CO2-Brine Model PVT Parameter Files Replaced by XML Attributes

[dkachuma]

CO2-Brine Model PVT Parameter Files Replaced by XML Attributes

Description of the Breaking Change

This is a discussion about #3777

This update removes the requirement for external text parameter files (phasePVTParaFiles and flashModelParaFile) for CO2-Brine fluid models CO2BrinePhillipsFluid, CO2BrineEzrokhiFluid, CO2BrinePhillipsThermalFluid, and CO2BrineEzrokhiThermalFluid. All parameters, including table ranges, discretization intervals, salinity, and model coefficients, are now specified directly within the XML input file. There is no need to specify external files (usually pvtgas.txt, pvtliquid.txt, co2flash.txt). This is a breaking change. Existing input files that reference those parameter files will no longer work and must be updated.

Key Changes

• Consolidated Grids: All fluid properties (solubility, density, viscosity, enthalpy) now share the same pressure and temperature discretization points. Previously, each property could have its own file with different ranges.
• XML Attributes: The fields phasePVTParaFiles and flashModelParaFile are removed. They are replaced by attributes like pressureCoordinates, pressureInterval, temperatureCoordinates, and temperatureInterval.
• Explicit Solubility Selection: Users can now explicitly choose between DuanSun (default), SpycherPruess, and Tables.

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1. Pressure and Temperature Table Points

Previously, the pressure and temperature discretization for each of the three model files was specified independently. Now a single common set of points is defined directly on the fluid XML element and applies to all internal tables (CO2 density, CO2 viscosity, brine density, brine viscosity, CO2 solubility).

The new attributes are:

Attribute	Description
pressureCoordinates	List of pressure values (Pa). If pressureInterval is positive, only the first and last values are used as bounds; intermediate values are ignored.
pressureInterval	Step size (Pa) for uniform pressure discretization. Set to 0 or omit to use coordinates as-is.
temperatureCoordinates	List of temperature values (K). Same rules as pressure.
temperatureInterval	Step size (K) for uniform temperature discretization. Set to 0 or omit to use coordinates as-is.

Two modes of grid specification are supported:

• Uniform spacing: Provide a min and max in *Coordinates and a positive *Interval. Points are generated as x_i = min + i * interval up to max.
• Explicit points: Set *Interval to 0 (or omit it) and list all desired points explicitly in *Coordinates. This is useful for non-uniform grids such as logarithmic spacing.

Example: Uniform grid (from thermalLeakyWell)

Old pvtgas.txt:

DensityFun SpanWagnerCO2Density     6.6e6 4e7 1e6 302.0 312.0 5
ViscosityFun FenghourCO2Viscosity   6.6e6 4e7 1e6 302.0 312.0 5

Old pvtliquid.txt:

DensityFun PhillipsBrineDensity     6.6e6 4e7 1e6 302.0 312.0 5 1.901285269
ViscosityFun PhillipsBrineViscosity 1.901285269

Old co2flash.txt:

FlashModel CO2Solubility            6.6e6 4e7 1e6 302.0 312.0 5 1.901285269

New XML:

<CO2BrinePhillipsFluid
  name="fluid"
  phaseNames="{ gas, water }"
  componentNames="{ co2, water }"
  componentMolarWeight="{ 44e-3, 18e-3 }"
  pressureCoordinates="{6.6e6, 4e7}"
  pressureInterval="1e6"
  temperatureCoordinates="{302.0, 312.0}"
  temperatureInterval="5"
  salinity="1.901285269" />

Example: Explicit (non-uniform) grid points

<CO2BrinePhillipsFluid
  name="fluid"
  phaseNames="{ gas, water }"
  componentNames="{ co2, water }"
  componentMolarWeight="{ 44e-3, 18e-3 }"
  pressureCoordinates="{1.000e+05, 2.512e+05, 6.310e+05, 1.585e+06, 3.981e+06, 1.000e+07}"
  temperatureCoordinates="{283.15, 305.15, 327.15, 349.15, 371.15, 393.15}"
  salinity="3.0" />

Note: when pressureInterval is omitted (or zero), the full list of values in pressureCoordinates is used directly as the grid points.

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2. Salinity

Previously, salinity was embedded as the trailing value on lines in pvtliquid.txt and co2flash.txt:

Old pvtliquid.txt:

DensityFun PhillipsBrineDensity     1.0e5 7.5e7 1e5 285.15 395.15 5 1.901285269
ViscosityFun PhillipsBrineViscosity 1.901285269

Old co2flash.txt:

FlashModel CO2Solubility            1.0e5 7.5e7 1e5 285.15 395.15 5 1.901285269

Now it is specified with the salinity attribute (in moles of NaCl per kg of brine). The default is 0, so salinity="0" may be omitted:

<CO2BrinePhillipsFluid
  ...
  salinity="1.901285269" />

Only a single salinity value can be specified and it will apply to all the functions CO2 solubility, brine density and brine viscosity.

