Modelling help for simulating a injection of FeCl3 into deep saline formation water.

[Sam OR]

Hi there,

I am currently part of a project to determine the stability of various electrolytes whenever injected into different formation waters.

I would like to identify how the stability of FeCl3 varies across a range of pHs (2-9) at a set Eh values. This has worked for the model I've posted below for Eh values exceeding 0.3V but anything more reduced than that and the program doesn't run with the error:

Newton-Raphson didn't converge after 999 iterations, max residual =           0, Xi = 0.0000
Largest residual(s):
                       Resid     Resid/Totmol   Cbasis
---------------------------------------------------------
---------------------------------------------------------
 

I'd like to be able to set the Eh value at more reducing conditions if possible and would appreciate any help regarding this.

Also, since I'm new to using GWB, if there are any errors I've made or improvements that you could suggest to more accurate represent the system I would greatly appreciate this also.

Thanks for the help!

Sam.

FeCl3 simulated injection.rea

[Jia Wang]

Hello Sam,

 

When you are trying to run a simulation over such a wide range of Eh values at different pHs, you are going to be outside of the water stability limits. When you are going too far beyond the water stability limits, the program is going to have a difficult time with convergence. Since you are starting from the reduced endpoint, it can help the program by swapping in reduced species into the Basis, HS- for SO4--, CH4(aq) for HCO3-, etc. I managed to start at a more reduced point (~-.3 V) with better suited basis species swapped in. Alternatively, you can try starting at the oxidized end point by setting your Eh to 1 in the Basis and keep your species as they are, slide to a more reduced end point. You can also try turning off precipitation (Config -> Options) for simplifying this run.

 

Nonetheless, I don't think you will be able to go from and Eh value of -1V to 1v for your full pH range, as again, the end points are too far beyond the stability of water limits. You can reference an Eh-pH diagram to see what range water is stable in. FYI, you can also create one in Act2 with a few clicks. For more information on activity diagrams, please see section 5 in the GWB Essentials Guide.

 

Hope this helps,

Jia Wang
Aqueous Solutions LLC

[Sam OR]

Hi there,

Thanks for getting back to me and helping with this. I attempted to rectify the errors here and posted about myself simplifying the model in this post: 

I was wondering if you would be able to take a look and help with the issues I have encountered.

Thanks again.

Sam.

On 3/8/2024 at 9:37 PM, Jia Wang said:

Hello Sam,

 

When you are trying to run a simulation over such a wide range of Eh values at different pHs, you are going to be outside of the water stability limits. When you are going too far beyond the water stability limits, the program is going to have a difficult time with convergence. Since you are starting from the reduced endpoint, it can help the program by swapping in reduced species into the Basis, HS- for SO4--, CH4(aq) for HCO3-, etc. I managed to start at a more reduced point (~-.3 V) with better suited basis species swapped in. Alternatively, you can try starting at the oxidized end point by setting your Eh to 1 in the Basis and keep your species as they are, slide to a more reduced end point. You can also try turning off precipitation (Config -> Options) for simplifying this run.

 

Nonetheless, I don't think you will be able to go from and Eh value of -1V to 1v for your full pH range, as again, the end points are too far beyond the stability of water limits. You can reference an Eh-pH diagram to see what range water is stable in. FYI, you can also create one in Act2 with a few clicks. For more information on activity diagrams, please see section 5 in the GWB Essentials Guide.

 

Hope this helps,

Jia Wang
Aqueous Solutions LLC

 

[Jia Wang]

Hello,

I apologize for the delay. I was revisiting the previous post and the new file posted. I can see that the files have slightly different chemistry and a sliding Eh path.

To start troubleshooting, I would suggest that you simplify your run by removing your sliding Eh path and perform a speciation calculation with just your basis composition. You can check the resulting H2(aq) concentration and see that it's very large, on the order of hundreds of thousands of molal.

You may also need to consider whether redox disequilibrium is necessary for any reactions. The software assumes by default that redox species are in equilibrium, as such the program will distribute the bulk concentration set across different redox species. You can disable redox equilibrium for the appropriate species pair and specify the bulk concentration for each oxidation as needed. Please see section 2.4 Redox couples and 7.3 Redox disequilibrium of the GWB Essentials Guide for more information.

Could you clarify what you are trying to do by swapping in FeCl3 into the basis? Reading your description, it sounds like you might want to titrate in FeCl3 or an FeCl3 solution to create a fluid which then needs to be evaluated at a range of Eh or pH. Is this what you are trying to do? If so, you would want to enter the constraints for your original fluid in the basis and then set up a titrate path in the Reactants pane for FeCl3 or FeCl3 solution. You can examine the concentration at the end of this titration simulation. Then, decide whether to slide Eh or pH from the end point, or perhaps to change assumed value for initial Eh or pH and rerun to compare.

The ionic strength of your solution is high. You might eventually want to consider using a virial activity model. Thermo.tdat is the default dataset and uses the bdot activity model. The bdot model typically loses accuracy as the fluid ionic increases pass ~ 0.3 molal. I would suggest you take a look at the virial models available (SIT or hmw) and consider them. You can view a summary of thermo datasets on the thermo page or find out more in section 2.3 Thermodynamic dataset in the GWB Essentials Guide.

Please consider posting to the same thread when you are following up on issues. It would be easier to keep track of progress when you post to the same thread.

Hope this helps,
Jia