BP says it has begun test of U.S. Gulf well cap

HOUSTON (Reuters) - BP Plc (BP.L) (BP.N) shut off the main route for oil gushing out of a new cap on its blown Gulf of Mexico well on Wednesday, kicking off a critical pressure test, a company executive said.

Kent Wells, BP's senior vice president for exploration and production, said the "ultimate" test would come when all flow is shut off on the seabed a mile below the ocean surface so scientists can accurately gauge pressure in the well.

But BP has closed the main valve in the middle of the cap, "and we no longer have flow out the top," Wells said.

That move came shortly after the retired Coast Guard Admiral Thad Allen, the top U.S. official overseeing the spill response, issued an order approving the test after a 24-hour delay to study whether it might exacerbate the leak.

"We'll be releasing an order to BP to proceed with the well integrity test, but we gave them some additional direction. We did this to be sure we were taking due care ... to make sure we didn't do any irreversible harm to the wellbore," Allen said.

Earlier in the day Doug Suttles, BP's chief operating officer for exploration and production, told CNN BP and U.S. government scientists decided on Tuesday afternoon to delay the test out of concern it could damage the stricken well and allow oil and gas to escape out the sides while the top was shut off.

But Allen said BP had provided information on piping in the stricken wellbore that assured officials of its structural strength. Taking the break for more discussion "makes sure we're getting this absolutely right," he said.

The six- to 48-hour test will entail shutting in all leaking crude to gauge pressures, Allen said.

SEA BED BREECH WOULD BE WORST CASE

BP and Allen have repeatedly said they don't know if the April 20 blowout damaged the pipe, or casing, and cement in the Macondo well. Such a breach could allow hydrocarbons to escape and possibly burst through the seabed if the cap were shut.

Allen said on Wednesday seismic data gathered on Tuesday "removed the possibility of a negative event," or hydrocarbons leaking out from the well beneath the seabed.

Once all flow is shut off, high pressure will indicate the well is intact. Low pressure would indicate a possible breach, and in that instance BP would "immediately" unleash the flow and proceed with oil-capture efforts, Wells said.

Allen said that BP is moving forward with expanding oil-capture capacity with four vessels on the water's surface later this month.

If the cap can shut in all the crude, the vessels will be on site to step in if a problem erupts, Allen said.

Nansen Saleri, former head of reservoir management for Saudi Aramco, the world's largest oil company, told Reuters earlier on Wednesday that BP should scrap the test and keep using surface vessels to collect leaking crude until a relief well intercepts and plugs the leak by mid-August.

Two vessels captured or burnt off 17,060 barrels of oil on Tuesday, and more are on tap to collect up to 80,000 barrels a day, according to BP.

"They already have a relatively robust remedial program in place. It's the safe option," said Saleri, now president and chief executive of Quantum Reservoir Impact in Houston.

Wells said the two vessels that had been collecting oil were shut down on Wednesday so all the flow would billow through the top of the well. Relief well drilling was also suspended pending the pressure test, he said.

"As soon as we feel really comfortable with the test, we can go back to those operations," Wells said.

(Reporting by Kristen Hays; Editing by Jerry Norton and Todd Eastham)