Abstract: With the exploration and development of oil and gas resources in China, heavy oil reservoirs account for more than 20% of the total reservoir development. The efficient exploitation of heavy oil resources is of significant importance to China's energy security. CO₂ injection technology has been proven as an effective method to enhance the recovery efficiency of heavy oil. Because of the extremely strong water sensitivity of the reservoir, large-scale water flooding of mid-deep heavy oil reservoirs is challenging, necessitating a shift in the development approach. The mechanism of CO₂ injection for mid-deep heavy oil reservoirs remains unclear, the injection/production parameters are not clearly determined, and there is a lack of experimental research to support these findings.. In this study, the mid-deep heavy oil reservoir in the Bei 10 well area of Xinjiang Oilfield was selected as the research object. The diffusion coefficient of CO₂ in the crude oil, the extraction mechanism by supercritical CO₂, and the high-temperature high-pressure long core huff-n-puff experiments were implemented to investigate the mechanisms of CO₂ injection in heavy oil reservoirs and the efficient CO₂ huff-and-puff development mechanisms.. The experimental results indicate that：1) The diffusion coefficient of CO₂ in heavy oil increases with temperature and pressure, with pressure having a greater influence on the diffusion coefficient than temperature. 2) With the increase of the number of extractions, especially in the first three rounds of extraction, the content of light components (C₆ to C₁₂) decreases significantly,while the content of heavy middle-components (C₁₄₊) gradually increases. It was also observed that CO₂ has a greater impact on the extraction of saturated heavy oil than the dead oil. 3) For heavy oil reservoirs, the recovery factor under depletion development is only 7.01%, whereas CO₂ huff-and-puff development significantly enhances the recovery factor, achieving a cumulative recovery factor of 36.94% and an oil exchange rate of 0.59 t/t（Oil production per ton of CO₂ injected）. 4) Via comprehensive comparative analysis, the optimal soak time is found to be 1-2 hours, and the optimal pressure depletion rate is 50 kPa/min. The findings of this study contribute to clarifying the mechanism of CO₂ injection for mid-deep heavy oil recovery, while providing guidance for the optimization of on-site CO₂ soak time and depletion development pressure drop rate in the Bei 10 block. This study also serves as a reference for the subsequent large-scale application of CO₂ huff-n-puff process, as well as insights for CO₂ injection development in similar reservoirs.

Key words: Mid-deep heavy oil reservoir, CO₂ huff-n-puff, CO₂ diffusion, CO₂ extraction, Operation parameters