Not sure I believe anything about oil forecasts. So many have been wrong since the recession.
I don't know about how realistic $20/bbl is but Saudi's OPEC minister was quoted as saying that they would sustain a price war to that level in the event of increased shale oil production. Some of the interesting points I heard this morning to support the Goldman analysis is that a lot of OPEC countries that aren't Saudi have grown so dependent on oil revenue to fund their governments that in the case of crashing prices they wouldn't be able to survive a severe production cut and would actually have to continue at max production levels. I think it's likely that Saudi would intervene at that point and fund them with some sort of hedge for their losses.
And again you also have to remember that one of the most oil-rich countries in the world was just reintroduced to the world economy due to the sanctions being reduced to Iran.
The U.S. is going to shut off its expensive Shale pumps for a year or two while we can get cheap oil from elsewhere. Then when Saudi bring prices back up above $60 they'll turn back on and equalize. But it's oil. Nobody really gets it. It's black magic.