Testing the stability of mixed CO₂/N2-Foam using new fluorosurfactant for enhanced oil recovery

Foam enhanced oil recovery techniques involving super critical CO₂ and N2 with surfactants are becoming popular these days due to the ability of foam to appreciably overcome problems like gravity override and viscous fingering commonly associated with gas injection. 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. It has been reported that CO₂ is unable to generate strong/stable foam as the pressure and temperature conditions are increased above its supercritical point (1100 psi, 31°C). On the other hand, N2 has been found to perform well in generating foam even at elevated pressures and temperatures. 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 35°C. Foam flooding experiments were then performed at a temperature of 50oC 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 70% to 95% by adjusting the individual gas and surfactant injection flow rates but keeping the total injection rate constant. 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 or more stable foam. It was also observed that pressure drop across the core is higher when the foam quality is increased from 70 to 80% but above foam quality of 90% pressure drop is again reduced indicating generation of weaker foam. Average steady state pressure drop for 70% foam quality was 60 psi at 0% N2 and 140 psi at 20% N2 whereas for 80% foam quality it was 130 psi at 0% N2 and 170 psi at 20% N2. However, at 90 and 95% foam qualities average steady state pressure drop reduced to 125 psi at 0% N2 and 150 psi at 20% N2. It can be concluded that effect of N2 was most profound for 70% foam quality and least for 90 and 95% foam quality. Also, foam quality of 80% exhibits highest pressure drop with and without addition of N2 indicating highest mobility reduction by foam around this value of foam quality. 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. Also, the surfactant shows good foaming ability even at very low concentrations, thus mitigating the high cost of these types of surfactants. Furthermore, this surfactant is a greener substitute to conventional surfactants as it does not contain environmentally harmful substances.