neqsim-electrolyte-systems

name	neqsim-electrolyte-systems
description	Electrolyte and brine chemistry guidance for NeqSim. USE WHEN: modeling produced water, scale prediction, CO2/H2S in aqueous systems, MEG/DEG injection, hydrate inhibitor dosing, or any system with ions, salts, or electrolytes. Covers SystemElectrolyteCPAstatoil setup, ion components, scale risk assessment, and brine handling patterns.
last_verified	2026-07-04

Electrolyte Systems Guide

Guide for modeling electrolyte/brine systems in NeqSim.

When to Use Electrolyte Models

• Produced water with dissolved salts (NaCl, CaCl2, BaCl4)
• Scale prediction (CaCO3, BaSO4, CaSO4)
• CO2/H2S solubility in brines
• MEG injection rate calculations
• Hydrate inhibitor dosing
• Seawater injection and mixing
• Desalination process modeling

EOS Selection for Electrolyte Systems

System	NeqSim Class	Mixing Rule
Water + salt + HC gas	SystemElectrolyteCPAstatoil	10
MEG/DEG + water + gas	SystemSrkCPAstatoil	10
Pure water + gas	SystemSrkCPAstatoil	10
Brine + multiple salts	SystemElectrolyteCPAstatoil	10

Basic Electrolyte Setup

import neqsim.thermo.system.SystemElectrolyteCPAstatoil;

// Create electrolyte system
SystemInterface brine = new SystemElectrolyteCPAstatoil(273.15 + 80.0, 200.0);

// Add gas components
brine.addComponent("CO2", 0.05);
brine.addComponent("methane", 0.80);

// Add water
brine.addComponent("water", 0.10);

// Add salt components (as ions)
brine.addComponent("Na+", 0.02);
brine.addComponent("Cl-", 0.02);
brine.addComponent("Ca++", 0.005);
brine.addComponent("SO4--", 0.005);

// Set mixing rule (MUST be numeric for CPA)
brine.setMixingRule(10);
brine.setMultiPhaseCheck(true);

Common Ion Components in NeqSim

Ion	NeqSim Name	Common Source
Sodium	"Na+"	NaCl
Chloride	"Cl-"	NaCl, CaCl2
Calcium	"Ca++"	CaCl2, CaCO3
Barium	"Ba++"	BaSO4, BaCl2
Strontium	"Sr++"	SrSO4
Sulfate	"SO4--"	Na2SO4, BaSO4
Bicarbonate	"HCO3-"	NaHCO3
Carbonate	"CO3--"	CaCO3, Na2CO3
Magnesium	"Mg++"	MgCl2
Potassium	"K+"	KCl
Iron(II)	"Fe++"	FeCl2, FeS

Produced Water Modeling

// Typical produced water composition
SystemInterface prodWater = new SystemElectrolyteCPAstatoil(273.15 + 80.0, 50.0);

// Dissolved gas
prodWater.addComponent("CO2", 0.01);
prodWater.addComponent("H2S", 0.001);
prodWater.addComponent("methane", 0.005);

// Water (dominant)
prodWater.addComponent("water", 0.90);

// Ions (typical North Sea produced water)
prodWater.addComponent("Na+", 0.03);
prodWater.addComponent("Cl-", 0.035);
prodWater.addComponent("Ca++", 0.003);
prodWater.addComponent("Ba++", 0.0001);
prodWater.addComponent("SO4--", 0.001);
prodWater.addComponent("HCO3-", 0.005);

prodWater.setMixingRule(10);
prodWater.setMultiPhaseCheck(true);

// Flash to get gas/liquid split
ThermodynamicOperations ops = new ThermodynamicOperations(prodWater);
ops.TPflash();
prodWater.initProperties();

Scale Risk Assessment

Higher-level scale helpers (prefer for screening)

Before hand-rolling the flash, note these ready-made helpers (see the neqsim-flow-assurance skill for full patterns):

• ElectrolyteScaleCalculator — activity-corrected SI for CaCO3, BaSO4, CaSO4, SrSO4 from ion mg/L (Davies + Ksp(T)).
• ScaleKinetics — induction time and reaction-vs-transport growth regime on top of an SI (SI says if, kinetics says how fast).
• BrineMixingScaleEvaluator — seawater/formation-water mixing sweep; finds the worst mixing fraction and mineral (sulphate scale often peaks mid-mix).
• ScaleDepositionAccumulator / PipeSegmentIntegrity — deposition and coupled corrosion+scale along a PipeBeggsAndBrills profile.
• Rigorous scale potential (saturation ratio SR = IAP/Ksp per salt): ops.checkScalePotential(phaseNumber) then ops.getResultTable(). This uses in-situ ion molalities, so it needs chemical reactions + speciation — call system.chemicalReactionInit() before createDatabase(true)/setMixingRule(10) and flash first (otherwise carbonate/bicarbonate/pH speciation is missing and the SR is wrong).

CaCO3 (Calcite) Scaling

Scale forms when the product of ion activities exceeds the solubility product:

$$ SI = \log_{10}\left(\frac{a_{Ca^{2+}} \cdot a_{CO_3^{2-}}}{K_{sp}}\right) $$

Where $SI > 0$ indicates supersaturation (scaling risk).

