NCG Parameters

Does anybody have this information:


What thermophysical parameters does T2VOC use in simulating a NCG?  Are these the same parameters TMVOC would use in simulating a NCG?

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  • Seth,

    The only NCG that T2VOC includes is the pseudo-component "air", whereas TMVOC allows the user to specify multiple NCGs (including the pseudo-component "air", but air could also be modeled in TMVOC as a mixture of N2, O2, CO2, Ar, etc.). "Air" in T2VOC has the same thermophysical properties as in all the EOSs that handle air (e.g., EOS3, EOS4, EOS7, EOS7R, EOS8, EWASG,etc.). For example, its molecular weight is 28.96 g/mol, and the air heat capacity is 733.0 J/(kg C). Note that T2VOC and most other EOSs use the ideal gas law, whereas in TMVOC the gas phase is treated as a mixture of "real" (not ideal) gases.

    Hope this helps,


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  • Hi Stefan,

    Thank you for the reply. We know that methane is not built-in to T2VOC like air, but Is it possible to use the Edit VOC Data menu under Global Properties to specify methane in T2VOC? Or is it necessary to get TMVOC to model methane?

    Thank you.
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  • Seth,

    While I don't see any fundamental reasons why you could not treat CH4 using the CHEMP block, I will have to leave it up to you to decide whether the treatment will be adequate for your application. Note, however, that you will still carry air as another NCG in this simulation, and you lose the opportunity to account for a VOC, which also means that you won't have a NAPL phase. In a sense, you are definitely using T2VOC for a purpose for which it was not designed. Most likely (again depending on the application you have in mind), TMVOC and EOS7C(A) would be more appropriate.


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    • Stefan Finsterle 

      Quite a late reply for this old post, but I would like to add something to the use of VOC in T2VOC (and TMVOC too).

      The standard use of a VOC in T2VOC assumes that the VOC can exist as a NAPL phase in the full range of temperatures encountered during a simulation. This is actually the same assumption used in TMVOC.  See TMVOC user's guide at pag. 8-9:

      The TMVOC approach has another limitation in that it cannot be used for organic chemicals whose critical temperature is lower than the
      maximum temperature reached during the simulation. In this case no vapor saturation pressure can be computed for the chemical and thus no equilibrium calculations are possible using the approach followed for a conventional VOC in TMVOC. Methane, in particular, has a critical temperature of - 82.6 °C, and ethane too has a low critical temperature of 32.35 °C. VOCs that are not condensible
      at prevailing temperature conditions can be treated as NCGs. Properties of the NAPL phase are assumed independent of the concentration of dissolved NCGs. 

      So, just including CH4 thermophysical parameters taken fro Reid et al. into the CHEMP input block is not enough to simulate CH4 as a VOC.

      Under single Aqueous and two-phase Aq+G conditions, T2VOC calls sub SATO which computes the NAPL phase saturation pressure at local temperature TX. If TX is greater than the VOC critical temperature TCRIT, SATO will print the message 'temperature out of range in SATO' and T2VOC will reduce the time step.

      The call is intended only to supply the saturation pressure of NAPL to be compared with the local partial pressure of VOC for the NAPL phase appearance test. Even though special values for parameters VPA,VPB,VPC and VPD could be given in CHEMP.2 input block to compute a dummy NAPL saturation pressure which is always greater than the CH4 partial pressure within the simulation, in order to avoid the appearance of a NAPL phase,  sub SATO will stop the simulation because of the low value of CH4 TCRIT.

      Giving a dummy value of TCRIT in the input file would not work, as TCRIT is also used in subs SUPSTO and COWATO called in two-phase Aq+G conditions for the calculation of gaseous VOC enthalpy.

      Thus, I think modeling CH4 as a VOC in T2VOC is not possible.


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  • Hi Stefan,

    I'm working with Seth on the project he mentions above. I'm evaluating EOS7C and having trouble with the initial conditions options. I want to start with initially zero (or very small) methane, residual water, and N2. The User's Guide says, "The intended application area of EOS7C is natural gas (methane, CH4) reservoirs. As such, whatever part of the gas phase that is not made up of water vapor, CO2,
    or gas tracer is assumed to be made up of CH4."

    However, when I input any non-zero NCG (N2 in this case) mass fraction into the initial conditions menu, I get the following error XMOLENCG OR XMOLECH4 ARE LESS THAN ZERO IN SUBROUTINE SOLIT. CHECK INITIAL CONDITIONS TO ENSURE MASS FRACTIONS ARE WITHIN SOLUBILITY LIMITS.

    Is it just not possible to start with almost all N2 and no methane in EOS7C?


    Thank you,


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  • Amy,
    I am not sure why this is happening. Can you send me your input file(s) including INCON and MESH?

    Thanks, -Curt
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