PHREEQC: high CO2 pressure modelling

[Gaus Irina <gaus@xxxxxxxxxxxxxxxx>]

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