Preparations for a key pressure test of the blown-out Gulf of Mexico oil well are under way, BP has said. The test, expected to yield information on how to seal the well permanently, is set to begin late on Wednesday and could last up to 48 hours.BP said valves were being positioned ready to close the cap so that pressure could build up and the well's strength and integrity be assessed.The test had earlier been delayed so experts could review the procedure. Scientists and officials feared the testing procedure could irreparably harm the well bore, hindering efforts permanently to seal it.