Alkaline flooding is a promising application for enhanced oil recovery through the in-situ formation of oil-in-water emulsions. However, the emulsification behavior for the improved oil sweeping efficiency varying from the total acid number (TAN) of crude oil is still vague, especially in porous media filled with high acidic oil. This study initially performs three-dimensional (3D) oil displacement experiments to investigate the effects of the TAN and the injected flow rate on the oil recovery performance using X-ray computed tomography. Consequently, the oil recoveries are confirmed to increase with the increasing acid concentration from multi-angle of the oil displacement patterns, fractal dimension, slice average concentration of the alkaline solution, and oil recovery curves. The optimum injected flow rate is confirmed to be 360 ml/h based on different series of flow rate experiments. Meanwhile, the fingering structure evolution is observed with respect to the immiscible fluid–fluid displacement. Nonlinear finger–finger interactions, such as tip-splitting, shielding, and coalescence, are also confirmed in the 3D porous media. Subsequently, two-dimensional pore-scale micromodel experiments are performed to verify the oil displacement mechanism that may occur in 3D porous media. With an increase in the acid concentration, the emulsification ability becomes stronger in the form of an emulsified transition from partial to full emulsification during the alkaline flooding process as a result of formed average emulsion droplet size ranging 156.1–78.8 μm. Partial emulsification is predominated by the snap-off, splitting, and division processes, leading to many oil ganglia and/or oil droplets formation that are bigger than pores (128 μm) and throats (39 μm). Full emulsification is characterized by the proper size of oil droplet formation by the shearing action mechanism. Far smaller oil droplets are effectively entrained in the aqueous phase and passed through the pores and throats as a continuous phase, resulting in the most favorable oil displacement. The study results suggest that high oil recovery could only be achieved under the condition of the proper sizes of emulsion droplets matched with the pore–throat structure. Finally, we propose herein that in engineering application, alkaline flooding has the highest potential of improving oil recovery in high-acidity oil reservoirs because of full emulsification.