Page 188 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 173

and the geometry of the grid. Do we need special grid options, such as radial
coning or local grid refinement, or will Cartesian coordinates be satisfactory?
If the study is designed to investigate near wellbore flow, it would be wise to
select a grid that provides good spatial resolution near the wellbore, for example,
radial coordinates. On the other hand, if the study is intended to provide an
overview of field performance, a coarse Cartesian grid may be satisfactory.
The level of complexity of the geology will influence grid definition, and
in the case of fractured reservoirs, the type of flow equations that must be used
[for example, see Reiss, 1980; Aguilera, 1980; Golf-Racht, 1982; and Lough,
et al, 1996]. A highly faulted reservoir or a naturally fractured reservoir is more
difficult to describe numerically than a homogeneous sand.
Model selection will be influenced by the types of processes and drive
mechanisms that dominate flow in the reservoir. Processes range from gas cap
drive and water drive under primary depletion, through water or gas injection
in pressure maintenance programs, to miscible or thermal flooding in enhanced
recovery projects. The choice of model will vary depending on the anticipated
process. For example, dry gas injection in a condensate reservoir is typically
modeled with a compositional simulator, while steam flooding a heavy oil
reservoir should be modeled with a thermal simulator.
A few guidelines are worth noting with regard to simulator selection.
Many novice modelers make the mistake of selecting models that are much more
complex than they need to be to satisfy the objectives of the study. According
to Coats [1969], the modeler should "select the least complicated model and
grossest reservoir description that will allow the desired estimation of reservoir
performance." This is a restatement of Ockham's Razor.
William of Ockham, a fourteenth-century English philosopher, said
"plurality must not be posited without necessity" [Jefferys and Berger, 1992],
Today this is interpreted to mean that an explanation of the facts should be no
more complicated than necessary. We should favor the simplest hypothesis that
is consistent with the data.
Ockham's Razor should be applied with care, however, because one of
the goals of a model study is to establish a consensus about how the reservoir
behaves. This consensus is political, to an extent, because the model must satisfy