Static reservoir pressures are required for all gas material balance calculations. There are no techniques currently available for using isochronal, transient pressure buildup tests to estimate original gas in place.

A method to extrapolate the slope of a line generated from two or more isochronal or short-term pressure buildup tests to initial conditions to estimate gas reserves has been developed. Simulation as well as theoretical studies prove the technique may be applied to gas reservoirs with a permeability of one md or greater. However, the extrapolations become skewed for gas reservoirs displaying a permeability of less than one md. The isochronal test method may not be used with a high degree of accuracy for these low permeability reservoirs. The divergence of the extrapolations was found to be caused by the μc product term evaluated at a subsequent pressure differing significantly from the pc product term evaluated at static conditions. This pressure drawdown effect was found to only be significant in reservoirs displaying a permeability of less than 1 md.

Analysis of the fundamental concepts has formulated a relationship between the diffusivity equation and the material balance equation to account for changing fluid properties during pressure buildup. A method has been developed from these relationships to adjust the real time isochronal interval for the changing gas fluid properties. The adjustment technique permits the transient pressure, isochronal method to be applied to very low permeability reservoirs as long as the pressure is 1,000 psia or greater.

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