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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapter
3 – P ressure Transient Analysis (PTA)- p86/743
From experience, this combination does not bring much. The problem with the Levitan method
is that each deconvolution will be the simple combination of a given build-up and a single
value of initial pressure. Even if we add one or two log-cycles between the build-up and the
deconvolution, all this addition will be coming from a single pressure value, and therefore
deconvolution will have no chance to pick-up intermediate behaviors.
3.D.4.b
Variant 2: Using Method 3 on all build-ups
Method 3 consists in picking the convergence time, selecting a build-up and running the
optimization. This can be successively applied to all build-ups.
Compared to the Levitan method, we get the same number of deconvolution responses and
the post-processing is the same. However it has the advantage of using a ‘proven’
convergence time instead of a wildly guessed initial pressure, and there is no reiteration. In
complement, each deconvolution uses positive information from all build-ups, and it is possible
to pick intermediate behaviors that are beyond the reach of the Levitan method.
There is no guarantee that the late behavior of all deconvolution curves will be the same. This
will be the case only if the material balance information is consistent. If not, it will point out
that the deconvolution process may not be valid, either because of poor data or because the
hypotheses behind this deconvolution are not valid
3.D.4.c
Remaining limitations
The deconvolution method presented in this paper clears some of the limitations of the von
Schroeter et al. method without some of the inconvenience of the Levitan method. However
limitations remain and should not be overlooked:
Deconvolution works if convolution, i.e. the principle of superposition is valid. This in
turn implies that the equations governing our system are linear. This rules out any
nonlinearity such as nonDarcy flow, multiphase flow, etc. In such case the optimization
will give ‘something’ but this something may be plain wrong and misleading.
For any practical purpose, the new deconvolution only works on shut-in periods. This
observation is the basis of this Method 3, which otherwise would not be valid.
Attempts to integrate interference wells in the deconvolution process have failed so far.
Deconvolution apparently adds one or two log cycles but there is no magic. We had this
information before. When interpreting a build-up, a properly trained engineer checks
the coherence of the model on the pressure history plot. If the simulation was
inconsistent it was the sign that ‘something’ had affected the long term response that
was not detected during this build-up. Adding this ‘something’ would often amount to
adding some late time boundary behaviors, in order for the model to follow the data
throughout the well history. The new deconvolution just does that, but in a very elegant
way.
Deconvolution is not a video game that turns bad data into good ones… When
successive build-ups are inconsistent, the deconvolution optimization will fail. At best it
will be specularly, but in the worst case it will look OK and will be misguiding.
With Method 3 different wellbore storage and skin factors may be handled. However the
rest of the model must remain constant. The idea of getting all shut-ins from ten years
of a permanent gauge, then run a deconvolution is perfectly ludicrous. In real life the
system changes and the convolution of a single model over several years just does not
make sense.