In order to create boundary conditions in a polygonal region the following workflow can be used:
Extract surface using ExtractSurface (a: Original subsurface mesh; b: Extracted surface)
Mark the mesh elements within the polygonal region deploying the tool ResetPropertiesInPolygonalRegions (a: Surface and polygon; b: Marked elements in polygonal region are colored yellow; c: Marked regions visualized by different colors)
Remove marked/unmarked mesh elements deploying tool removeMeshElements (Resulting patches visualized by different colors and z-translations)
Compute the associated area for the nodes of the surface mesh deploying tool ComputeNodeAreasFromSurfaceMesh
The surface mesh patches (created until step 3) can be used as input for OGS-6 simulations. For OGS-5 simulations only the additional step 4 has to be performed.