If the shale oil process produces at $20, then it is profitable as long as oil on the open market is above that.
That may make it a lower-profit supply, but profitable nonetheless. Government guarantees to purchase 2 billion barrels a year from shale for the next 20 years at $30 barrel would be a good use of government money from an energy security perspective.
If market prices stayed above $30, it wouldn’t cost the government a dime because the producers would sell on the open market rather than take the government’s $30. But domestic production would be guaranteed a profit in case OPEC tried to manipulate market prices for pumped crude lower to squeeze them out. It would effectively eliminate the risk to capital investment that developing the shale oil will require.
Gov’t subsidy won’t work that way. Besides that, nobody would build that much plant for zero profit when there is oil to be pumped at decent profit.