Compressibility On/Off in ECO2M

Hello folks,

I posted a similar thread on TOUGH2, but here in TOUGH3, there seems to be an option under MOP2 where we can turn on/off the effect of compressibility on porosities.

My question is where does compressibility play a role during model initialization? Say after the hydrostatic equilibrium is reached, we have the shallower cells to show lower reservoir pressure than the deeper cells. So, if we have two cells with different initial reservoir pressure, how would the constant value of compressibility be distributed across these cells? How does this specific MOP2 option affect the answers to these questions?

Thank you!


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  • Pore compressibility describes how porosity changes with pressure change.  You can turn it off by setting COM in the ROCKS block to zero.  Then porosity will be constant (unless you have precipitation or some other process affecting porosity).  If COM is non-zero, the new porosity appears in the SAVE file.  After you run your hydrostatic equilibrium, check the porosities in the SAVE file.  If pressure has changed, they should be slightly different than the porosity you specified in the ROCKS block (and bigger pressure changes will result in bigger porosity changes).  Usually COM is small so this is a small effect.  For some EOS's, the new porosity appears as part of the main output, but if porosity changes are small, there may not be enough significant figures to see them, so looking at the SAVE file is better.  If you then use this SAVE file as an INCON file in a subsequent simulation, the new porosities will be used instead of the original porosity specified in ROCKS.  The MOP2(15) option does not turn off compressibility entirely, it just controls which porosity (original or new) you use in the energy equation. 

    I recommend you set up a simple problem (maybe just one column) and let the pressure change via hydrostatic equilibration or injection/production and look at the new porosity values in the SAVE file.  Experiment with different values of COM to make the changes bigger or smaller. 

    Mathematicallly, I think the equation is COM = (1/por)*d(por)/dP, where por is porosity and P is pressure.  So you can see that even with a constant value of COM, different pressure changes will result in different porosities. 

    Hope this helps, Christine

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