Officials reported progress in the latest effort to place a cap over the well that would funnel at least some of the oil and gas to a ship at the surface.

As engineers made headway on Thursday in containing the oil leak at the bottom of the Gulf of Mexico, crews on two floating rigs flanking the spot where the Deepwater Horizon exploded and sank were doing what rig crews normally do: drilling wells.

The two wells, aimed at the bottom of the runaway well that has spewed millions of gallons of oil into the gulf, represent the most conventional solution to the disaster and the one that experts say is all but certain to succeed. Once either of the relief wells strikes pay dirt, the plan is to pump heavy drilling mud and cement down it to bring the blowout under control and permanently seal the damaged well.

Officials reported progress on Thursday in the latest effort to place a cap over the well that would funnel at least some of the oil and gas to a ship at the surface. Earlier, 20-foot-long shears were used to snip the damaged riser pipe at the wellhead, and technicians began to lower the cap over it.

Late Thursday, Adm. Thad W. Allen of the Coast Guard, who is commanding the federal response to the disaster, announced that the cap had been put in place, but warned that “it will be some time before we can confirm that this method will work and to what extent it will mitigate the release of oil into the environment.” Among the concerns was that the cap would not fit tightly and would allow seawater into the oil. That could lead to the formation of icelike hydrates that could block the flow.

Live video feeds from the sea bed appeared to show oil spewing from valves at the top of the cap, as planned. As oil gradually begins to flow up through a pipe to the drillship, these valves would be closed. “It's looking hopeful,” a BP spokesman said.

Also late Thursday, President Barack Obama told Australian and Indonesian leaders that he was cancelling a trip to those countries planned for later this month. White House officials had indicated earlier in the day that Mr. Obama might cancel the trip because of the oil spill.

Given the string of engineering problems so far — the shears were used, for example, because a diamond-laced wire saw had become dull, perhaps from objects pumped into the well as part of an earlier failed effort — the relief well plan has faced its share of scepticism. Doubters have pointed to past problems with relief wells, including one drilled during a blowout off southern Mexico 30 years ago that was unable to stop the gusher for three months after it was completed, and another off Australia last fall that did not hit its target until the fifth try.

BP officials say that the first relief well already extends more than 12,000 feet below sea level, about halfway to the target, but because drilling gets slower as a well gets deeper, it is not expected to be finished before August. The second well was started later and is not yet as deep. President Obama said federal officials ordered BP to drill the second well as a backup shortly after the rig exploded on April 20; the company said it was planning two wells anyway.

The wells cost about $100 million each and are being drilled from rigs owned by Transocean, the company that owned the Deepwater Horizon.

The work could be delayed by hurricanes or by equipment or drilling problems, and the wells might initially miss the target, causing further delays as the drill bits are backed up and redirected. But BP officials and outside experts say that the relief wells will work. It is a matter of when, they say, not if.

“This is the answer,” said Walt Warchol, a retired drilling engineer in Houston. “It's just going to take some time.” The previous engineering efforts at the sea bed have felt like a mash-up of “Groundhog Day” and “Armageddon” without the Hollywood ending, and each operation has come with the caveat that it has never been tried a mile under water.

By contrast, relief wells are within an engineer's comfort zone. They have been drilled many times in many places, and except for having a target, are not much different than normal oil wells. And engineers have drilled plenty of normal wells in 5,000 feet of water.

The wells start out on the vertical, but then are drilled at an angle to intersect the existing well, which was drilled vertically. This directional drilling, which uses steerable drill bits and other specialised equipment and software, is commonplace, although with relief wells, engineers must take care to create gentler turns to allow more efficient pumping of the mud used to stop the blowout.

Although the drilling process involves locating the original well's bottom casing — a 7-inch-diameter steel pipe roughly 18,000 feet below sea level — it is not a typical needle-in-a-haystack problem. Because the existing well was constantly surveyed while it was being drilled, engineers know with a high degree of certainty where in the haystack the needle is.

“Essentially, they have every inch of the well measured,” said John Hughett, a petroleum engineer in Dallas who is not involved in the effort.

That certainty, plus the constant surveying that is being done on the relief wells, should make it relatively easy for the engineers to locate the existing well using three-dimensional display software and other tools, Mr. Hughett and others said. “It would sort of surprise me if they don't get it on the first try,” Mr. Hughett said.

An oil industry expert familiar with the relief well effort said the data on the existing well was much better than the information that was available for the Australian blowout last fall. That well was an older one that was surveyed less often, making it harder for the relief well to intersect, said the expert, who spoke on condition of anonymity because he was not authorised to comment.

The technology used in surveying and drilling is also much better now than it was 30 years ago when the blowout at the rig off Mexico, Ixtoc 1, occurred, said Mr. Hughett. He said he drilled some relief wells in the 1970s “where we kind of poked around” and it took time to find the existing well.

BP began drilling the first relief well on May 2 and the second one two weeks later. Last week, the company decided to suspend operations on the second well, saying that some equipment on the rig would be better used in the containment effort. That decision was overruled by the government, and drilling has continued. In the United States, there is no requirement that oil companies drill relief wells pre-emptively as a precaution should an accident occur. — New York Times News Service

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