Detailed experimental data of the phase behavior of a rich gas displacing a Middle Eastern crude are given. These data include constant composition expansion, differential liberation, saturation pressure, and multiple contact PVT data as well as slim tube data at several pressures. The phase behavior of the reservoir oil / rich gas system was modelled using a cubic equation of state (EOS) with fifteen components. The critical properties of the heavy fractions, the binary interaction parameters and the volume translation parameters were adjusted by regression to match the experimental data. A good match of the experimental data was obtained. With the aid of pseudo-ternary diagrams and pressure-composition diagrams generated from the EOS the mechanism of oil recovery was interpreted as a condensing / vaporizing process with significant upper phase extraction (vaporization). The EOS fluid description was then utilized in a compositional simulator to model the experimental slim tube results. A good match of the experimental data was achieved by including the effect of interfacial tension (IFT) on relative permeabilities.
Once the primary recovery mechanisms were identified, the fifteen components were reduced to six using the technique described by Nutakki et al1 . The calculations using six components were almost identical to the calculations with 15 components for both the PVT and the slim tube data.
The results clearly show that for a condensing / vaporizing process the traditional interpretation of slim tube recoveries is not valid. The break in the slim tube recovery curve with increasing pressure does not indicate multiple contact miscibility, but rather a region of reduced IFT. To model properly this behavior in a compositional simulator, the effect of interfacial tension on relative permeability must be accounted for. Because of the reduced IFT, it is possible to obtain high recovery even though the displacement process is not miscible.