Discussed herein are three aspects of fluid phase equilibrium as they relate to enhanced oil recovery processes and to the simulation of such processes. First, experimental phase equilibria measurements show that the addition of n-butane can lower saturation pressures relative to those of base oil + CO2 mixtures. Enrichment of the injection gas with only 10 mol·% n-butane reduces the saturation pressure at the L-V critical point by 1400 psia [9.65 MPa] and by nearly 2250 psia [17.51 MPa] for a 20 mol·% addition. Likewise, addition of 6 mol·% n-butane reduces the slim tube minimum miscibility pressure by approximately 550 psia [3·79 MPa]. Thus, a n-butane enriched CO2 injection gas can achieve multiple contact miscible displacements in lower pressure reservoirs. It would not be possible to miscibly flood those low pressure reservoirs with pure CO2 injection.
Second, phase distribution curves from constant composition volumetric expansion experiments illustrate the solvent capacity of the injection gas for the base oil.
Third, a set of equation of state parameters describing the fluid PVT and phase behavior is presented and used to represent the suite of measured properties. To mimic miscibl·e-like behavior during multiple contacting, severe compromises are required in the representation of base oil and first contact base oil + injection gas mixture properties.