CO2 solubility

[Ricardo Borba]

Dear Dr. David Parkhurst

I am simulating a real situation of CO2 solubility at 600 atm and 65ºC. Under these conditions, in the aquifer, there are no bubbles, the solution is monophasic. I tried to model this situation in Phreeqc in different ways and obtained different CO2 solubilization capacities for the same P and T conditions. For simulations 1, 3 and 4 the CO2 was completely dissolved, for the others there was only its partial dissolution. Could you help me understand why the simulations showed different results?

SOLUTION 1
    temp      60
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Cl        10
    Na        10
    -water    1 # kg
REACTION 1
    CO2(g)     1
    90 moles in 10 steps
GAS_PHASE 1
    -fixed_pressure
    -pressure 600
    -volume 1
    -temperature 65
END
SOLUTION 1
    temp      60
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Cl        10
    Na        10
    -water    1 # kg
REACTION 1
    CO2(g)     1
    90 moles in 10 steps
GAS_PHASE 1
    -fixed_pressure
    -pressure 600
    -volume 1
    -temperature 65
    CO2(g)    0
END
SOLUTION 1
    temp      60
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Cl        10
    Na        10
    -water    1 # kg
REACTION 1
    CO2(g)     1
    90 moles in 10 steps
EQUILIBRIUM_PHASES 1
    CO2(g)    100 0
GAS_PHASE 1
    -fixed_pressure
    -pressure 600
    -volume 1
    -temperature 65
END
SOLUTION 1
    temp      60
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Cl        10
    Na        10
    -water    1 # kg
REACTION 1
    CO2(g)     1
    90 moles in 10 steps
GAS_PHASE 1
    -fixed_pressure
    -pressure 600
    -volume 1
    -temperature 65
    CO2(g)    1
END
SOLUTION 1
    temp      60
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Cl        10
    Na        10
    -water    1 # kg
REACTION 1
    CO2(g)     1
    90 moles in 10 steps
EQUILIBRIUM_PHASES 1
    CO2(g)    100 0
REACTION_PRESSURE 1
    1 600 in 10 steps
END
SOLUTION 1
    temp      60
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Cl        10
    Na        10
    -water    1 # kg
REACTION 1
    CO2(g)     1
    90 moles in 10 steps
EQUILIBRIUM_PHASES 1
    CO2(g)    100 0
REACTION_PRESSURE 1
    1 600 in 10 steps
GAS_PHASE 1
    -fixed_pressure
    -pressure 1
    -volume 1
    -temperature 65
    CO2(g)    1
END

[dlparkhurst]

I'm not going to go through all the permutations. I'll just make a few comments.

In your first simulation, you did not include CO2(g) in the GAS_PHASE definition. So all 90 moles go into solution with a calculated pH of 87. 10 to 20 moles of solute is pretty much the limit, even for pitzer.dat. Results would be reasonable if you included CO2(g) in the GAS_PHASE.

"CO2(g) 100 0" in EQUILIBRIUM_PHASES indicates a bubble would form when the partial pressure in the solution reached 10^100 atm. I think that is probably black-hole kind of pressures. For your conditions,  CO2(g)  2.78 0 would represent 600 atm.

You probably do not want CO2(g) as an EQUILIBRIUM_PHASE and in a GAS_PHASE. Pick one or the other.

[MichaelZ20]

Hi David!
I paid attention to a strange behavior of gas phase and pH at small addition of CO2 (1 mole) when H2O(g) is considered.
Could you please comment?

SOLUTION 1
    temp      65
    pH        7  charge
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Cl        10
    Na        10
    -water    1 # kg
END

USE SOLUTION 1
   
REACTION 1
    CO2(g)     1
    1 moles in 100 steps
   
GAS_PHASE 1
    -fixed_pressure
    -pressure 600
    -volume 1e-9
    -temperature 65
     CO2(g) 0
     H2O(g) 0
   
USER_GRAPH 1
-headings RXN Gas_volume pH
-axis_titles "CO2 addition, mol" "Gas volume, L" "pH"
10 GRAPH_X RXN
20 GRAPH_Y GAS_VM * GAS("CO2(g)")
30 GRAPH_SY -LA("H+")
END

[dlparkhurst]

That is a bug. All the gas should be dissolved. I'll have to check with Tony on how to fix the problem.