A preliminary study of CO2 effects on heavy oil recovery in CO2-assisted thermal injection

CO₂-assisted thermal injection incorporates the merits of non-thermal CO₂ injection and thermal injection methods on heavy oil recovery. However, the effects of CO₂ on heavy oil recovery are unclear considering the extremely high temperatures. In this work, we studied the CO₂-heavy oil interactions and oil displacement by CO₂ under high temperatures and pressures. The carbon number distribution and four groups (asphaltene, colloid, saturates, and aromatics) of heavy oil were tested. The oil displacement by CO₂ was captured using T₂ NMR and NMR imaging analyses. Our results showed that the light hydrocarbons were extracted under an elevated temperature of 333.15 K, while the heavy hydrocarbons were extracted under a low temperature of 313.15 K. As the CO₂ exposure pressure increased, the extraction effect was improved by expanding extractable carbon numbers and vaporing more saturates. The oil displacement efficiency by CO₂ increased as the temperature rose, but it did not strictly increase with increasing pressure. The spatial oil recovery of a sample, determined by NMR imaging, was unevenly distributed, i.e., it was larger on one side of the sample than on the other side. This phenomenon could be related to the gravity effect, under which the oil flows downward during the CO₂ injection experiment. This study helps to provide a full picture of CO₂ roles in CO₂-assisted thermal injection for heavy oil recovery.