Preparations for a key pressure test of the blown-out Gulf of Mexico oil well are under way, BP has said.

英国石油公司发言人称，公司正在准备对墨西哥湾泄露油井进行一次关键性气压试验。

The test, expected to yield information on how to seal the well permanently（永久地） , is set to begin late on Wednesday and could last up to 48 hours.

BP said valves（阀门，瓣膜） 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（不能恢复地） harm the well bore（钻孔） , hindering（阻碍，打扰） efforts permanently to seal it.

"There is a tremendous sense of urgency," said Adm Thad Allen, the US incident commander. But he warned that nobody wants to make "an irreversible mistake".

It had been expected that with the new containment cap in place BP would start the vital pressure test early on Tuesday.

Following the procedural review, the "well integrity test" will now include seismic（地震的） , acoustic（声学的） and temperature monitoring of the well and the surrounding area.

The purpose of testing is to check whether oil will leak out of other areas if valves on the blowout preventer and new cap are closed.

"Upon completion of the tests, the federal government will possess valuable data regarding both the condition of the well - important when action is taken to ultimately kill the well with the relief well efforts - as well as an understanding of our capacity to shut the well in for brief periods if needed to prepare for a hurricane," the White House said in a statement.

The blown-out well has been spewing（喷涌，呕吐） oil into Gulf waters since April.

During the test process, BP will monitor the pressure of the oil in the well.

High pressure will mean the oil has been contained inside the wellhead.

But if the tests show there is low pressure, this may indicate that oil is leaking elsewhere in the well.

BP officials said that if the pressure proved to be low, the cap would not be kept shut and ships on the surface of the Gulf would continue collecting oil.

BP has temporarily suspended work on drilling its two relief wells - which offer the only prospect of permanently plugging the leaking well - because of the planned test.