BP said its capped-off well appeared to be holding steady Friday morning, almost midway into a white-knuckle waiting period in which engineers watched the pressure gauges for signs of a leak.
Results monitored from control rooms on ships at sea and hundreds of miles away at the company's U.S. headquarters in Houston showed the oil staying inside the cap, rather than escaping through any undiscovered breaches, BP PLC vice president Kent Wells said on a conference call.
Two underwater robots scoured the sea floor looking for signs of new leaks.
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President Barack Obama said BP's capping of the spill was "good news" but cautioned that testing continued.
There was no evidence of a leak in the pipe under the sea floor, Wells said, one of the main concerns. Wells spoke 17 hours after valves were shut to trap oil inside the cap, a test that could last up to 48 hours.
He said pressure continued to rise inside the tight-fighting cap, a good sign that oil was not getting out somewhere else. The pressure was above the minimum they were hoping to see, but not yet in the high range they were hoping for.
"The pressures we've seen so far are consistent with the engineering analysis work that BP has done," Wells said. "It's been a very steady build."
Wells also said work would resume on a relief well, the oil giant's more permanent solution meant to plug the leak for good underground to end one of the nation's worst environmental catastrophes.
That's also a sign that things were going well. Engineers had stopped drilling one of the wells Thursday in case that bore hole deep underground could be affected by the oil cap effort.
Engineers and scientists continue to monitor the cap's pressure. When the test is complete, more sea floor mapping will be done to detect any damage or deep-water leaks.
BP finally stopped oil from spewing into the sea Thursday for the first time since an April 20 explosion on the BP-leased Deepwater Horizon oil rig killed 11 workers and unleashed the spill 5,000 feet (1,500 meters) beneath the water's surface. The well has spilled up to 184 million gallons (697 million liters) since then.
The accomplishment was greeted with hope, high expectations — and, in many cases along the beleaguered coastline, disbelief. BP Chief Operating Officer Doug Suttles urged caution and warned the flow could resume, saying it wasn't a time for celebration.
It's not clear yet whether the oil will remain bottled in the cap after the test, or whether BP will use the device to funnel the crude into four ships on the surface.
BP said the decision on whether to reopen the well after the test would be made by the government's national incident command, run by retired Coast Guard Adm. Thad Allen.
The cap is a temporary measure. Even if it holds, BP needs to plug the gusher with cement and mud through a relief well deep underground, where the seal will hold more permanently than any cap from above could.
The 48-hour watch period started at 3:25 p.m. EDT (1925 GMT) when the last of three valves in the 75-ton cap was slowly throttled shut.
It came after repeated attempts to stop the oil — everything from robotics to different capping techniques to stuffing the hole with mud and golf balls. The week leading up to the moment where the oil cloud ended was a fitful series of starts and setbacks.
BP officials have said repeatedly they were right to take a step-by-step approach to trying to shut off the geyser over the last three months, to make sure they didn't make the disaster worse. They have also pointed out that the current cap system in place took time to design and build and to make sure it could withstand the massive water pressures a mile below the sea.
BP removed a previous, looser cap last weekend, at which point oil flowed freely into the water. Robotic submarines swarmed the site to unbolt a busted piece of pipe and install a connector atop the spewing well bore — and by Monday the cap, a stack of lines and valves, was latched onto the damaged well.
After that, engineers spent hours creating a map of the rock under the sea floor to spot potential dangers, like gas pockets. They also shut down two ships collecting oil above the sea to get an accurate reading on the pressure in the cap.
As the oil flowed up to the cap, two valves were shut off like light switches, and the third dialed down like a dimmer switch until it too was choked off. And just like that, the oil stopped.
The Gulf Coast has been shaken economically, environmentally and psychologically by the hardships of the past three months. That feeling of being swatted around — by BP, by the government, even by fate — was evident in the wide spectrum of reactions to news of the capping.
The fishing industry in particular has been affected by the spill. Surveys of oyster grounds in Louisiana showed extensive deaths of the shellfish. Large sections of the Gulf Coast — which accounts for 60 to 70 percent of the oysters eaten in the United States — have been closed to harvesting.
The saga has also devastated BP, costing it billions in everything from cleanup to repair efforts to plunging stock prices. BP shares, which have lost nearly half their value since the disaster started, jumped in the last hour of Thursday trading on Wall Street after the oil stopped. But they were down again more than 3 percent Friday morning.
Long after the well is finally plugged, oil could still be washing up in marshes and on beaches as tar balls or disc-shaped patties. The sheen will dissolve over time, scientists say, and the slick will convert to another form.
There's also fear that months from now, oil could move far west to Corpus Christi, Texas, or farther east and hitch a ride on the loop current, possibly showing up as tar balls in Miami or North Carolina's Outer Banks.
The National Oceanic and Atmospheric Administration expects to track the oil in all its formations for several months after the well is killed, said Steve Lehmann, a scientific support coordinator for the federal agency.
Once the well stops spewing oil, the slicks will rapidly weather and disappear, possibly within a week, and NOAA will begin to rely more heavily on low-flying aircraft to search for tar balls and patties. Those can last for years, Lehmann said.
"Eh, the damage is done. The oil's everywhere now," he said. "You ain't never gonna get it out of the water."