Geochemist's Workbench Support Forum

# Calculating mineral SI without knowing pH

## Recommended Posts

Hello,

I made an reactive experiment with CO2-enrich brine and minerals under pressure. I have fluid sample information like most cation concentrations and the amount of CO2 exsolved at atm pressure (converted to CO2 fugacity). However I have no pH measurements at high pressure.

I would like to use GWB to calculate mineral saturation index, but I'm a bit confused about setting up pH and HCO3-. The presents of CO2 and carbonate (dolomite) should both effect pH and HCO3-, so which should I put to swap with which..?

I really appreciate if someone can show me the right way to set up the condition and calculate mineral (dolomite) SI.

Many thanks,

Jin

##### Share on other sites

Hi Jin,

You have two original basis entries (H+ and HCO3-) and two species you can swap in and constrain (set the fugacity of CO2(g) and the mass of Dolomite). The reactions for CO2(g) and Dolomite-ord include both H+ and HCO3-, so it doesn't matter here what gets swapped for what.

Here's an example from section 19.4, Dolomitization of a limestone, in Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text.

T = 60

swap CO2(g) for H+

swap Dolomite-ord for HCO3-

f CO2(g) = 1

1 free mol Dolomite-ord

Na+ = .1 molal

Cl- = .1 molal

Ca++ = .01 molal

Mg++ = .01 molal

You could alternatively swap the CO2(g) for the HCO3- and the Dolomite-ord for the H+ and arrive at the same results.

Hope this helps,

Brian Farrell

Aqueous Solutions LLC

##### Share on other sites

Hi Brian,

When I use Dolomite as a swap, it means it is forced to be in equilibrium? I think in my solution Dolomite is still dissolving, so I want to calculate SI for it as well. Can I only swap CO2 with H+, and use HCO3- to equilibrate charge balance?

Best,

Jin

Hi Jin,

You have two original basis entries (H+ and HCO3-) and two species you can swap in and constrain (set the fugacity of CO2(g) and the mass of Dolomite). The reactions for CO2(g) and Dolomite-ord include both H+ and HCO3-, so it doesn't matter here what gets swapped for what.

Here's an example from section 19.4, Dolomitization of a limestone, in Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text.

T = 60

swap CO2(g) for H+

swap Dolomite-ord for HCO3-

f CO2(g) = 1

1 free mol Dolomite-ord

Na+ = .1 molal

Cl- = .1 molal

Ca++ = .01 molal

Mg++ = .01 molal

You could alternatively swap the CO2(g) for the HCO3- and the Dolomite-ord for the H+ and arrive at the same results.

Hope this helps,

Brian Farrell

Aqueous Solutions LLC

##### Share on other sites

Hi Jin,

Yes, when you swap a mineral into the basis you are setting the fluid to be in equilibrium with it. If you're working with an incomplete water analysis (or a low quality analysis) then you can get misleading results when using charge balancing. I would compare the simulation assuming equilibrium with Dolomite with a second simulation in which you set HCO3- as the charge balancing ion.

Hope this helps,

Brian

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.