Material Balance methods for estimating gas reserves are widely used and, in general, are much preferred over volumetric methods where conditions are favorable and production information is available. Although plotting pressure divided by compressibility, P/Z, against cumulative production on
cartesian coordinates is generally accepted, the log-log plot is of historical interest, but the method has serious disadvantages-and some compensating advantages-when applied to new wells in new reservoirs. Regardless of the type of plot used to interpret P/Z and, cumulative production data, pressures representative of the true reservoir pressure are a necessity.
The simple material balance method ilustrated is virtually useless for reservoirs that have active water drives. Also applying the method to wells in low-permeability reservoirs is difficult where pressure builds slowly after the well is shut in. Liquid production and accumulating in the wellbore add to the difficulties.
The effect of water influx on P/Z-cumulative-gas-production curves has been described in detail and has been reviewed. The performance of a gas reservoir containing 469 billion Ft3 of gas in place has been calculated for aquifer sizes 1.5 and 10 times the size of the gas reservoir.
After the calculated production of 300 billion ft3, or 64 % of the gas in place, theP/Zcumulative production curve for the gas reservoir in contact with the small aquifer indicates gas in place of 475 billion ft3, or 101.3% of the gas actually in place. For a gas reservoir. The curve would indicate 990 billion ft3 or 211% of the actual gas in place. Thus it was concluded that it is dangerous to extrapolate P/Z cumulative production curves by using a straight line without considering the possibility of water influx.
Unfortunately, similar apparent behaviour can be caused early in the productionn life of a well by combination of flow rates, unsteady-state behaviour, and insufficient time for buildup of the shut-in pressure. This behavior is characteristic of wells in reservoirs with low permeability. At first, the well is put on production at very high rates of flow. The short-term shut-in pressures show rapid decrease from the original shut-in pressure. Then, the operator cuts the production rate and shut-in pressures tend to stabilize. Again, interpreting the P/Z cumulative production curve brcomes difficult. Regardless of reservoir characteristics, stabilization time, or producing techniques, a minimum of 5% of the gas in place should be produced before confindende can be placed in a material balance forecast.