Viscosity Models

This guide documents the viscosity calculation methods available in NeqSim for gas, liquid, and multiphase systems.

Table of Contents

• Overview
• Available Models
  • LBC (Lohrenz-Bray-Clark)
  • Friction Theory
  • PFCT (Pedersen)
  • Chung Method
  • Polynomial Correlation
  • Reference Fluid Methods
• Model Selection Guide
• Tuning Parameters
  • CSP PFCT Parameter Tuning
• Usage Examples

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Overview

Viscosity describes a fluid’s resistance to flow. NeqSim provides several viscosity models suitable for different applications:

Units:

• Default output: Pa·s (Pascal-seconds)
• Alternative: cP (centipoise), where 1 cP = 0.001 Pa·s

Setting a viscosity model:

fluid.initPhysicalProperties();
fluid.getPhase("gas").getPhysicalProperties().setViscosityModel("friction theory");
fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("LBC");

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Available Models

LBC (Lohrenz-Bray-Clark)

The Lohrenz-Bray-Clark (1964) method is widely used in reservoir simulation. It combines a dilute gas correlation with a dense fluid polynomial correction.

Class: LBCViscosityMethod

Equation: \(\eta = \eta^* + \frac{(\eta_r - 0.0001)^4}{\xi}\)

where:

• $\eta^*$ is the dilute gas viscosity
• $\eta_r$ is a function of reduced density
• $\xi$ is the inverse viscosity parameter

The dense fluid contribution uses a polynomial: \(\eta_r = a_0 + a_1\rho_r + a_2\rho_r^2 + a_3\rho_r^3 + a_4\rho_r^4\)

Default parameters: {0.10230, 0.023364, 0.058533, -0.040758, 0.0093324}

Applicable phases: Gas, Oil

Best for:

• Reservoir simulation
• Light to medium oils
• Systems where tuning to lab data is available

Usage:

fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("LBC");

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Friction Theory

The Quiñones-Cisneros and Firoozabadi (2000) friction theory relates viscosity to the repulsive and attractive pressure contributions from the equation of state.

Class: FrictionTheoryViscosityMethod

Equation: \(\eta = \eta_0 + \kappa_a P_a + \kappa_{aa} P_a^2 + \kappa_r P_r + \kappa_{rr} P_r^2\)

where:

• $\eta_0$ is the dilute gas viscosity (Chung low-density term + Wilke mixture rule)
• $P_a$ is the attractive pressure from EoS
• $P_r$ is the repulsive pressure from EoS
• $\kappa$ are friction coefficients (EoS-dependent)

Applicable phases: Gas, Oil (any EoS-based phase)

Best for:

• Wide pressure/temperature ranges
• Near-critical conditions
• When using SRK or PR EoS

Automatic EoS detection: The method automatically selects SRK or PR constants based on the phase type.

Implementation notes in NeqSim (2026 update):

• SRK and PR friction constants are selected automatically.
• The dilute-gas baseline now uses Wilke mixing, which is the standard state-of-practice for mixture dilute viscosities.
• The Chung correction factor $F_c$ includes acentric factor, dipole moment, and component viscosity correction term to better handle polar species.
• If friction theory is called for a non-EoS phase, NeqSim falls back to total pressure as repulsive contribution and zero attractive contribution, with a warning in logs.

Usage:

fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("friction theory");

Custom constants (for other EoS):

FrictionTheoryViscosityMethod viscModel =
    (FrictionTheoryViscosityMethod) fluid.getPhase("oil")
        .getPhysicalProperties().getViscosityModel();

// Set custom friction theory constants
viscModel.setFrictionTheoryConstants(kapac, kaprc, kaprrc, kapa, kapr, kaprr);

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PFCT (Pedersen)

The Pedersen Friction Corresponding States Theory uses methane as a reference fluid with shape factors for mixture calculations.

The model can be selected with either "PFCT" or "CSP". The "CSP" alias is intended for PVT workflows and external reports that label the Pedersen/PFCT viscosity model as CSP viscosity.

