Supercritical CO2-oil miscible flow and residual oil formation in tight sandstone using micro-nano CT

Supercritical CO 2 -oil miscible flow behavior is crucial in both CO 2 -enhanced oil recovery and geological sequestration processes, and is primarily governed by the balance between viscous and capillary forces, particularly in tight sandstone reservoirs. In this study, an SC-CO 2 miscible flooding model was developed based on micro-nano computed tomography (CT) of tight sandstone to systematically investigate the seepage mechanisms of SC-CO 2 miscible flooding under different capillary number (log 10 Ca ) conditions. The results indicate that at low capillary numbers (log 10 Ca =-5.5), SC-CO 2 miscible displacement is dominated by capillary forces, leading to pronounced capillary fingering, a narrow CO 2 -oil miscible zone, and high residual oil saturation. Conversely, at high capillary numbers (log 10 Ca =-4.2), viscous forces prevail, capillary fingering is markedly suppressed, and the miscible zone widens significantly; residual oil saturation decreases by 48.2% compared with the low capillary number case, while displacement efficiency increases by 31.4%. Compared with water flooding, SC-CO 2 miscible flooding reduces residual oil saturation by 40.8% and enhances displacement efficiency by 21.6%. As the capillary number increases, the morphology of residual oil progressively shifts toward smaller-scale columnar and droplet forms. This study offers key theoretical insights into the complex multiphase flow mechanisms in porous media, contributing to a deeper understanding of residual oil mobilization during SC-CO 2 enhanced oil recovery and geological sequestration in tight sandstone reservoirs.