| Summary: | Carbon dioxide flooding is a strategic replacement technology for greatly enhancing oil recovery in low-permeability oilfields, which includes social benefits resulting from carbon emission reduction and economic benefits owing to the improvement of oil recovery. Therefore, it is of great significance to develop and apply the technology of CO<sub>2</sub> flooding and storage in the petroleum industry. In reservoir conditions, CO<sub>2</sub> is usually under a supercritical state, presenting both low viscosity and high diffusivity of a gaseous state and high density of a liquid state. The special phase behavior of CO<sub>2</sub> directly affects its extraction capacity, resulting in the change of miscible behavior between CO<sub>2</sub> and crude oil. In this paper, the ultra-high-pressure–high-temperature pressure–volume–temperature (PVT) system was used to evaluate the phase characteristics of CO<sub>2</sub> during the process of reservoir development. The phase behaviors of the CO<sub>2</sub>/CH<sub>4</sub>/N<sub>2</sub> crude oil system were compared and analyzed. Moreover, the matching mechanism between supercritical CO<sub>2</sub> characteristics and oil–gas system miscibility was investigated and defined. This work deepened the understanding of the phase characteristics of CO<sub>2</sub> in the process of miscible flooding, providing both theoretical guidance for the application of CO<sub>2</sub> injection on oilfields and the essential scientific basis for the implementation of CCUS-EOR technology.
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