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modern seawater and kinetic carbonates


Rosario
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I am trying to model the modern carbonate banks by mixing modern seawater composition with aragonite and calcite. The results I am getting are not what we see in modern carboante banks. I am using calcite kinetic rates for both calcite and aragonite. One problem that I have is that all the kinetic rates are in mole/m2 s and surface area in m2/g. In react all units are in cm2. If I don't change the units the model runs but is not realistic if I convert the units the model does not converge. The expected results should be the precipitation of dolomite and anhydrite and calcite dissolution. Thanks

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Hi Rosario:

 

I don't think the rate parameters are entirely behind the issue- kinetics is still driven by equilibrium. In your model, initial equilibration of seawater with calcite and aragagonite drives both minerals from undersaturation (dissolution) to supersaturation (precipitation). Given that the kinetic rates for both are identical, the mineral with higher thermodynamic stability (calcite) precipitates. Additionally, the rates you've set are so high that the model proceeds according to equilibrium and you have simple conversion of aragonite to calcite.

 

I would fix this model by looking at two issues:

 

1. set realistic kinetic rates for calcite and aragonite for the timeframe you've specified.

2. try to set up initial equilibrium between seawater and aragonite/calcite in such a way that they are not initially supersaturated.

 

I hope that helps,

 

Tom Meuzelaar

RockWare, Inc.

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Hi Rosario:

 

I don't think the rate parameters are entirely behind the issue- kinetics is still driven by equilibrium. In your model, initial equilibration of seawater with calcite and aragagonite drives both minerals from undersaturation (dissolution) to supersaturation (precipitation). Given that the kinetic rates for both are identical, the mineral with higher thermodynamic stability (calcite) precipitates. Additionally, the rates you've set are so high that the model proceeds according to equilibrium and you have simple conversion of aragonite to calcite.

 

I would fix this model by looking at two issues:

 

1. set realistic kinetic rates for calcite and aragonite for the timeframe you've specified.

2. try to set up initial equilibrium between seawater and aragonite/calcite in such a way that they are not initially supersaturated.

 

I hope that helps,

 

Tom Meuzelaar

RockWare, Inc.

 

 

 

Tom,

thank you. In order to do #2 would you run the model without the kinetic to equilibrate it. then pick up the fluids and interact it with the rocks again??? The kinetic rates are a problem because there is limited info. since it is always assumed that they are at local equilibrium.

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