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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