The paper is devoted to show how different estimations of the interfacial tension between oil and gas phases can really affect the overall production evaluations. Obviously such effects are significant only for rich gas condensate reservoirs (liquid drop out >10%).

Commercial reservoir simulators usually calculate interfacial tensions by the parachor method, which is known to underestimate low interfacial tensions between almost equal phases. In fact the parachor method fails just for those values (low values) which are critical.

Besides gradient theory, which is founded on statistical mechanics and therefore completely different from the parachor method, the other procedures are mainly derived from the original parachor method. Such new methods, suitable for calculating low interfacial tensions, can be grouped into two main families: a first group where pure component parachors are modified with respect to the original ones, and a second group where the critical exponent is changed, while parachor values of pure components are just the classical ones. Moreover two kind of mixing rules can be considered for this kind of methods: one founded on the principle of corresponding states and a second one, the Weinaugh-Katz parachors mixing rule.

After a short overview of the methods for the evaluation of the interfacial tension for mixtures and a discussion on the so- called critical exponent, each of them is used for the evaluation of the interfacial tension of mixtures, whose experimental values have been found in literature.

Moreover a real fluid has been taken in account. A synthetic reservoir has been considered and gravity drainages corresponding to the different interfacial tension evaluations, obtained by using the different methods, has then been performed on each of these fluids. All the results are then compared. The comparison shows the importance of the selection of a proper calculation method for the interfacial tension, also to consider a reliable recovery estimation.

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