Experimental exploration of CO2 dissolution, diffusion, and flow characteristics in bound water conditions during oil displacement and sequestration

The mass transfer and diffusion characteristics of CO2 are essential for its storage efficiency in geological formations and effectiveness in enhancing oil recovery. However, there is a lack of comprehensive analysis on CO2 diffusion and flow behavior during oil displacement across different pore sizes, and the macroscopic characteristics of CO2 storage during diffusion are still not well understood. This study integrates microfluidic and core diffusion experiments to investigate CO2 diffusion behavior in water across varying pore sizes, analyze its flow characteristics during oil displacement, and assess CO2 storage efficiency and mechanisms at a macroscopic scale. The results indicated that in pure water with a 300 mu m pore size, CO2 exhibited an average diffusion rate of 3.9 mu m/s, which was 2.3 times higher than that in saline water. Smaller pore sizes were found to accelerate the mass transfer process. Under non-miscible conditions, asymmetric velocity distribution and the formation of vortex zones impacted CO2 storage and oil recovery. Conversely, under miscible conditions, CO2 and oil demonstrated multiple contact miscibility, although instability was more pronounced in larger pores. The CO2 retention rates under miscible and low-permeability conditions were 1.15 and 1.42 times higher, respectively, than those observed under high-permeability, non-miscible conditions. In low-permeability cores, CO2 storage was primarily through dissolution, whereas in high-permeability cores, structural trapping prevailed. These findings deepen our understanding of CO2 dissolution and diffusion behaviors within geological structures.