Page 179 - Principles of Applied Reservoir Simulation 2E

164 Principles of Applied Reservoir Simulation

small blocks. An LGR grid is an example of a flexible or unstructured grid. A
flexible grid is made up of polygons in 2D (polyhedra in 3D) whose shape and
size vary from one subregion to another in the modeled region.
Although many grid preparation options are available, improving grid
preparation capability is an on-going research and development topic. For
example, not all flow simulators use a finite difference formulation. Some are
based on a control volume finite element formulation that use triangular meshes
in 2D (tetrahedral meshes in 3D). Finite difference grids typically display global
orthogonality in which the grid axes are aligned along orthogonal coordinate
directions. Examples of globally orthogonal coordinate systems include the
Cartesian x-y-z system and the cylindrical r-Q-z system. Grids with global
orthogonality may be distorted to fit local irregularities such as faults using
corner-point geometry. By contrast, finite element grids display orthogonality
in which gridblock boundaries are perpendicular to lines joining gridblock nodes
on opposite sides of each boundary. An example of a locally orthogonal grid
is a perpendicular bisector (PEBI) grid. Aziz [ 1993], Chin [1993], Heinemann
[ 1994], Verma and Aziz [ 1997], and Heinemann and Heinemann [ 1998] provide
additional discussion of grid preparation research.

16.3 Model Types

Models may be classified into three different types: full field models,
window area models, and conceptual models. Full field models are used to match
performance of the entire field. They take into account the interaction between
all wells and layers. The results of full field models are already matched to field
scale and require no further scaling. The disadvantage of using full field models
is that the number of grid blocks may need to be large or the grid size may need
to be relatively coarse to include the entire field.
Window area models are designed to look at smaller areas of the field.
These models are often constructed from a full field description. Window area
models allow finer grid resolution or shorter turnaround time if the model runs
faster than a full field model. The window area models are useful for studying
recovery mechanisms and for determining reasonable grid preparation criteria