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 CO₂ flooding and storage in the petroleum industry. In reservoir conditions, CO₂ 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 CO₂ directly affects its extraction capacity, resulting in the change of miscible behavior between CO₂ 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 CO₂ during the process of reservoir development. The phase behaviors of the CO₂/CH₄/N₂ crude oil system were compared and analyzed. Moreover, the matching mechanism between supercritical CO₂ characteristics and oil–gas system miscibility was investigated and defined. This work deepened the understanding of the phase characteristics of CO₂ in the process of miscible flooding, providing both theoretical guidance for the application of CO₂ injection on oilfields and the essential scientific basis for the implementation of CCUS-EOR technology.

The reservoir proportion with deep and low permeability, where oil and gas exist widely in nanopores, has been discovered increasingly in China. Affected by the nano-confinement effect, the phase behavior of fluid in nanopores varies with nanopore size rather than being constant. In this paper, the density, viscosity, and saturation pressure of pure and mixed fluids in nanopores are studied by molecular dynamics (MD) simulation combined with statistical physics. The feasibility of using the MD method to investigate fluid behavior in nanopores was verified with laboratory data. It was found that the fluids’ phase behavior parameters in nanopores are lower than those in the bulk phase due to the confinement effect. The boundary of confinement effect (BCE) is defined as a size range of nanopores that separates the pore scale into the confinement scale and bulk scale. Each fluid has a specific BCE influenced by the molecular size of fluid. The phase behavior of crude oil in shale and tight oil reservoirs is most affected by the molecular adsorption and interaction energy between the fluid molecule and pore wall. Clarifying a specific BCE in shale reservoirs can significantly enhance the understanding of reservoirs and guide reservoir development strategies.