SolutionGasWaterRatio calculates a standard-volume ratio of gas dissolved in an aqueous phase to water. The result is reported as Sm³ gas per Sm³ water. Use the class for reproducible screening and sensitivity studies; qualify the selected thermodynamic method against laboratory data before reservoir, emissions, or facility decisions.

The current public class offers McCain, Søreide–Whitson, and electrolyte-CPA paths. They do not share the same physical model or failure behavior, so a method name alone is not a validation statement.

Public input and result contract

Input or result Public API Unit and implementation boundary
Temperature setTemperaturesAndPressures, calculateRsw K
Pressure setTemperaturesAndPressures, calculateRsw bara
Salinity setSalinity(double) mol NaCl per kg water
Salinity with unit setSalinity(double, String) molal, mol/kg, wt%, weight%, ppm, or mg/l
Calculation path setCalculationMethod enum or recognized method-name string
Vector result getRsw() Sm³ gas per Sm³ water
Indexed result getRsw(int) Sm³ gas per Sm³ water

The temperature and pressure arrays must have equal lengths. The implementation stores the provided arrays and calculates points by matching index; callers should not mutate them during a calculation. It does not validate null arrays, empty arrays, negative salinity, or a method-specific temperature/pressure range.

The ppm and mg/l conversions are approximate and assume water density near 1 kg/L. wt% is converted to molality using 58.44 g/mol for NaCl. Treat salinity as NaCl equivalent unless the selected model and input system have been independently qualified for the actual brine.

Select a calculation path

Method Current implementation Use boundary
MCCAIN Culberson–McKetta methane-in-water polynomial plus the implemented McCain salinity correction The source gas composition is ignored. Use only as a methane/brine correlation screen inside its documented source range.
SOREIDE_WHITSON Builds a new SystemSoreideWhitson, normalizes non-water source components, adds excess water and NaCl, applies mixing rule 11, and performs a multiphase TP flash Requires a meaningful positive source-gas inventory. A caught flash exception returns zero.
ELECTROLYTE_CPA Uses SystemSrkCPAstatoil at zero salinity or SystemElectrolyteCPAstatoil with explicit Na⁺/Cl⁻ at positive salinity, applies mixing rule 10, and performs a multiphase TP flash Model availability is not evidence of accuracy for every gas, salt, temperature, or pressure. Flash exceptions propagate.

The EoS-based paths identify an aqueous phase by phase type or, as a fallback, by water mole fraction above 0.5. They return zero when no aqueous phase or no dissolved non-water gas is found. For Søreide–Whitson, zero can also mean that the caught TP flash failed. A returned zero is therefore not distinguishable from a physical zero without independent phase and convergence diagnostics.

The class defaults to ELECTROLYTE_CPA. Select the method explicitly in auditable workflows.

Complete McCain screening example

This Java 8 program is extracted, compiled, and executed from this page by the repository test suite. It checks implementation behavior for pure methane: finite positive results, increasing Rsw over a bounded pressure series, and reduced Rsw after adding NaCl. Those checks are regression evidence, not laboratory validation.

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import neqsim.pvtsimulation.simulation.SolutionGasWaterRatio;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkCPAstatoil;

public final class SolutionGasWaterRatioReferenceExample {
  private static final Logger logger =
      LogManager.getLogger(SolutionGasWaterRatioReferenceExample.class);

  private SolutionGasWaterRatioReferenceExample() {}

  public static void main(String[] args) {
    SystemInterface methane = new SystemSrkCPAstatoil(350.0, 100.0);
    methane.addComponent("methane", 1.0);
    methane.setMixingRule(10);

    SolutionGasWaterRatio calculator = new SolutionGasWaterRatio(methane);
    calculator.setCalculationMethod(SolutionGasWaterRatio.CalculationMethod.MCCAIN);
    calculator.setSalinity(0.0);

    double[] temperaturesK = {350.0, 350.0, 350.0};
    double[] pressuresBara = {50.0, 100.0, 150.0};
    calculator.setTemperaturesAndPressures(temperaturesK, pressuresBara);
    calculator.runCalc();

    double[] pressureSeries = calculator.getRsw();
    assert pressureSeries.length == pressuresBara.length;
    for (double value : pressureSeries) {
      assert Double.isFinite(value);
      assert value > 0.0;
    }
    assert pressureSeries[0] < pressureSeries[1];
    assert pressureSeries[1] < pressureSeries[2];

    calculator.setTemperaturesAndPressures(
        new double[] {350.0}, new double[] {100.0});
    calculator.setSalinity(0.0);
    calculator.runCalc();
    double pureWaterRsw = calculator.getRsw(0);

    calculator.setSalinity(3.5, "wt%");
    calculator.runCalc();
    double salineWaterRsw = calculator.getRsw(0);

    assert salineWaterRsw > 0.0;
    assert salineWaterRsw < pureWaterRsw;

    logger.info(
        "Rsw at 50/100/150 bara: {}/{}/{} Sm3/Sm3; at 100 bara pure/saline: {}/{}",
        pressureSeries[0],
        pressureSeries[1],
        pressureSeries[2],
        pureWaterRsw,
        salineWaterRsw);
  }
}

Run Java with assertions enabled (-ea) when using this example outside the test suite.

McCain equation implemented by the class

The implementation converts kelvin to degrees Fahrenheit and bara to psia, then evaluates

\[R_{sw,mathrm{pure}}=A(T_F)+B(T_F)P_{mathrm{psia}}+C(T_F)P_{mathrm{psia}}^2\]

and applies

\[R_{sw,mathrm{brine}}=R_{sw,mathrm{pure}}10^{-C_s(T_F,P_{mathrm{psia}})S_{mathrm{wt%}}}\]

before multiplying the scf/STB result by 0.178108 to report Sm³/Sm³. The class source documents the methane-correlation range as 60–350 °F and up to 10,000 psia. That source boundary is not a substitute for checking the original correlation, pressure convention, fluid composition, and project data.

EoS result basis

For the EoS paths, the implementation sums non-water, non-ion mole fractions in the selected aqueous phase and divides by the water mole fraction. It converts that molar ratio to a standard-volume ratio using NeqSim’s standard-state temperature and reference pressure, an ideal-gas molar volume, and a fixed water density of 1000 kg/m³.

This result basis does not model gas liberation through a separator train, stock-tank shrinkage, brine-density variation, mineral precipitation, chemical reaction, or uncertainty. Confirm phase identity, material balance, convergence, composition, salt representation, and standard conditions before comparing Rsw values across tools or datasets.

Validation checklist

Before engineering use:

  1. state gas composition, water analysis, NaCl-equivalent assumption, temperature in K, pressure in bara, and standard-volume basis;
  2. preserve the selected calculation method with the result;
  3. inspect phase identity and convergence for EoS calculations;
  4. compare pressure, temperature, and salinity trends rather than accepting a single point;
  5. benchmark against traceable laboratory data over the intended range;
  6. treat zero, non-finite, or discontinuous values as diagnostics requiring investigation.

The enabled SolutionGasWaterRatioTest protects public method selection, units, array-length handling, positive McCain results, pressure monotonicity, and salting-out behavior. Its broader verification suite provides exploratory comparisons but does not establish universal method accuracy.