Geochemist's Workbench Support Forum

# Unable to understand the reason for increase in fCO2

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Roserun.reaHello,

I want to calculate the equilibrium between a given brine and given initial fugacity of CO2 (simulating dissolution of CO2 in brine). To do that I specified the brine composition in REACT and swapped CO2(g) with HCO3- in the basis. I mentioned the initial fugacity as 100 atm and let it run. The equilibrium fugacity of CO2(g) is about 175 atm.

I am using GWB 8.0 version.

I am attaching here the REACT input file that I am using.

Vivek.

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Hi Vivek,

Fugacity is initially set in your calculation as a constraint. By setting such a high fucacity CO2(g), you are making the concentration of CO2(aq) very high. This (along with high Ca++, Mg++) makes the water highly supersaturated with respect to a number of minerals. The second block of results is the true equilibrium state, which would occur if those minerals were allowed to form. The reaction Ca++ + HCO3- -> Calcite + H+ explains your results. Carbonate minerals form, and the pH decreases. As the pH decrease, the CO2(aq)/ HCO3- buffer tips to CO2(aq). The output fugacity is that "which would be found in a gas phase that is in equilibrium with the system, if such a gas phase were to exist" (Geochemical and Biogeochemical Reaction Modeling, 3.3.7). So, high CO2(aq), high CO2(g).

This may not be the best configuration to model dissolution of CO2(g) into a brine. Here you are starting with high CO2(g), but a neutral pH. What you want is to mix a low CO2 brine with high CO2 fluid. You might try mixing two fluids, or instead try a sliding fugacity path, in which the CO2(g) increases from a small original value (the brine) to a high value due to reaction with injected CO2(g).

Hope this helps,

Brian Farrell

Aqueous Solutions LLC

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