Page 89 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 28

( a ) ( b )
p i p i
Z i Z i

p p
Z Z

p
Z ab

G / G RF (RF) comp G p G
p
Fig. 1.10 Graphical representations of the material balance for a volumetric depletion
gas reservoir; equ. (1.35)

Figure 1.10(a) shows how the recovery factor (RF) can be determined by entering the
ordinate at the value of (p/Z) ab corresponding to the abandonment pressure. This
pressure is dictated largely by the nature of the gas contract, which usually specifies
that gas should be sold at some constant rate and constant surface pressure, the latter
being the pressure at the delivery point, the gas pipeline. Once the pressure in the
reservoir has fallen to the level at which it is less than the sum of the pressure drops
required to transport the gas from the reservoir to the pipeline, then the plateau
production rate can no longer be maintained. These pressure drops include the
pressure drawdown in each well, which is the difference between the average reservoir
and bottom hole flowing pressures, causing the gas flow into the wellbore; the pressure
drop required for the vertical flow to the surface, and the pressure drop in the gas
processing and transportation to the delivery point. As a result, gas reservoirs are
frequently abandoned at quite high pressures. Recovery can be increased, however,
by producing the gas at much lower pressures and compressing it at the surface to
give the recovery (RF) comp, as shown in fig. 1.10(a). In this case the capital cost of the
compressors plus their operating costs must be compensated by the increased gas
recovery.

Figure 1.10(b) also illustrates the important techniques in reservoir engineering,
namely, "history matching" and "prediction". The circled points in the diagram, joined by
the solid line, represent the observed reservoir history. That is, for recorded values of
the cumulative gas production, pressures have been measured in the producing wells
and an average reservoir pressure determined, as described in detail in Chapters 7
and 8.

Since the plotted values of p/Z versus G p form a straight line, the engineer may be
inclined to think that the reservoir is a depletion type and proceed to extrapolate the
linear trend to predict the future performance. The prediction, in this case, would be
how the pressure declines as a function of production and, if the market rate is fixed, of
time. In particular, extrapolation to the abscissa would give the value of the GIIP which