Roles of CO ₂ in Crude Oil Properties and the Stability of W/O Emulsion during CO ₂ Flooding: A Review

In the past few decades, CO 2 flooding has become one of the primary technologies for oil recovery enhancement. The CO 2 dissolved in crude oil can lead to expansion of crude oil volume, reduction in crude oil viscosity, and oil–water interfacial tension. Meanwhile, asphaltenes can also be precipitated and deposited from crude oil due to the CO 2 extraction effect, which seriously threatens crude oil production and flow assurance. In addition, the oil recovery process is always accompanied by crude oil emulsification, especially for the formation of water-in-oil (W/O) emulsion, obviously increasing fluid viscosity and decreasing transportation efficiency. Therefore, it is of significance to summarize the effect of CO 2 flooding on the physical properties of crude oil and stability of emulsion. Compared with previous reviews, this review focuses on the role of CO 2 dissolved in crude oil in the physical properties of crude oil and stability of the W/O emulsion, rather than the effect of traditional chemical additives or natural surface-active substances in crude oil. First, a summary was made of the effect of CO 2 flooding on crude oil physical properties, such as the viscosity, expansion coefficient, CO 2 solubility, minimum miscible pressure (MMP), bubble point pressure, asphaltene aggregation characteristic, and interfacial tension. Then, the role of CO 2 in the W/O emulsion was emphasized under the production pressure. Finally, the effect of CO 2 bubble kinetics, including bubble rising and growth, on W/O emulsion stability under depressurization was reviewed systematically. The relevant viewpoints aim to reveal the roles of CO 2 in the physical properties of produced crude oil and the stabilization of the W/O emulsion.