Hydrogen dissolution in brine

[ylg]

Hi all,
I am a reservoir engineer aiming at modeling interactions between hydrogen and rocks. As such, I ma not very proficient with Phreeqc.
We set up experiments to measure H2 dissolution in carbonated water first at atmospheric conditions. Experiment was done using 125 mL glass bottles, closed and sealed which contains 60 mL of formation water plus 30 mM HCO3- supplied by NaHCO3 was added as carbon source. The rest of the volume is H2(g)
The experiment results shows CO2(g) and no CH4 (g) generation and pH ~ 8.

When I set up my phreeqc model (using phreeqc.dat database), the results shows CH4 and no CO2. The only way I manage to change this is by setting a very small CO2 partial pressure in EQUILIBRIUM_PHASES (as shown in the attached file). This is somewhat a numerical tricks.
Is there a more rigourous way to reproduce the experimental results?

In other runs, I observed very large initial and instantaneous hydrogen decrease, probably due to equilibrium with the solution, which is not observed in the experiment. This seems to indicate that there is something missing in my definition of the initial conditions or its equilibrium
Thanks for your advices to solve this issue