TX 75083-3836, U.S.A., fax 01-972-952-9435.Abstract CO 2 mixed with NGL is being evaluated as a method to enhance viscous oil recovery from Schrader Bluff and other oil reservoirs at Milne Point, Alaska. The sequestration of CO 2 is a secondary objective of these proposed projects. Mixtures of CO 2 and NGL with the crude oil show a large three-phase liquid-liquid-gas region, so three and four-phase flow may occur in the reservoir when water is alternated with the miscible injectant. A compositional EOS simulator has been used to simulate the oil recovery for both three-phase and four-phase flow cases. The objective was to understand how important it is to simulate four-phase flow rather than use a three-phase flow approximation. Two methods were used to model three and four-phase relative permeabilities from twophase relative permeabilities as well as to evaluate the sensitivity of the results to the relative permeability parameters. A two-dimensional vertical cross-section of the reservoir was modeled with a stochastic permeability field to approximate heterogeneities. These simulations clearly show that under these conditions, four-phase flow is significant. Four-phase flow occurs over a significant part of the reservoir and affects both the sweep efficiency and the injection and production rates when the wells are pressure constrained and the production pressure corresponds to the three-phase region of the phase diagram. The results are also very sensitive to the relative permeability parameters regardless of which relative permeability model is assumed. The project life is particularly sensitive to the relative permeability and is longer for fourphase flow than predicted by a three-phase flow approximation because the relative permeability of each phase is lower when four phases are modeled.