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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
r 4 – R ate Transient Analysis (RTA)- p143/743
4.D
The case of dry gas
In this paragraph we present the way the methods created for oil production analysis are
modified and adapted to take into account the gas properties specificity.
The original methods specific to gas (and only to gas) production analysis are directly
presented in the chapters ‘Old Stuff’ and ‘Right Stuff’.
4.D.1
Diffusion of real dry gas
As mentioned in the ‘Theory’ chapter, in order to extend the methodology of Dynamic Data
Analysis to gas cases, we introduce a function of the pressure called the pseudopressure. It is
given by:
Gas pseudopressure:
dp
Z
p
pm
p
0
2
Using the gas pseudo pressure instead of the pressure, the diffusion equation remains valid
the same methodology presented above can apply.
In addition to these methods, the gas case presents few particularities presented here.
4.D.2
The old Stuff
4.D.2.a
Fetkovich
It is worth noting that specific methods or extensions have been studied for gas production,
such as the Carter type-curve. When using the Fetkovich type curve with gas, the expression
of the rate match is modified to use the pseudo pressure m(p), in a similar manner to what is
done in Pressure Transient Analysis. In addition, Fraim and Wattenbarger suggested the use of
a normalized time rather than time itself, with the following definition:
dt
pcp
c
t
t
t
i t
n
0
The gas diffusion equation can then be re-written in terms of pseudo pressure m(p):
)(
0002637
.0
)(
2
pm
c
k
t
pm
t
When the pressure varies significantly, the term
t
c
and therefore the diffusion term varies.
If we introduce the pseudotime:
t
wf
n
d pI
t t
0
)) ( (
)(
where
pcp
c
pI
t
i t
)(
The diffusion equation becomes:
)(
0002637
.0
)(
2
pm
ic
k
t
pm
t
n
Note: the use of the normalised pseudo time requires the knowledge of the average pressure
at each step, that makes its use difficult.