New Orleans, Louisiana (CNN) -- A highly anticipated test designed to measure pressure within BP's ruptured Gulf of Mexico oil well began Thursday after a delay caused by leaking equipment.
A short time later, BP Senior Vice President Kent Wells announced that for the first time in months, no oil was flowing into the Gulf. This was part of the test, as BP measures pressure in the well to see how it's holding. Higher pressure readings mean the well is containing the oil, while lower pressure means some is leaking out.
The data is being particularly closely scrutinized at six-hour intervals, so a key time will occur later Thursday night, after the first six hours.
The "well integrity" test could end after six hours, if the results are disappointing. But it could go on for 48 hours. The longer it goes, the better indications are that the well is holding with a custom-made sealing cap.
BP's stock jumped on word that the oil flow had been cut off, as part of the test. The stock rose $2.74 a share, more than 7 percent, to close at $38.92.
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The cap, lowered in place earlier this week, has never been deployed at such depths or under such conditions and therefeore, there were no guarantees on how well it would contain the oil, BP said.
Earlier Thursday, BP replaced what is known as a choke line after a leak was discovered the day before when the company first attempted the crucial pressure test, said Senior Vice President Kent Wells.
BP plans to close off -- one by one -- the valves on the cap system through which oil can escape, said retired Adm. Thad Allen, the federal government's disaster response manager.
Scientists and engineers will monitor the pressure every six hours and evaluate the situation, Allen said. If at any time, the pressure is deemed too low -- meaning that oil is escaping through another source in the breached well -- the testing will stop, Allen said.
Allen compared low pressure in the well to a leaky garden hose that dribbles out water with your thumb pressed hard on the nozzle, Allen said.
If the pressure readings are sufficiently high, the valves on the stacking containment cap could remain closed and signal a beginning of the end to the catastrophe that began when the Deepwater Horizon drilling rig exploded April 20, killing 11 workers and triggering the relentless spill.
Allen said the cap was not designed to permanently shut in the well -- it was meant to move to a four-vessel containment system and assure redundancy in the event of a hurricane. But he said there could be a huge side benefit if the oil can be contained -- a "twofer," as he called it.
Allen said the more permanent solution to the spewing oil remains the two relief wells BP is drilling and expects to have them finished in August.
BP pumped drilling mud into those relief wells to mitigate risks during the pressure testing. The two wells intersect with the Macondo.
Oil recovery was stopped Wednesday ahead of the integrity test but resumed while BP was fixing the problem with the leaking choke line. It was stopped again with the testing under way.
Wells said BP collected 537,600 gallons of oil Wednesday. Government scientists estimate between 1.5 million to 2.5 million gallons are flowing into the Gulf every day.
A key question over the pressure tests was whether shutting the well was worth the risk, or whether they might cause fresh damage to the blowout preventer.
If it gives them enough time to drill the relief wells then we should be in good shape. I have heard that the wells might actually be up at the end of the month. Four months later and hopefully, we can get some end in sight.
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