Effects of CO2-foam stability, interfacial tension and surfactant adsorption on oil recovery by alkaline-surfactant-alternated-gas/CO2 flooding

In this study, experimental investigations were carried out to develop optimum chemical formulations for alkaline-surfactant-alternated-gas/CO2 (ASAG) flooding based on the combined effect of forming strong/stable foam, achieving ultra-low interfacial tension (IFT), and lowering surfactant adsorption. First, laboratory experiments were aimed to formulate the chemical slugs on the basis of CO2-foam stability, IFT measurements, phase behavior tests, and dynamic adsorption studies. Second, formulated chemical slug was tested with and without alternated gas/CO2 injection in field reservoir cores through core flooding experiments for a better insight into the oil recovery processes. The results indicated that CO2-foam stability decreased with the increased in temperature and maximum stability was achieved at specific surfactant concentrations. Adding alkali at optimum concentrations decreased the oil-water IFT values of the alkali-surfactant (AS) formulations to ultralow values (0.0068 mN/m and 0.0087 mN/m). 70 % of formation brine salinity was identified as the optimal salinity through phase behavior tests, where the equilibrium IFT was also the lowest. The results of the dynamic adsorption studies showed that adding alkali could reduce surfactant adsorption from 1.41 mg/g to 0.52 mg/g. Additionally, preflush with a secondary surfactant (black liquor, BL) at its CMC value further reduced surfactant adsorption to 0.21 mg/g. ASAG flooding resulted in incremental oil recovery of 8.35 % original oil in place over AS flooding which involved only chemical slug injection. The study suggests that alternate injection of AS solution and gas/CO2 in short slugs into low permeability reservoir cores can result in higher additional oil recovery due to in-situ foam formation and ultra-low IFT environment. Additionally, preflush with a low-cost surfactant significantly reduces primary surfactant loss due to adsorption thereby improving oil recovery by ASAG flooding.