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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
r 4 – R ate Transient Analysis (RTA)- p144/743
They demonstrated that the real gas constant pressure solution, when plotted using
normalized time, exhibits a boundary-dominated period that matches the exponential decline.
The major drawback however, is that the normalized time expression requires an a priori
knowledge of the average pressure, hence of the reserves. The method is thus iterative in
nature.
Note: if the normalized time is not used a gas response in depletion mode will not necessarily
follow the exponential decline.
Using Fetkovich type-curves
It is important to remember that the Fetkovich type-curve is based, for the depletion part, on
the Arps decline curves. Like the decline curves it suffers some limitations:
-
It is assumed that the bottom-hole pressure is fairly constant. Fetkovich suggests that if
the pressure is smooth, and uniformly decreasing, one could use a
p normalized rate.
-
The well behavior is assumed constant, e.g. no change in skin with time.
The drainage area of the considered well is constant, i.e. the producing behavior of
surrounding wells must also be stabilized.
4.D.2.b
Gas material balance
ZP
vs Q plot
This method is specific to gas. It is not an ‘adapted’ oil method to gas, it is presented in the
chapter ‘The old stuff’.
4.D.3
The right stuff
4.D.3.a
Blasingame plot
The cornerstone of the Blasingame plot is the linearity between the normalized rate
q t
p p t
i
w
and
1
t
e
during boundary dominated flow. This relation, valid for a slightly
compressible fluid, does not apply to gas unless the rate is normalized with respect to
m p
and the time is modified as follows:
t
t
g
g
g
ti gi
egas
dt
pcp
tq
q
c
t
0
) ( ) (
)(
)0(