Reaction path modeling...mixing two fluids with "flash scenario"

[Kirkoff Xn]

 

Hi

When modeling injection of fluid A (oxidizing) into an aquifer having fluid B using flash model, I am a bit confused seeing the results yielded.

The native groundwater has high TDS while the injectate is a freshwater.

When plotting mineral saturation versus mixing ratio, there is a pattern of mineral saturation decrease with calcite dissolution (more likely)[I assume it's quite sound geochemically].

However, when plotting components in fluids that would define mineral stability being seen, I find some inconsistency. I have high Cl- concentration

at 0% mixing fraction, implying that Cl- concentration is higher in the injectate water than in the native groundwater—which is not the case!

Part of what I am trying to demonstrate is how groundwater get refreshed, and the hydrochemical facies evolution being seen in piper diagram can be

backed up by the mixing using  "flash model".

Also, evaporites (gypsum and anhydrite) are more likely to increase with mixing ratio since the mixing ratio increases towards the native groundwater. In GSS file, It is possible to know the saturation index of minerals and these two minerals have high SI in the native groundwater than in the surface freshwater.

Also, if I mix 50% fluid A with 50% fluid B in GSS (analysis > mix sample >mix by volume), I am wondering if the results yielded is the same as at 0.5 mixing fraction in react.

 

Attached, you'll find both the GSS & react files. 

Thank you

 

[Jia Wang]

Hello,

With the flash diagram setting, the fluid defined in the Reactants pane is mixed with the fluid defined in the Basis pane at all fractions. This means that at a mixing fraction of 0.4, this is a mixture of 40% of the fluid in your Reactants pane (freshwater) and 60% of the fluid in the Basis pane (groundwater). At a mixing fraction of 0, it is simply the fluid defined in the Basis pane. To see a detailed example of the flash configuration, please see the Fluid Mixing and Scaling lesson on the GWB Academy and section 3.8 Flash diagrams in the GWB Reaction Modeling User Guide.

If you would like to compare mixing between GSS and React, I would suggest that you make sure the correct proportions are used in GSS. I noticed that a reactant times factor of 100 was set in your React run. The mixing fraction calculation would account for the reactant times factor in React but you would need to set the volume (or mass) of each fluid directly in GSS. If the mixture is in the same proportion between GSS and React, they should show the same results. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[Kirkoff Xn]

Hi Jia

I don't know if I misunderstood what's offered in GWB online academy https://academy.gwb.com/mixing_and_scaling.php

Here's what's said about proportions "The horizontal axis shows the extent of mixing from purely the first fluid (seawater, on the left) to purely the second (brine, on the right); a mixing value of 0.4, for example, represents a mixture of 60% of the first fluid and 40% of the second."

The first fluid is the reactant and the 2nd is in the basis pane...It looks like what you wrote is the reverse of what's written from the GWB online academy. Can you further explain this?

 

And the Cl- concentration in fluid component looks not "geochemically sound"...I don't know if the flash scenario is the appropriate one...but that's what I think!

Thank you

 

 

[Jia Wang]

Hello,

Thank you for pointing out the confusion with the wording here. In this case, the first fluid is referring to the seawater composition in the Basis pane, which contains a high amount of sulfate in solution . The second fluid is referring to the barium rich solution in the Reactants pane. It is clearer if we examine the concentration vs mixing fraction plot, which shows that high sulfate concentration is present at a mixing fraction of zero. At a mixing fraction of 1, we see a high concentration of barium and practically no sulfate. We will update the Academy in the near future to clarify the explanation.

Just an additional note, it is not typical for users to set a reactant times factor when creating a flash diagram. In this case, your  injectate is scaled by a factor of 100 whereas your groundwater fluid is not. Perhaps this is causing some unexpected behavior in your model? 

Best regards,
Jia