Hysteretic RELP and CAP


Just a quick question: Is it possible to use IPR 12 with ICP 9 ? Following the manual, it should only be implemented with ICP 12, since ICP 12 uses some parameters from IPR 12, but what if I want to put capillary pressure off (even with no physical meaning)? Indeed I tried and it works! I also checked the source code and seems nothing is in conflict.


Thanks for your answer, and sorry for asking this basic question!


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  • 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.


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      • Ramin
      • Ramin.1
      • 2 yrs ago
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      Christine Doughty 

      Dear Chris, many thanks for your time and reply.

      I am using TOUGH2 and the script for ICP 9 is:

          9 CONTINUE

      The reasons I am doing this is that I have hysteretic functions all over my domain (both capillary and RELP). With an aim to include the well block (about 25 grid block) (high permeability/high porosity) I tried different non-hysteric functions (assigned to well domain), but it didn't work (even though I played with lot of things, it did not converge) (using EOS7c). The only way it converged was to use IPR 12 in combination with ICP 9 (zero capillary pressure). As you said, it is not reasonable physically to use zero CAP with hysteretic REPL, but in fact, I don't want any of those concepts in my well block.


      Thanks for your useful comments


  • Hmm, I have had trouble with convergence in EOS7c from time to time also.  If you have not already tried it, I suggest trying IRP=12 and ICP=12 and turning off the hysteresis by setting RP(3)=Sgrmax=0.  Also, you could try IRP=7 for a non-hysteretic version of the van Genuchten relative permeability, which should be compatible with PC=0.

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