Error

[Chans_803]

Hi all,

 

I am trying to model the siderite dissolution in a brine under 50 atm CO2.

But, the program gives an error saying' N_R didnt converge after 999 iterations. What could be the cause? I am a newbie on this program.

Can someone help me out?

 

 

Thank you in advance.

[Brian Farrell]

Hi Chans,

 

Could you please post the React script that you are using?

 

Thanks,

Brian Farrell

Aqueous Solutions LLC

[chance]

Thank you Brian,

 

I did get the run. But this time I want the CO2 fugacity at 50 atm. But i dont know how to define it in the model. There are 2 options as far as i understood: 1. fixed fugacity (I guess it is fixed at only atmospheric CO2. 2. Sliding fugacity.

 

I dont want varying fugacity. How can I define set the CO2 fugacity at 50 atm.

I am attaching the react file.

 

Thanks again.

Regards,

 

Chans

siderite(kinetics)_exp1.rea

[Brian Farrell]

Hi Chance,

 

There are a few things that you should think about here. A CO2(g) fugacity of 50 is awfully tough to maintain at pH 8.5. At that pH, most of the carbonate system (the HCO3- component) is present as HCO3- and CO3--, but there is very little CO2(aq), so there can't be a very high CO2(g) fugacity (in a gas reservoir in equilibrium with the system). If you react CO2(g) with your initial water (by sliding the fugacity) you will lower the pH and dissolve Siderite, at least in an equilibrium system. You want to make sure that your numerical model reflects your conceptual model correctly.

 

As for your kinetic rate law, the rate constant is very, very low for the time span considered, and so the reaction rate would be incredibly slow.

 

Hope this helps,

Brian

[chance]

Hi Brian,

 

Thank you for looking at my file. The pH=8.5 is the pH of initial brine. As the CO2(g) dissolves in the initial brine, I see that, in the model, the pH value is dropping to around 5 and more CO2 (aq) are being produced. So defining the CO2 fugacity as sliding at 50 atm is right?

 

In our work, no siderite dissolution was observed. The model results shows no siderite dissolution either. Do you think our numerical model does not really reflect the conceptual model correctly?

 

Also one more question to make sure one thing?? I thought fugacity was something related to the CO2 gas not to the CO2 (aq) species? Do you mean that as CO2 gas dissolves in the water, the CO2 fugacity increases?

 

Many thanks!!!! You have been so helpful!!!

 

Regards,

 

Chans

[Brian Farrell]

Hi Chance,

 

I don't know exactly what your system is, but it sounds like you have a pH 8.5 brine which is reacted with CO2, causing the water to acidify and Siderite to (possibly) dissolve. If this is the case, then a sliding f CO2(g) path seems to be appropriate. If you observe no Siderite dissolution in your experiments, then your kinetic rate law could be just fine. I simply assumed you had observed dissolution in an experiment and were trying to replicate the data in a reaction model. It sounds like you have your model set up well, but it is always good to make sure that the way you conceptualize your problem and the way you model it match up.

 

As for fugacity, is is roughly the partial pressure of the gas phase which would be in equilibrium with your fluid (whether or not it is actually present). The reaction CO2(g) = CO2(aq) holds the activity of CO2(aq) proportional to fugacity CO2(g). To answer your last questions, CO2(g) will dissolve to form CO2(aq) because of a high CO2(g) fugacity. A different example would be a titration of some sort, where acid is added to a fluid with dissolved CO3-- or HCO3-. As acid is added, reactions such as CO3-- + H+ = HCO3- and HCO3- + H+ = CO2(aq) occur, causing CO2(aq) to build up, and thus the CO2(g) fugacity increases. When the fugacity exceeds atmospheric pressure, the fluid would bubble off CO2(g) in real life.

 

Hope this helps,

Brian

[chance]

Thank you Brian. You helped a lot!!

We didnt see any siderite dissolution in our lab work although we were expecting some dissolution. And then we tried to model our experiment. The model observation was same too. But i was not sure if i was conceptualizing the problem correctly with the sliding fugacity option of the program. Now I see it is the appropriate one after i discussed it with you. Now the fugacity of a gas is very clear to me. Thank you again for your kind help.

 

 

 

Regards,

Chans.