Salting out effect and confusion about true concentration of ions in brine

[Mojo]

Hi 

I am trying to model salting out effect that happens during dry gas injection. I used thermo.tdt in React to model the reactions between the rock and brine at 135 C. The model works just fine when there is no evaporation. But when I considered evaporating >99% of the brine, the model outcomes does not look trustworthy to me as the brine ionic strength is super high (like around 500,000 PPM!). I doubt any brine at this salinity can be stable. 

I have already read again section 3.2 of the manual and I cannot use thermo_kmw.tdat as the ionic species and minerals I have in my system are not defined in this dataset. Do you have any suggestions how one can model salting out effect in this case? And why model shows such a high salinity brine? 

 

Also when I am calculating ionic strength based on the data from "Components in fluid", it does not match what I get from "Ionic strength" listed in "Chemical Paramnters" dropdown menu. 

 

And lastly, I am confused between the difference of "components in fluid" and "species concentration". They do not give the same number for any ion (for example check H+). Why is that? Which one shows the true concentration of ions (in particular H+) in brine?

 

Many thanks and kind regards,

Mojtaba

20 yrs injection.rea

[Jia Wang]

Hello Mojtaba,

The dataset, thermo.tdat, used in your attached script is a dataset published by Lawrence Livermore National Laboratory. This dataset uses an extension of the Debye-Huckel equation, the Bdot Model, to calculate the activity coefficients for aqueous species. The Bdot equation is widely applied in geochemical models designed to operate over a range of temperatures. This model is reasonably accurate for Na+ and Cl- ions up to about an ionic strength of 3 molal and other species to an ionic strength of ~ 0.3 molal. For modeling high ionic strength solutions, the virial models are almost always more accurate. The thermo_hmw dataset uses the Harvie-Moller-Weare formalism of the Pitzer equations. This method however is not easily extensible and data is often limited at 25C. You can read more about activity coefficients in section 7.4 in the GWB Essentials User Guide. For a deeper dive into the topic, you can refer to chapter 8 Activity Coefficients in the Geochemical and Biogeochemical Reaction Modeling textbook. 

A good starting point for you may be using the Yucca Mountain Project dataset (thermo_ymp.R2.tdat). This dataset follows the HMW formalism of the Pitzer equation and contains more components than thermo_hmw.tdat. This dataset also includes some provision for extrapolating to higher temperatures. You can find the dataset installed with your GWB software or you can find it on the GWB thermo dataset webpage. 

Reference for the dataset: Jove-Colon, C., Wolery T, Rard, J, Wijesinghe, A, Jareck, and R, Helean K: 2007. Pitzer database development: Description of the Pitzer geochemical thermodynamic database data0.ypf.R2. Appendix I. In In-Drift Precipitates/Salts Model, Report ANL-EBS-MD-000045 REV 03, DOC.20070306.0037 Las Vegas, Nevada: Sandia National Laboratories.

"Components in fluid" and "Species concentration" are not the same thing. This is because in geochemical modeling we separate free species concentration from bulk component concentration. For example, the bulk sodium component concentration is going to be distributed amongst various species containing sodium (e.g. NaCl, Na+, NaCO3-, etc). If you want to find the concentration of the free ion Na+, then you would look under "Species concentration". If you want the total concentration of sodium in fluid, then you would look under "Components in fluid" for sodium. If you would like an example and further explanation, see section 7.2 Equilibrium models in the GWB Essentials User Guide. 

The ionic strength reported by the GWB program is the true ionic strength, which accounts for the role of complexing in reducing the number of free ions in solution. If you want to check the ionic strength by hand, you would want to use the concentration given under "Species concentration" and not the components. 

Hope this helps,
Jia Wang
Aqueous Solutions