the co2 lightens up the oil (making it less viscous) and increases the pressure in the reservoir ( provides energy to drive the oil to the producing wells) the big problem is the co2, being much less viscous than the oil it is displacing, wants to flow to the producing wells  bypassing the oil instead of displacing it  the same reason  waterflooding is relatively inefficient   and the reason the doe has put such a large number (  90 billion barrels) on "potential" oil reserves from co2 eor  
Thank you both for the insights