PhreeqcUsers Discussion Forum
Processes => Reactive transport modelling => Topic started by: zsszabo86 on May 14, 2019, 03:09:34 PM

Hello
I have run into two problem which I did not see before. I run reactive transport simulations and want to model a scenario when CO2 containing water flows into a geological formation. I have problems with both ways I figured out:
1. I start the code like this:
SOLUTION 0
temperature 135
pressure 272.8
units mol/kgw
pH 5.029
pe 5.059
Al 1.92E06
C(4) 4.99E01
Ca 2.03E03
Cl 4.93E01
Fe 4.26E06
K 4.05E02
Mg 2.16E03
Na 4.63E01
S 5.13E02
Si 5.45E03
GAS_PHASE 1
fixed_pressure
pressure 272.8
volume 5000
temperature 135
CO2(g) 272.8
SAVE solution 0
END
USE solution 0
etc.
then it seems that the gas phase can be completely used up and then there is no more CO2 coming into my cells with solution 0 (even though I believed that the saved solution 0 is CO2saturated). other words: it seems that not the new saved solution 0 is used and it also seems that the gas phase is used not only for calculating the solution 0 composition but it is in all cells
it would be great to get some explanation of what is happening
2. The other way was to simulate the CO2containing solution in a different model then to copy paste the result as the solution 0. it seems ok to do that but then no reaction happens. and looking at the output I see that the solution 0 is not flowing into cell1. see below 0.000e+00 Solution 0 and also see in attachment.
Mixture 1.
0.000e+00 Solution 0 Solution after simulation 2.
6.677e01 Solution 1 Solution after simulation 2.
3.323e01 Solution 2 Solution after simulation 2.
Again, I cannot figure out why the solution 0 is not flowing in and advice would be helpful.
Thank you in advance!

Assuming you are using TRANSPORT, solution 0 has the additional CO2 needed to produce a partial pressure of 272.8, and it will be advected into the column with each shift, provided you have specified a flux boundary condition at cell 1.
If you specify diffusion only, solution 0 will not be advected into the column.
Not sure about a constant boundary condition, but I think cell 1 would be maintained at the solution 1 composition, and solution 0 would not affect the calculation.
Alternatively, you can specify GAS_PHASE 1, which would continue to react with cell 1 throughout the calculation, although the CO2(g) pressure will continuously decrease. You may want to specify CO2(g) in EQUILIBRIUM_PHASES 1, which would maintain the CO2(g) partial pressure throughout the simulation (provided you give a large amount of moles in the definition).

Many thanks for the useful reply! It finally opened my eye, what the problem is (it is diffusion only!!!) and how to solve it.