Page 92 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 31

mathematical model to describe the reservoir performance based on insufficient history
data can produce erroneous results when used to predict future reservoir performance.

If production-pressure history is available it is possible to make an estimate of the GIIP,
16
in a water drive reservoir, using the following method as described by Bruns et.al . The
depletion material balance, equ. (1.34), is first solved to determine the apparent gas in
place as
G
G = p (1.42)
a
1 − E /E i
If there is an active water drive, the value of G a calculated using this equation, for
known values of E and G p, will not be unique. Successive, calculated values of G a will
increase as the deviation of p/Z above the depletion material balance line increases,
due to the pressure maintenance provided by the aquifer. The correct value of the gas
in place, however, can be obtained from equ. (1.40) as

G — W E
G = p e (1.43)
1E /E i
−
where W e is the cumulative water influx calculated, using some form of mathematic
aquifer model, at the time at which both E and G p have been measured.

W - too small
e
G a W - correct model
e

W - too large
e

G

W E/ (1−E / E) i
e

Fig. 1.12 Determination of the GIIP in a water drive gas reservoir. The curved, dashed
lines result from the choice of an incorrect, time dependent aquifer model;
(refer Chapter 9)

Subtracting equ. (1.43) from equ. (1.42) gives

WE
G = G + e (1.44)
a
1 − E /E i
If the calculated values of G a, equ. (1.42), are plotted as a function of W eE/(1−E/E i) the
result should be a straight line, provided the correct aquifer model has been selected,