You also don't need to drill so deep if all you're interested in is lower grade heat. For example, at Cornell in Ithaca, NY they've just drilled a 3 km deep hole to evaluate getting heat (at ~90 C) to make hot water for heating the campus.
Note the wireline logs. You can see the radioactivity (which is where the heat comes from) go way up as they reach the crystalline basement rocks.
The technical issue that's going to determine if this is feasible is how quickly water from an injection well can get to a withdrawal well. This depends on the how fractured the rocks are.
> In Finland, the temperature rises by 18 degrees per kilometer.
The geothermal gradient at Ithaca is a bit higher than that.
Also, correct me if I'm wrong, but isn't Finland mostly underlain at fairly shallow depth by hard crystalline rocks that are more expensive to drill through? At Ithaca, there's a 3 km thickness of sedimentary rocks before hitting that basement rock.
The caveat is that you will cool the hole extracting the heat, exhausting usable heat the sooner the more you extract. Whether the sustainable heat energy per unit of invested money is attractive depends on circumstances.
