AbstractThis study was conducted to investigate the phase behaviour of CO2–brine and CO2–oil systems under various operating conditions. Through this study, CO2 solubility measurement tests were carried out for CO2–water, CO2–brine, and CO2–oil mixtures at various equilibrium pressures ranging Peq = 0.7–10.3MPa and temperatures ranging Texp = 21–40°C. Additionally, series of oil swelling/extraction tests were conducted at aforementioned experimental conditions using a see-through high- pressure cell to determine the oil swelling factor at various equilibrium conditions. CO2 solubility measurement tests showed that at constant temperatures, an increase in CO2 solubility value was observed for CO2–water, CO2–brine, and CO2–oil mixtures when the equilibrium pressure increases. Furthermore, as it was expected for all mixtures, the solubility of CO2 reduces with increased temperature. In this study, it was also found that at a constant temperature, the oil swelling factor, SF, increases up to a pressure so called extraction pressure, Pext, at which majority of the light to medium hydrocarbon groups in the oil phase are extracted by CO2 and vaporized into the CO2-rich phase. Additionally, it was observed that for the pressures higher than the extraction pressure, the oil swelling factor reduced with equilibrium pressure because more hydrocarbon components were extracted at higher pressures. The extraction pressure was determined at different experimental temperatures and results revealed that the extraction pressure increases by increasing experimental temperature. Comparison of the CO2 solubility values in oil at extraction pressures corresponding to different experimental temperatures also showed that the major hydrocarbon extraction occurs when a certain amount of CO2 has dissolved in the oil phase which is called threshold CO2 solubility, χth. The defined threshold CO2 solubility was found to be approximately the same for the CO2–oil mixture under this study at different temperatures.