Page 235 - Hydrocarbon

222 Reservoir Simulation

reservoir, the more influence the gravity force will have, meaning that high dip
reservoirs are more likely to yield stable displacement. The above is an example of
a rate dependent process, in which the displacement rate affects the shape of the
displacement front, and possibly the UR. Physical effects such as this are the reason
for limiting the offtake rate from producing fields.

9.5. Reservoir Simulation

Reservoir simulation is a technique in which a computer-based mathematical
representation of the reservoir is constructed and then used to predict its dynamic
behaviour. The reservoir is gridded up into a number of grid blocks. The reservoir
rock properties (porosity, saturation and permeability), and the fluid properties
(viscosity and the PVT properties) are specified for each grid block.
The number and shape of the grid blocks in the model depend on the objectives
of the simulation. A 100 grid block model may be sufficient to confirm rate-
dependent processes described in the previous section, but a full field simulation to
be used to optimise well locations and perforation intervals for a large field may
contain up to a million grid blocks. The larger the model, the more time-
consuming to build, and slower to run on the computer.
The reservoir simulation operates based on the principles of balancing the three
main forces acting upon the fluid particles (viscous, gravity and capillary forces), and
calculating fluid flow from one grid block to the next, based on Darcy’s law. The
driving force for fluid flow is the pressure difference between adjacent grid blocks.
The calculation of fluid flow is repeatedly performed over short time steps, and at
the end of each time step the new fluid saturation and pressure is calculated for
every grid block (Figure 9.18).
The amount of detail input, and the type of simulation model depend on the
issues to be investigated, and the amount of data available. At the exploration and
appraisal stage it would be unusual to create a simulation model, since the lack of
data make simpler methods cheaper and as reliable. Simulation models are typically
constructed at the field development planning stage of a field life, and are
continually updated and increased in detail as more information becomes available.

One Dimensional Cartesian Radial Three Dimensional
Model Model
Two Dimensional Models
Figure 9.18 Typical grid block con¢gurations for reservoir simulation.