Classes:

• PFCTViscosityMethodMod86 - Standard Pedersen method (1987)
• PFCTViscosityMethodHeavyOil - Extended for heavy oils

Equation: \(\eta_{mix} = \eta_{ref} \cdot \frac{f_\eta \cdot \alpha_{mix}}{\alpha_0}\)

where:

• $\eta_{ref}$ is the reference fluid (methane) viscosity
• $f_\eta$ is the shape factor ratio
• $\alpha$ are molecular weight correction factors

Applicable phases: Gas, Oil

Best for:

• Petroleum mixtures with characterized fractions
• Heavy oil systems (use "PFCT-Heavy-Oil")
• Wide-range predictions
• PVT viscosity matching with four CSP correction factors

Usage:

// Standard Pedersen
fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("PFCT");

// Equivalent CSP alias
fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("CSP");

// Heavy oil variant
fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("PFCT-Heavy-Oil");

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Chung Method

The Chung method (1984, 1988) is a corresponding states method for dilute gas and dense fluid viscosity.

Class: ChungViscosityMethod

Equation (dilute gas): \(\eta_0 = \frac{40.785 F_c \sqrt{M T}}{\Omega_v V_c^{2/3}}\)

where:

• $F_c$ is the correction factor for polar/associating fluids
• $\Omega_v$ is the collision integral
• $V_c$ is critical volume
• $M$ is molar mass

Applicable phases: Primarily gas phase

Best for:

• Dilute gas mixtures
• Polar gases
• As baseline for other methods

Usage:

fluid.getPhase("gas").getPhysicalProperties().setViscosityModel("Chung");

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Polynomial Correlation

Uses component-specific polynomial coefficients from the database.

Class: Viscosity (liquid), GasViscosity (gas)

Equation: \(\ln(\eta) = A + \frac{B}{T} + C\ln(T) + DT\)

where A, B, C, D are component-specific parameters from COMP database.

Database columns: LIQVISC1, LIQVISC2, LIQVISC3, LIQVISC4

Applicable phases: Primarily liquid

Best for:

• Pure components with available parameters
• Simple mixtures with mixing rules

Usage:

fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("polynom");

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Reference Fluid Methods

Specialized high-accuracy methods for specific fluids:

Methane (Reference)

Class: MethaneViscosityMethod

• Uses Setzmann & Wagner reference equation
• High accuracy for pure methane
• Used as reference in PFCT calculations

CO₂

Class: CO2ViscosityMethod

• Fenghour et al. correlation
• Covers gas, liquid, and supercritical regions

Hydrogen

Classes: MuznyViscosityMethod, MuznyModViscosityMethod

• High-accuracy hydrogen viscosity
• Important for hydrogen energy applications

Usage:

fluid.getPhase("gas").getPhysicalProperties().setViscosityModel("MethaneModel");
fluid.getPhase("gas").getPhysicalProperties().setViscosityModel("CO2Model");
fluid.getPhase("gas").getPhysicalProperties().setViscosityModel("Muzny");

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Model Selection Guide

Application	Recommended Model	Notes
Reservoir simulation	LBC	Tunable, industry standard
Wide P-T range	Friction Theory	Good near critical
Heavy oils	PFCT-Heavy-Oil	Extended for high MW
Characterized crudes	PFCT	Works with pseudo-components
Gas processing	Chung	Good for gases
Pure CO₂	CO2Model	High accuracy
Pure H₂	Muzny	Reference accuracy
Aqueous systems	Salt Water	Water correlation

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Tuning Parameters

LBC Parameter Tuning

The LBC method is commonly tuned to match laboratory viscosity data:

// Get current parameters
LBCViscosityMethod lbc = (LBCViscosityMethod)
    fluid.getPhase("oil").getPhysicalProperties().getViscosityModel();

// Set all 5 parameters
double[] params = {0.1023, 0.023364, 0.058533, -0.040758, 0.0093324};
fluid.getPhase("oil").getPhysicalProperties().setLbcParameters(params);

// Or tune individual parameters
fluid.getPhase("oil").getPhysicalProperties().setLbcParameter(0, 0.105);

Tuning procedure:

1. Measure viscosity at multiple P-T conditions
2. Run flash and calculate properties
3. Adjust parameters to minimize error
4. Validate at other conditions

CSP PFCT Parameter Tuning

The PFCT/CSP viscosity model exposes four dimensionless CSP correction factors. All factors default to 1.0, preserving the historical NeqSim calculation unless a user supplies tuned values. External PVT tools may report fitted vectors such as 0.6232, 1.1507, 1.0000, 1.0000; these can be supplied directly as the four-value CSP viscosity parameter vector.

