Mobility concept in TOUGH (Parameter MOP(11) )
Hi dear group, I hope this wouldn’t be a silly question
I am working with an unsaturated column composed of sandstone and mudstone layers. As first step (and using EOS9), I modeled the gravity-capillary equilibrium with the water table, which I represented as a block of huge volume and small nodal distance located on the base of the column. This block is fully saturated and has the same hydraulic properties as the penultimate layer of sandstone. Then, I let it equilibrate for 10 years, employing the weighting scheme MOP(11)= 2. Naturally, none of the column elements reached the saturated conditions after that time. Therefore, I tough that setting a different option that also considers the upstream weighting scheme for mobility would not affect the water amount in the grid elements. However, when I set MOP(11) = 0 and WUP=1, results varied, being the saturations higher for this last case.
Now, I am having a hard time trying to understand what does mobility exactly means for Tough codes. Is it not the equivalent of water relative permeability for EOS9? So, Why do water saturations change when applying either MOP(11) = 0 or 2 if both consider an upstream weighting scheme for water mobility (and I am not dealing with absolute permeability yet?
Thanks in advance for your comments
Mobility generally refers to the product of absolute permeability and relative permeability. If you have different absolute permeabilities for sandstone and mudstone, I believe that MOP(11)=0 and MOP(11)=2 should produce different results during the transient period when water is moving around. But I believe the steady-state result of gravity-capillary equilibrium should not depend on MOP(11), since fluid is not moving. Are you sure 10 years is long enough to have reached steady state? To check, you can set kdata=2 so you get flow information in your output file and make sure flow rates are very small (~1e-13) at the end of your simulation.
P.S. Not a silly question at all. MOP(11) was one of the hardest things for me to figure out in TOUGH, and I still may not always get it right. I find doing one-column simulations and looking carefully at the results is a good strategy.
Dear Dr. Doughty
Thanks a lot for your detailed and rapid answer. Now the parameter have more sense for me, being able to understand more in deep the weighting schemes developed by Tsang and Pruess (1990) (Actually, I highly recommend other users who may face the same problem in the future to read this paper in order to follow the reasons behind the different schemes in TOUGH.)
Furthermore, you are also right about the needed time to reach steady state in the model, Dr. Doughty. When I set kdata=2, I realized that flow continues taking place at considerable rates. Therefore, I increased it in my model. Thanks again for your advice.