Black-Oil Simulations

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Introduction

When fluids flow from a petroleum reservoir to the surface, pressure and temperature decrease. This affects the gas/liquid equilibrium and the properties of the gas and liquid phases. The blackoil model enables estimation of these, from a minimum of input data.

The black-oil model employs 2 pseudo components:

  • Oil which is usually defined as the produced oil, at stock tank conditions.
  • Gas which then is defined as the produced gas at atmospheric standard conditions.

The basic modeling assumption is that the gas may dissolve in the liquid hydrocarbon phase, but no oil will dissolve in the gaseous phase. This implies that the composition of the gaseous phase is assumed the same at all pressure and temperatures.

The black-oil model assumption is reasonable for mixtures of heavy and light components, like many reservoir oils. The assumption gets worse for mixtures containing much of intermediate components (propane, butane), and is directly misleading for mixtures of light and intermediate components typically found in condensate reservoirs.

In DWSIM, a set of models calculates properties for a black oil fluid so it can be used in a process simulation. Black-oil fluids are defined in DWSIM through a minimum set of properties:

  • Oil specific gravity (SGo) at standard conditions
  • Gas specific gravity (SGg) at standard conditions
  • Gas-to-oil ratio (GOR) at standard conditions
  • Basic Sediments and Water (%)

Black oil fluids are defined and created through the Compound Creator tool. If multiple black-oil fluids are added to a simulation, a single fluid is calculated (based on averaged black-oil properties) and used to calculate stream equilibrium conditions and phase properties.

The Black-Oil Property Package is a simplified package for quick process calculations involving the black-oil fluids described above. All properties required by the unit operations are calculated based on the set of four basic properties (SGo, SGg, GOR and BSW), so the results of the calculations cannot be considered precise in any way. They can exhibit errors of several orders of magnitude when compared to real-world data.

For more accurate petroleum fluid simulations, use the petroleum characterization tools available in DWSIM together with an Equation of State model like Peng-Robinson or Soave-Redlich-Kwong.

Setup

Create Black-Oil compounds and load them into DWSIM using the Compound Creator.

Create a new simulation. Add Black-Oil compounds to it.

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Add a Black-Oil Property Package to the simulation. The Flash Algorithm setting is irrelevant because the Black-Oil PP has its own flash algorithm.

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Add a Material Stream to the simulation and set the amount of the black oil compounds in the mixture.

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When the Stream is calculated, the equilibrium phases will be displayed with all the required properties calculated, so you can go forward simulating your process with black-oil fluids.

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If you inspect fluid properties through the Pure Compound Properties Utility, you'll notice that the Black-Oil fluids only have black-oil properties defined. That's why they won't work at all with regular Property Packages.

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