Download Heptanes-Plus Characterization Introduction

Heptanes-Plus Characterization
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Introduction
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Molar distribution
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Experimental Analysis
Mathematical modeling
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Grouping (defining Pseudo components)
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Properties estimation (Pseudo components)
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The properties for Needed for EOS calculations are:
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Tc and Pc
Acentric factor
Volume-translation coefficient
Binary Interaction parameter
Z-factor (viscosity)
Exercises
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TBP data in monograph
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Effect of changing Gamma Distribution Parameters
Distribution and specific gravity correlation
Grouping
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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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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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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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Molar Distribution of C7+ (Recommendations)
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GC-analysis to C20+ or (C36+) is a more or less standard measurement used
for all fluid samples.
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TBP Distillation is the best existing method to obtain measure component
distribution and physical properties (Mw, Sg and TB) of each cut.
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Advantages
Relatively cheap
Require little fluid samples
Advantages
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 TBPanalysis for (1) oil reservoir that may be a candidate for gas injection and (2)
most gas condensate reservoirs.
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TBP-Distillation
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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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TBP-Distillation
 Example of data from a TBP-distillation
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Gas Chromatography (GC) Simulated Distillation

GC distillation is based on selective
separation of components as
temperature is increased ion a
capillary tube.
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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. KatzFiroozabadi Generalized properties
often assumed. Specially to convert
from mass to moles.
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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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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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Grouping
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Amount
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Molecular Weight
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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.
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C7-C10
C11-C15
C16-C22
C23-C35
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C36+
The general recommendation is to split
the C7+ in fractions in about equal
mass.
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Properties Estimation
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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.
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Watson characterization factor is often
used to estimate how aromatic a
component is.
K W  Tb1 / 3 / 
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KW  4.5579 M 0.15178 0.84573
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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Properties Estimation (Specific Gravity)

The specific gravity for the individual
pseudo components can be estimated
from the Soreide correlation
  0.2855  C f ( M i  66)0.13
 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=0.2855 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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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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Properties Estimation (EOS Parameters)
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Use the Twu correlation (or Lee-Kesler) to estimate Tc and Pc
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Chose Acentric factor to match Tb
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Determine the volume-translation coefficients (si) to match the specific gravities for
each of the pseudo components
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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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