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Calculating mineral SI without knowing pH


maybright
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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

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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

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Hi Brian,

 

Thanks for your reply.

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

 

 

 

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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

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