This paper presents an analytical derivation that constitutes a sound theoretical background for gas flow equation. This gas flow equation is based on a new pseudotime function which, unlike the computation of material balance pseudotime, is insensitive to time step-size and purely analytical. This new pseudotime function offers a simpler approach to handle viscosity-compressibility variations since viscosity-compressibility ratio is a function of cumulative production. Unique to the proposed approach is that the flowing material balance method utilizes pseudocumulative, which is not a function of material balance pseudotime.

Currently, the analytical derivation given in the literature for gas flow equation involving material balance pseudotime has created the perception that material balance pseudotime is intuitive. An analytical derivation is given to show that material balance pseudotime function has sound theoretical basis.

Presently, iterative scheme, algorithms and graphical techniques involving a number of plotting functions have been proposed to solve gas-in-place. This paper presents a direct approach to solve gas-in-place when early pseudosteady state line is observed. Additionally, a technique is given to validate computed initial-gas-in-place; thus initial-gas-in-place computed by any method can be verified with the proposed technique.

The proposed analysis yields initial-gas-in-place, pseudosteady state constant and drainage area. Two simulated and one field published examples are presented to validate our proposed analysis.

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