Index	Regression parameter	Corrected PFCT term
0	VISCOSITY_CSP_1	Critical-temperature scaling exponent
1	VISCOSITY_CSP_2	Critical-pressure scaling exponent
2	VISCOSITY_CSP_3	Molar-mass scaling exponent
3	VISCOSITY_CSP_4	Alpha/molecular-weight correction exponent

Use PhysicalProperties.setCspViscosityParameters(double[]) to set all four parameters, setCspViscosityParameter(int, double) to tune one parameter, and getCspViscosityParameters() to read the active values. The longer setCspViscosityCorrectionFactors and getCspViscosityCorrectionFactors names are equivalent aliases.

For automated fitting, add viscosity measurements to PVTRegression with addViscosityData(...), then register RegressionParameter.VISCOSITY_CSP_1 through VISCOSITY_CSP_4 individually or call addCspViscosityRegressionParameters() to fit all four together. Viscosity observations are expected in Pa s and can target gas, oil, liquid, aqueous, or water phases.

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Usage Examples

Basic Viscosity Calculation

import neqsim.thermo.system.SystemSrkEos;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

// Create and flash fluid
SystemInterface fluid = new SystemSrkEos(350.0, 150.0);
fluid.addComponent("methane", 0.70);
fluid.addComponent("ethane", 0.10);
fluid.addComponent("propane", 0.05);
fluid.addComponent("n-heptane", 0.10);
fluid.addComponent("n-decane", 0.05);
fluid.setMixingRule("classic");

ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
ops.TPflash();

// Initialize physical properties
fluid.initPhysicalProperties();

// Get viscosities
double gasVisc = fluid.getPhase("gas").getViscosity("cP");
double oilVisc = fluid.getPhase("oil").getViscosity("cP");

System.out.println("Gas viscosity: " + gasVisc + " cP");
System.out.println("Oil viscosity: " + oilVisc + " cP");

Comparing Viscosity Models

String[] models = {"LBC", "friction theory", "PFCT"};

for (String model : models) {
    SystemInterface fluid = createFluid();
    ops.TPflash();
    fluid.initPhysicalProperties();

    fluid.getPhase("oil").getPhysicalProperties().setViscosityModel(model);
    fluid.initPhysicalProperties();

    double visc = fluid.getPhase("oil").getViscosity("cP");
    System.out.println(model + ": " + visc + " cP");
}

Viscosity vs Temperature

SystemInterface baseFluid = createFluid();
baseFluid.initPhysicalProperties();

double[] temps = {300, 320, 340, 360, 380, 400};  // K

for (double T : temps) {
    SystemInterface fluid = baseFluid.clone();
    fluid.setTemperature(T, "K");

    ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
    ops.TPflash();
    fluid.initPhysicalProperties();

    double visc = fluid.getPhase("oil").getViscosity("cP");
    System.out.println("T=" + (T-273.15) + "°C: " + visc + " cP");
}

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Mathematical Details

Dilute Gas Viscosity (Chapman-Enskog)

For dilute gases, viscosity is calculated from kinetic theory:

\[\eta_0 = \frac{5}{16} \frac{\sqrt{\pi m k_B T}}{\pi \sigma^2 \Omega^{(2,2)*}}\]

where:

• $m$ is molecular mass
• $\sigma$ is Lennard-Jones diameter
• $\Omega^{(2,2)*}$ is the collision integral

Mixing Rules

Most models use molar fraction-weighted mixing:

\[\eta_{mix} = \exp\left(\sum_i x_i \ln \eta_i\right)\]

or the more rigorous:

\[\eta_{mix} = \frac{\sum_i x_i \sqrt{M_i} \eta_i}{\sum_i x_i \sqrt{M_i}}\]

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References

1. Lohrenz, J., Bray, B.G., Clark, C.R. (1964). Calculating Viscosities of Reservoir Fluids from Their Compositions. JPT.
2. Quiñones-Cisneros, S.E., Firoozabadi, A. (2000). One Parameter Friction Theory. AIChE J.
3. Pedersen, K.S., et al. (1987). Viscosity of Crude Oils. Chem. Eng. Sci.
4. Chung, T.H., et al. (1988). Generalized Multiparameter Correlation for Nonpolar and Polar Fluid Transport Properties. I&EC Res.