Is the role of carbon dioxide when injected in the oil-bearing formation limited to the swelling of oil only or it has another role, such as reducing viscosity or anything else?
Let's structure things simply. The first role of CO2 in oil revery is the role of sweep agent and pressure maintenance (for which CO2 can compete with immiscible gas or water). Secondary roles include swelling of residual oil and viscosity reduction of displaced oil both of derive from CO2 miscibility in oil. Other more complex and least ascertain effects can also exist. If establishing comparisons one need to make sure to isolate effects. As CO2 assisted recovery is sometimes operated in tertiary mode (after primary depletion and sweep by dry gas or water) one need to be a bit careful to sort attribute impact to one role or the other.
When it comes to CO2 flooding, there are a number of mechanisms including interfacial tension reduction, oil swelling, oil viscosity reduction, light hydrocarbons extraction. These mechanisms could play more crucial roles in enhanced oil recovery, based on the fact that the CO2 displacement is miscible or immiscible. Like for instance, in miscible condition, IFT reduction and oil viscosity reduction play more vital roles in CO2-EOR.