Reaction Path

[Kirkoff Xn]

Hi Jia and Brian,

I am running a reaction path model (flush) to ensure whether the aquifer minerals (calcite and dolomite) would dissolve. Dissolution is more likely considering that the injectant is less saturated in both mineral phases. While the reaction path shows calcite dissolution (see figure below), I am a bit perplexed considering that the saturation indices of both minerals show they are saturated throughout the reaction path (see 2nd figure attached). Is it possible for a mineral to dissolve while being stable throughout the reaction path within the system? How can this be explained? Can you advise how I can go with this? I am attaching the react input file.

Thank you,

                  

 

 

 

Trace Rx Path.rea

[Brian Farrell]

The minerals actually become undersaturated after each increment of added fluid (and removal of the mixed fluid), but because this is an equilibrium model, the minerals dissolve in response until the fluid re-equilibrates with them (i.e., until they're no longer undersaturated) at the end of the step. 

If that's not obvious, you can play around with your model to see what's happening. If you use the dump command, which removes minerals from the system before tracing the reaction path, you'll see how the flush model you've configured causes calcite and dolomite to change from saturated to undersaturated when there is no mineral mass available to buffer the fluid chemistry. Or, you can add kinetic rate laws for the Calcite and Dolomite and set their rate constants to small values (for slow reaction), or even 0 (for no reaction), and then if you disable precipitation for other minerals, you'll again see that the flushing fluid causes the minerals to become undersaturated. But since the dissolution rates set are so slow, the minerals don't dissolve to any significant extent. Assuming progressively faster kinetics, however, you should approach the equilibrium behavior in which dissolution is fast enough that the fluid remain in equilibrium with the minerals after each step.

Hope this helps,

Brian Farrell
Aqueous Solutions