Page 180 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 165

for use in full field models, especially with regard to layering. Full field models
require sufficient layering to track fluid contact movement or other depth de-
pendent information that is needed to achieve study objectives. Window area
models have the disadvantage of not being able to accurately model flux across
window area boundaries. This means that effects of wells outside the window
area are not taken into account except through boundary conditions. Some
commercial simulators will output time-dependent boundary conditions for use
in window area models. Although this information is helpful, the process is
cumbersome and does not necessarily yield accurate results. Field history can
be used to guide development of the window area model, but has only limited
utility as a criterion for validating window model performance. Heinemann
[1995] has discussed further concepts and applications of a dynamic windowing
technique that is designed to minimize the difficulties of preparing and applying
window area models in conjunction with full field models.
One of the most useful types of models is the conceptual model.
Conceptual models can be built quickly and require only an approximate
description of that part of the reservoir that is relevant to the conceptual study.
Computer resource requirements are relatively small when compared with full
field or window area models. Results of the conceptual model are qualitative
and best used for comparing concepts such as vertical layering. They can also
be used to prepare pseudo curves for use in full field or window area models.
For example, the saturation of a block in a model with a transition zone depends
on the depth of the center-point of the block (see Chapter 6). As a result, a grid
that is vertically coarse may have only a rough approximation of the transition
zone. More accurate modeling of saturation gradient in a transition zone requires
vertical grid refinement or use of pseudo curves. Conceptual models are useful
for preparing such pseudo curves. The disadvantage to conceptual models is that
their results do not apply directly to the description of a particular field. Since
there is no history match, conceptual model results should be viewed as
qualitative rather than quantitative estimates of field performance. They do
provide useful qualitative information that can be applied to specific fields in
window area and full field models.