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3. Solubility Model Selection

A new solubilityModel attribute allows explicit selection of the CO2 solubility model. The available options are DuanSun (default), SpycherPruess, and Tables.

3.1 Duan and Sun (default)

This is the default and does not need to be set explicitly. CO2 solubility in brine is computed; water vaporization into the gas phase is zero.

Old co2flash.txt:

FlashModel CO2Solubility 1.0e5 5.e7 1e6 283.15 343.15 2.5 0.0

New XML (equivalent solubilityModel omitted as DuanSun is the default):

<CO2BrinePhillipsFluid
  ...
  pressureCoordinates="{1e5, 5.e7}"
  pressureInterval="1e6"
  temperatureCoordinates="{283.15, 353.5}"
  temperatureInterval="2.5" />

3.2 Spycher and Pruess

This model accounts for both CO2 solubility in brine and water vaporization into the gas phase. Previously it was selected via a trailing keyword in the flash file:

Old co2flash_thermal_Spycher.txt:

FlashModel CO2Solubility 1.0e5 5.e7 1e6 283.15 343.15 2.5 0 1.0e-10 SpycherPruess

New XML:

<CO2BrinePhillipsThermalFluid
  ...
  pressureCoordinates="{1e5, 5.e7}"
  pressureInterval="1e6"
  temperatureCoordinates="{283.15, 353.5}"
  temperatureInterval="2.5"
  solubilityModel="SpycherPruess" />

3.3 Tables (user-supplied solubility tables)

When solubilityModel="Tables" is set, solubility data is read from externally generated lookup tables provided as TableFunction XML elements. The table names are referenced via the solubilityTableNames attribute.

• If one table name is provided, it is treated as the CO2 solubility table; water vaporization is zero.
• If two table names are provided, the second is treated as the water vaporization table.

Each table must be a TableFunction with inputVarNames="{ pressure, temperature }", with coordinates provided via coordinateFiles and values via voxelFile.

New XML:

<Constitutive>
  <CO2BrinePhillipsFluid
    name="co2BrinePhillipsMixture"
    phaseNames="{ gas, water }"
    componentNames="{ co2, water }"
    componentMolarWeight="{ 44e-3, 18e-3 }"
    pressureCoordinates="{1e6, 50e6}"
    pressureInterval="1e6"
    temperatureCoordinates="{258.0, 380.0}"
    temperatureInterval="5.0"
    solubilityModel="Tables"
    solubilityTableNames="{ co2Solubility, h2oVapourisation }" />
</Constitutive>

<Functions>
  <TableFunction
    name="co2Solubility"
    inputVarNames="{ pressure, temperature }"
    coordinateFiles="{ pvtData/pres.txt, pvtData/temp.txt }"
    voxelFile="pvtData/co2Solubility.txt" />

  <TableFunction
    name="h2oVapourisation"
    inputVarNames="{ pressure, temperature }"
    coordinateFiles="{ pvtData/pres.txt, pvtData/temp.txt }"
    voxelFile="pvtData/h20Vapourisation.txt" />
</Functions>

Where pres.txt and temp.txt contain the pressure and temperature coordinate values respectively, and co2Solubility.txt and h20Vapourisation.txt contain the corresponding solubility values on the 2D grid (in row-major order).

Note: When using solubilityModel="Tables", the pressureCoordinates/temperatureCoordinates attributes still control the grid for the CO2 and brine property tables (density, viscosity). The solubility coordinate grid is defined by the coordinate files inside the TableFunction elements.

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4. Phillips Brine Density and Viscosity Model

The CO2BrinePhillipsFluid (and its thermal variant CO2BrinePhillipsThermalFluid) use the Phillips et al. (1981) correlation for brine density and viscosity. No additional coefficients need to be specified beyond salinity and the common pressure/temperature grid.

Old pvtliquid.txt:

DensityFun PhillipsBrineDensity     1.0e5 7.5e7 1e5 285.15 395.15 5 0
ViscosityFun PhillipsBrineViscosity 0

New XML (using CO2BrinePhillipsFluid):

<CO2BrinePhillipsFluid
  name="fluid"
  phaseNames="{ gas, water }"
  componentNames="{ co2, water }"
  componentMolarWeight="{ 44e-3, 18e-3 }"
  pressureCoordinates="{1.0e5, 7.5e7}"
  pressureInterval="1e5"
  temperatureCoordinates="{285.15, 395.15}"
  temperatureInterval="5"
  salinity="0" />

---

5. Ezrokhi Brine Density and Viscosity Model

The CO2BrineEzrokhiFluid (and CO2BrineEzrokhiThermalFluid) use the Ezrokhi correlation (Zaytsev and Aseyev, 1993) to correct pure water density and viscosity for salinity and dissolved CO2. This requires three empirical coefficients for each.

