How do I know which module I need?

Each Equation-of-State (EOS) module describes the thermophysical properties of specific combinations of phases and components. The user has to decide which are the components (e.g., water, CO2) of interest for a given application. One should also consider the range of pressure and temperature covered by an EOS module. Finally, check whether you need to be able to handle special processes, or whether you need inverse modeling capabilities, etc. Refer to the licensing page for available modules, and consult the related documentation, e.g., Section 6 of the TOUGH2 User's Guide.