This paper describes a technique to model the behavior of three phase low temperature CO2 systems using two phase PVT algorithms, and its application to two field CO2 projects.

An approximate tuning procedure has been developed for using a two phase equation of state to characterize the displacement behavior of CO2 in the three phase domain. This characterization technique results in good prediction of laboratory slim tube displacement tests and the minimum miscibility pressure (MMP) for both pure and impure CO2 streams. Laboratory corefloods and field scale displacements can also be modeled to a practical level of accuracy. With this pseudo two-phase representation, complex three-phase flash calculations and the need for four-phase relative permeabilities are avoided.

This approach was used in forecasting the North Ward Estes CO2 project. Volume of mixing effects characteristic of low temperature CO2 systems are modeled in a vertical, fine resolution geostatistical cross section with a compositional simulator. Reasonable agreement with observed field performance is obtained. An immiscible CO2 huff-n-puff test at the McElroy field was also modeled, using a 2-D radial, compositional model. The cross section was derived from geostatistics. The simulation also provided a good approximation of field observation.

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