Abstract: The purpose of this paper is to set up a strict method for complete phase diagram calculations in oil-gas systems and evaluate the adaptability of equation-of-state (EOS) so that the phase characteristics in critical and two phase regions are obtained. Based on the analysis of phase equilibrium thermodynamic model in the hydrocarbon system, the complete pressure
-composition (p-X) phase diagram calculation method has been discussed and set up in gas injection process, which is displayed in geometric proportion of molar mass and volume. Applying the theory in conjunction with experimental data, two kinds of displays of the complete p-X phase diagrams are cal-
culated in various miscible gases (rich gas, dry gas, N₂, CO₂) injection processes. Results show that the solution prepared here is reliable and can describe phase properties well. It has practical significance for the further determination of miscible conditions and mechanisms of oil and injected solvent by the phase diagram.

Key words: gas injection, miscible drive, phase equilibrium, p-X phase diagram