No Oil Leaking As BP Conducts Critical Pressure Tests In Gulf Oil Well

New Orleans, Louisiana (CNN) -- A highly anticipated test designed to measure pressure within BP's ruptured Gulf of Mexico oil well finally began Thursday, and for the first time in nearly three months, oil stopped flowing into the Gulf. The move was lauded by a variety of officials as a positive step, accompanied by a strong note of caution that the cutoff was simply part of the test, as BP and government experts try to assess how the well is holding up. The test got under way after two days of delays, first as government scientists scrutinized testing procedures and then as BP replaced a leaking piece of equipment known as a choke line. The data are being particularly closely scrutinized at six-hour intervals. Higher pressure readings mean the well is containing the oil, while lower pressure means some is leaking out. The "well integrity" test could end after one of the six-hour periods, 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 cautioned that the oil cutoff, while welcomed, isn't lilkely to go beyond the 48 hours. Valves are expected to open after that to resume siphoning oil to two ships on the surface, the Q4000 and Helix Producer, as government and BP officials assess the data and decide what to do next. Two more ships are due to join them in coming weeks, bringing containment capacity to 80,000 barrels of oil a day, more than high-end estimates of how much oil had been leaking. "It felt very good to see no oil going into the Gulf of Mexico," BP Senior Vice President Kent Wells said in a briefing. He said company officials are "obviously very encouraged" but they are "trying to maintain a strict focus" on remembering the whole purpose of the test, which is to gather data and decide how to proceed.