BP Oil Well in the Gulf of Mexico Declared Dead

The final plugging of the well was a somewhat underwhelming denouement to one of the great engineering challenges in modern times. After a number of missteps, BP was able to seal the well in mid-July with a temporary custom cap.

Once the oil had stopped flowing, experts embarked on a slow, careful, multistep process to ensure that it would be shut in for good: In early August, the seal was improved with a shot of drilling mud and cement from the top. Later, crews swapped out the old blowout preventer — the safety device that failed during the April 20 blowout of the Deepwater Horizon oil rig, killing 11 workers — with a newer, stronger cap.

Throughout the process, Thad Allen, the federal spill response chief, asserted that the well could be considered dead only when the outer ring of the well, called the annulus, was also plugged with cement from deep underground.

After testing to ensure that they would do no harm, crews on Thursday drilled into the annulus nearly 18,000 feet below the ocean's surface, then began filling it with cement to ensure that oil would never again flow from the reservoir below.

Pressure tests were conducted late Saturday night that showed the cement job had been a success. On Sunday morning, Allen declared the well "effectively dead."