A unified treatment of materials balance computations to estimate hydrocarbons in place is presented. This method generalizes the normal procedures for gas reservoirs to all fluids of interest to the petroleum industry. We show that a plot of average pressure divided by a two-phase deviation factor can be used to compute hydrocarbons in-place. A simple procedure to estimate the required two-phase deviation factors for black oils and volatile oils is discussed. This extends the method of Vo et al. for gas condensates to these other systems. We concluded, in particular, that data above the phase envelope (single phase information) can be extrapolated and combined with data inside the phase envelop (two-phase information) to obtain initial hydrocarbons in place. The need for accurate fluid property description is emphasized and potential information that is obtainable from accurate fluid descriptions is noted. It is shown that data above the phase envelope can be extrapolated to obtain initial hydrocarbons in place.

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