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At its capacity, the SPR holds about 35 days of normal usage for the US. In a real emergency, that number would be about 70 days, maybe even 100.
Currently the SPR holds 696 million barrels.
http://www.spr.doe.gov/dir/dir.html
The actual capacity is 727 million barrels.
http://energy.gov/fe/services/petroleum-reserves
Recently in June 2011, the President directed a sale of 30 million barrels of crude oil to offset disruptions in supply due to Middle East unrest. The United States acted in coordination with its partners in the International Energy Agency (IEA). IEA countries released all together a total of 60 million barrels of petroleum.
http://energy.gov/fe/services/petroleum-reserves/strategic-petroleum-reserve
The US consumes about 16 mmbpd (million barrels per day) of refined product. Total petroleum product is higher at +19 mmbpd but that includes natural gas liquids and ethanol.
http://www.eia.gov/dnav/pet/pet_cons_psup_dc_nus_mbblpd_m.htm
Many people do the math of 696 ÷ 16 = 43.5 days and claim that is our time for using the SPR. But that is foolish for several reasons.
First of all, if there was something like an OPEC embargo again, Texas, North Dakota and the like are not going to shut down there wells. We don't import 16 mmbpd of oil, we import ~7.5 mmbpd.
http://www.eia.gov/dnav/pet/pet_move_impcus_a2_nus_epc0_im0_mbblpd_m.htm
Even if there is a full out world war, I don't see Canada or the like stopping there sale of oil (and our flow of dollars) in any event. All of our OPEC imports total about 3.5 mmbpd. We get a tiny 0.03 mmbpd from Russia.
So what is the total need the SPR would have to replace? Maybe 4 mmbpd?
That number is also good in that the SPR can only withdraw 4.4 mmbpd. There is only so many pumps and pipes, and there are certainly limits of being able to load ships.
So 696 ÷ 4.4 = 158 days.
This is still likely faster than we would need it. If OPEC or others are not selling oil to us, they are going to look for other customers. Any major crisis is going to raise not lower their expenses. If they sell to someone who normally buys from someone else, that original supplier is going to have surplus oil for sale.
Wait, you mean oil is fungible? ;-)