Qualify Furst electrolyte H2–NaCl brine flash: hydrogen-rich dense oil root at 323 K and 100 bara

[EvenSol]

Use case

Found while building notebooks/reservoir/hydrogen_brine_pore_network_paper_comparison.ipynb in EvenSol/NeqSim-Colab for #3734. The suggested SystemFurstElectrolyteEos needs qualification before supplying hydrogen–brine flow properties.

Reproducer

Tested source: 87f3c41266ed6ce6dc4ec3879411ae7da59b5eca (current master inspected 2026-09-16); all main Java sources compiled with JDK 17, --release 8.

SystemInterface f = new SystemFurstElectrolyteEos(323.15, 100.0);
f.addComponent("hydrogen", 10.0);
f.addComponent("water", 1.0 / 0.01801528);
f.addComponent("Na+", 1.0);
f.addComponent("Cl-", 1.0);
f.setMixingRule(4);
f.setMultiPhaseCheck(true);
f.init(0);
f.init(1);
new ThermodynamicOperations(f).TPflash();
f.initProperties();
for (int i = 0; i < f.getNumberOfPhases(); i++) {
  System.out.println(f.getPhase(i).getPhaseTypeName() + " "
      + f.getPhase(i).getDensity("kg/m3"));
}

Observed phase types/densities: oil ≈92.1958 kg/m3 and aqueous ≈1023.1673 kg/m3. Aqueous H2 mole fraction ≈7.37575e-6. With the same composition and electrolyte CPA/rule 10 the phases are gas ≈7.20314 kg/m3 and aqueous ≈1003.81368 kg/m3; aqueous H2 mole fraction ≈0.00111991. Dry hydrogen SRK density is ≈7.09214 kg/m3 at these conditions. The model-to-model solubility comparison is a diagnostic, not experimental validation.

Expected / impact

A qualified hydrogen–brine workflow should return a physically defensible hydrogen-rich gas and brine state, or explicitly report unsupported/unqualified parameter coverage. A dense nonaqueous root would contaminate buoyancy, pore-flow and storage estimates. The notebook rejects this candidate and does not propagate it into accepted calculations.

Investigation and acceptance

• Determine whether H2 ion/water parameter coverage, root selection, or electrolyte phase stability causes this state; do not force a gas label onto the same root.
• Add an exact reproducer with component/charge conservation, phase composition, density, fugacity residual and repeatability diagnostics.
• Qualify nearby T/P/salinity points and compare to public hydrogen–brine measurements before recommending this model.
• Preserve existing methane/electrolyte cases; document supported model selection and limitations.

Searched existing hydrogen/brine and Furst/hydrogen issues; no matching specific issue found. Related umbrella: #3734.