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# Defining ِDirichlet / Neumann boundary conditions in ECO2N

Hello

I am trying to define different types of boundary conditions in TOUGH2 with ECO2N module to see the effect of B.C on the simulation. I think the TOUGH2 code simulates Neumann conditions by default which there is no flow crossing the model domain.

I have built a 2-D mesh and need to assign very large volumes to the grid blocks adjacent to the boundary, to have the specified boundary conditions instead of no flow boundary.

I wonder if anybody can help me to define Dirichlet boundary conditions with this module.

Thank you all

George

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• George,

Now that you have elements with large volumes along your boundaries, simply give them (as initial conditions through blocks INDOM or INCON) the thermodynamic state you want to see fixed along the boundary - they won't change during the simulation, as any fluid (or heat) flowing in or out into won't be enough to increase the pressure, saturation, mass fraction or temperature of the element because of its very large volume (in fact, the fluid/heat will simply "disappear" due to numerical cancellation).

Also note that no flow is a special case of a Neumann boundary condition, and it is indeed the default condition at a boundary. If flow is not zero, specify sinks/source using block GENER.

Stefan

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• Thank you Stefan.

Best

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• Hello Stefan and George,,

I just saw this post and I am also dealing with this problem. I am confused because according to the TOUGH2 Sample Problems (page 24) Dirichlet boundary conditions is default for TOUGH code. It assigns very large volumes to the elements adjacent to the model boundary, so there will be no change in fluid/heat exchange in Dirichlet BCs.

The second type of BCs is Neumann and represents no flow crossing the boundary of the model. (TOUGH2 user guide, page 62)

Actually, I defined three 2-D models with different domain radius (5km, 75km, and 100km) but same hydraulic properties and injection rates. In all cases, I got the same results and I think it should be due to the fixed BCs. The outer elements in all cases represent very large volumes.

I wonder if I am right or not and if it is OK to assign zero injection rates (in block GENER) to the boundary cells in order to define the no flow boundary conditions.

Thank you very much

Ramin

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• Ramin,

I don't know which report you are referring to, but your understanding of how TOUGH2 works regarding boundary conditions is simply not correct. TOUGH2 does no such thing as assigning large volumes and Dirichlet b.c. to elements at the boundary (in fact, TOUGH2 does not even know (or care) which elements are at the boundary). Assigning zero fluxes in GENER is the same as not having a GENER block at all for these elements, so this would not change anything.

Please re-read my previous post, which explains the concept, or - much better - go through the TOUGH sample problems and see how boundary conditions are implemented in each case.

Good luck!

Stefan

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