Here is a method for modifying an existing three-phase simulator so that it may be used to forecast miscible flood performance. The simulator is capable of modeling the essential features of miscible displacement while leaving the fine structure of unstable miscible flow unresolved, making it possible to represent the reservoir by a fairly coarse numerical grid. possible to represent the reservoir by a fairly coarse numerical grid. Introduction When designing or evaluating a miscible displacement project, one would like to be capable of accurately project, one would like to be capable of accurately forecasting reservoir performance for a variety of operating conditions. In the past, physical model studies have provided the bulk of the data used in these evaluations. Unfortunately, proper scaling of miscible displacement is difficult to obtain and model construction is time-consuming and expensive. Consequently, considerable effort has been devoted in recent years to the development of numerical reservoir simulators capable of predicting miscible flood performance. In this paper, a method is described for modifying an existing three-phase simulator so that it may be used to simulate miscible flooding. The model employed is unique in that the essential characteristics of miscible displacement are described without reproducing the fine structure of unstable frontal advance. This feature is of particular importance as it allows the reservoir to particular importance as it allows the reservoir to be represented by a fairly coarse numerical grid. We shall describe here three- and four-component versions of the miscible flood simulator. In the three-component version, wetting (water) and nonwetting (hydrocarbon) phases are considered with two-component miscible flow of oil and solvent in the hydrocarbon phase. With the four-component version, slug as well phase. With the four-component version, slug as well as continuous solvent injection may be examined. The validity of the miscible simulator is demonstrated by comparing predicted performance with experimental results from both linear and areal displacement in laboratory models. Predictions are also compared with reported results of miscible displacements carried out in a shallow, water-bearing reservoir. Finally, possible application of the simulator is demonstrated by its use in the evaluation of alternative flooding configurations and operational policies for a field example. policies for a field example. Simulator Requirements A common characteristic of miscible recovery processes is unstable frontal advance, in the form of either viscous fingering or gravity tonguing. These instabilities are the natural result of the highly adverse viscosify ratio and large density difference that generally exist between an oil and the displacing solvent. Fig. 1, for example, depicts the swept zone at solvent breakthrough for a five-spot pattern flood as actually observed in a Hele-Shaw (parallel plate) model. The model is simulating the miscible displacement of oil by solvent at an adverse mobility ratio of about 15. The effect that an unstable frontal advance exerts upon oil recovery might be considerably worse than actually observed if it were not for the fact that solvent disperses in the oil, which renders the effective viscosity and density differences less extreme than those of the pure components. Thus, a successful miscible-flood simulator should allow for the possibility of unstable frontal advance and must describe possibility of unstable frontal advance and must describe the dispersion phenomenon, at least as related to the determination of effective fluid properties. P. 874