CO2 injection is a commonly used technique for recovering residual oil from mature reservoirs. It gains more and more interest due to the importance of carbon dioxide capturing and sequestration. The increasing CO2 availability and the easy implementation make this Enhanced Oil Recovery (EOR) method favorable in times of low oil prices. In most reservoirs injected or native CO2 exists in its supercritical state (scCO2) because the critical point of carbon dioxide is at a lower pressure and temperature than the reservoir conditions. Its miscibility behavior makes scCO2 a good solvent that generates high oil recovery rates. The swelling of the hydrocarbon phase, and consequently the efficiency of a flood, strongly depends on the miscibility of the residual oil and the solvent. Therefore, immiscibility of scCO2 with the crude oil leads to poor recovery, which poses some limitations to CO2 EOR procedures. Additionally, viscosity or gravity effects could decrease the sweep efficiency and therewith the recovery rate.
The miscibility of the injection fluid and the residual crude oil can be improved by solubilizing an appropriate additive in the scCO2. In this work the compatibility of different additives with scCO2 is investigated in order to identify compounds that enable an easy co-injection of the additive within the flow of the CO2. Phase behavior studies under typical reservoir pressures and temperatures with a (model) crude oil show the solubilization behavior of the recovery fluid CO2 and the effect of the different additives on the mutual miscibility.
Information about the influence of different additives on the crude oil's miscibility and swelling with the injected CO2 allows conclusions about the best procedure for each reservoir. The additives can also interact with the formation water and build stable CO2-foams downhole. These foams exhibit a higher viscosity than the pure gas which also results in improved sweep efficiency.
Using an appropriate additive can lead to a change of the miscibility behavior of a reservoir and make it more favorable for a CO2 EOR process. A former near-miscible or even immiscible application can become a miscible flood. Furthermore, the additives can influence the performance of water alternate gas (WAG) and foam methods and lead to higher recovery rates.