TOUGH2 allows 3 options for assigning the tortuosity coefficient and the the diffusivities used to compute the diffusion fluxes (see pag. 159 of the User's guide).
In PetraSim only the first two can be used:
1. assign TORTX different than 0.
2. leave TORTX=0 (Millington & Quirk model).
The third option (use diffusivities <0) cannot be used as PetraSim does not accept negative values. A possibility could be to assign the absolute values in PetraSim, modify the input file assigning negative values and then run directly the TOUGH2 executable without running it from inside PetraSim, as it would overwrite the input file.
TORTX can be assigned from:
Material Data > Additional Material Data > Misc(ellanea)
The diffusivities can be assigned from:
Properties > Global Properties > EOS > Molecular Diffusion