New Attribute	Description
ezrokhiDensityCoefficients	Three-element array {a0, a1, a2} for the density correction A(T) = a0 + a1*T + a2*T^2
ezrokhiViscosityCoefficients	Three-element array {b0, b1, b2} for the viscosity correction B(T) = b0 + b1*T + b2*T^2 (defaults to {0, 0, 0})

Old pvtliquid_ez.txt:

DensityFun EzrokhiBrineDensity   0.1033 -2.2991e-5 -2.3658e-6
ViscosityFun EzrokhiBrineViscosity 0 0 0

New XML:

<CO2BrineEzrokhiFluid
  name="fluid"
  phaseNames="{ gas, water }"
  componentNames="{ co2, water }"
  componentMolarWeight="{ 44e-3, 18e-3 }"
  pressureCoordinates="{1.0e5, 6e7}"
  pressureInterval="1e5"
  temperatureCoordinates="{283.15, 393.5}"
  temperatureInterval="5"
  salinity="1.901285269"
  ezrokhiDensityCoefficients="{0.1033, -2.2991e-5, -2.3658e-6}"
  ezrokhiViscosityCoefficients="{0, 0, 0}" />

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6. Common Pressure and Temperature Grid: Migration Guidance

This is the most impactful change for results.

Previously, the three parameter files (pvtgas.txt, pvtliquid.txt, co2flash.txt) each independently defined their own pressure and temperature ranges and step sizes. Now a single common grid is used for all properties (CO2 density, CO2 viscosity, brine density, brine viscosity, CO2 solubility).

Case 1: All files had the same ranges

When all three old files used identical pressure and temperature ranges and step sizes, migration is straightforward, read the values directly from any of the three files and use them in the new attributes.

Example from thermalLeakyWell (all files matched):

Old pvtgas.txt:

DensityFun SpanWagnerCO2Density     6.6e6 4e7 1e6 302.0 312.0 5
ViscosityFun FenghourCO2Viscosity   6.6e6 4e7 1e6 302.0 312.0 5

Old pvtliquid.txt:

DensityFun PhillipsBrineDensity     6.6e6 4e7 1e6 302.0 312.0 5 1.901285269
ViscosityFun PhillipsBrineViscosity 1.901285269

Old co2flash.txt:

FlashModel CO2Solubility            6.6e6 4e7 1e6 302.0 312.0 5 1.901285269

All three used pressure [6.6e6, 4e7] step 1e6 and temperature [302.0, 312.0] step 5: use them directly:

<CO2BrinePhillipsFluid
  ...
  pressureCoordinates="{6.6e6, 4e7}"
  pressureInterval="1e6"
  temperatureCoordinates="{302.0, 312.0}"
  temperatureInterval="5"
  salinity="1.901285269" />

Case 2: Files had different ranges

When the old files used different ranges, you must choose a combined grid that covers all of them. The strategy is to take the widest range across all three files to ensure that no pressure or temperature value encountered during the simulation falls outside the table bounds. At the same time, avoid an unnecessarily fine step size, as a finer grid increases memory usage and initialization time.

A real example of this is the gravitySegregation case. The old files were:

Old pvtgas.txt:

DensityFun SpanWagnerCO2Density     1e6  1.5e7 5e4 367.15 369.15 1
ViscosityFun FenghourCO2Viscosity   1e6  1.5e7 5e4 367.15 369.15 1

Old pvtliquid.txt:

DensityFun PhillipsBrineDensity     1e6  1.5e7 5e4 367.15 369.15 1 0
ViscosityFun PhillipsBrineViscosity 0

Old co2flash.txt:

FlashModel CO2Solubility            5e5  1.5e7 5e4 367.15 369.15 1 0

The pressure minimum differs: pvtgas.txt and pvtliquid.txt started at 1e6 Pa, while co2flash.txt started at 5e5 Pa. The temperature range and step were identical across all three. To cover all cases the widest pressure range must be used, taking 5e5 as the lower bound:

<CO2BrinePhillipsFluid
  ...
  pressureCoordinates="{5e5, 1.5e7}"
  pressureInterval="5e4"
  temperatureCoordinates="{367.15, 369.15}"
  temperatureInterval="1" />

Note: Because the grid is now shared, results may differ slightly from previous runs even when the ranges were the same, if any of the old files used different step sizes for different properties. This is expected and is a consequence of the unified discretization approach.

Important: GEOS uses constant extrapolation when pressure or temperature exceed the table bounds, without issuing a warning. Always choose bounds that fully cover the expected pressure and temperature range of your simulation, including in wells.