Abstract
To address the scattered distribution of remaining oil and the difficulty of quantitatively characterizing its microscopic occurrence after long-term water flooding in medium-to-high-permeability sandstone reservoirs, this study investigated representative core samples from the Songnan Oil and Gas Block of Jilin Oilfield. High-pressure mercury intrusion, nuclear magnetic resonance (NMR) movable-fluid testing, cryogenic molding–based full-field map scanning, and three-dimensional (3D) reconstruction were used to characterize differences in pore structure, variations in movable fluid during water flooding, and the microscopic occurrence of remaining oil. The results show that: (1) The pore-throat systems of the sandstone reservoirs are highly heterogeneous and are dominated by medium-sized pore throats, with widely developed micropore throats and locally enlarged pore throats. (2) During water flooding, the peak amplitudes of the NMR T2 spectra generally decrease, and the spectral distributions gradually change from bimodal to unimodal. Oil in well-connected pores is preferentially mobilized. After displacement, the remaining oil is still concentrated mainly in medium-sized pores, which constitute both the principal pore domain for oil mobilization and a key site of remaining-oil retention at the late stage. (3) The 2D image-analysis and 3D reconstruction results show that, after water flooding, the remaining oil occurs as locally enriched clusters, connected bodies, and dispersed residues. Across the four imaged samples, cluster-type remaining oil accounts for 54–80%, whereas the combined proportion of discontinuous forms—including multi-pore, columnar, droplet-like, and film-like oil—accounts for 20–46%. These proportional differences reflect variations in oil-phase connectivity and retention under the combined influence of water-flooding state and pore-network characteristics. The results support an integrated characterization approach combining NMR movable-fluid testing, cryogenic molding–based full-field map scanning, and 3D reconstruction, providing a new technical route for detailed identification of remaining oil at the high-water-content stage and for identifying potential targets for further oil recovery.