maybright Posted November 16, 2017 Posted November 16, 2017 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
Brian Farrell Posted November 22, 2017 Posted November 22, 2017 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
maybright Posted November 27, 2017 Author Posted November 27, 2017 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
Brian Farrell Posted November 28, 2017 Posted November 28, 2017 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
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now