Pore-scale flow mechanisms and residual shale oil trapping in water and supercritical CO2 flooding

In CO2-enhanced oil recovery (EOR) and geological sequestration processes, multiphase flow mechanisms play a critical role in controlling both displacement efficiency and CO2 storage security. In this study, a high-temperature, high-pressure microfluidic platform was employed to investigate pore-scale flow mechanisms and residual shale oil trapping during water and supercritical CO2 (scCO2) immiscible flooding. The K-means clustering algorithm was employed to quantitatively characterize fluid saturations. Furthermore, high-magnification Zeiss microscopy was utilized to identify residual oil trapping types and elucidate their formation mechanisms at the pore scale. The results indicate that during scCO2 flooding, capillary fingering and Haines jumps are pronounced. As the capillary number increases, the capillary fingering effect diminishes while CO2 channeling becomes increasingly pronounced; in contrast, capillary fingering is relatively weak during water flooding and at higher capillary numbers, residual oil initially transforms into small-scale columnar forms. These findings advance the fundamental understanding of multiphase flow and residual oil trapping in unconventional reservoirs and provide essential theoretical guidance for optimizing CO2-EOR strategies and improving the efficiency of geological CO2 storage.