Controlling Effect of Heterogeneity in High-Permeability Reservoirs on Waterflood Sweep Characteristics and Remaining-Oil Distribution

High-permeability reservoirs at the extra-high water-cut stage commonly exhibit preferential flow, limited sweep expansion, and complex remaining-oil occurrence. To clarify the pore-scale mechanisms controlling waterflood sweep and remaining-oil retention, this study integrates CT-assisted core flooding and microfluidic chip visualization using a high-permeability sandstone core from the Guantao Formation in the Bohai Bay Basin. The CT-assisted core flooding experiment was used to quantify the stage-wise evolution of pores swept by the water phase, while the microfluidic experiment was used to visualize displacement pathways, local bypassing, and remaining-oil morphology under controlled pore-network conditions. The results show that waterflood sweep exhibits clear stage-wise evolution. During the low water-cut stage, injected water preferentially advances through large pore channels, resulting in limited sweep efficiency. With increasing water cut, pores newly swept by the water phase gradually shift from large pores to medium and small pores, accompanied by increasing displacement pressure. Under the present experimental conditions, the lower radius limit of pores newly swept by the water phase is approximately 7.54 μm, corresponding to a capillary force of about 0.9 MPa. When the injected volume exceeds approximately 2.5 PV, the sweep efficiency approaches a plateau and increases only from 0.72 to 0.75 at 5.0 PV, indicating that approximately 25% of the pore space remains difficult to be effectively swept. Image-based classification indicates that remaining oil can be divided into six occurrence types: clustered, porous, columnar, dead-end, film-like, and granular. Clustered and porous are the dominant occurrence types, accounting for a combined 59.7% of the total remaining oil. Pore-structure heterogeneity controls the microscopic sweep boundary through the combined effects of intra-unit structural dispersion and cross-unit structural contrast, which together regulate capillary resistance, seepage resistance, preferential flow, local bypassing, and remaining-oil retention. Microfluidic observations further show that permeability contrast and displacement velocity affect pore-scale displacement pathways and remaining-oil morphology. These findings provide experimental evidence for understanding the lower sweep-radius limit and remaining-oil occurrence mechanisms in high-permeability heterogeneous reservoirs at the extra-high water-cut stage, while the chip-scale velocity effects should be interpreted as pore-scale mechanistic evidence and require further validation before field-scale application.