// Rigorous route: mix formation water + seawater, solve speciation, check scale
SystemInterface mixed = new SystemElectrolyteCPAstatoil(273.15 + 80.0, 50.0);
// Add components from both waters (water, Na+, Cl-, Ca++, Ba++, SO4--, HCO3-, CO2 ...)
mixed.chemicalReactionInit();   // MANDATORY for pH + carbonate speciation + SR
mixed.createDatabase(true);
mixed.setMixingRule(10);
mixed.setMultiPhaseCheck(true);

ThermodynamicOperations ops = new ThermodynamicOperations(mixed);
ops.TPflash();                  // solves aqueous speciation
mixed.initProperties();

// (1) Scale potential per salt: saturation ratio SR = IAP/Ksp (>1 => scaling)
int aq = mixed.getPhaseNumberOfPhase("aqueous");
ops.checkScalePotential(aq);
String[][] sr = ops.getResultTable();     // rows: {saltName, SR, ""}
double pH = mixed.getpH();                // speciated in-situ pH from same flash

// (2) Precipitation amount: a supersaturated brine drops a solid phase
if (mixed.hasPhaseType("solid")) {
    double solidMoles = mixed.getPhase("solid").getNumberOfMolesInPhase();
    double solidMassKg = solidMoles * mixed.getPhase("solid").getMolarMass();
}

BaSO4 (Barite) Scaling

Most problematic scale in oil production — forms when Ba++ meets SO4--.

// Formation water (high Ba++)
SystemInterface formWater = new SystemElectrolyteCPAstatoil(273.15 + 90.0, 200.0);
formWater.addComponent("water", 0.95);
formWater.addComponent("Na+", 0.025);
formWater.addComponent("Cl-", 0.020);
formWater.addComponent("Ba++", 0.001);  // High barium
formWater.addComponent("Sr++", 0.0005);
formWater.setMixingRule(10);

// Injection water (high SO4--)
SystemInterface injWater = new SystemElectrolyteCPAstatoil(273.15 + 20.0, 200.0);
injWater.addComponent("water", 0.96);
injWater.addComponent("Na+", 0.015);
injWater.addComponent("Cl-", 0.015);
injWater.addComponent("SO4--", 0.01);   // High sulfate
injWater.setMixingRule(10);

MEG Injection Calculations

// Wet gas with MEG injection for hydrate prevention
SystemInterface wetGas = new SystemSrkCPAstatoil(273.15 + 5.0, 100.0);
wetGas.addComponent("methane", 0.85);
wetGas.addComponent("ethane", 0.08);
wetGas.addComponent("propane", 0.03);
wetGas.addComponent("water", 0.03);
wetGas.addComponent("MEG", 0.01);  // Monoethylene glycol
wetGas.setMixingRule(10);
wetGas.setMultiPhaseCheck(true);

// Check hydrate temperature with MEG
ThermodynamicOperations ops = new ThermodynamicOperations(wetGas);
ops.hydrateFormationTemperature();
double hydrateT = wetGas.getTemperature() - 273.15;  // °C

// Compare with and without MEG to get suppression

CO2 Solubility in Brine

Important for CCS and EOR projects:

// CO2 solubility decreases with salinity (salting-out effect)
SystemInterface co2Brine = new SystemElectrolyteCPAstatoil(273.15 + 50.0, 100.0);
co2Brine.addComponent("CO2", 0.10);
co2Brine.addComponent("water", 0.80);
co2Brine.addComponent("Na+", 0.05);
co2Brine.addComponent("Cl-", 0.05);
co2Brine.setMixingRule(10);

ThermodynamicOperations ops = new ThermodynamicOperations(co2Brine);
ops.TPflash();
co2Brine.initProperties();

// CO2 in aqueous phase
double co2InWater = co2Brine.getPhase("aqueous").getComponent("CO2").getx();

// In-situ pH — getpH() has a built-in acid-gas fallback, so a CO2/H2S brine
// returns an acidic pH even WITHOUT chemicalReactionInit() (no more silent 7.0).
double pH = co2Brine.getpH();               // ~3.9 for CO2-saturated water
// double pH = co2Brine.getPhase("aqueous").getpH("acidgas"); // force the estimate

For a rigorous speciated pH in a buffered brine (explicit H3O+/HCO3-/OH-), run system.chemicalReactionInit() before createDatabase(true) / setMixingRule(10). The getpH("acidgas") fallback is a screening estimate that ignores bicarbonate buffering and salt-ion alkalinity — see the electrolyte-CPA model docs and the flow-assurance skill for corrosion use.

Key Units and Conversions

Quantity	Unit	Conversion
Ion concentration	mol fraction	ppm = x × MW_solution / MW_ion × 1e6
TDS	mg/L	Sum of all dissolved ion concentrations
Salinity	wt% NaCl equiv	Based on Na+/Cl- content
pH	dimensionless	From H+ activity

Common Pitfalls

1. Charge balance: Total positive charges must equal total negative charges
2. Mixing rule must be numeric 10: Not "classic" — CPA requires numeric mixing rule
3. Ion names are case-sensitive: "Na+" not "na+" or "NA+"
4. Multi-phase check: Always enable for electrolyte systems (setMultiPhaseCheck(true))
5. Temperature limits: Electrolyte models may have narrower valid T range than HC models
6. Convergence: Electrolyte flashes can be slow — be patient or reduce component count
7. Missing counter-ions: Always add both cation and anion (e.g., Na+ with Cl-)