Drill stem testing of low permeability reservoirs is challenging because high pressure drawdown around the wellbore readily lowers fluids below saturation pressure and creates two-phase flow into the wellbore. The fluids produced at surface no longer represent the original reservoir fluid. This paper shows the benefits of a careful methodology of data selection and equation of state (EOS) modeling to validate data used to characterize the reservoir fluid. Cases are examined where standard methods of capturing and characterising the reservoir fluid failed either because of sampling difficulties caused by two-phase flow within the reservoir. The first example is a lean gas condensate where there was a discrepancy between surface and bottom hole sample compositions. By an extended analysis of Gas: Oil ratio (GOR) over all the flow tests and taking a broader perspective of the welltest data, it was demonstrated the surface data were superior to the bottom hole samples. The second study is of a disguised volatile oil which had previously been classified as a rich gas-condensate based on welltest GOR. However, a review of the GOR versus rate and EOS modeling of reservoir saturation pressures gave a consistent indication that the native fluid was not a condensate but a volatile oil. In both examples the methodology of careful data selection followed by consistency of EOS modeling gave interpretations contrary to rule of thumb assumptions and revealed the true reservoir fluid compositions.

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