Performance comparison of novel surfactant-scCO₂ systems for oil recovery in high salinity carbonate reservoirs

Recent work on CO₂ Enhanced Oil Recovery (CO ₂EOR) has focused on the design of fluid systems that have the ability to overcome the deficiencies typically associated with CO₂ injection. However, applicability remains a challenge due to the instability of the developed fluid systems at reservoir conditions. The objective of this work was to evaluate and compare novel surfactant-CO₂ systems that improve oil recovery. For this purpose, multiple core-flooding experiments were conducted at high temperature (90°C) and high pressure (1800 psi) and in high saline conditions (TDS>200,000 ppm) to mimic actual reservoir conditions. Three novel surfactant solutions (an amphoteric fluoro-surfactant, a hydrocarbon surfactant and a visco-elastic surfactant) were co-injected with sc-CO₂ in foot long carbonate cores to compare their stability and effectiveness in incremental oil recovery. Interfacial Tension (IFT) experiments were also performed with the most performant surfactant at the same conditions and surfactant concentration to further highlight its contribution in increasing oil recovery. Results from this comparative core-flooding study clearly demonstrate that when combined with sc-CO₂, the fluoro-surfactant provided a much higher oil recovery than the hydrocarbon or the visco-elastic surfactant (6-9% increase). The pressure difference between the outlet and inlet of the core (ΔP) during flooding was also much higher for the fluoro-surfactant system indicating possible foam generation. The use of surfactants with CO₂ also delayed the breakthrough of CO₂ which allowed for longer contact between the oil and CO₂. IFT results also showed a significant drop in interfacial tension between reservoir oil and the injected fluid after adding the fluoro-surfactant. This work provides a novel option for surfactant-scCO₂ flooding that overcomes the shortcomings of CO₂ gas injection leading to an increase in total oil recovery. The fluoro-surfactant performs exceptionally well even at the very low concentration used in this study which also reduces the overall cost of the design.