Compositional model
In a compositional model, instead of a simple 2-component description (black oil models), there are many components as we need to describe the fluid. Typically this is somewhere between 7 and 20 components, with 10 or less being most common.
These components range from methane (C1), ethane (C2), and propane (C3), all the way to the heavy components. The lighter components may be individually modeled, while some of the heavier components may be modeled as a group.
In all compositional models, phase behavior is a function of pressure, temperature and composition.
Complicated processes involving the exchange of components and complicated phase behavior (miscibility, for example) can’t be described with a simple black oil approach, and require a compositional model. Many other simpler processes also require a compositional approach. These include situations in which we know that phase behavior is a strong function of pressure and composition, and situations in which fluid composition varies with time and space.
Examples include:
- highly volatile oil
- gas condensate cycling
- near critical fluid
- gas injection
- CO2 or N2 injection
All of these require a compositional model to resolve the PVT into components.