Experimental and Numerical Investigation of Pore-Scale Remaining Oil Mobilization Mechanisms Under Variable-Rate Waterflooding

In the late high water cut stage of medium- and high-permeability sandstone reservoirs, remaining oil becomes highly dispersed, and rational optimization of the water injection rate is crucial for further recovery improvement. However, a quantitative relationship between pore-scale remaining oil mobilization and macroscopic displacement efficiency is still lacking, limiting mechanistic guidance for field-scale rate optimization and water cut control. To address this issue, variable-rate waterflooding experiments were conducted on medium- and high-permeability sandstone cores to establish the relationships among displacement rate, injected pore volume multiples, and oil displacement efficiency and to identify the optimal rate. In addition, an X-CT-based heterogeneous pore-scale geometric model and a series of idealized pore-throat models were used for oil–water two-phase flow simulations. Combined with an orthogonal experimental design, the effects of displacement rate, wettability, oil viscosity, interfacial tension, and pore structure parameters on remaining oil mobilization and displacement efficiency were systematically evaluated. The results indicate an optimal displacement rate of 2–3 mL/min. At low rates, remaining oil cannot be effectively mobilized, whereas excessively high rates intensify fingering and channeling along high-capacity pathways, reducing the final displacement efficiency. Simulations further show that a moderate rate increase markedly lowers remaining oil saturation, while overly high rates induce severe channeling. Orthogonal analysis reveals that displacement rate and wettability are the dominant factors, with normalized weights of 46.3% and 30.5%, respectively. Larger pore diameters, smaller pore-throat ratios, and higher coordination numbers reduce mobilization pressure and remaining oil saturation. This work elucidates pore-scale remaining oil mobilization kinetics under variable-rate waterflooding and provides a quantitative microscopic basis for rate optimization and fine-scale remaining oil exploitation in mature waterfloods.