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Fluid Mixing and Gas Exsolution


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I am interested in determining the exsolution of CO2 due to the mixing of two different fluids. The natural system I study is artesian, so the interaction between fluids happens as they migrate upwards and pressure decreases. Obviously the CO2 exsolves primarily as a function of the pressure decrease but some interesting things happen presumably due to the interaction between aquifers and source waters.


Is there a way that I could determine this using React?

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There are a few ways to approach this problem. With a sliding fugacity model, you can account for the decrease in pressure as fluids migrate upwards. For example, Section 2.2.3 of the Geochemical and Biogeochemical Reaction Modeling text describes an example in which carbonate minerals form as CO2(g) exsolves from fluid flowing out of overpressured sediments.


Section 15.2 in GBRM is a very simple example of fluid mixing, in which an acid is added to a fluid in equilibrium with calcite, causing some calcite to dissolve and CO2 to be produced.


If you're interested in the kinetics of CO2 exsolution, you should look into section 4.5 of the GWB Reaction Modeling Guide.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC

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OK Thanks.


I am setting up my initial fluid system. I have a few more questions...


Can I set up the initial and final pressure conditions? For the first simulation I would like to simply take a single fluid and move it to different pressure and temperature conditions to determine potential precipitation of minerals and exsolution of gas.


As far as I understand it is that the system will "react" the fluid to reach the defined final fugacity....?


When setting the system to a sliding fugacity, must I know the final fugacity value? Is there a way to have the system determine these values based on the HCO3 concentration, pH and initial/final pressure conditions?


If I want to prevent precipitation of, for example CaCO3, should I "config>suppress>calcite"?

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You can set the fugacity of gases, or you can compile a thermo dataset at the pressures of interest, but you cannot set a confining pressure for your system.


By definition, for a sliding fugacity path you must set the target value. In a non-sliding path, changes in temperature or chemical composition will affect the fugacity over the course of a reaction path, however.


If you believe a mineral is unlikely to form, you would use the suppress function to remove it from consideration in the model. You could suppress quartz and some of the other more stable silica polymorphs at low temperature so that amorphous silica could precipitate, for example.





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