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.