Pore-Scale Dynamics of Oil-Water Displacement in Bentheimer Sandstone via 4D Micro-CT

Pore-scale immiscible displacement governs macroscopic injectivity, trapping and recovery in subsurface engineering applications such as enhanced oil recovery and geological CO 2 storage, yet the observation of transient interfacial events in natural rocks remains limited by laboratory imaging speed. Here, laboratory-based 4D micro-CT imaging is utilized to monitor oil-displacing water in Bentheimer sandstone, and a voxel-wise phase-transition timing analysis is employed to quantify fluid invasion pathways, saturation evolution, and dynamic connectivity changes throughout the pore network. The conclusion is as follows: the displacement exhibits a reproducible three-stage evolution: initiation, rapid displacement, and terminal seepage. These stages are clearly identified by slope changes in the oil saturation and injected-volume curves. During initiation, the nonwetting oil preferentially invades larger pores under capillary control and remains weakly connected. Rapid displacement is dominated by capillary fingering and intermittent pore throat breakthroughs, producing the peak increase in oil saturation and the formation of a spanning connected pathway. In the terminal seepage stage, invasion is confined to constricted throats and poorly connected domains, leading to bypass flow and a diminishing efficiency. Time-resolved reconstructions indirectly evidence transient phenomena including Haines jumps, backflow, and nonwetting phase disconnection/snap-off, which drive nonmonotonic connectivity evolution. The results demonstrate that capillarity-driven interfacial reconstruction controls connectivity and therefore the macroscopic displacement efficiency in sandstone. This work provides a quantitative, time-resolved experimental basis for constraining pore-scale models and improving injection and recovery strategies in porous materials.