Dynamic Evolution of Pore Structures in Unconsolidated Sandstone Reservoir during High-Rate Polymer Flooding

In unconsolidated sandstone reservoirs, polymer flooding triggered major changes in physical properties, including porosity and permeability under high-rate, high pore volume conditions, leading to time-variation reservoir properties. Offshore oil reservoirs often exhibit higher displacement multiples and rates, leading to a more pronounced phenomenon of time-variation physical properties. This study employed a combination of X-ray diffraction (XRD), casting thin-section analysis, scanning electron microscopy (SEM), high-pressure mercury intrusion porosimetry (HMIP), and CT scanning to comprehensively analyze changes in mineral composition, porosity, permeability, and pore structure before and after polymer flooding. Prolonged high-rate flooding scoured clay minerals, enlarged pore–throat radii (average and maximum), and expanded medium/small throats into larger ones, amplifying core heterogeneity. Geometrically, throats became more regular, voluminous, and elongated. The results indicated that after high-rate and high pore volume polymer flooding, the fractal dimension decreased, and the shape of the pore network became simplified. The simplification of the pore strongly correlated with the smoothing of pore walls observed in SEM images, revealing the impact mechanism of polymer flooding on the pore structure.