In field operations of volatile oil and gas condensate reservoirs, separator conditions often change because these volatile fluids are usually separated through three and more separation stages, when wellhead pressure allows. The well stream is usually rerouted to lower pressure separators at later stages of the field life.

Although the reservoir simulation is widely used for recoverable reserves calculations, material balance equation is still used as a fast and easy method for reserves and forecast calculations. The conventional material balance equation does not allow accurate calculations for hydrocarbons in-place when separator conditions change during the history of the field production, since PVT properties are usually calculated for specific separator conditions in black-oil and modified black-oil approaches.

In this work, a new material balance equation was developed. This equation can be easily used and will allow accurate calculations of hydrocarbons in-place when separator conditions change during the production history. The inputs to the equation can be easily obtained from the equation of state model (EOS) which is used to generate the black oil or modified black oil PVT tables. In some cases, PVT properties can be also calculated at different separator conditions using newly published correlations that take separator conditions into consideration.

To validate the new equation, many reservoir fluids covering a wide range of black and volatile oil reservoirs (and their associated PVT tables) were used. The new equation was applied to calculate oil in-place using the production and pressure data from many test cases generated by fully compositional simulation (note that compositional simulation takes into consideration the effects of changing separator conditions on production data during the simulation time). The new material balance was found to give results with close agreement to the compositional simulation model.

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