Question about Gas Relative Permeability ($k_{rg}$) calculation for IRP=7 in TOUGHREACT v4.13 (EOS3)

Hello everyone,

I am a university student and recently started using TOUGHREACT v4.13 (with the EOS3 module).

I have a question about the gas relative permeability (k_rg) formula used when IRP = 7 (Mualem-van Genuchten model) is specified, particularly under the condition where RP(4) = 0 and RP(5) > 0.

I understand that the liquid relative permeability (k_rl) is calculated using the standard Mualem-van Genuchten formula:

k_rl = S_e^(1/2) × [1 - (1 - S_e^(1/λ))^λ]²

However, I am trying to determine the corresponding formula used for the gas relative permeability (k_{rg})

I have tried hand-calculating k_{rg} using common models (like the standard Corey model, k_{rg} = (1 - S_e)^{n} or similar variants), but my results do not match the values written to the flowdata.tec output file.

Could someone please clarify what specific equation TOUGHREACT uses to calculate k_{rg} under this exact set of parameters (IRP=7, RP(4)=0, and RP(5)>0)?

Thank you very much for your help.