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# Pore Compressibility in ECO2M

Hi guys,

I have a 3D cartesian model where I assign constant porosity across all elements in the model. In TOUGH2, I think we can assign only a constant value of pore compressibility.

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?

Would appreciate any insights! Thanks!

Nur

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• Dear Nur,

pore compressibility changes the rock porosity as function of pressure change occurring in time since the start of simulation with respect to the initial pressure.

Thus, if you want to simulate a steady state as a proxy for an initial state of your system, if the compressibility coefficient is greater than zero in TOUGH2, then you will come up with different element porosities (with respect to the constant value assigned through the ROCKS domain) as function of local P change.

As the simulation to reach a steady state might be in some way arbitrary, I would suggest not to simulate the effects of compressibility (and I would add those of thermal expansivity) during steady state runs, but just to include these effects in subsequent simulations.

The steady state results should be the same, with or without compressibility, as the porosity value would not have effects on flow field and P&T distribution. This unless permeability is linked in some way to the change of porosity, such as with a Kozeny-Carman like relationship.

In TOUGH2 you need to set compressibility to zero if you want to neglect its effects. In TOUGH3 this may be obtained by switching off the compressibility through an input option, but the result should be the same.

Regards,

Alfredo

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Alfredo Battistelli said:

since the start of simulation with respect to the initial pressure

As for the "initial pressure", are you referring to the initial pressure of each respective gridblock resulting from the steady state run? In other words, would this mean that the deeper and shallower elements will need to experience the same amount of pressure change to reach the same extent of porosity change?

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• For 'initial pressure' I do mean the P of each element at the beginning of a simulation run. If you need a steady state run and you activate the pore compressibility (>0.), then the porosity of each element will be changed according to the local P change to reach the final steady-state P. Then, when you restart from the steady-state conditions, each element will have a different porosity.

For this reason I suggest not to activate the pore compressibility in steady-state runs, but only for subsequent runs. May be others could have a different view.

Regards,

Alfredo

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