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Starting modelling with GWB (simulation of CO2-charged flux into a basaltic aquifers)


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Hello,

I am Ali and we have GWB pro in our centre, but I am actually not so familiar with the steps to run a scenario.

I want to run a model to monitor water-rock interactions in a shallow basaltic aquifer exposed to a CO2-charged fluid (leakage scenario). Could you please tell me regarding the procedure step by step? (for instance, putting water samples in React, then find out which minerals in host aquifer in equilibrium and then put them all as a basis for X1t or x2d simulations,...)

any help high appreciated. 

 

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Hello Ali,

It sounds like you are on the right track on getting this started. Here are some suggestions that may help. If you aim to build a reactive transport model, then starting in React to build a reaction model, checking if the reactions are behaving in the way you expect before moving to one of the reactive transport modeling apps is a good idea.

In React, you would set up the basis pane with the composition of the fluid you wish to perform reactions with. This is also where you would swap in any constraints, such as minerals or gas buffers, that you can assume your fluid is in equilibrium with. You can use the go initial command to check that the program can successfully calculate mass distribution. If successful, you can specify various types of reactions (e.g. kinetic reactions, sliding activity and fugacity paths, etc) in the Reactants pane.

If your work requires a reactive transport model, you would need to select whether X1t or X2t is best suited for your needs. X1t and X2t are for 1D and 2D modeling respectively. In these apps, you would set up your domain discretization and flow parameters. You can always check that the flow parameters are correct by setting an inert fluid (e.g. just Na+ and Cl-) and running the simulation. Once you have verified that these are correct, you can add the geochemical reactions that you have set up and tested in React. You can simply click and drag the Basis pane from React into the corresponding pane in the RTM app.

There are a lot of resources available to help you get started with The GWB. The software is accompanied by six user guides which will explain various features. React is described extensively in the Geochemical Reaction Modeling User Guide. If this is the your first time using the GWB software suite, I would also recommend reviewing the SpecE8 section in the GWB Essentials User Guide, which explains how chemical mass distribution is figured. Usage of X1t and X2t are described in detail in the Reactive Transport Modeling User Guide. In addition to the user guides, there is also the GWB Academy, which offers a series of lessons on a wide variety of geochemical modeling topics.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

 
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Dear Jia,

Thanks for your response and clarification.

In React or SpecE8, when we want to swap for example a mineral, we need to know it is in equilibrium or not, right? (then, how I can assume this? I thought to find which minerals are in equilibrium with the system, add the water analysis result in basis pane in SpecE8 or React and then run and check the result to find out which minerals are in equilibrium (SI values 0), then use them for the final run and swap them). 

Yes, the final goal is running a reactive transport modelling to show the changes occur in a shallow basaltic aquifer exposed to a CO2-charged solution leakage. The available data are water analysis, chemical composition and mineralogy of host rock, and we then hypothesize a leakage solution. 

As I understand, first of all, I need to use water analysis to find out which mineral are in equilibrium with the system of interest, then for next run, swap those minerals in the basis inputs in X1t or X2t (I will decide which one is better reach my favourite results). The question I have is when and where I need to add chemical or mineralogy of host rock (basalts)? (I guess, the CO2-charged solution should add as a reactant to monitor chemical reactions, right?)

 

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Hello,

It sounds like you have a wealth of information regarding your system. What you described may work, but unless you suppress more stable minerals in from in React, the software will consider all possible minerals given your composition. Minerals that are most stable would be swapped into React in its calculation of the equilibrium state, which won't necessarily be the minerals that you expect according to your mineral assemblage analysis. Please note with React, you should see two Xi = 0 text blocks any time an initial system contains supersaturated minerals and precipitation is not disabled. The first block shows you the metastable equilibrium state of your aqueous speciation after you have initialized your run, where no supersaturated minerals have been precipitated. SpecE8 on the other hand only calculates the metastable equilibrium state of your fluid. More details regarding this can be found in the GWB Essentials Guide and the Reaction Modeling Guide.

I would suggest using the mineral assemblage you have taken from your aquifer as a starting point. If you suspect that a mineral is in equilibrium with the composition of your fluid, you can swap in that mineral and then run a speciation calculation to see if the concentration calculated in the fluid matches that component in your chemical analysis. Any other information regarding your system's conditions (e.g. gas buffers) that can help you figure the controls on the fluid composition would be helpful as well.

In terms of inputting minerals from the basaltic aquifer, this would depend on the types of reaction. Minerals that react very quickly within your system can be swapped in your Basis pane and treated as equilibrium reactions. Minerals that react slowly but measurable over the course of the time scale of your system can be constrained kinetically. You can add a kinetic mineral reaction (as well as a variety of other types of kinetic reaction) in your Reactants pane. You can also set a mineral that is initially in equilibrium with your system but allowed to react according to kinetics through the simulation. For more information on kinetic reactions, see section 4 in the GWB Reaction Modeling Guide. If certain reactions are relatively slow and does not impact your fluid composition over the time scale of interest, then you might consider excluding them from your model.

In a reactive transport model, you would set the composition of the fluid entering the domain in the Fluids pane in X1t and X2t. For more information on boundary fluids, please see section 2.8 of the Reactive Transport Modeling user guide.

Hope this helps,
Jia

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