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Activity models for modeling brine/rocks interactions in relatively high (>100℃) temperature


Yumeng
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Hello guys,

As a user of the GWB, I am currently working on a project to model the chemical reactions between high salinity (about 6 molal NaCl solution) brines and rocks that are mainly composed of silicate minerals in a sedimentary basin. 

Now I have some questions here expecting for explanation and clarification with respect to activity models in the REACT module of the GWB.

After reading the book "Geochemical and biogeochemical reaction modeling", I find that the widely used B-dot method (the extended Debye-Huckel method) is regarded to be valid only at relatively low salinity (less than 3 molal NaCl solution) environment. The virial method can keep reliable at very high salinity environment, but it is not well extensible to situations beyond room temperature, and besides, silicate(Si-Al) minerals are not well supported. So the modelers who have to model brine/rock interactions are in a dilemma: if choosing the B-dot method, it is not accurate and reliable enough; if choosing the virial method, it is unusable to model complex systems involving silicate minerals beyond room temperature.

 

So my question is what I can do if I have to model reactions between high salinity brine and sedimentary/crystalline rocks that are composed of silicate minerals? As we all know, the high saline brines in sedimentary basins are quite common, and this kind of situation should be very normally seen. Is there any convention to deal with this problem if there is no direct answer?

 

In addition, I am quite interested in the upcoming thereda2020.tdat database, which seems to be useful for the modelers like me, can you give a brief introduction to this database?

 

Thanks in advance for your help,

 

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Hello Yumeng,

please look here: https://www.thereda.de/en/datenabfrage/download

Please note that depending on the particular system the valid temperature range for Pitzer coefficients is different. This is marked in the (quite bulky) parameterfile. 

Note 2: Pitzer coefficients for Si- and Al-Species are only estimates and valid for 25°C only. However, if the solubility of the solid phase in question is very low, then the database might give a good estimate. Whether the assumption of thermodynamic equilibrium between a silicate mineral and an aqueous solution is valid needs to be ascertained in any particular case.

If you apply THEREDA to real systems for which experimental data are available, we are always very interested in your results!

Best regards,

Helge 

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Hello Helge,

Thanks so much for your reply. 

After reading your explanation and checking the link you sent to me, I find that the Pitzer coefficients for Si- and Al- species are only valid for 25 ℃ in Thereda database, which may not be enough for geochemical modeling of brine-involved hydrothermal processes. Could you explain me why the thermodynamic data for Si- and Al- species are only valid at room temperature (25 ℃), and cannot be extended to relatively high temperature? Is this because doing experiments on silicate minerals to regress Pitzer coefficients beyond room temperature are not available now?

Best regards,

Yumeng

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As to my knowledge there are experimental data at higher temperature which could allow for the determination of Pitzer coefficients for Si- and Al-Species. But this work (compiling suitable experimental data, critical assessment, fitting procedure) simply has not been done yet. A project to pursue this goal has been outlined and we are waiting for funding. 🙂

Best regards,

Helge

 

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