Hysteretic RELP and CAP

Hi Ramin,

What version of TOUGH are you using?  My TOUGH3 does not have ICP=9 as an option.  In looking at the source code, I see that if you pick an ICP that does not exist, no call to the PCAP function is made.  I don't think this is a good idea, because later the code will look for a value of capillary pressure, and it will not have been defined.  In my experience, this situation gives unpredictable results, because there is no way of knowing what value is in that memory position.

But to answer your question, you should not try to use IRP=12 without using ICP=12.  The main point of hysteretic relative permeability is that the residual gas saturation for each grid block depends on the the saturation history of that grid block.  The history information is calculated in the ICP=12 capillary pressure function and then used in the IRP=12 relative permeability function.  If you want the capillary pressure function to be non-hysteretic, set CP(12)=1.  I do this frequently, and it works fine. If you actually want no capillary pressure (Pcap=0, so Pgas=Pliquid), then I recommend setting capillary pressure strengths, CP(3) and CP(7), to a very small number (I have never tried this, but it might work).  I do not recommend setting CP(3) to 0.  Note that CP(7)=0 is a flag to set CP(7) equal to CP(3).  

Be careful that your model is physically reasonable.  Does it make sense to have a hysteretic relative permeability and zero capiillary pressure?  If you think of the pore scale, any non-linear relative permeability (hysteretic or non-hysteretic) arises because of capillary forces.

Chris