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Hi,

I am trying to calculate the maximum solubility of CO2 in a saline brine at surface and subsurface conditions.

I have experimented with a number of different ways of adding CO2 to the system using React, including swapping CO2 for HCO3- or pH in the basis, and adding free CO2 as a reactant.   However the results appear to indicate that the programme responds by simply reacting/dissolving as much of the gas as is initially stated, even beyond the likely point of saturation.

Please can you advise me on how best to go about performing this calculation using GWB?

Many thanks, 

Steph

 

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Hello Steph,

The React program begins by calculating the system's initial equilibrium state based on the constraints set in the basis pane. Then the program changes the system by adding or removing reactants to the system via a reaction path. I am not sure how your model is set up, but you can constrain the CO2 fugacity in your basis pane and the program will calculate the amount of dissolved CO2 based on the equilibrium constant and the CO2 gas fugacity or partial pressure you have set. In the Reactants pane, you can set up a reaction pathway to alter different aspects of your system, such as changing pH, temperature, or reactants, to see how the dissolved CO2 concentrations changes. If you want to model the system at changing CO2(g) fugacity, you can set up a sliding fugacity reaction path. To see some examples of setting a CO2 fugacity, you can see the Gas solubility example in React on the GWB diagrams webapge. If you are interested in modeling gas solubility by varying two variables, you can check out the Solubility contours example in Phase2. 
 
For more information in general regarding React, please see chapter 2 in the GWB Reaction Modeling User Guide. For an example of sliding fugacity, please see section 3.6 Sliding activity and fugacity in the same user guide. 
 
If you would like someone to take a closer look at a specific issue in your model, please attach your input file along with the thermo dataset. 
 
Hope this helps,
Jia Wang
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Hi Jia, 

Many thanks for your response.

I had actually tried the sliding fugacity method but, while it shows how the dissolved CO2 concentration changes, there seems to be no constraint on how much can be dissolved.  I have tried adding as much as 10 molar CO2, all of which dissolves - something which isn't possible!

I am attaching files to demonstrate what I have been trying, along with a spreadsheet illustrating the difference between the results I generated with GWB and those I obtained using Phreeqc.  I hope these help clarify the problem I am having, but please do let me know if not and I'll try to explain myself better.

Many thanks, 

Steph

GWB comparison.xlsx 4722_dilute_test_CO2_fixed.rea 4722_dilute_test_CO2_sliding.rea thermo_hmw.tdat

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