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Black Oil Characterization in Symmetry

Introduction

In certain circumstances, users may have very limited information to characterize a fluid because a laboratory analysis may not have been measured or the data may have been artificially generated. In these cases, a Black Oil Characterization provides a way of calculating the fluid’s compositional information based on limited data.

The minimum input required for a Black Oil Characterization is the gas gravity (relative to air), the oil specific gravity (relative to water), the solution gas to oil ratio and the water cut (if water is present in the system).

The objective of this communication is to present the implementation of the Black Oil Characterization option in Symmetry. This characterization procedure takes advantage of the PIONA Characterization technique and Oil Source unit operation to create the oil’s components slate and find the composition that matches the given experimental data.

Black Oil Characterization

The Black Oil Characterization offers users an opportunity to take a limited input specification (known as Black Oil input) for a condensate or an oil to generate a compositional analysis.

To do a Black Oil Characterization in Symmetry, Natural Gas components like Nitrogen, Methane, Ethane, Propane, Iso-Butane and n-Butane must be added to the Thermodynamic Model. If the Water Cut is to be calculated, then Water must be also added.

The minimum required input is the gas gravity (relative to air), the oil specific gravity (or oil density or API gravity) and the solution Gas to Oil Ratio (GOR) or the Gas to Liquid Ratio (GLR) at reference conditions. GOR is the volume of gas produced at surface standard conditions divided by the volume of oil produced at reference conditions. GLR is the volume of gas produced at surface standard conditions divided by the volume of total liquid (oil + water) produced at reference conditions. In lieu of the gas gravity, a Gas Analysis can be provided, in this case, the gas composition must be provided.

If water is part of the system, then the Water Cut must be also provided, the Water Cut is defined as the ratio of water produced compared to the volume of total liquids (oil + water) produced.

In addition, physical properties of the oil like density, viscosity, PIONA and SARA distributions as well as the Saturation Pressure of the bulk fluid at Reservoir Temperature can also be added.

Black Oil Application in Oil Source

The Oil Source unit operation contains the means to perform a Black Oil Characterization. To start a new Black Oil Characterization, the Black Oil Application option must be selected in the Summary tab of the Oil Source.

The Black Oil inputs can be entered from the Summary Tab. Here, the user can select which experimental variables will be used to characterize the Black Oil.

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Black Oil Analysis Input

The available variables for a Black Oil input are Gas Specific Gravity, Oil Density (or Oil API or Specific Gravity), GOR, GLR, Water Cut and Saturation Pressure of the fluid. To add the variables to the characterization, the Active box of each one must be selected and the Specified value must be entered. The Scale column shows a factor used to weight the variables during the regression process.

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Options

The following options can be used to customize the Black Oil characterization.

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Gas Analysis

Checking this box will enable a tab to enter the gas composition that is used to calculate the Gas Specific Gravity. The user has the option to include or not in the analysis of acid gas components. The C6 and C7+ fraction can be entered as zero if that is the case. If the C7+ fraction is given, the MW must be provided as well.

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Spec. Gas Contaminants

This option is available only when the Gas Specific Gravity is Activated. Checking this box enables a tab to specify the mole fraction of Gas Contaminants like N2, H2S, CO2, H2 and CO. The user has the option to include or not acid gas components.

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Oil Density Unit As

Defines the Oil Density input type (Mass density, API Gravity or Specific Gravity) to be used in the Black Oil characterization.

Reference Conditions

The liquid reference temperature and pressure can be added here. The reference conditions can be set for the current unit operation (Local) or for the entire flowsheet (Global).

Saturation T

This is the temperature at which the saturation pressure of the fluid will be calculated.

Additional Data

Here PIONA and SARA distributions can be added as well as oil density and viscosity vs temperature curves.

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Example

The following example will go through the necessary steps to characterize a Black Oil.

The property package employed in this example is Advanced Peng-Robinson, once the package is selected the following components need to be added:

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Then, a PIONA slate is built, to create the slate the new “Black Oil” characterization style can be used.

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This new style is very simple to use, the only parameter that must be provided is the Fluid Type which can be set to Oil, Heavy Oil and Bitumen. Depending on the selection, the final boiling point of the slate will be internally set. The initial boiling point is set to 0 C.

For Black Oil characterization it is important to create a low number of pseudo-components; the number of components per PIONA family can vary from 3 to 5 depending on the selected Fluid Type. For this example, the Fluid Type is set to Oil, the P, I, N and A families are selected and the Create Slate button is clicked to obtain the PIONA pseudo-components.

Now, go to the Flowsheet environment and add an Oil Source unit operation. The first thing that we need to do is to select the new Black Oil application type.

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The Black Oil Application will bring new grids that can be used to enter Black Oil inputs like Gas and Oil Specific Gravity, Gas to Oil Ratio and Water Cut.

The Oil Specific Gravity is technically the Oil Density at Reference Conditions, if this value is given in density or API gravity units, then it can be entered in those units by selecting the unit type in the drop-down menu from the Options grid.

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The Options grid also can be used to enter the Reference Conditions used in the Oil Specific Gravity, Gas to Oil Ratio and Water Cut calculations.

Now, enter the following Black Oil input:

  • Gas Specific Gravity = 0.65
  • Oil Specific Gravity = 0.88 (54.88 lb / ft3)
  • Gas to Oil Ratio = 950 SCF/bbl
  • Water Cut = 75%

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Note that the Gas to Oil Ratio and Water Cut are matched out of the box, we need only need to adjust the oil and gas composition to match the oil and gas specific gravity.

Go to the Settings tab and observe that the Regression Parameters are set to the default values for a Black Oil Fluid, change the regression tolerance to 1.0 E-3 and then click on Regress Parameters a couple of times to find the best combination of compositions that match the Black Oil input.

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Observe that after the regression the calculated and experimental data are the same.

To test the results, we can connect a Material Stream to the Oil Source and separate the Oil, Gas and Water at the reference conditions (60 F and 1 atm) using a three-phase separator; then, the gas and oil stream properties can be compared to the experimental data.

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To learn more about Black Oil Characterization in Symmetry please contact your local support office.

Herbert Loria, Ph.D., P.Eng., VMG Calgary

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