Evaluation of fluorosurfactant performance with super-critical CO₂ flooding for high salinity carbonate reservoirs

Super-Critical CO₂ flooding combined with surfactants is one of the latest methods being used for Enhanced Oil Recovery. This overcomes the shortcomings associated with CO₂ gas injection like gravity override and viscous fingering to an appreciable extent. It restricts the mobility of the injected fluid leading to higher contact with the resident crude resulting in better sweep -efficiency. A number of surfactants have been tested with CO ₂ to appraise performance in different scenarios. However there have been problems of surfactant instability at real reservoir conditions i.e. at high temperature and high salinity. Adsorption of surfactants on the rock surface is another issue that usually decreases the effectiveness of the system. In the core-flood experiments performed in this work, an amine oxide-based amphoteric fluorosurfactant has been injected with super critical CO₂ for the first time on foot long carbonate cores saturated with high saline formation water (Total Dissolved Solids > 200,000 ppm). High temperature (90°C) and pressure (2500 psi) were applied coupled with 5 days of aging time with reservoir crude to recreate actual reservoir environment. Different injection strategies were studied including coinjection of super-critical CO₂ and surfactant solution, and alternating injection of super-critical CO₂ and surfactant solution in different ratios, and then compared to find out the optimum injection strategy. Results from the study exhibit a significant increase in the oil recovery due to this CO ₂-surfactant system as well as foam generation in high saline environment for the co-injection scheme. This research provides a new and viable option for CO₂-Surfactant flooding especially for high salinity carbonate reservoirs. It displays the usefulness of the surfactant even at very low concentrations, thus mitigating the high cost of this type of surfactants. Furthermore, this surfactant does not contain environment harmful substances making it a greener substitute to conventional hydrocarbon surfactants.