When needed, as in the aftermath of Katrina, the oil reserves are sold to US markets.
You can learn more about the SPR at http://fossil.energy.gov
Nancy Pelosi has strong feelings on releasing at least some of the SPR to help lower the prices of gas...
LINK: Peloshi to Bush: Free our Oil
“Americans convinced the Bush Administration to not keep filling the SPR a number of weeks ago. We are calling upon them now to free our oil, call the White House, e-mail the White House so that we can increase the supply to reduce the price at the pump. The President’s number is (202) 456-1414, and his e-mail is comments@WhiteHouse.gov.”