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

Basic Electrolyte Setup

Pitzer hydrate and mixed-chloride route

For Pitzer water activities use SystemPitzer with setMixingRule("classic") and setMultiPhaseCheck(true). The hydrate onset temperature/pressure/curve operations use PitzerHydrateFlash and a matched liquid-water reference. See docs/thermo/pitzer_hydrate_equilibrium.md for the tested setup and limits. applyPhreeqcCo2ChlorideParameters(Map<String, Double>) loads the pinned PHREEQC Na/K/Ca/Mg-Cl and CO2 lambda rows, but requires an explicit CO2-cation-Cl zeta for every present cation. Zero is an explicit screening assumption, not a fitted catalog coefficient. It remains unqualified. When both K+ and Mg++ are present (including zero-amount components), the pinned catalog also lacks their theta row. Use applyPhreeqcCo2ChlorideParameters(zeta, potassiumMagnesiumTheta) with an explicit finite value; the one-argument overload rejects that topology. For concentration sweeps retain the same components and neutral parameter dataset at zero salt. See the guide's 57-point reference assessment: numerical convergence and a passing regression suite do not mean every experimental comparison passed the 1 K accuracy criterion. Do not silently fill missing tuples or equate parameter coverage with scientific validation.

Hand off to neqsim-flow-assurance / flow.assurance with the configured fluid, salt mole basis, dataset identity, supplied zeta/theta values, pressure grid and operating temperature. Carry forward the Henry-domain lower limit (CO2 274.19 K; methane 275.46 K) and onset-only scope; subzero hydrate predictions require a qualified low-temperature aqueous gas reference.

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

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).

Once the SI/SR is known, the inhibitor side (minimum inhibitor concentration, residual SI after treatment, chemical compatibility, and what a pH adjuster or H2S scavenger does to the brine) lives in neqsim-production-chemistry (ScaleInhibitorPerformance, ScaleControlAssessor, ProductionChemicalScaleScenario). A StreamChemistryAdapter / fromStream(...) helper reads the ion table straight off the electrolyte stream built here — do not retype it.

Activity-consistent calcium-sulfate equilibrium

Use the pure-mineral operation when the engineering question is gypsum/anhydrite equilibrium rather than screening SI:

MultiSaltPrecipitationResult solids =
    new ThermodynamicOperations(brine).precipitateScales("CaSO4_A", "CaSO4_G");

CalciumSulfatePhaseBoundaryQualification evidence =
    new ThermodynamicOperations(brine).qualifyCalciumSulfatePhaseBoundary();

CaSO4_G is hydrated gypsum: the authoritative operation includes $a_{\mathrm{w}}^2$, removes or returns two water moles per formula unit, and reports hydrated mass. CaSO4_A remains water-free anhydrite. The qualification object separates this mineral-standard-state evidence from the Pitzer or electrolyte-CPA aqueous activity model. It is currently fail-closed against the CC BY 4.0 Voigt–Freyer pure-water and NaCl crossing envelopes and does not qualify high-pressure use; do not fit or reinterpret Pitzer interactions to hide a mineral-correlation mismatch. See docs/pvtsimulation/scale_prediction_api.md for the source matrix, numerical residuals, and limits. The object registers the NIST ThermoML CaCl2 pressure series as independent finite-concentration evidence, but explicitly leaves the dilute limiting-volume term unresolved; do not treat the presence of pressure evidence as high-pressure calcium-sulfate qualification.

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);

Cathodic (high-pH) scaling in seawater electrochemical devices

Electrochlorination cells, seawater electrolysers and impressed-current cathodic protection all evolve hydrogen at the cathode, which lifts the boundary-layer pH far above the bulk seawater value of ≈ 8.1. The deposit there is brucite Mg(OH)₂ together with CaCO₃, not the sulphate minerals produced-water work usually screens for. Set the boundary-layer pH, not the bulk pH:

ElectrolyteScaleCalculator calc = new ElectrolyteScaleCalculator()
    .setTemperatureCelsius(8.0).setPressureBara(1.013)
    .setPH(10.5)                       // cathode boundary layer, NOT the bulk 8.1
    .setCations(412.0, 0.02, 7.9, 1290.0, 10780.0, 399.0, 0.002)  // S = 35 seawater, mg/L
    .setAnions(19350.0, 2710.0, 142.0, 0.0);
calc.calculate();
double siBrucite = calc.getBruciteSaturationIndex();   // Mg(OH)2 - two decades per pH unit
double siCalcite = calc.getCaCO3SaturationIndex();
double aOH = calc.getHydroxideActivity();

Bulk S = 35 seawater at 8 °C is already supersaturated in calcite (SI ≈ +0.58, Ω ≈ 3.8) but strongly undersaturated in brucite (SI ≈ −4.8); brucite crosses SI = 0 near pH 10.5 at 8 °C and near pH 10.1 at 14 °C (retrograde solubility, so it is worse in warm water).

getpH() on a loaded SystemElectrolyteCPAstatoil can return a flat 7.0 without an explicit reaction/ion setup — for boundary-layer work set the pH explicitly on the scale calculator instead of reading it back from a flash.

Cell voltage is an electrolyte-transport question. For a current-controlled device the applied voltage floats with seawater resistivity, so a fixed voltage limit (vendor maximum, Ex/ATEX certification) has a temperature-dependent margin. Use neqsim.process.chemistry.electrochlorination.SeawaterElectrolyteConductivity (UNESCO/PSS-78) for conductivity, resistivity and the ohmic voltage:

SeawaterElectrolyteConductivity sw = new SeawaterElectrolyteConductivity()
    .setSalinityPsu(35.0).setTemperatureCelsius(8.0);
sw.calculate();
double kappa = sw.getConductivitySPerM();                       // ≈ 3.62 S/m
double ratio = sw.ohmicVoltageRatioVersusTemperature(15.0);     // ≈ 1.19 vs a 15 °C design basis

Chain: this skill → neqsim-flow-assurance (scale kinetics, remediation) for deposition rate and dissolver selection; → neqsim-standards-lookup for NORSOK S-001 / ISO 13702 when the consequence is firewater or seawater-system availability.

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

Gas-solubility reference assessments

Match reported gas partial pressure and final aqueous salt molality. Account for solvent evaporation: 1 kg feed water need not leave 1 kg aqueous water. Rebuild independent trial fluids, compensate feed water, and bracket from valid liquid-bearing states. Verify component inventory, ion confinement and molecular fugacity equality before using a trial in a pressure root. Check that changing the finite gas-reservoir size leaves the matched result unchanged. Use init(1) for fixed-T/P fugacities, retaining accuracy deviations as evidence. See KClCO2SolubilityReferenceAssessmentTest and the benchmark notes.

Common Pitfalls

1. Charge balance: Total positive charges must equal total negative charges
2. Mixing rule is model-specific: CPA requires numeric 10; SystemPitzer uses "classic"
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-)