension and thus promote microscopic stripping and emulsification. However, its limited mobility control makes large-scale enrichment difficult. The surfactant–polymer (SP) binary compound flooding can reconstruct microscopically dispersed oil droplets into macroscopic oil clusters and form a highly continuous enrichment zone at the displacement front. BPPG-assisted heterogeneous flooding further establishes a synergistic mechanism of viscoelastic action and profile control, thereby achieving full-field reconfiguration of the pore-scale flow field. On this basis, a dimensionless residual-oil scale was introduced to construct a limit chart for the mobilization and enrichment of microscopic residual oil under different chemical flooding modes. This study provides a theoretical basis for the design of enhanced oil recovery schemes in complex reservoirs.