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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapter
3 – P ressure Transient Analysis (PTA)- p69/743
3.B.8
IPR & AOF
A common measure of the performance of any gas, or oil, well is its ability to deliver against
atmospheric pressure. It is certainly unrealistic if we consider the bottom hole pressure;
however this measurement provides an input value for IPR calculations, and it is an accepted
‘universal indicator’ for gas wells. It is called the ‘Absolute Open Flow’ potential of the well. The
concept of AOF is more useful if one uses wellhead instead of bottomhole pressures, as this
indicates now the maximum achievable flowrate.
In order to evaluate the AOF, the well is tested at multiple rates. The bottom hole pressure is
measured during each drawdown and buildup. These data are plotted on the adequate plot in
order to deduce the leading parameters values for the equations governing the rate and the
stabilized flowing pressure relationship. The most classical plot is ∆m(p).vs q, on a log-log
scale , leading to the C and n parameters values of the Rawlings and Schellhardt equation:
n
wf
pmpmCQ
))
( )( (
The same equation is used to describe the Inflow Performance Relationship and to create the
very useful IPR plot m(p
wf
) versus q.
Fig. 3.B.11 – AOFP plot
Fig. 3.B.12 – IPR plot
The AOF is obtained by extrapolating the deliverability curve to a ∆m(p) value corresponding
to a flowing bottom hole pressure of 14.7 psi.
Cullender and Katz improved later the method in order to reduce the well test duration by
using short and non stabilized flow period data. The various well test types and IPR and AOF
methods are detailed in the chapter on Well Performance Analysis.