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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
r 4 – R ate Transient Analysis (RTA)- p130/743
Above all, it is important to stress that decline curves have many limitations:
The bottom-hole pressure must be fairly constant.
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.
A refinement can be made for the case where the decline in the oil rate is caused by an
increase in the water cut, mostly in water drive reservoirs with unfavourable mobility ratio. If
one replaces the oil rate by the oil cut, f
o
the Arps equation can be used for wells with variable
gross production. The same plots can be made and extrapolated: log(f
o
) vs t, f
o
vs Q and
log(f
o
) vs Q.
4.B.2
Fetkovich
In 1980, Fetkovich introduced a type-curve combining the theoretical response of a well in a
closed reservoir, and the standard Arps decline curves. The motivation behind this work was to
come up with a loglog matching technique applicable to both the transient part of the data and
the boundary dominated flow period. By presenting both periods, the Type-Curve would avoid
incorrectly matching transient data on decline curves.
A determining preliminary step was that the exponential decline can be shown to be the long-
term solution of the constant pressure case. The Fetkovich type-curve is derived assuming a
slightly compressible fluid and constant flowing pressure. Extension to gas can be made with
the appropriate dimensionless rate expression, as described below. The original type-curve
presented by Fetkovich displayed rate only. A composite presentation including the cumulative
was later introduced to bring more confidence in the matching process and to reduce the effect
of the noise.
Fig. 4.B.3 – Fetkovich type curve