Wax Characterization

Documentation for wax modeling and characterization in NeqSim.

Table of Contents

• Overview
• Wax Formation Theory
• WaxCharacterise Class
• Wax Models
• Usage Examples
• Flow Assurance Applications

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Overview

Package: neqsim.thermo.characterization

Wax precipitation is a major flow assurance concern in oil production, particularly in:

• Subsea pipelines
• Cold climate operations
• Production restarts after shutdown

NeqSim provides wax characterization and thermodynamic modeling capabilities based on the Pedersen model and related approaches.

Key Classes

Class	Description
WaxCharacterise	Main wax characterization class
WaxModelInterface	Interface for wax models
PedersenWaxModel	Pedersen’s wax precipitation model

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Wax Formation Theory

What is Wax?

Wax consists of high molecular weight n-paraffins (typically C18+) that crystallize when crude oil is cooled below the Wax Appearance Temperature (WAT).

Key Temperatures

Temperature	Description
WAT (Wax Appearance Temperature)	First crystals appear
Pour Point	Oil stops flowing
Gel Point	Oil becomes semi-solid

Wax Precipitation Mechanism

1. Nucleation: Wax molecules cluster as temperature drops
2. Crystal Growth: Wax crystals grow on nuclei
3. Deposition: Crystals deposit on cold surfaces (pipe walls)
4. Aging: Deposited wax hardens over time

Thermodynamic Model

The solid-liquid equilibrium for wax is described by:

\[\ln\left(\frac{x_i^L \gamma_i^L}{x_i^S \gamma_i^S}\right) = \frac{\Delta H_f}{R} \left(\frac{1}{T_f} - \frac{1}{T}\right) + \frac{\Delta C_p}{R} \left(\frac{T_f}{T} - 1 - \ln\frac{T_f}{T}\right)\]

where:

• $x_i^L, x_i^S$ = mole fractions in liquid and solid
• $\gamma_i^L, \gamma_i^S$ = activity coefficients
• $\Delta H_f$ = heat of fusion
• $T_f$ = fusion (melting) temperature
• $\Delta C_p$ = heat capacity change upon fusion

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WaxCharacterise Class

Creating a Wax Characterization

import neqsim.thermo.system.SystemSrkEos;
import neqsim.thermo.characterization.WaxCharacterise;

// Create oil system with plus fraction
SystemSrkEos oil = new SystemSrkEos(323.15, 50.0);
oil.addComponent("methane", 0.40);
oil.addComponent("ethane", 0.10);
oil.addComponent("propane", 0.08);
oil.addComponent("n-butane", 0.05);
oil.addComponent("n-pentane", 0.04);
oil.addComponent("n-hexane", 0.03);
oil.addTBPfraction("C7", 0.10, 95.0 / 1000, 0.72);
oil.addTBPfraction("C10", 0.08, 135.0 / 1000, 0.78);
oil.addTBPfraction("C15", 0.06, 210.0 / 1000, 0.82);
oil.addTBPfraction("C20", 0.04, 280.0 / 1000, 0.85);
oil.addTBPfraction("C30", 0.02, 420.0 / 1000, 0.88);
oil.setMixingRule("classic");

// Create wax characterization
WaxCharacterise waxChar = new WaxCharacterise(oil);

Setting Wax Parameters

// Set wax model parameters
// Parameters depend on the model used

// For Pedersen model, typical parameters:
double[] waxParams = new double[3];
waxParams[0] = 1.0;    // Parameter A
waxParams[1] = 0.0;    // Parameter B  
waxParams[2] = 0.0;    // Parameter C

waxChar.setWaxParameters(waxParams);

// Set individual parameter
waxChar.setWaxParameter(0, 1.05);

Heat of Fusion Parameters

// Set heat of fusion correlation parameter
waxChar.setParameterWaxHeatOfFusion(0, 0.0);

// Set triple point temperature parameter
waxChar.setParameterWaxTriplePointTemperature(0, 0.0);

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Wax Models

Pedersen Wax Model

The default model based on Pedersen’s work, which correlates wax properties with carbon number.

Heat of Fusion Correlation

\[\Delta H_f = A + B \cdot MW + C \cdot MW^2\]

where MW is the molecular weight of the n-paraffin.

Triple Point Temperature

\[T_{tp} = A_1 + A_2 \cdot \ln(CN) + A_3 \cdot CN\]

where CN is the carbon number.

