EOS7 - Brine Definition & Tracing flow of brines
I am looking for more clarity on the definition of brine in EOS7. In the TOUGH2 user's guide V2, it states that, "The brine is modeled as NaCl solution, while the non-condensible gas is air, although the treatment could be adapted, with minor modifications, to other brines and gases." (Sec. 6.6, pg. 40).
1. What exactly does it mean NaCl solution? (for example 90% NaCl and 10% freshwater)
2. I want to model the flow of brines of different origins, how can I trace these two different origins brines (e.g. flows, mixing, transport etc)?
Petrasims seems to only allow a tracer to one brine (the EOS7 brine, which I am not sure it's true definition and if it is possible to modify and split into separate brines).
As stated by the TOUGH2 user's guide "EOS7 represents the aqueous
phase as a mixture of (pure) water and brine". In practice the aqueous phase is a mixture of a reference brine (with mass fraction Xb) and pure water (with mass fraction 1-Xb).
The reference brine is a water-NaCl solution with a fixed salt mass fraction of 0.2498 and density of 1185. kg/m3 at reference conditions.
You can have additional details on EOS7 looking at Pruess (1991) EOS7, An Equation-of-State Module for the TOUGH2 Simulator for Two-Phase Flow of Saline Water and Air, LBL-31114.
I think you cannot track two different brines in EOS7. If you have only liquid conditions, you might try to track just one of the two brines with a tiny concentration of air.
I suggest to consider the use of EOS7R.
Basically it adds the modeling of two radionuclides into EOS7. You can use the radionuclides as tracers added to the two brines you intend to model. If you switch off the radionuclides decay and adsorption, they can be used as conservative tracers to track the two brines. This approach should work well if you have to model single-liquid conditions only. In case of evaporation of the liquid phase, you should elaborate a bit about the most appropriate partitioning of the radionuclides between liquid and gas phases.
PS: Oldenburg and Pruess (1995). EOS7R: Radionuclide Transport for TOUGH2, LBL-34868.
I think you have to consider the intrinsic limitations of EOS7 approach to model a brine of variable salinity. Strong evaporation processes may increase the brine fraction Xb over 1, if the equivalent NaCl content of aqueous solution becomes higher than the salinity of the reference brine.
In addition, aqueous phase properties are computed with a simplified approach which is usually adequate around the chosen P&T reference conditions. If your simulation experiences large P&T changes, EOS7 approach might loose its accuray.
Some tests with extremely simplified models replicating P&T conditions and evaporation processes you expect might give you an idea if the EOS7/EOS7R approach is reliable for your needs.
EWASG would be probably a better choice for the modeling of evaporation processes (full evaporation is allowed) and the evaluation of brine thermophysical properties as function of PTX, but you would not be able to explicitly track two different brines, apart from their possible different NaCl content and the modeled flow field.
I'm not sure about other available EOS for TOUGH2/3 that could do a better job. Others may probably give you additional suggestions.
PS: TMGAS can simulates NaCl brines with the updated EWASG formulation plus a user's specified number of dissolved solids modeled as tracers. Two of them could be used to track the two brines. The only limitation is that using dissolved solids (as tracers) full evaporation of the brine is not allowed. The brine could also be tracked by using different NCGs dissolved into each of the two brines.
But TMGAS is not available to the public.