CO2 foam is an effective method to reduce CO2 mobility and improve displacement efficiency in CO2 enhanced oil recovery (EOR) and CO2 storage applications. Foam strength and stability are key parameters that influence the efficiency of the foam which depend on several factors including the presence of oil, injection velocity and rock type. The aim of this work was to evaluate the effect of rock type on CO2 foam strength and stability by conducting corefloods with sandstone and carbonate rocks at reservoir conditions. The effect of injection velocity and the presence of residual oil on the foam generation and displacement efficiency was also investigated.
Steady-state CO2 injections revealed differences in foam generation, strength and stability in sandstone compared to carbonate based on the calculated apparent viscosities. Results showed that the strongest foam was generated in sandstone compared to carbonates because of higher absolute permeability. Drainage-like co-injections with increasing gas fraction showed the relation between rock permeability and the limiting capillary pressure and co-injection at different injection velocities revealed shear-thinning foam rheology in both rock types. Despite stronger foam generation in sandstone, unsteady-state CO2 injections showed similar oil displacement efficiency in both rock types. CO2 foam increased oil recovery by 200% in both rocks compared to CO2 injection without foam. In addition, foam showed a significant impact on water displacement compared to pure CO2 injection which is advantageous for CO2 storage applications. Water recovery during CO2 EOR was 60% in sandstone and 88% in limestone. Dissolution of calcite was observed in limestone, which increased pore space and the CO2 storage capacity. Overall, the results indicate that CO2 foam generation, stability and coalescence are sensitive to rock permeability and pore geometry in the conducted experiments.