Page 192 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 177

gridblocks between each gridblock containing a well), but this may mean loss
of vertical definition. A way to resolve the problem is to set up one or more
cross-section models that represent different parts of the field. Vertical confer-
mance effects in these regions are modeled in detail by calculating flow
performance with the cross-section models. The flow performance of a detailed
cross-section model is then matched by adjusting relative permeability curves
in a model with fewer layers. The resulting pseudo-relative permeability curves
are considered acceptable for use in an areal model
Another aspect of model initialization is equilibration. This is the point
at which fluid contacts are established and fluid volumes are calculated.
Resulting model volumes should be compared with other estimates of fluid in
place, notably volumetric and material balance estimates. There should be
reasonable agreement between the different methods (for example, within two
percent). Finally, the history match can begin.

18.1 Illustrative History Matching Strategies

A universally accepted strategy for performing a history match does not
exist. History matching is as much art as science because of the complexity of
the problem. Nevertheless, there are some general guidelines that can help move
a history match toward successful completion. These guidelines have been
presented by such authors as Crichlow [1977], Mattax and Dalton [1990],
Thomas [1982], and Saleri, et al. [1992]. One set of guidelines is presented in
Table 18-1. The first two steps in the table take precedence over the last two,
If the first two steps cannot be achieved, there is a good chance the model is
inadequate and revisions will be necessary. An inadequate model may be due
to a variety of problems: for example, the wrong model was selected, the
reservoir is poorly characterized, or field data is inaccurate or incomplete.
Among the data variables matched in a typical black oil or gas study are
pressure, production rate, water-oil ratio (WOR), gas-oil-ratio (GOR), and tracer
data if it is available. More specialized studies, such as compositional or thermal
studies, should also match data unique to the process, such as well stream
composition or the temperature of produced fluids.