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CO2 solubility (2)


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Hi Jia, 

Many thanks for your earlier response regarding problems with modelling CO2 solubility in brine.

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.  I'd really appreciate your help with this.

Many thanks, 

Steph

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

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Hello Steph,
 
Taking a look at your files, there are a few things that I would suggest you check in your simulations to make sure that you are comparing the same simulations.
 
The default fugacity coefficient model in thermo_hmw.tdat is the Tsonopoulos model. I believe PhreeqC may use the Peng-Robinson model instead. Please check if you are using the same fugacity coefficient model. If not, you can change to use the Peng-Robinson model in React. For more information, please refer to the press_model command in the GWB Command user guide.
 
It seems like you used CO2 fugacity for your GWB model but used partial pressure for PhreeqC. The GWB programs can calculate the fugacity coefficient and use the partial pressure to calculate the fugacity for your fluid. To do that, you can swap CO2(g) into the basis pane and then set the partial pressure instead of the fugacity. You should use either fugacity or partial pressure in both models to make the best comparison.
 
It might be best to start with this to either enter a fugacity or partial pressure into the basis pane and calculate speciation to make sure the results are expected before using sliding fugacity.
 
Please note that gases are unlike minerals, there is not a saturation limit in the GWB apps for gases. I am not sure how PhreeqC differs in this regard but it might be good to check if the buffer you are applying in your PhreeqC model is fixed throughout the whole simulation.
 
Hope this helps,
Jia Wang
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