Field data and simulated models have revealed that in some cases waterdrive gas reservoirs can be mistakenly misidentified using material balance methods as depletion drive gas reservoirs causing a significant overestimation in gas reserves.

The famous straight-line plot of p/z vs. Gp has been traditionally used to estimate original gas in place (and gas reserves) for depletion-drive gas reservoirs. A gas reservoir in contact with an aquifer in transient phase (unsteady-state) and producing under a certain production schedule can plot as a straight-line on a p/z plot masking the existence of an active aquifer and causing a significant overestimation in gas reserves.

The authors in this paper simulate synthetic cases of gas reservoir/aquifer models using a forward model and an inverse model that were programmed in visual basic to show that the combination of certain rate schedules and the unsteady state nature of aquifers can cause a straight-line p/z plot in waterdrive gas reservoirs.

The authors also demonstrate that the Havlena-Odeh1,2  Plot (also known by some authors in the liteariture as the Cole3  Plot) for those same cases will also mask the existence of an active transient aquifer giving the same value of overestimated origianal gas in place (OGIP) as that obtained from the p/z plot.

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