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Mixing of GW and thermal water


EEasley
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I am working on a problem that involves the mixing of groundwater with a thermal water of mixed composition (it is not a thermal end-member, it has been mixed with GW). I am using REACT and first entering the groundwater end-member in the Basis, running to get the speciation, and picking up the fluid as the reactant. Then I entered the mixed thermal water as the basis and reacted the two. My main question is in handling the temperatures. The groundwater is ~ 11C and the mixed thermal water is ~66C. When I first ran the groundwater and picked it up, does the program keep the 11C temperature that is specified initially, or do I have to re-enter the temperature on the Basis page? I tried setting the third option under the Temp tab, and putting the initial T as 66C and the reactants T as 11C. The purpose of this reaction path is to extrapolate the chemical composition of the pure thermal end-member, but I am not sure if I am going about this the right way. I was hoping to look at the behavior of Cl, HCO3, and SO4 as the fluids are mixed and figure out the saturation limits of these components. I've attached the file below. I'm getting some weird results when I plot the components (Cl, SO4, and HCO3) vs. temperature. Thanks so much. Also, below are the original basis for the GW and thermal water. I set the charge balance on Bicarb b/c I used a field titration to get the data and it seems to be adding a significant amount of error to the charge balance. I also have the flash option on.

 

Basis Groundwater (in mg/kg)

Ca++ 16

Mg++ 2.85

Na+ 2.707

K+ 0.4699

HCO3- 47

SO4-- 16

Cl- 1.176

F- 0.109

pH 8.22

O2(aq) 8.45

Temp 11 C

 

Basis- Mixed Thermal Water

Ca++ 8.277

Mg++ 0.409

Na+ 148.6

K+ 8.776

HCO3- 117

SO4-- 170.7

Cl- 13.8

F- 16.11

pH 8.22

O2(aq) 1.36

Temp 66 C

EEasley Mixing.rea

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  • 1 month later...

Hi Elisabeth:

 

Your temperature settings are correct, but the 'flash' option is preventing proper polythermal mixing from occurring. Can you explain a little more about what the purpose of this model is?

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.

 

 

Tom-

 

The purpose of the model is to predict the pure thermal end-member water composisiton in order to apply geothermometers (I want to know the composition of the thermal water before it is mixed with groundwater). Dilution and boiling can introduce significant errors to the equilibration temperatures, and so it is important to apply geothermometers to the most thermal end-member to ensure that there are no significant errors. All the water samples that I have collected appear to be mixed with groundwater. I guess I am a little confused about the purpose of using the flash option. For the file I posted earlier, I tried setting it up as a forward model to see how the system would behave if I mixed the sample until it reached essentially the pure groundwater end-member. Then I created an inverse model that "unmixed" the groundwater (I just used negative values for the groundwater components in the reactants)to try and reach the thermal end-member composition. I can get the inverse model to work (flash option off)), but I can only react it one time. Increasing the # of times reacted to > 1 results in nonconvergence. I would like to see what happens if I reacted the system, say 50 or 100 times so that I can see a longer reaction pathway, but I think the problem is that the concentration of the basis species are too low and result in negative concentrations when you subtract out the groundwater components. I've attached the inverse "unmixing" model, with a few mineral phases suppressed. Thanks.

Waunita inverse model minerals suppressed.rea

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Hi Elisabeth:

 

The purpose of a flash diagram is to predict scale and mineral saturation state changes that arise when two fluids are mixed in varying proportions. What GWB actually does is remove fluid from the system in the same proportion that new fluid (in your case, end member groundwater) is titrated in. This allows you to see a range of mixing ratios between your end member groundwater fluid and your mixed fluid. So this approach may not work for your inverse model, as you would essentially be removing water of different compositions twice over.

 

Keep in mind that the water you are 'unmixing' is still likely affected by scaling and volatile removal during fluid ascent. Reconstructing reservoir conditions is a tricky problem that all modelers face. A few things you might add to your approach:

 

  1. Assume that your end member fluid was in equilibrium with local host rock at depth (the Tomichi intrusion, I believe?). If you know the composition of this host rock, you can partially constrain the composition of your thermal end member by titrating these minerals into water at reservoir P/T.
  2. If you know that a gas cap exists within the reservoir, you can titrate these gases to saturation into your surface water. With CO2, this will help restore pH/alkalinity. If you have no knowledge of the existence of a gas cap, another approach is to titrate in flashed (exsolved; note that 'flash' here is a different term than what is used in the model) gases. But this requires a measured gas/water ratio, and gas compositions.
  3. If you have compositions for surface scale, you can titrate these minerals back into your surface water. There are some difficulties with this, but eventually, your surface water should approach the various saturation states you estimate for your end member thermal water.

 

As far as your model, it doesn't look like you are driving any species concentrations negative. You have some negative components (H+, O2), but that's a mathematical and not a physical phenomena.

 

I hope that helps,

 

Tom

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