1. Heptanes-Plus Characterization
Introduction
Molar distribution
Experimental Analysis
Mathematical modeling
Grouping (defining Pseudo components)
Properties estimation (Pseudo components)
The properties for Needed for EOS calculations are:
Tc and Pc
Acentric factor
Volume-translation coefficient
Binary Interaction parameter
Z-factor (viscosity)
Exercises
TBP data in monograph
Effect of changing Gamma Distribution Parameters
Distribution and specific gravity correlation
Grouping
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2. Introduction Heptanes-Plus Characterization
The Heptanes plus in a reservoir fluid contains hundreds of different components. It is impossible with chemical separation techniques to identify these components.
Even if we had identified them, it would not be possible to measure the critical properties and other EOS parameters for fluids heavier than C20.
This problem is solved practically by making approximate characterization of the heavier compounds with experimental and mathematical methods.
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3. Introduction Heptanes-Plus Characterization
The approximate procedure can be split into three main tasks.
Dividing the C7+ into a number of fractions with known molar compositions.
Defining the molecular weight, specific gravity, and boiling point of each fraction.
Estimating the critical properties (Tc,Pc), Acentric factor, volume shift (Si), and the BIP`s for each of the fractions.
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4. Molar Distribution of C7+
The molar distribution of the C7+ can be found from two experimental analysis
TBP Distillation
GC chromatography
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5. Molar Distribution of C7+ (Recommendations)
GC-analysis to C20+ or (C36+) is a more or less standard measurement used for all fluid samples.
Advantages
Relatively cheap
Require little fluid samples
TBP Distillation is the best existing method to obtain measure component distribution and physical properties (Mw, Sg and TB) of each cut.
Measured physical properties of each distillation cut.
GC-analysis provides the necessary information for C7+ characterization for most oil reservoirs. We recommend however at least one complete TBP-analysis for (1) oil reservoir that may be a candidate for gas injection and (2) most gas condensate reservoirs.
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6. TBP-Distillation
In a TBP distillation the separation of fractions are done by vaporization. Each distillation cut has a range of boiling points.
For each of the distillation cuts the following physical properties are measured.
Molecular weight
Specific gravity (density)
The physical properties of the last fraction is often from material balance.
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7. TBP-Distillation Example of data from a TBP-distillation
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8. Gas Chromatography (GC) Simulated Distillation
GC distillation is based on selective separation of components as temperature is increased ion a capillary tube.
The mass distribution of the fluid produced out of the capillary tube is measured. The mass is “converted” to components based on the temperature (time) known pure components are produced.
No information about the physical properties are provided. Katz-Firoozabadi Generalized properties often assumed. Specially to convert from mass to moles.
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9. Gas Chromatography (GC) Simulated Distillation
The GC-apparatus require proper maintenance to be reliable. It is specially sensitive to shift in baseline.
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10. Mathematical modeling
The heptanes plus distribution is usually modeled using the three parameter Gamma distribution model.
 is the shape (typical range 0.5 – 2.5) =1 is the exponential distribution. =1 is recommended if no GC or TBP-data is available.
 can physically interpreted as the minimum molecular weight found in the distilled fraction (is often 90 for C7+)
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11. Grouping
The heptanes plus fraction contains a more or less continues distribution of different components.
The continues distribution of components is usually grouped into 3-5 pseudo components.
Each pseudo components represents a range of hydro carbons e.g.
C7-C10
C11-C15
C16-C22
C23-C35
C36+
The general recommendation is to split the C7+ in fractions in about equal mass.
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12. Properties Estimation
Most of the correlations used for estimating the EOS properties of the C7+ fractions are based on the molecular weight, specific gravity, and boiling point. Because this reflect the chemical makeup of the fluid.
Watson characterization factor is often used to estimate how aromatic a component is.
Kw varies roughly from 8.5 to 13.5
Paraffinic compounds 12.5<Kw<13.5
Napthnic compounds 11.0<Kw<12.5
Aromatic compounds 8.5<Kw<11.0
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13. Properties Estimation (Specific Gravity)
The specific gravity for the individual pseudo components can be estimated from the Soreide correlation
Cf typically has a value between 0.27 and 0.31 and is determined for reservoir fluid(s) by matching the specific gravity of STO or C7+.
The ”constants” Sgo= and the exponent = 0.13 should only be modified if TBP data is available and/or fluid samples with different Molecular weights and specific gravities.
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14. Properties Estimation (Boiling Point)
The correlations for the EOS parameters depends on the specific gravity and the boiling point of each pseudo components. The recommended correlation for the boiling point based on the specific gravity and the molecular weight is the Soreide correlation.
We reefer to the SPE Phase monograph Chapter 5 for other correlations.
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15. Properties Estimation (EOS Parameters)
Use the Twu correlation (or Lee-Kesler) to estimate Tc and Pc
Chose Acentric factor to match Tb
Determine the volume-translation coefficients (si) to match the specific gravities for each of the pseudo components
Binary Interaction Parameters (BIPS) library between the pseudo components and the non-hydrocarbons (Chapter 4 in SPE Phase Monograph). BIPS between C1 and C7+ pairs we recommend calculated using the modified Cheu-Prausnitz equation.
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