Water-to-Rock Ratio Calculation

[Benjamin]

Is there a convenient way to perform and plot water-to-rock ratio  calculations in React?  In other words, repeatedly reacting 1 kg of solution with fixed composition (i.e., seawater) with 1 kg of "rock" of fixed mineralogy to achieve ever-increasing water-to rock ratios and plotting the precipitated minerals (y axis) vs. water-to-rock mass ratio (x-axis).  This is a typical calculation to do with EQ3/6 and it is intuitively possible with GWB, but I can't come up with a way of doing it that isn't either "backwards", i.e., repeatedly reacting a rock (reactants tab) with a solution of fixed initial composition (Basis) or time-consuming (setting up many reaction files).  Any help would be greatly appreciated.

[Brian Farrell]

Dear Benjamin,

There are probably a variety of ways to conceptualize the problem, but combining React’s “flush” configuration with the “reactants times” feature is probably one of the more elegant ways to do so. A flush model is traced from the reference frame of the rock through which a fluid migrates. The migrating fluid displaces existing fluid from the system.

Please note there is no “water-to-rock ratio” variable in the plot output. However, the “H2O reacted” (solvent) or “Mass reacted” (solvent + solutes) variables available under Reactants properties (or Chemical parameters for GWB releases before 12) are probably the best ways to describe the progress of reaction. If you export the plot as an enhanced metafile to PowerPoint, it’s a simple matter to adjust the label.

For more information and an example of the flush configuration, please see section 3.3, Flush model, in the GWB Reaction Modeling Guide. More good examples can be found in section 3.4, Polythermal reaction paths, of the Reaction Modeling Guide, as well as section 19.4, Dolomitization of a limestone, in Craig Bethke’s Geochemical and Biogeochemical Reaction Modeling text.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC
 

[Benjamin]

Hi Brian,

Thanks for the response.  The only problem I see with this solution is that  it requires the basis to be in equilibrium with the initial mineralogy (quartzite  in Ex. 3.3), which is not possible for  minerals with no stability in the presence of water at the reaction conditions you are examining.  This is how I ended up with the "backward" configuration (minerals reacting with water, rather than water reacting with minerals).  Is there a workaround for this?

Thanks!

Ben

[Brian Farrell]

Hi Ben,

You’re very welcome. One idea would be to add a kinetic rate law for any mineral present but not in equilibrium with the initial system. Of course, you’d need meaningful kinetic parameters.

If you prefer the “backwards” configuration, you can do that too. In any titration path (minerals added as simple reactants to a fluid, for example) the reactant mass is added incrementally throughout the calculation (100 steps by default). So if you react 1 kg of a mineral with ~1 kg of solution, you’ll first add 10 g, then another 10 g, and so on, until you’ve reacted the entire mass. In this way, you’re effectively starting at an infinite water:rock ratio and decreasing it to 1 at the end of the reaction path. And as before, you can plot the amount of mass reacted on your x or y axis. This is obviously different conceptually from the previous suggestion, but I think the term leaves a lot of room for interpretation.

I'm not positive how EQ3/6 conceptualizes the problem, but after a quick search I found an excerpt from Low-Grade Metamorphism of Mafic Rocks edited by Peter Schiffman and Howard W. Day. The description on page 90 seems pretty similar to what I've described in this reply.

Hope this helps,
Brian