A tilted model

Download another model from github:

wget https://raw.githubusercontent.com/OPM/opm-upscaling/master/tests/input_data/grids/PeriodicTilted.grdecl

The file is 658 KB, and as before contains a corner-point grid model with porosity and permeability:

As you can see, the layers here are not more-or-less at the same depth, but tilted. Edit upscale.param to say filename=PeriodicTilted.grdecl and run again (remember, we are still using periodic boundary conditions). You will see something like the following (rounded):

472.99569      8.9836861e-06   -144.89391 
1.0247864e-05  549.99607       0.00027845773
-144.89414     -1.6279639e-06  254.09423

As you can see, there are now large off-diagonal values corresponding to X-Z communication, as could be expected from looking at the model. Changing to fixed boundary conditions (boundary_condition_type=0) gives the result:

409.268455330211 0 0
0 549.996060654282 0
0 0 238.516324893972

The Y permeability is the same (approximately 550 mD), but the X, Z and off-diagonal elements are very different.

For this case, only the results from periodic boundary values can be considered correct, and this will hold for any tilted model. We therefore recommend using periodic boundaries in general, unless you for some reason require diagonal tensors only. In that case you should take care to align the model grid with the layers to avoid making large errors.