Pore-structure evolution and microscopic residual-oil distribution following polymer flooding in the Daqing reservoirs, Songliao Basin

To address the development challenge in the “dual ultra-high” stage of the Daqing Oilfield, where more than 30% of oil remains unrecovered after polymer flooding, this study investigates the evolution of pore structures and residual oil occurrence in sandstones of different sedimentary facies. A multi-scale quantitative characterization was conducted using high-pressure mercury intrusion, micro-computed tomography, scanning electron microscopy, and UV fluorescence thin section analysis to analyze pore structure and residual oil distribution before and after polymer flooding. The results show that polymer flooding induces a pronounced “dual-directional reconstruction” of pore structures. In dominant flow channels, erosion by high-viscosity polymer solutions leads to fragmentation and detachment of framework minerals, resulting in a significant increase in the volume fraction of macropores (>50 μm). In contrast, in branched channels, polymer adsorption/bridging effects and clay mineral migration cause pore–throat shrinkage, increasing the proportion of micropores (<1 μm) by 10-15% and reducing their contribution to permeability by 16-20%. Sedimentary facies exert a strong control on pore evolution: channel sandstones are dominated by macropore expansion and enhanced heterogeneity, whereas sheet sandstones exhibit increased microporosity and specific surface area, further restricting effective flow pathways. In terms of residual oil distribution, a “mobilization-recapture” microscopic model was proposed. Polymer flooding effectively mobilizes large-cluster residual oil, reducing its proportion by 15-20%; however, it simultaneously promotes oil migration toward grain surfaces and micropores, increasing the proportions of grain-adsorbed and film-type residual oil. These results indicate that polymer flooding enhances displacement efficiency while strengthening oil-rock interfacial adsorption, providing theoretical support for polymer flooding optimization and subsequent profile control strategies in heterogeneous reservoirs.