Page 140 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 125

Table 13-3
Examples of Cubic Equations of State
RT aiT Vl
Redlich-Kwong P
V-b V(V+b)
RT a(T)
Soave-Redlich-Kwong P
V-b V(V+b)
RT a(T)
Peng-Robinson P
V-b V(V+b) + b(V-b)

RT a(T)IT V2
Zudkevitch-Joffe P
V-b(T) V[V+b(T)]

Equations of state are valuable for representing fluid properties in many
situations. For example, suppose we want to model a system in which production
is commingled from more than one reservoir with more than one fluid type. In
this case the most appropriate simulator would be a compositional simulator
because a black oil simulator would not provide as accurate a representation of
fluid behavior.
The two most common types of reservoir fluid models are black oil
models and compositional models. Black oil models are based on the assumption
that the saturated phase properties of two hydrocarbon phases (oil and gas)
depend on pressure only. Compositional models also assume two hydrocarbon
phases, but they allow the definition of many hydrocarbon components. Unlike
a black oil simulator, which can be thought of as a compositional simulator with
two components, a compositional simulator often has six to ten components. By
comparison, process engineering simulators that are used to model surface
facilities typically require up to 20 components or more. The cost of running
a compositional simulator increases dramatically with increases in the number
of components modeled, but the additional components make it possible to more
accurately model complex fluid phase behavior. If compositional model results
are to be used in a process engineering model, it is often necessary to compro-
mise on the number of components to be used for each application.