This paper provides PVT and compositional data for seven CO2/synthetic-oil systems. The oils were composed of light paraffins plus heavy hydrocarbons selected from one or two of the three main chemical types; namely, paraffins, naphthenes, and aromatics. The PVT data were used to generate pressure-composition phase diagrams and swelling index profiles. The compositional data were smoothed to produce equilibrium phase compositions, K-values, and extraction indexes. Results for the different oils are contrasted using the comparison basis of equivalent molar averaged normal boiling point. This parameter is easy to calculate and provides a means by which chemical effects can be quantified in the presence of oil volatility effects.

The results demonstrate that two oil compositional effects control the extent of hydrocarbon extraction by CO2, and subsequently the development of CO2/synthetic-oil miscibility. The major effect is usually the oil volatility, but effects of hydrocarbon chemical type can be dominant. For example, a paraffinic-aromatic synthetic oil exhibited considerably greater miscibility with CO2 than a paraffinic-naphthenic synthetic oil of similar molecular weight and volatility. The substitution of naphthenes into a paraffinic mixture resulted in no appreciable increase in CO2 solubility for the heavy paraffins present, even though the volatility of the oil was increased. A similar substitution of aromatics enhanced the extraction of heavy paraffins by CO2. The data suggest a trend for the influence of chemical type on oil miscibility with CO2; namely, paraffins are beneficial, aromatics are nearly as beneficial as paraffins, and naphthenes are detrimental. The effect of chemical type on oil swelling is also significant. At near-miscible conditions, swelling was most extensive in the presence of aromatics. Swelling was insensitive to paraffins, with naphthenes showing intermediate behavior. The results also illustrate that near-critical liquid-liquid K-values exhibit trends similar to those reported in the literature for liquid-vapor K-values.

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