Page 173 - Principles of Applied Reservoir Simulation 2E

158 Principles of Applied Reservoir Simulation

a much finer grid than can be used in a reservoir simulator. For example, a
computer mapping program such as that described by Englund and Sparks [ 1991 ]
or Pannatier [1996] may use a grid with a million or more cells to represent the
reservoir, yet reservoir simulation grids are usually 100,000 blocks or less. This
means that the reservoir representation in the computer mapping program must
be scaled up, or coarsened, for use in a reservoir simulator. Although many
attempts have been made to find the most realistic process for scaling up data,
there is no widely accepted up-scaling method in use today [for example, see
Christie, 1996; Dogru, 2000],

16.2 Grid Preparation

Reservoir grids may be designed in several different ways. For a review
of different types of grids, see Aziz [1993]. Definitions of coordinate system
orientation vary from one simulator to another and must be clearly defined for
effective use in a simulator. Reservoir grids can often be constructed in one-,
two-, or three-dimensions, and in Cartesian or cylindrical coordinates. Horizontal
ID models are used to model linear systems that do not include gravity effects.
Examples of horizontal ID models include core floods and linear displacement
in a horizontal layer. Core flood modeling has a variety of applications, including
the determination of saturation-dependent data such as relative permeability
curves, A dipping ID reservoir is easily defined in a model by specifying
structure top as a function of distance from the origin of a grid.
Figure 16-1 is an example of a 2D grid. Grids in 2D may be used to model
areal and cross-sectional fluid movement. Grid orientation in 2D is illustrated
by comparing Figure 16-lc and Figure 16-2. Although Figure 16-lc has fewer
blocks, which is computationally more efficient, Figure 16-2 may be useful in
some circumstances. For example, Figure 16-2 is more useful than Figure 16-1 c
if the boundary of the reservoir is not well known or an aquifer needs to be
attached to the flanks of the reservoir to match reservoir behavior.
The use of 2D grids for full field modeling has continued to be popular
even as computer power has increased and made large 3D models practical.
Figure 16-3 shows a simple 3D grid that is often called a "layer cake" grid.