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# Hi everybody, I have a question regarding a particular simulation (I am using Petrasim). Can you help me?

Hi everybody,

I have a question regarding a particular simulation (I am using Petrasim). Can you help me?

I am trying to simulate the heat and fluid flow within a volcanic edifice.

I built a non-symetric mesh that represent one flank of the volcano. The mesh is 3D and has the following dimensions: X = 0-8950 m, Y = 0-250 m and Z ranges from 0 to 4950 m. Each block is 100 x 100 x 100 m.

The reason I use a third dimension (Y-axis) is the following. Later on, I want to inject a magmatic fluid through the blocks X = 8550-8950 m, Y= 150 m and Z= 50 m. The two planes at Y= 50 m and Y = 250 m will be assigned fixed conditions of T and P, so they will act as a thermal insulator, in order to cool down the fluid that will rise through the Y = 150 m plane.

Anyhow, as a first step (Sim. 1), I just want to have a good hydrostatic pressure throughout all the mesh (I am not distinguishing yet between any of the y-planes). So, I asign a atmospheric pressure and T =25 C for all the upper z blocks (that represent the surface). I use fixed conditions for all those blocks.

I was expecting to have a total pressure at X = 8950 m and  Z = 50 m of around 500 bar (pressure induced by a 5 km water column at 25C), but instead of that I get a pressure of 363 bar.

To check if it was not due to the assymetry of the mesh, I built a symmetric mesh but I get the same results.

I suspect that lateral water flux towards the left part of the mesh (lower altitude topography) might cause some variations in the hydrostatic P profile, but I don't believe this can explain a decrease in more than 100 bars regarding the hydrostatic pressure I should obtain.

Then (Sim. 2), what I did was to isolate the 9 vertical columns at the left of the mesh ( X= 8750-9950 m, Y = 50-150 m, Z= 50 - 4950 m) by setting a low permeability value to the blocks located aside on the left (X= 8450-8650 m, Y = 50-150 m, Z= 50 - 4850 m). As a result, I got a total pressure at depth in this isolated part of  the mesh of 483 bar, much closer to the hydrostatic pressure I was expecting.

In the rest of the mesh, I still obain pressures that are too low.

What do you think is the reason?

Thanks,

Nathalie.

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• I attached the figures and  .sim files of the two simulations (sim 1 and sim 2) so you can vizualise better what I am saying

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• Nathalie,

From your figures, it is apparent that you have atmospheric pressure set along your sloping top boundary.  Given this boundary condition, and the lateral and vertical permeability of the material, you should not expect the pressure to be 500 bars underneath the deepest part of the model.  The reason is that even this deepest part is in communication with the entire sloping atmospheric boundary.  Your second image reveals this well, i.e., when you isolate the column under the highest part of the model, you get a high hydrostatic pressure representative of the depth of the grid blocks.  But just outside of the isolated column, the pressure again responds to the sloping boundary atmospheric pressure, and is not everywhere equal to hydrostatic based on depth of grid block.

You could further demonstrate this by setting the horizontal permeability to zero, in which case all of the bottom grid blocks should attain hydrostatic pressure based only on the their depths, i.e., no communication with the sloping boundary except at the top of each column of grid blocks.

-Curt

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