The start of BP Plc's "integrity test" came after the US government, which had delayed the procedure for more than 24 hours over safety concerns, gave its final go-ahead on the condition that BP take some extra precautions.
The test could take up to 48 hours and is designed to discover whether a tight-fitting cap placed on the ruptured wellhead late Monday can truly hold down the tens of thousands of barrels of oil spewing from the bottom of the ocean.
BP began by closing a middle ram on the top of the so-called capping stack installation early Wednesday evening. It would then slowly close other valves on the cap, forcing the oil back into the well, and carefully monitor pressure readings with the hope that the pressure continues rising.
US Coast Guard Admiral Thad Allen said the delay was ordered amid concerns about whether the well could withstand the pressure build-up that would be caused by plugging the leak. Engineers fear capping the main leak might expose additional leaks in other parts of the well.
"This is a very significant event and a significant test in the evolution of this response," said Allen, who heads the government's response to the worst oil spill in US history. "If the pressure stays low, that will tell us that the oil may be going somewhere else."
If the test is successful, BP said it hoped the 5.5-metre, 68-tonne cylinder newly installed atop the well will stop all the estimated 30,000 to 60,000 barrels of oil from leaking into the Gulf every day.
But BP Vice President Kent Wells said the unprecedented nature of the test - as with all of BP's previous efforts to stop the leak - meant there could be delays in shutting the valves along the way. A low pressure reading also meant the test could be quickly abandoned.
"If we see that the pressure stays at a low pressure, and is flat or declining, we would immediately open up the well," Wells told reporters. "That would be a very strong indicator that we do not have integrity in the well."
Even if the pressure readings are high, the solution is temporary. As a result, BP is continuing to drill a relief well that it hopes would intersect the existing well shaft at the end of this month, allowing a permanent closure of the well some time in August.
The drilling has been halted until the integrity test is complete. The leak has spewed undersea pollution since the Deepwater Horizon oil rig exploded April 20, causing the biggest spill in US history and laquering the coasts and coastal wildlife of nearby Louisiana and other states.
Since April, a series of attempts by BP to contain the oil flow have failed amid the complexity of capping a ruptured wellhead at unprecedented ocean depths of about 1.5 kilometres.