# GAS-WATER: flow condition to initialize a study with constant pressure and saturations¶

In the Gas-Water model there are three possible states in any region:

1. Undersaturated water: only the aqueous phase is present.

2. Undersaturated gas: Vapour phase only present.

3. Saturated: both phases are present.

For a region in the undersaturated water state, you must specify the pressure of the aqueous phase, and the mole fraction concentration of the dissolved gas as in the following example:

FLOW_CONDITION initial
TYPE
LIQUID_PRESSURE dirichlet
GAS_IN_LIQUID_MOLE_FRACTION dirichlet
TEMPERATURE dirichlet
END
LIQUID_PRESSURE 1.20D7 Pa
GAS_IN_LIQUID_MOLE_FRACTION 1.D-6
TEMPERATURE 45.0d0 C
END


Note that the mole fraction does not include the salt in the brine - this will be moles of CO2/ (moles of water+moles of CO2)

For a region in the unsaturated vapour state, you must specify the pressure of the vapour phase, and the mole fraction concentration of the water vapour as in the following example:

FLOW_CONDITION initial
TYPE
GAS_PRESSURE dirichlet
WATER_IN_VAPOR_MOLE_FRACTION dirichlet
TEMPERATURE dirichlet
END
GAS_PRESSURE 1.20D7 Pa
WATER_IN_VAPOR_MOLE_FRACTION 1.D-5
TEMPERATURE 45.0d0 C
END


For a region in the saturated state, you must specify the pressure of the aqueous phase, and the saturation of the gas phase as in the following example:

FLOW_CONDITION initial
TYPE
LIQUID_PRESSURE dirichlet
GAS_SATURATION dirichlet
TEMPERATURE dirichlet
END
LIQUID_PRESSURE 1.20D7 Pa
GAS_SATURATION 0.5
TEMPERATURE 50.0 C
END