C7+ Characterization

Overview

Heavy hydrocarbon fractions (C7+) are not single components but complex mixtures of hundreds of molecules. Since equations of state require specific critical properties ($T_c$, $P_c$, $\omega$) for each component, the C7+ fraction must be characterized — broken into pseudo-components with estimated properties.

C7+ characterization is often the most important step in EoS modeling because:

• C7+ typically represents 20-60% of reservoir fluid by mole
• Phase behavior predictions are highly sensitive to heavy fraction properties
• Small changes in C7+ characterization can shift bubble/dew points by hundreds of psi

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Property Estimation

Input Data

The minimum data needed for C7+ characterization:

Correlation Methods

Kesler-Lee (1976)

Estimates $T_c$, $P_c$, $\omega$ from $T_b$ and $\gamma$:

$$T_c = 341.7 + 811\gamma + (0.4244 + 0.1174\gamma)T_b + \frac{(0.4669 - 3.2623\gamma) \times 10^5}{T_b}$$

Best for: General petroleum fractions, widely validated.

Twu (1984)

Uses a perturbation approach starting from n-alkane properties:

1. Calculate n-alkane properties at the same $T_b$
2. Apply perturbation corrections based on $\gamma$ deviation from n-alkane

Best for: Heavy fractions, better extrapolation to high molecular weights.

Riazi-Daubert (1987)

Simple two-parameter correlations:

$$\theta = aM_w^b \gamma^c$$

Where $\theta$ represents $T_c$, $P_c$, or $T_b$, and $a$, $b$, $c$ are correlation-specific constants.

Best for: Quick estimates, widely used in refining applications.

Comparison

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Splitting Methods

Purpose

A single C7+ pseudo-component is insufficient for accurate phase behavior. The C7+ fraction should be split into multiple pseudo-components that represent the molecular weight distribution.

Whitson Gamma Distribution

The molar distribution function follows a three-parameter gamma distribution:

$$p(M) = \frac{(M - \eta)^{\alpha-1}}{\beta^\alpha \Gamma(\alpha)} \exp\left(-\frac{M - \eta}{\beta}\right)$$

Where:

• $\eta$ = minimum molecular weight (typically 84 for C7)
• $\alpha$ = shape parameter (controls distribution shape)
• $\beta = (M_{w,C7+} - \eta) / \alpha$

Katz Splitting

Assumes exponential decay of mole fractions with carbon number:

$$z_n = z_7 \exp[-A(n - 7)]$$

Where $A$ is fitted to match the measured C7+ mole fraction and molecular weight. This is simpler than the Whitson method but less flexible.

Number of Pseudo-Components

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Lumping

Purpose

After splitting, the total component count may be too large for efficient simulation. Lumping groups similar components to reduce the count while preserving phase behavior.

Guidelines

1. Group by volatility — components with similar K-values
2. Preserve C1, CO2, N2, H2S as individual components
3. Keep C2-C6 as pure components or small groups
4. Lump C7+ pseudo-components into 2-5 groups

Typical Lumped Composition

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Tuning Strategy

Matching Measured Data

After initial characterization, the EoS model must be tuned to match laboratory PVT data:

Recommended Tuning Order

1. Match saturation pressure by adjusting C7+ molecular weight ($\pm$5-10%)
2. Match liquid density by tuning volume translation
3. Match gas-oil ratio by adjusting C7+ splitting
4. Fine-tune $k_{ij}$ for non-hydrocarbon components

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Related Topics

• EoS Overview — Role of characterization in EoS workflow
• Peng-Robinson EoS — How component properties feed into EoS
• Flash Calculations — Sensitivity of flash to characterization
• Phase Envelope — Impact on predicted phase boundaries

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References

1. Whitson, C.H. (1983). "Characterizing Hydrocarbon Plus Fractions." SPE Journal, 23(4), 683-694. SPE-12233-PA.

2. Kesler, M.G. and Lee, B.I. (1976). "Improve Prediction of Enthalpy of Fractions." Hydrocarbon Processing, 55(3), 153-158.

3. Twu, C.H. (1984). "An Internally Consistent Correlation for Predicting the Critical Properties and Molecular Weights of Petroleum and Coal-Tar Liquids." Fluid Phase Equilibria, 16(2), 137-150.

4. Riazi, M.R. and Daubert, T.E. (1987). "Characterization Parameters for Petroleum Fractions." Industrial & Engineering Chemistry Research, 26(4), 755-759.

5. Pedersen, K.S., Christensen, P.L., and Shaikh, J.A. (2015). Phase Behavior of Petroleum Reservoir Fluids, 2nd Edition. CRC Press.