Abstract: To address the insufficient understanding of the microscopic mechanism of enhanced oil recovery (EOR) by CO₂ flooding in high-temperature high-pressure and low-permeability reservoirs,a high-temperature high-pressure micro-visualization experiment was conducted.Using ImageJ threshold analysis,the microscopic oil displacement mechanisms of CO₂ miscible and immiscible flooding were studied,the residual oil of CO₂ miscible and immiscible flooding was classified and characterized,and the effects of CO₂ injection volume,injection rate,injection pressure,and reservoir heterogeneity on the final recovery degree were investigated.The study shows that the leap displacement and gas-oil-gas composite displacement occur,and CO₂ compression-release easily occur in narrow throats during CO₂ flooding.In both CO₂ miscible and immiscible flooding processes,as the CO₂ injection volume increases,the displacement efficiency rises significantly and then stabilizes.With increasing displacement velocity,displacement efficiency first increases and then decreases.When the displacement pressure exceeds the minimum miscibility pressure (MMP),the recovery of residual oil in low-permeability reservoirs can be enhanced.Compared with immiscible flooding,CO₂ miscible flooding yields better results and the final recovery degree is less affected by reservoir heterogeneity.This research provides an important theoretical basis and practical guidance for the design of CO₂ flooding development plans in low-permeability reservoirs.

Key words: low-permeability reservoir, high-temperature high-pressure, micro-mechanism visualization experiment, CO₂ miscible flooding, enhanced oil recovery (EOR), CO₂ immiscible flooding, residual oil