Summary
Approaches are presented for reducing the computation time spent on flash
calculations in compositional, transient simulations. In a conventional flash
calculation, the majority of the simulation time is spent on stability
analysis, even for systems far into the single-phase region. A criterion has
been implemented for deciding when it is justified to bypass the stability
analysis. With the implementation of the developed time-saving initiatives, it
has been shown for a number of compositional, transient pipeline simulations
that a reduction of the computation time spent on flash calculations by
approximately 85 to 90% can be achieved.
Introduction
Modeling of oil production requires simulations of transient flow. Examples
are miscible-gas displacement in petroleum reservoirs and multiphase flow in
pipelines, including slug formation and startup scenarios. In a typical
transient-flow simulation, the system is discretized into a number of cells or
sections. Phase amounts and phase properties are needed for each cell or
section to solve the conservation equations in the model, and specifically for
transient pipeline simulations to calculate such properties as heat loss to the
surroundings, liquid holdup, and pressure drop. If the overall composition is
constant during the simulation, the phase properties can be stored in
precalculated tables listing the needed properties as a function of pressure
and temperature (Bendiksen et al. 1991; Xu and Shea 2001). This is referred to
in the following sections as a noncompositional, table-based simulation.
When simulating miscible-gas displacement in reservoirs, the assumption of a
constant overall composition is not adequate because the injection gas will
dissolve in the reservoir fluid and vice versa. Similarly, in many typical
multiphase pipelines, the fluid composition will vary because of velocity
differences between phases, interfacial mass transfer, and merging networks. In
these situations, a compositional approach is useful. A compositional model has
the drawback that the computation time is much higher than that of a
noncompositional table-based approach. The phase amounts and properties must be
evaluated in each cell or section in each timestep. Furthermore, phase
compositions are required to calculate the interfacial mass transfer.
Nevertheless, the increased accuracy in the fluid description makes the
compositional approach attractive to represent multiphase pipelines and
reservoirs with large compositional variations.
Flash Equilibrium Calculations in Compositional, Transient Simulations
The physical properties of a fluid in a cell or section depend on whether
the fluid is present as a single phase or splits into several equilibrium
phases. A flash calculation is therefore required in each timestep to determine
the number of equilibrium phases and their amounts and compositions. Even with
relatively few mixture components, the computation time of a compositional,
transient simulation far exceeds that of a corresponding noncompositional,
table-based simulation; furthermore, the computation time increases with an
increased number of components.
© 2006. Society of Petroleum Engineers
View full textPDF
(
440 KB
)
History
- Original manuscript received:
29 October 2003
- Meeting paper published:
5 October 2003
- Revised manuscript received:
6 October 2005
- Manuscript approved:
8 October 2005
- Version of record:
20 February 2006
View Cart
