Solid/liquid ratio in X1t model

[Zhidi Wu]

Hello,

We built a reactive transport model using X1t, where CO2 was introduced at 47 hours and continued until 82 hours. However, the model shows unexpected results: pH went down immediately once CO2 was introduced (expected) but quickly recovered back to 7 (unexpected) (see screenshot below). Also, we didn't see increases in Ca and Mg, which we expected to occur due to the dissolution of calcite and other carbonates. We think this might have something to do with the solid/liquid ratio. I looked through the X1t manual, and it looks like we can confirm the solid/liquid ratio by using the volume of minerals in Reactants and the fluid volume from flow and permeability parameters. Do you agree with me, or is there a better way to calculate the solid/liquid ratio?

I also attached our model and thermo database.

Thanks,

Zhidi

Test_1_05142024.x1t AGI_20e_4-11-2024_debug.tdat

[Jia Wang]

Hello Zhidi,

Thank you for attaching your input file and database. When examining the minerals present in your system, particularly in node 5, the breakthrough curve indicates that calcite precipitates in large amounts as your second fluid enters the system. The assumption that calcite is continuously dissolving and thus releasing Ca does not align with the results shown. Additionally, you should consider the effects that other Ca-containing minerals, such as anhydrite, will have on your system. The concentration in the fluid reflects the net effect of the reactions taking place in the system.

If the results do not reflect your assumptions, I suggest checking whether your fluid chemistries and various constraints are set up correctly. Your Fluid 2 chemistry shows a higher bulk concentration for the carbon component, but the pH remains the same. Is this accurate? All components in this fluid are the same as your initial fluid, except for the bulk concentration of the carbonate component. Also, note that swapping in a species like CO2(aq) for HCO3- does not set the free concentration for CO2(aq). To set a free concentration, you need to select the dropdown menu on the unit and choose "free." Please refer to section 7.2, Equilibrium Models, in the GWB Essentials User Guide for a more detailed explanation of "free" vs. "bulk" concentration.

It would be simpler to troubleshoot your fluid chemistry in React, and I suggest you begin there. You can also use one of the reaction pathways in React to create a fluid by titration for use in your simulation.

You can plot and export mineral volume and fluid volume of your system at each time step of the simulation. You can find both variables under "Physical parameters", plot them, and then go to Edit -> Copy as -> Spreadsheet and paste the data into Excel.

Hope this helps,
Jia Wang
Aqueous Solutions LLC