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

Table of Contents

Overview

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

Units:

Setting a viscosity model:

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

Select the model after the flash and initial property calculation, then reinitialize each changed phase as shown. Model keys are case-sensitive. Unlike conductivity selection, an unsupported viscosity key leaves the current model unchanged; verify the selected key in tests rather than assuming a fallback.

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:

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:

Usage:

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

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:

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

Best for:

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

Implementation notes in NeqSim (2026 update):

Usage:

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

Custom constants (advanced use):

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

// Values must come from an independently validated parameter set.
viscModel.setFrictionTheoryConstants(kapac, kaprc, kaprrc, kapa, kapr, kaprr);

Here kapac, kaprc, kaprrc, and kaprr are scalar double values; kapa and kapr are double[][] matrices. This API replaces the model’s complete parameter set and is not a general fitting shortcut.

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:

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

where:

Applicable phases: Gas, Oil

Best for:

Usage:

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

Use "CSP" as an equivalent alias for "PFCT", or select "PFCT-Heavy-Oil" for characterized heavy fractions. Reinitialize the phase after any selection.

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:

Usage:

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

Reference Fluid Methods

Specialized high-accuracy methods for specific fluids:

Methane (Reference)

Class: MethaneViscosityMethod

CO₂

Class: CO2ViscosityMethod

Hydrogen

Classes: MuznyViscosityMethod, MuznyModViscosityMethod

The implemented case-sensitive keys and their constraints are:

Key Intended fluid
MethaneModel Pure methane
CO2Model Pure CO₂
Muzny, Muzny_mod Pure hydrogen
KTA, KTA_mod Pure helium
Salt Water Aqueous water/brine phase

For example, select and recalculate a pure-hydrogen gas phase with:

fluid.getPhase("gas").getPhysicalProperties().setViscosityModel("Muzny");
fluid.getPhase("gas").initPhysicalProperties();

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
General gas processing Friction theory or PFCT Validate against representative data
Pure CO₂ CO2Model High accuracy
Pure H₂ Muzny Reference accuracy
Pure helium KTA Reference-fluid correlation
Aqueous systems Salt Water Water/brine correlation

Tuning Parameters

LBC Parameter Tuning

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

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

// 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);
fluid.getPhase("oil").initPhysicalProperties();

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.

Usage Examples

Basic Viscosity Calculation

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

// Create and flash fluid
SystemInterface fluid = new SystemSrkEos(298.15, 4.0);
fluid.addComponent("n-heptane", 0.5);
fluid.addComponent("nC10", 0.5);
fluid.setMixingRule("classic");

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

// Initialize physical properties
fluid.initPhysicalProperties();
fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("LBC");
fluid.getPhase("oil").initPhysicalProperties();

// Get viscosity
double oilVisc = fluid.getPhase("oil").getViscosity("cP");
if (!(oilVisc > 0.0) || !Double.isFinite(oilVisc)) {
    throw new IllegalStateException("Expected positive finite oil viscosity");
}

Comparing Viscosity Models

String[] models = {"LBC", "friction theory", "PFCT"};
double[] viscositiesCp = new double[models.length];

for (int i = 0; i < models.length; i++) {
    SystemInterface fluid = new SystemSrkEos(298.15, 4.0);
    fluid.addComponent("n-heptane", 0.5);
    fluid.addComponent("nC10", 0.5);
    fluid.setMixingRule("classic");
    ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
    ops.TPflash();
    fluid.initPhysicalProperties();

    fluid.getPhase("oil").getPhysicalProperties().setViscosityModel(models[i]);
    fluid.getPhase("oil").initPhysicalProperties();
    viscositiesCp[i] = fluid.getPhase("oil").getViscosity("cP");
}

Viscosity vs Temperature

SystemInterface baseFluid = new SystemSrkEos(300.0, 20.0);
baseFluid.addComponent("nC10", 1.0);
baseFluid.setMixingRule("classic");

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

for (int i = 0; i < temps.length; i++) {
    SystemInterface fluid = baseFluid.clone();
    fluid.setTemperature(temps[i], "K");

    ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
    ops.TPflash();
    fluid.initPhysicalProperties();
    fluid.getPhase("oil").getPhysicalProperties().setViscosityModel("LBC");
    fluid.getPhase("oil").initPhysicalProperties();
    viscositiesCp[i] = fluid.getPhase("oil").getViscosity("cP");
}

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:

Mixing Rules

Mixing behavior is model-specific. Common forms include logarithmic molar weighting:

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

or the more rigorous:

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

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.