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Chapte
r 4 – R ate Transient Analysis (RTA)- p141/743
Only problem is that the time function used by the first plot,
ta
involves the reservoir average
pressure hence it requires an estimate of the reserves.
In this Topaze example, the complete procedure follows:
(1) Estimate Gi beforehand.
(2) By selecting a time range where the system is believed to be in pseudo steady state, the
software performs an automatic regression to determine b using equation (1) and the method
defined above.
(3) Then the straight line method will be applied in the plot
Zp
/
versus Q, Pi/Zi can be input
to find Gi (STGIIP).
Like most of the methods extrapolating a behavior the constraint is that the well status and
production conditions must be constant in the interval used for the analysis.
4.C.5
P-Q diagnostic plot
The two methods presented above require selecting data set during the P.S.S. flow period.
Kabir et al. presented a cartesian plot of P vs q which provides a simple way to perform a
diagnosis.
The life of a production well can be divided in three type of behavior:
1.
The infinite acting radial flow
2.
A period during which the production is maintained and imposed by the completion.
3.
The Pseudo Steady State, when the well behavior is boundary dominated.
During the P.S.S. it is demonstrated that the slope dp/dq is governed by the equation:
dt
dq
Ahc
Bq
dq
dp
t
wf
2339 .0
In a closed system, the rate has an exponential decline, therefore, the slope dp/dq will be
constant and function of the drained volume.
A typical P vs Q plot behaviour would be:
Fig. 4.C.10 – Typical P vs Q plot