A comparative study of oil compositional variations during CO2 and carbonated water injection scenarios for EOR

With the significant increase in the atmospheric concentration of CO2, the interest for a combination of Enhanced Oil Recovery (EOR) and safe underground storage of CO2 in oil reservoirs has been increased. Two applicable methods that can address these concerns are CO2 flooding and carbonated water injection ( CWI). Mutual interactions taking place between oil and free CO2 or carbonated water (i.e. dissolve CO2 in the aqueous phase) would possess a fundamental difference. For free CO2 and oils, a combination of condensing/ vaporizing type of mass transfer would occur. However, in the case of carbonated water, a one- way mass transfer of CO2 from the aqueous phase to the oil can take place. This would lead to significant differences in the oil properties after dynamic interactions with free CO2 or carbonated water. In this study, for the first time, we are aiming to comprehensively address these concerns. An integrated approach was conducted to perform a series of carefully designed high-pressure and high-temperature direct visualizations (micromodel) as well as contact (PVT) experiments using a representative live oil. The results of pore-scale visualization experiments revealed an adverse strong extraction taking place during CO2 injection. Due to this extraction, the remaining oil after CO2 flooding was heavier than the original oil and its production was even more difficult. Contrary to that, during CWI the resident oil became lighter (lower viscosity) and no evidence of substantial extraction or oil downgrading was observed. To confirm pore-scale observations, PVT experiments were performed that revealed a strong adverse compositional change happening during CO2-oil single contact test. The oil contacted with CO2 was heavier than the original oil and a heavy condensate formed mainly from the medium components of oil was produced. However, during CW-oil multiple contacts test, the oil composition did not change and only after extensive cell volumes of CW comes in contact with the live oil; a slight change in the oil composition was observed. This slight change was due to the small extraction of light components of the oil into the new gaseous phase and it was observed in the form of a condensate. The produced condensate was lighter than the one produced from CO2-oil single contact test that reveals even after significant cell volumes of CW comes in contact with oil, the compositional change in oil is negligible compared to that of CO2 injection. The results of this study will shed some lights on the complex fluid-fluid interactions and oil compositional variations that take place during conventional CO2 and carbonated water injection in oil reservoirs.