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Black Oil

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Fluid Types: Black OilVolatile OilGas condensateWet gasDry gasFluid Types Comparison

Black oil: Crude oil, or distilled crude containing the fractions heavier than middle distillates.
It also applies to Black oil model used in reservoir simulation

Black oil model

The black oil model is a simple fliud model, two-component description used for reservoir simulation. In the most classic black oil model we have:

  • stock tank oil - reservoir oil with separator gas dissolved in it
  • separator gas - the vapor phase under reservoir conditions

In the “classic” black oil model, the reservoir oil may be a mixture of stock tank oil and separator gas, but the separator gas is always gas only.

In all black oil models, phase behavior is entirely a function or pressure.
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.

Phase diagram

BO phase diagram.jpg
From the shape of the phase diagram, black oil is a fluid type made up of lots of heavy end components. (the phase diagram for heavy components is elongated, exists over a broad range of temperatures and pressures, and lays down to the right.)
The quality lines within the phase envelope for black oil are evenly spaced. This indicates that when moving from a liquid to a gas, a consistent increment of pressure drop changes the ratio of liquid to gas by a constant amount.


Reservoir state liquid
Initial GOR less than 2000 scf/STB
Producing GOR increases when reservoir pressure drops below bubble point
Initial STO (McCain) less than 45oAPI, and decreasing over time, then ultimately back up
Color very dark, brown or black
Bo at bubble point less than 2.0 rb/STB
C7+ Mole Fraction greater than 20% (+/- 2%)
Other produced gas is usually a dry gas

See also

Compositional model