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Re: PHREEQC: high CO2 pressure modelling




Let me try to explain the two models that you have generated. First, using
GAS_PHASES, you have taken a liter of gas with a pressure of 52 atmospheres
of CO2, a set of phases, and a liter of water and let them react to
equilibrium. I haven't run the calculation, but I assume most of the CO2 is
consumed dissolving the minerals. Note the initial gas phase has a fixed
amount of CO2 that you can calculate with the ideal gas law PV = nRT, I
think it about 2 moles. When the CO2 reacts, the volume of the gas phase is
reduced and I suspect actually disappears.

When you add the CO2 with EQUILIBRIUM_PHASES, you are allowing up to 10000
moles of CO2 to react to try to attain a PCO2 of 52 atmospheres. Thus, the
main difference in the two calculations is the amount of CO2 that reacts
with a liter of water and the corresponding minerals. If you allow an
unlimited supply of CO2 (EQUILIBRIUM_PHASES approach) you can dissolve more
minerals, but continue to have sufficient CO2 to raise the pCO2 to 52
atmospheres, thus you get a much larger amount of CO2 per kilogram of
water.

The key to the calculation is primarily the amount of CO2 you add per liter
of water and minerals and the extent that the minerals react with the CO2.
Equilibrium with all the minerals is probably not realistic, but is an
end-member case contrasted to completely unreactive minerals.

David


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled



                                                                                                                
                      Gaus Irina                                                                                
                      <gaus@xxxxxxxxxx         To:      "'dlpark@xxxxxxxx'" <dlpark@xxxxxxxx>                   
                      rgm.fr>                  cc:                                                              
In-Reply-To: <D965434E9D6BD511AE3500306E01C8BE01ED9713@xxxxxxx>
                                               Subject: PHREEQC: high CO2 pressure modelling                    
                      08/05/02 08:49                                                                            
                      AM                                                                                        
                                                                                                                
                                                                                                                



Dear Dr Parkhurst

We are currently working on a geochemical modelling project on CO2
sequestration. For that I am trying to model the impact of  the injection
of
dissolved CO2 gas on shales. Therefore I need to define a constant CO2
pressure. I assumed this could be done in two ways. The first whereby the
CO2 is modelled as an gas phase, the second whereby the  pressure of CO2 is
included in an equilibrium phases block :

First approach :

SOLUTION 1 Caprock formation water (based on results CAPROCK1 model)
        units   mol/kgw
        pH      7.67
        pe      -4.071
        density 1.023
        temp    37.0
             Al          3.50E-08
             Ba          1.247e-005
             C           6.922e-005
             Ca          1.771e-001
             Cl          4.787e-001
             Fe          2.475e-007
             K           1.419e-004
             Mg          1.111e-002
             Na          1.060e-001
             S           4.807e-004
             Si          2.520e-004
             Sr          9.664e-005


GAS_PHASE 1                    #Assuming a fugacity of 0.52 at 100 bars of
CO2 at
37C
-fixed_pressure
-pressure   52
-volume     1
-temperature 37
CO2(g)      52

EQUILIBRIUM_PHASES 1
Chalcedony                                 0                       196.0
Illite                                     0                       35.21
Kaolinite                                  0
K-feldspar 4.08
#K-feldspar                                0                       4.08
Calcite                                    0                       5.61
Smectite-high-Fe-Mg            0                       11.93
Albite                                     0                       25.59
Clinochlore-7A                       0                       3.99
Pyrite                                     0                       8.62
Gypsum                                     0                       0.0
Siderite                                   0                       0.0
Dawsonite                                  0                       0.0
Dolomite                                   0                       0.0
Barite                                     0                       0.0

EnD

The second approach :

SOLUTION 1 Caprock formation water (based on results CAPROCK1 model)
        units   mol/kgw
        pH      7.67
        pe      -4.071
        density 1.023
        temp    37.0
             Al          3.50E-08
             Ba          1.247e-005
             C           6.922e-005
             Ca          1.771e-001
             Cl          4.787e-001
             Fe          2.475e-007
             K           1.419e-004
             Mg          1.111e-002
             Na          1.060e-001
             S           4.807e-004
             Si          2.520e-004
             Sr          9.664e-005


EQUILIBRIUM_PHASES 1
Chalcedony                                 0                       196.0
Illite                                     0                       35.21
Kaolinite                                  0
K-feldspar 4.08
#K-feldspar                                0                       4.08
Calcite                                    0                       5.61
Smectite-high-Fe-Mg            0                       11.93
Albite                                     0                       25.59
Clinochlore-7A                       0                       3.99
Pyrite                                     0                       8.62
Gypsum                                     0                       0.0
Siderite                                   0                       0.0
Dawsonite                                  0                       0.0
Dolomite                                   0                       0.0
Barite                                     0                       0.0
CO2(g)                                     1.71600334        10000

End

The resulting amount of dissolved CO2 in the solution seems to be hugely
different (5.06 10-5 mole/kg in the first, 7.14 moles/kg in the second
approach). Apparently I modelled two different things here. The first value
seems too low taking into account such a high CO2 pressure, while the
second
value largely overestimates the experimentally estimated value of
approximately 1.1 moles/kg (although this value was measured in the
solution
without interaction with the sediment).

I would be very grateful if you could give me a hint in where I am going
wrong here.

Many thanks



Dr Irina Gaus

Water modelling section
Bureau des Recherches Geologiques et Minieres (BRGM)
i.gaus@xxxxxxx






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