Theoretical vs realistic precipitation

[Mannes]

Hi all!

I have been doing some simulations regarding precipitation av barite and celestite when mixing and cooling produced water and seawater. After help here on this forum, I used EQUILIBRIUM_PHASES, and put in zero for both Si and AMOUNT:
EQUILIBRIUM_PHASES 1
    Barite    0 0
    Celestite 0 0

The thing is, this leads to much bigger amounts of celestite than barite precipitated (because of larger amounts of Sr than Ba in the solutions), even though the SI of celestite is only in the area around 0.25-0.14, while SI for barite is ranging from 1.4-2.2.

I have discussed my results with professionals who deal with these scales, calculates the amounts of inhibitor needed and so on. They have told me that "in real life" you need a SI quite much higher than zero to actually get any precipitation, and that in my case, we would actually get practically no celestite precipitated, only barite.

What I'm wondering is, how can I incorporate this into my simulations, to get as realistic results as possible?

Would the best idea be to simply insert for example SI=0.2? Is there another way of getting more realistic outputs?

[dlparkhurst]

First, the magnitude of the SI does not necessarily indicate the rate or amount of precipitation, only that thermodynamically, the phase should precipitate.

It's OK for starting precipitation, but is SI=0.2 is reasonable after precipitation occurs, or does the solution approaches an SI of 0, thermodynamic equilibrium.

If the former, then use 0.2 as the target SI. If the latter, you probably need to use KINETICS and RATES.

[Mannes]

Okay. I spoke to the guy after the initial post here on the forum, and he said that as long as SI is lower than approx. 0.8, we do not get noticeable precipitation. He also said that scales with higher SI normally only precipitated down to 0.6-0.8, and stopped there.

So, based on that info and your response, I guess setting the target SI=0.6, or thereabouts, should give me decent answers.

Thanks a lot for your help!

[John Mahoney]

Sorry for getting into this so late.  I think you friend is correct when it comes to barite.  I have consistently seen water samples from all over that are oversaturated with respect to barite by about an order of magnitude.  It doesn't seem to matter too much whether the solution is really loaded in sulfate like 10,000 mg/L or if the barium is elevated and sulfate is low.  Even some literature look for Ted Eary's paper (older one in Applied Geochemistry) on Pit lake equilibrium I think his Ba figure shows something of the same effect, maybe a little smaller but still some oversaturation. 

So something is happening with barite but I cannot tell exactly what is happening.