Table of Contents
As described in the TOUGH2 User's Guide, the thermophysical properties of fluid mixtures needed for assembling the governing mass- and energy-balance equations are provided by "equation-of-state" (EOS) modules. Each equation of state uses a different set of primary variables (such as pressure, temperature, and mass fractions) to define each possible phase condition. When the phase changes, the EOS switches and initialized new primary variables.
PetraSim supports the following EOS options in TOUGH2, Figure 4.1. PetraSim also supports TOUGHREACT, TMVOC, and TOUGH-Fx/HYDRATE. For the options available for these different versions, go to File->Preferences....
To select an EOS, go to File->Preferences.... On this dialog, you can select the TOUGH version and a particular EOS supported by that version. After making the selection, you must then select File->New to create a new model with the new EOS. Your preference will be remembered by PetraSim, so that next time you create a new model, it will start with the EOS you previously selected as your preference.
There can be only one EOS in an analysis.
Each EOS has different options that define the assumptions used in the analysis. For EOS3, this includes whether to include heat transfer (isothermal or non-isothermal) and whether to include molecular diffusion. These options are selected by selecting Properties->Global Preferences... or 
on the toolbar. On the EOS tab all available options will be displayed.
The user is referred to the TOUGH2 User's Guide [Pruess, Oldenburg, and Moridis, 1999] for detailed descriptions of the options available for each EOS.