neqsim-eos-regression

EOS Parameter Regression Workflow

Guide for fitting equation-of-state parameters to experimental data using NeqSim.

When to Use This Skill

• Fitting binary interaction parameters (kij) to VLE/LLE data
• Matching saturation pressure (bubble/dew point) to lab measurements
• Tuning C7+ characterization to PVT experiments (CME, CVD, DL)
• Validating EOS predictions against NIST or experimental data
• Selecting the best EOS for a specific fluid system

Regression Strategy

Step 1: Select Base EOS

Fluid System	Recommended EOS	Reason
Light hydrocarbons (C1-C6)	SRK or PR	Well-characterized, reliable kij
Oil with C7+	PR with volume correction	Better liquid density
Water + hydrocarbons	SRK-CPA	Handles hydrogen bonding
CO2 + hydrocarbons	PR or SRK	Good CO2 fugacity
Glycol systems (MEG, TEG)	SRK-CPA	Polar + associating
Electrolytes	Electrolyte-CPA	Ion interactions

Step 2: Gather Experimental Data

Required data types by priority:

1. Saturation pressure (bubble/dew point) at reservoir temperature — most critical
2. Liquid density at reservoir conditions — validates volume translation
3. GOR/Rs from separator tests — validates phase split
4. Viscosity — validates transport property correlations
5. Compositional data from CVD/DL — validates K-values

Step 3: Set Up Regression in NeqSim

// Create fluid
SystemInterface fluid = new SystemPrEos(273.15 + 100.0, 200.0);
fluid.addComponent("methane", 0.70);
fluid.addComponent("ethane", 0.10);
fluid.addComponent("propane", 0.05);
fluid.addComponent("n-heptane", 0.15);
fluid.setMixingRule("classic");

// Access binary interaction parameters
fluid.setMixingRule("classic");

// Get current kij
double kij = fluid.getPhase(0).getMixingRule()
    .getBinaryInteractionParameter(0, 3);  // methane-nC7

// Set modified kij
fluid.getPhase(0).getMixingRule()
    .setBinaryInteractionParameter(0, 3, 0.02);
fluid.getPhase(1).getMixingRule()
    .setBinaryInteractionParameter(0, 3, 0.02);