Page 161 - Principles of Applied Reservoir Simulation 2E

146 Principles of Applied Reservoir Simulation

usually consists of a multivariable table relating surface pressure to such
parameters as flow rate and GOR. The tables are often calculated using a separate
program that performs a nodal analysis of wellbore flow. Well models typically
assume that fluid phases are folly dispersed and that the block containing the
well is perforated throughout its thickness. Some commercial simulators will
let the user specify a perforation interval under certain conditions,
The different types of well controls include production and injection well
controls, and group and field controls for a surface model. The production well
model assumes the user specifies one option as the primary control, but may also
specify other options as targets for constraining the primary control. For
example, if oil rate is the primary control, then the produced GOR may be
restricted so that the oil rate is decreased when GOR exceeds the specified value.
This provides a more realistic representation of actual field practice.
Injection well controls assume that initial injection well mobility is given
by total gridblock mobility. This makes it possible to inject a phase into a block
that would otherwise have zero relative permeability to flow.
Allocation of fluids in a well model depends on layer flow capacity and
fluid mobility. The fluid allocation procedure in WINB4D is discussed in
Chapter 30. Simulators can also describe deviated or horizontal wells depending
on how the well completions and parameters are specified.
Well, group and field controls can be specified in commercial simulators
with a surface facilities model. The user specifies a hierarchy of controls that
most realistically represent how the field is being operated. For example, well
production may be constrained by platform separator and storage capacity, which
in turn is constrained by pipeline flow capacity. The ability to integrate reservoir
and surface flow technology using a single simulator is an area of research that
is receiving increasing attention [for example, see Heinemann, et al., 1998].

15.4 Simulator Solution Procedures

Fluid flow equations are a set of nonlinear partial differential equations
that must be solved by computer. The partial derivatives are replaced with finite
differences, which are in turn derived from Taylor's series [for example, see Aziz