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184 Principles of Applied Reservoir Simulation

Modelers must resist being drawn into the "one more run" syndrome. This
occurs when a modeler (or member of the study team) wants to see "just one
more run" to try an idea that has not yet been tried. In practice, a final match is
often declared when the time or money allotted for the study is depleted.

18.5 History Match Limitations

History matching may be thought of as an inverse problem. An inverse
problem exists when the dependent variable is the best known aspect of a system
and the independent variable must be determined [Oreskes, et al, 1994]. For
example, the "dependent variable" in oil and gas production is the production
performance of the field. Production performance depends on input variables
such as permeability distribution and fluid properties. The goal of the history
match is to find a set of input variables that can reconstruct field performance.
In the context of an inverse problem, the problem is solved by finding a
set of reasonable reservoir parameters that minimizes the difference between
model performance and historical performance of the field. As usual, we must
remember that we are solving a non-unique problem whose solution is often as
much art as science. The uniqueness problem arises from many factors. Most
notable of these are unreliable or limited field data and numerical effects.
Advances in hardware and software technology have made it possible to
minimize the effects of numerical problems, or at least estimate their influence
on the final history match solution. Data limitations are more difficult to resolve
because the system is inherently underdetermined: we do not have enough data
to be sure that our final solution is correct.

Test of Reasonableness
A model may be considered reasonable if it does not violate any known
physical constraints. In many cases, a model may be acceptable if it is reason-
able. In other situations, not only must physical constraints be satisfied, but
approved processes for evaluating data must also be followed. Thus a model may
be reasonable, but if it is based on an innovative technique that is reasonable but
not approved, the model will be unacceptable. The modeler may use a method