Testing the stability of Sc-CO₂-foam in sandstone using new fluorosurfactant

Foam enhanced oil recovery techniques involving super critical CO₂ and 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. Since CO₂ at super critical conditions is unable to generate strong foam several surfactants have been tested with sc-CO₂ for foam stability at actual reservoir conditions. In this work, an amine oxide-based amphoteric fluorosurfactant has been injected with sc-CO₂ for the first time in sandstone cores initially saturated with 0.15 vol% surfactant solution. The concentration of the surfactant solution was kept above its critical micelle concentration (CMC) of 0.10 vol% which was determined through interfacial tension (IFT) measurements between sc-CO₂ and surfactant solution at 1500 psi and 35°C. Co-injection of sc-CO₂ and 0.15 vol% surfactant solution was performed at varying temperature conditions (35°C, 50°C and 80°C) and constant pressure of 1500 psi. In-situ foam quality was varied from 50% to 95% by varying the individual gas and surfactant injection flow rates to observe its effect on foam mobility. Each foam quality was maintained until steady state conditions prevailed. Strong foam or weak foam was characterized based on pressure response across the core. The results of this study show that foaming ability of the surfactant with sc-CO₂ increases from 35°C to 50°C. But as temperature is increased to 80°C its foaming ability decreases. In all cases, strongest foam is generated at highest foam quality of 90%. Maximum steady state pressure drop for 90% foam quality was 16 psi at 35°C and 220 psi at 50°C whereas at 80°C it was 60 psi indicating foam became weaker at the same foam quality at high temperatures. Also, 0.90 was found to be the critical foam quality (fg∗) in all cases above which if foam quality is increased foam seems to weaken indicated by decline in pressure drop across the core. 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 alternative for CO₂-foam flooding highlighting the usefulness of the surfactant 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.