Foam enhanced oil recovery techniques involving super critical CO2 with surfactants are becoming popular these days due to the ability of foam to appreciably overcome problems like gravity override and viscous fingering. Foam lowers the mobility of the injected fluid consequently increasing the sweep efficiency. Several studies have been conducted to study the differences between CO2-foam and N2-foam which report that CO2 is unable to generate strong/stable foam above its supercritical point (1100 psi, 31°C) whereas N2-foam is unaffected by the increase in pressure and temperature. However, an in-depth investigation to test the stability of mixed CO2/N2–foam has not been carried out yet especially in sandstone porous media. In this work, a novel mixed CO2/N2-foam system was tested for stability for the first time in foot long sandstone cores using an amine oxide-based amphoteric fluorosurfactant.

The concentration of the surfactant solution was kept at 0.15 vol% which is above its critical micelle concentration (CMC) of 0.10 vol% that was determined through interfacial tension (IFT) measurements between sc-CO2 and surfactant solution at 1500 psi and 50°C. Foam flooding experiments were then performed at a temperature of 50°C and back-pressure of 1500 psi in which co-injection of sc-CO2, N2 and 0.15 vol% surfactant solution was carried out. N2/CO2 ratio was varied between 0-20% and in-situ foam quality was varied from 0.70 to 0.95. Each foam quality was maintained until steady state conditions prevailed. Strong foam or weak foam was characterized based on pressure drop across the core.

The results of this study show that as N2/CO2 ratio is increased from 0 to 20% an increased pressure drop across the core is observed even when the total injection rate is held constant leading to the conclusion that addition of N2 to sc-CO2 was able to generate a stronger (more viscous) foam. Average steady state pressure drop for 0.70 foam quality was 60 psi at 0% N2 and 140 psi at 20% N2 whereas for 0.80 foam quality it was 130 psi at 0% N2 and 170 psi at 20% N2. However, at 0.90 and 0.95 foam qualities average steady state pressure drop reduced to 125 psi at 0% N2 and 150 psi at 20% N2 implying that effect of N2 was most profound for 0.70 foam quality and least for 0.90 and 0.95 foam quality. Also, foam quality of 0.80 exhibits highest pressure drop with and without addition of N2. Steady state pressure drop was stable for about 0.5 PV of injection at all foam qualities indicating high foam stability with this surfactant.

This study provides a new and viable alternative for sc-CO2-foam flooding highlighting the effectiveness of addition of N2 to the foam system even at small proportions. The tested mixed CO2/N2-foam system could strengthen the potential of sc-CO2 EOR.

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