Study on PVT Dissolution Characteristics of Different Gases in Heavy Oil under High Temperature and High Pressure

CO₂-EOR is a promising technology for improving heavy-oil recovery while enabling geological carbon storage. In this study, the dissolution behaviors of N₂, CO₂, and flue gas in six heavy oils were investigated by using high-temperature and high-pressure PVT experiments and visual micromodel foamy-oil experiments. The bubble point pressure and oil formation volume factor increased with the dissolved gas-oil ratio and temperature. Gas dissolution reduced oil viscosity, and the viscosity reduction rate increased with the dissolved gas-oil ratio but decreased at higher temperatures because gas solubility was reduced. Among the tested gases, CO₂ showed the highest dissolved gas-oil ratio, the strongest oil-swelling capacity, and the most pronounced viscosity reduction effect, whereas N₂ mainly contributed to pressure support and flue gas exhibited intermediate behavior controlled by its CO₂ fraction. Higher-viscosity oils required higher pressure to achieve comparable gas dissolution, which is consistent with stronger mass-transfer resistance and lower available free volume. Microscopic observations showed that CO₂ foamy oil generated more uniform and smaller bubbles than did N₂ foamy oil. Increasing the temperature accelerated bubble coalescence and shortened the stable foamy-oil stage. These results provide a quantitative gas-selection basis for gas-assisted cold production and CO₂ storage in heavy-oil reservoirs.