Kent Wells, a BP PLC vice president, said engineers glued to an array of pressure, temperature, sonar and other sensors were seeing no evidence of oil escaping into the water or the sea floor. Undersea robots were also patrolling the well site for signs of trouble.
A new breach underground was a major concern going into the test, because oil breaking out of pipes in the bedrock would be harder to control and could endanger plans for a permanent plug.
"We're feeling more comfortable," Wells said on a morning conference call, but cautioned: "The test is not over." BP and the federal point man for the disaster, retired Coast Guard Adm. Thad Allen, have said they may decide to reopen the cap at least partly after the 48-hour test period ends on Sunday, although it's not clear what conditions would prompt them to do so.
That call will be made by Allen, Wells said.
BP shut valves in the cap Thursday, stopping the flow of oil into the Gulf for the first time since the April 20 explosion on the BP-leased oil rig Deepwater Horizon killed 11 workers and unleashed the spill 5,000 feet below the sea.
With the cap working like a giant cork to keep the oil inside the well, scientists kept watch in case the buildup of pressure underground caused new leaks in the well pipe and in the surrounding bedrock that could make the disaster even worse.
Pressure readings after 41 hours were 6,745 pounds per square inch and rising slowly, Wells said, below the 7,500 psi that would have shown the well was not leaking. He said pressure continued to rise by around 2 psi per hour, compared to a range between 2 and 10 psi BP and the government provided Saturday. A low pressure reading, or a falling one, could mean the oil is escaping.
Wells said bubbles seen on video feeds from the well were common underwater, but said they would take samples to make sure they aren't gas escaping from the well.
The cap is designed to prevent oil from spilling into the Gulf, either by keeping it bottled up in the well or by capturing it and piping it to ships on the surface. It is not yet clear which way the cap will be used if it passes the pressure test.
Either way, the cap is a temporary measure until a relief well can be completed and mud and cement can be pumped into the broken well deep underground to seal it more securely than the cap.
BP is drilling two relief wells, one of them as a backup.
Wells said work on the first one was far enough along that they expect to reach the broken well's casing, or pipes, deep underground by late this month. Then the job of jamming it with mud and cement could take "a number of days through a few weeks." Even with the cap holding, no one has declared victory - or failure. President Barack Obama on Saturday cautioned the public "not to get too far ahead of ourselves," warning of the danger of new leaks "that could be even more catastrophic." BP and government scientists at sea and in a faraway control room at BP's US headquarters in Houston have been keeping a nerve-wracking vigil around the clock since the cap was shut.
One mysterious development was that the pressure readings were not rising as high as expected.
Allen said Saturday that two possible reasons were being debated by scientists: The reservoir that is the source of the oil could be running lower three months into the spill. Or there could be an undiscovered leak somewhere down in the well. Allen ordered further study but remained confident.
"This is generally good news," he said. But he cautioned, "We need to be careful not to do any harm or create a situation that cannot be reversed." Even if the cap passes the test, more uncertainties lie ahead: Where will the oil already spilled go? How long will it take to clean up the coast? What will happen to the region's fishermen? And will life on the Gulf Coast ever be the same again? There was no end in sight to the cleanup in the water and on shore. Somewhere between 94 million and 184 million gallons have spilled into the Gulf, according to government estimates.
