Gas reservoir performance data were generated numerically to study the magnitude of three energy sources in abnormal pressured reservoirs. It was found that pressured reservoirs. It was found that peripherial water influx and formation peripherial water influx and formation expansion made the greatest long-term energy contribution. Based on the generated data, a technique is presented for determination of gas-in-place. Attempts were made to apply the technique to conventional field data with inconsistent success.


The petroleum industry has become indoctrinated into evaluating all gas reservoirs based on the P/Z - Gp Plot. This indoctrination has been so complete that gas reservoirs that deviates from a straight line indicates water-influx or gas-efflux. During the entire period of indoctrination, it has been known that very restrictive assumptions were made in order to justify the P/Z - Gp straight line relationship. These assumptions are:

- No loss or gain of energy from outside forces;

- The connate water does not expand;

- The pore space does not contract;

- The gas is always single phase in the reservoir; and

- The pressure used in the equation represents the volumetric average reservoir pressure.

In many field examples these limitations placed little restriction on the accuracy of the P/Z - Gp plot. With the advent of low permeability, high condensate, high rock compressibilities, or limited water influx the P/Z - G curve began to yield apparently correct, but unfortunately erroneous answers in the early life of a gas reservoir. This divergence from the norm has resulted in considerable re-evaluation of existing techniques of gas reservoir analysis. This re-evaluation has been most noticeable in the area of abnormal pressured gas reservoirs.

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