Volatile oil and gas condensate reservoirs were uncommon before 1930s. Since then they have been discovered with increasing frequency. This is attributed to the increasing drilling depths. Recently there has been a growing interest in these near critical fluids reservoirs. Large number of discoveries made it imperative to implement the relevant methods to deal with such reservoirs.

Usual industrial practice in Pakistan is to use the P/Z plot to estimate the GIIP (Gas initially in place). This is the simplest method and does not require the set of PVT properties below the saturation pressure. However the P/Z method is limited in its application to only volumetric depletion type, dry gas, reservoirs. The general material balance equation gives reliable GIIP estimates, since it accounts for all drive mechanisms. Usually general material balance methods are not preferred in gas condensates, because these require standard PVT properties below the saturation pressure. These properties may be computed using Walsh-Towler algorithm, Whitson-Torp (K-value flash) method or Cubic Equation of State (EOS).

This paper demonstrates the application of Walsh-Towler algorithm and K-value flash method (Whitson-Torp method) to calculate the standard PVT properties below the saturation pressure for a gas condensate reservoir. Then the application of Havlena-Odeh and Cole plot methods is exhibited to estimate the GIIP. The results are then compared with that of P/Z plot and an 11% over-prediction by P/Z plot was observed.

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