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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
r 3 – P ressure Transient Analysis (PTA) -p123/743
The classical way of determining the skin versus rate relationship is to use semilog analysis of
each flow period if this analysis is valid. Then plot the resulting skin versus the corresponding
rate as illustrated in the figure below. This will return the skin without turbulence ie: the
mechanical skin, the intercept of the assumed straight-line, S
0
and the rate dependency D
derived from the slope. Here it is important to remember that a flow/after flow test with only
one single shut-in may not produce the required results as a semilog analysis of a producing
period may be impossible due to the inherent fluctuation of the rates masking the pressure
response where semilog analysis is valid.
It is therefore recommended to use the type of tests that have intermediate shut-ins,
isochronal or modified isochronal, where we stand a better chance of obtaining the skin from
the analysis of these shut-ins. The plot of skin versus rate is done automatically in Saphir. The
results can then automatically be transferred and the model generated with regression to
improve the model match as necessary.
If the skin versus rate plot fails because semilog analysis is not valid, the rate dependency
parameters cans still be set in the model dialog using an initial guess. Then the model match
can be refined by regression.
Fig. 3.J.5 – Skin versus rate plot
The figures below show the comparison between a simulation run with a constant skin and a
rate dependant skin:
Fig. 3.J.6 – Isochronal test matched with a constant skin