Model Interface

import neqsim.thermo.characterization.WaxModelInterface;

// Get the wax model
WaxModelInterface model = waxChar.getModel();

// Add TBP fractions as wax-forming components
model.addTBPWax();

// Get wax parameters
double[] params = model.getWaxParameters();

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Usage Examples

Basic Wax Appearance Temperature Calculation

import neqsim.thermo.system.SystemSrkCPAstatoil;
import neqsim.thermodynamicoperations.ThermodynamicOperations;
import neqsim.thermo.characterization.WaxCharacterise;

// Create waxy crude oil
SystemSrkCPAstatoil oil = new SystemSrkCPAstatoil(323.15, 50.0);
oil.addComponent("methane", 0.30);
oil.addComponent("ethane", 0.08);
oil.addComponent("propane", 0.05);
oil.addComponent("n-hexane", 0.10);
oil.addTBPfraction("C10", 0.15, 0.140, 0.78);
oil.addTBPfraction("C20", 0.12, 0.280, 0.84);
oil.addTBPfraction("C30", 0.10, 0.420, 0.87);
oil.addTBPfraction("C40", 0.07, 0.560, 0.89);
oil.addTBPfraction("C50+", 0.03, 0.700, 0.91);
oil.setMixingRule(10);

// Characterize wax
WaxCharacterise waxChar = new WaxCharacterise(oil);
waxChar.getModel().addTBPWax();

// Calculate WAT (Wax Appearance Temperature)
ThermodynamicOperations ops = new ThermodynamicOperations(oil);
try {
    ops.calcWAT();
    double wat = oil.getTemperature() - 273.15;  // Convert to Celsius
    System.out.println("WAT: " + wat + " °C");
} catch (Exception e) {
    System.out.println("WAT calculation failed: " + e.getMessage());
}

Wax Amount vs Temperature

// Calculate wax precipitation curve
double[] temperatures = {50, 45, 40, 35, 30, 25, 20, 15, 10};  // °C
double pressure = 50.0;  // bara

System.out.println("Temperature (°C) | Wax (wt%)");
System.out.println("--------------------------");

for (double tempC : temperatures) {
    SystemSrkCPAstatoil system = oil.clone();
    system.setTemperature(tempC + 273.15);
    system.setPressure(pressure);
    
    ThermodynamicOperations ops = new ThermodynamicOperations(system);
    ops.TPflash();
    
    // Get wax amount if solid phase exists
    if (system.hasPhaseType("wax") || system.hasPhaseType("solid")) {
        double waxFraction = system.getWtFraction(system.getPhaseIndex("solid"));
        System.out.printf("%8.1f         | %5.2f%n", tempC, waxFraction * 100);
    } else {
        System.out.printf("%8.1f         | %5.2f%n", tempC, 0.0);
    }
}

Complete Wax Characterization Workflow

import neqsim.thermo.system.SystemSrkCPAstatoil;
import neqsim.thermo.characterization.WaxCharacterise;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

public class WaxCharacterizationExample {
    
    public static void main(String[] args) {
        // Step 1: Create fluid from PVT data
        SystemSrkCPAstatoil fluid = createFluidFromPVT();
        
        // Step 2: Characterize wax components
        WaxCharacterise waxChar = new WaxCharacterise(fluid);
        waxChar.getModel().addTBPWax();
        
        // Step 3: Tune wax parameters to match experimental data
        // (Example: adjust heat of fusion parameter to match experimental WAT)
        tuneWaxParameters(waxChar, 35.0);  // Target WAT = 35°C
        
        // Step 4: Calculate wax precipitation curve
        calculateWaxCurve(fluid);
        
        // Step 5: Export results
        exportResults(fluid);
    }
    
    private static SystemSrkCPAstatoil createFluidFromPVT() {
        SystemSrkCPAstatoil fluid = new SystemSrkCPAstatoil(323.15, 100.0);
        
        // Add light components
        fluid.addComponent("nitrogen", 0.5);
        fluid.addComponent("CO2", 1.5);
        fluid.addComponent("methane", 35.0);
        fluid.addComponent("ethane", 8.0);
        fluid.addComponent("propane", 5.0);
        fluid.addComponent("i-butane", 1.5);
        fluid.addComponent("n-butane", 3.0);
        fluid.addComponent("i-pentane", 1.5);
        fluid.addComponent("n-pentane", 2.0);
        
        // Add characterized heavy fractions (potential wax formers)
        fluid.addTBPfraction("C6", 3.0, 0.086, 0.69);
        fluid.addTBPfraction("C7-C9", 8.0, 0.107, 0.74);
        fluid.addTBPfraction("C10-C15", 12.0, 0.160, 0.79);
        fluid.addTBPfraction("C16-C20", 8.0, 0.250, 0.83);
        fluid.addTBPfraction("C21-C30", 6.0, 0.380, 0.86);
        fluid.addTBPfraction("C31-C40", 3.0, 0.520, 0.88);
        fluid.addTBPfraction("C41+", 2.0, 0.700, 0.91);
        
        fluid.setMixingRule(10);
        return fluid;
    }
    
    private static void tuneWaxParameters(WaxCharacterise waxChar, 
                                          double targetWAT) {
        // Iterative tuning to match experimental WAT
        // This is a simplified example
        double[] params = waxChar.getWaxParameters();
        
