Fluid-Rock Interaction in X1t

[agion18]

Hello,

I am trying to model reactive transport flow in X1t, wherein there is a fluid flowing through a given mass of rock with known composition.  I would like to simulate the transport of mass out of the system by the fluid.  I have explored the reactions in React and everything appears to be ok. However,  in X1t it is not clear to me what the mass of the initial rock in the system is. I have set up my rock composition (really a glass with no mineral phases) in the "Reactants" page as simple oxides and maintained the default "reactants times" value of one, as well as the the initial fluid composition in the "Initial" page. I have noticed that in this format additional reactants are added with time, similar to React. Instead I would like to have a set starting composition and mass and simulate a flow-through/leaching calculation for a given domain. Any help on setting up the model correctly would be greatly appreciated.

 

-Austin

[Jia Wang]

Hello Austin,

It sounds like you've built your chemical model in a single-node system and want use it to build a reactive transport model. First, I just want to note that oxide type entries do not carry the same properties as a mineral reaction in the GWB. In the GWB, a mineral reaction is accounted for in the thermodynamic database with valid equilibrium constants for a temperature range. Oxides do not carry this information. When you add a simple oxide as a reactant in React, you are simply changing the chemical composition available in your system. Therefore, if you wish to set a solid composition in your starting system, whether it be in React or X1t, you will need to specify the composition as minerals rather than oxides. You can view the thermodynamic database loaded for your current run by going to File -> View -> select the file that ends with the extension .tdat.

There are a couple of ways to set minerals in the initial system depending on the type of reactions desired.

If a mineral is assumed to be in equilibrium or can be treated as in equilibrium with the initial fluid, you can use the swapping feature to swap it in for a component in the Initial pane. Doing so will tell the software to solve for the component concentration in equilibrium with the mineral swapped in. This mass will be set as part of your initial domain. Please see section 7.2 Equilibrium models in the GWB Essentials Guide for more information and examples on swapping.

In the case where a mineral reaction should be accounted for kinetically, you can add the mineral in your Reactants pane by going to add -> Kinetic -> Mineral... Any mass specified here will also be counted as present in your system initially. Note that the software allows you to specify a mineral that's in equilibrium with your system initially (e.g. swapped into the Initial basis) but as a kinetic reaction when you start your simulation. If you would like to do that, you can swap the mineral into the basis and set 0 for the mass in the Reactants pane for the kinetic mineral. For examples of how kinetic reactions works in the GWB, please review section 4 Kinetic Reaction Paths in the GWB Reaction Modeling Guide.

The Reactive Transport Modeling guide is a great resource for getting started with reactive transport modeling with the GWB. You can also search the Forum to see if other users had posted similar queries previously. It might not be the exact issue that you have but oftentimes, the solutions can share a lot of similarities. If you have specific technical issues with your model, please attach your input file and any relevant thermodynamic file so we can take a closer look.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[agion18]

Jia,

 

Thank you for the response.  I've been exploring the guides you've mentioned and am already familiar with the thermodynamic databases and swapping. The problem that I am having is that my initial system contains no crystalline phases, but a glass with a known bulk composition, hence why I was trying to work with the oxides. I would like to be able to enter that glass composition into the initial system and have the flowing fluid react with that glass.  I know I can add a glass to the database, but I would need to know the equilibrium constants for its dissolution and am not sure the best way to do this or this there is an alternative.

Edit: I have been looking at trying to calculate the equilibrium constants from the amorphous components (siiica, Al(OH)3 etc). Is there a GWB compatible database that would be best for these amorphous phases?

-Austin

[Jia Wang]

Hello Austin,

The program is installed with datasets for modeling a variety of reactions, you can browse through the  thermo webpage to see a summary of the datasets available. Datasets may have some of the reactions that you need but you may need to add ones that are not available.

Other users have also modeled glass dissolution in the past. Below is a link to a quick search on the Forum. You may want to check the threads to see if there are resources posted by other users.

https://forum.gwb.com/search/?q=glass&quick=1

Hope this helps,
Jia

[agion18]

Jia,

I have been searching through the literature and the forums and was able to add my glass of interest to a custom database. However, now when I try to perform any calculations I receive errors, for example as "       -- Error: Reaction for Alunite has a charge imbalance of   0.00124" or that "residuals are too large" and the calculation fails to converge at the initial point. I've tried to swap species and slightly change the composition of my initial systems, but have not been able to find the cause of the problem. Any further help would be greatly appreciated. I've attached my input files and thermo database as well.

 

Regards,

Austin

ProblemScript.rea.x1t logKcalc.tdat

[Jia Wang]

Hello Austin,

Thank you for attaching your scripts.

To troubleshoot, I would check if your initial system converged before any type of reactions or transport takes place. This is the most easily done by running a "go initial" command in the command pane or selecting the same option under the Run menu. This triggers the X1t to run a speciation calculation for the initial equilibrium state. When I did this, I saw that the program fails to converge. This suggests that the chemical constraints that you have set up for your initial fluid are not quite right or some critical component is missing.

Looking at the Basis and I see that most component concentrations are set at 1 mg/l except for O2(g) and Alunto glass is set in equilibrium with the initial fluid. Using your description above, it doesn’t sound like the glass is in equilibrium with the initial fluid, in that case, you should prescribe the mass in the Reactants tab, under the kinetic rate law set for the glass reaction. I unswapped Alunto glass from the Initial pane and set a small amount of SiO2(aq) in the fluid and then set 50 volume% for Alunto glass in the Reactants pane. Making these couple of changes allowed me to run the full simulation in X1t.  

Some additional information regarding setting up entries in the software. The GWB programs allow you to decouple redox pairs (e.g. Fe++ and Fe+++), which will allow you to add them as separate entries in your Basis. You can choose the redox couple to disable in the "Redox Couples…" dialog under the Config menu. I am not sure what treatment of redox species would be appropriate for your system but want to let you know that this is available. The oxidation state set by your O2(g) affects the mass distribution between all redox species unless otherwise specified. You can learn more about redox coupling in the GWB is section 2.4 Redox couples of the GWB Essentials Guide.

Hope this helps,
Jia