Novel cationic surfactants for CO₂-foam flooding in carbonate reservoirs
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A majority of oil throughout the world is contained in carbonate reservoirs. Alkaline-Surfactant-Polymer (ASP) floods cannot be applied in many carbonate reservoirs for three main reasons: conventional alkali are not compatible with divalent ions, adsorption of anionic surfactants is high in the absence of alkali, and the permeability of the rock is often low for polymers to pass through the pores. One alternative to ASP flooding is CO₂-foam flooding. Foam flooding reduces the mobility of the CO₂ and increases the sweep efficiency. To overcome the adsorption of surfactant on the carbonate surface, cationic surfactants can be used rather than anionic surfactants. The objective of this research is to study two novel cationic surfactants for foam flooding applications. These surfactants are gemini surfactants, containing two head groups and two tail groups. The bulk foam stability in the presence and absence of oil was studied for these surfactants and compared to conventional surfactants; these gemini surfactants showed comparable bulk foam stability to other cationic surfactants. Corefloods in the absence of oil were performed at reservoir conditions to prove foam formation in porous media and to determine the optimum ratio of CO₂ to surfactant injection ratio. Both water-wet and oil-wet coreflood experiments were performed for the gemini surfactants. The water-wet corefloods for both surfactants recovered 6-16 %OOIP after the waterflood. The pressure drop during the water-wet foam floods was not too high, less than 15 psi/foot which is reasonable for a low permeability carbonate core. The corefloods showed results comparable with a polymer flood, with no injectivity issues, indicating that these surfactants can be used in place of polymer flooding in carbonate reservoirs. The oil-wet experiment also resulted in foam flood recovery of 13% OOIP, despite the poor wettability alteration results seen with calcite chips. With better foam stability in the presence of oil and enhanced wettability alteration, this new class of cationic surfactants could be a viable option for enhanced oil recovery in carbonate reservoirs.