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Modeling with several minerals with greatly differing kinetic rates


Mattm
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Hello,

I'm putting a reactive transport model together in X1t. The aquifer I'm modeling contains carbonates, clays, and silicates. I'm utilizing custom kinetic rate laws which are sensitive to changes in pH. I have been trying to run simulations using kinetic constraints for all the minerals, but my model has been getting stuck and crashing and/or running incredibly slow.

I have read that it is best to avoid simultaneously using kinetic constraints for minerals with greatly differing kinetic rates, and it is a better approach to instead set the more rapidly reacting minerals as equilibrium species. I'm wondering how I might do that? Does that mean I would just add the rapidly reacting minerals as "simple" minerals? Since the "simple" minerals are "added to the model over the course of the simulation", I'm not sure how I would approach setting their wt% values with respect to the single wt% value I have from XRD data. Likely, I am misunderstanding the meaning of the "simple" minerals. 

I'm also wondering, at what point do I draw the line? For example, I am including carbonates with reaction rates around 10^-5, chlorite with a reaction rate around 10^-12, anorthite with a reaction rate around 10^-14, and clays with reaction rates as low as ~10^-20 in basic conditions. I think I should be using equilibrium to model carbonate and perhaps chlorite dissolution, but between anorthite and the clays, I still have several orders of magnitude difference in their kinetic rates. I guess my question is at what difference in kinetic rate magnitude would you avoid utilizing kinetics. 

Thanks

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  • 2 weeks later...

Hello Matt,

I apologize for the delayed response. Simple minerals are not typically used in reactive transport model in the way described above. If you want to set a mineral in equilibrium with the fluid, you should use the swap feature. For examples on swapping, see section 7.2 Equilibrium models in the GWB Essentials User guide.

With regards to order of magnitude differences, there is no rule or hard cutoff. The choice depends on the problem you're modeling and. The use of kinetic rate laws is typical for reactions that progress slowly through time but not so slow that it’s not relevant on the time scale of interest. If you are starting with an all kinetic model, you can compare various reaction rates and saturation indices of kinetic minerals. If the fastest reactions remain in equilibrium with the fluid or close to it, then you can use that as a starting point to swap the mineral into the basis. If any of the rate laws are for minerals that will become saturated at some later point in the simulation and will precipitate relatively quickly, then they shouldn't be swapped into the initial system. You can just let them precipitate at equilibrium should they become saturated. You may also want to turn on “explain steps” and “follow output” in X1t (under Config > Stepping, check "explain steps" and on the Results pane "Follow Output”) to report the time limiting factors of the run.

Hope this helps,
Jia Wang

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