        // Adjust parameters to match target WAT
        // In practice, this would involve optimization
        params[0] = 1.0 + (targetWAT - 30.0) * 0.01;
        waxChar.setWaxParameters(params);
    }
    
    private static void calculateWaxCurve(SystemSrkCPAstatoil fluid) {
        // ... implementation
    }
    
    private static void exportResults(SystemSrkCPAstatoil fluid) {
        // ... implementation
    }
}

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Flow Assurance Applications

Pipeline Wax Deposition Prediction

// Pipeline conditions
double inletTemp = 60.0;   // °C
double outletTemp = 15.0;  // °C (cold seabed)
double pressure = 80.0;    // bara

// Calculate wax deposition risk
SystemSrkCPAstatoil fluid = createWaxyOil();
WaxCharacterise waxChar = new WaxCharacterise(fluid);
waxChar.getModel().addTBPWax();

// Check if operating below WAT
ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
ops.calcWAT();
double wat = fluid.getTemperature() - 273.15;

if (outletTemp < wat) {
    System.out.println("WARNING: Operating below WAT!");
    System.out.println("WAT: " + wat + " °C");
    System.out.println("Outlet temp: " + outletTemp + " °C");
    System.out.println("Subcooling: " + (wat - outletTemp) + " °C");
    
    // Estimate wax buildup potential
    double subcooling = wat - outletTemp;
    String risk = subcooling > 20 ? "HIGH" : 
                  subcooling > 10 ? "MEDIUM" : "LOW";
    System.out.println("Deposition risk: " + risk);
}

Wax Inhibitor Evaluation

// Test effect of pour point depressant (PPD)
SystemSrkCPAstatoil fluidWithPPD = createWaxyOil();

// Modify wax properties to simulate PPD effect
WaxCharacterise waxCharPPD = new WaxCharacterise(fluidWithPPD);
double[] params = waxCharPPD.getWaxParameters();
params[0] *= 0.85;  // Reduce wax formation tendency
waxCharPPD.setWaxParameters(params);

// Compare WAT with and without PPD
ThermodynamicOperations opsBase = new ThermodynamicOperations(createWaxyOil());
opsBase.calcWAT();
double watBase = opsBase.getThermoSystem().getTemperature() - 273.15;

ThermodynamicOperations opsPPD = new ThermodynamicOperations(fluidWithPPD);
opsPPD.calcWAT();
double watPPD = opsPPD.getThermoSystem().getTemperature() - 273.15;

System.out.println("WAT without PPD: " + watBase + " °C");
System.out.println("WAT with PPD: " + watPPD + " °C");
System.out.println("WAT reduction: " + (watBase - watPPD) + " °C");

Restart Analysis

// Analyze restart conditions after cold shutdown
double ambientTemp = 4.0;  // °C (seabed temperature)
double shutdownTime = 48.0;  // hours

// Check gel formation risk
if (ambientTemp < pourPoint) {
    System.out.println("CRITICAL: Gel formation likely!");
    System.out.println("Ambient: " + ambientTemp + " °C");
    System.out.println("Pour point: " + pourPoint + " °C");
    System.out.println("Margin: " + (pourPoint - ambientTemp) + " °C");
    
    // Recommend restart procedure
    System.out.println("\nRecommended actions:");
    System.out.println("1. Chemical treatment before restart");
    System.out.println("2. Hot oil circulation");
    System.out.println("3. Controlled pressure buildup");
}

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Parameter Estimation

From Experimental Data

Wax model parameters can be estimated from experimental data:

Data Type	Use
WAT	Primary parameter tuning
Wax content vs T	Validate precipitation curve
Pour point	Confirm gel behavior
n-Paraffin distribution	Component characterization

Typical Parameter Ranges

Parameter	Typical Range	Effect
A (heat of fusion)	0.8 - 1.2	Higher = higher WAT
B (MW coefficient)	-0.01 to 0.01	Shape of curve
C (MW² coefficient)	0 to 0.001	High MW behavior

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See Also

• Asphaltene Characterization - Asphaltene modeling
• Flow Assurance - Complete flow assurance guide
• PVT Characterization - Fluid characterization
• TBP Fractions - Plus fraction handling