If a problem crops up, he said, collection systems could be restarted, some within a few hours. In a few weeks there should be enough capacity to collect more than the high estimate of 60,000 barrels a day. But Mr. Suttles said that if valves on the cap were reopened to restart collection, oil would pour anew into the gulf for up to three days.

Image Workers skimmed for oil in Perdido Pass, Orange Beach, Ala., during a heavy rain storm on Sunday. Credit... Dave Martin/Associated Press

If the well is not reopened, it could mean that the precise volume of oil that leaked — the well has been estimated to be flowing at a rate of 35,000 to 60,000 barrels a day — may never be known. That raises the question of whether the company might escape some liability for the spill.

It has been an encouraging several days for BP, but it comes after many engineering efforts that produced little but a lexicon of strange terms, all defining failure: containment dome, junk shot and top kill among them.

Even the good news about the test and the new cap, which was installed last week, left many wondering why the project could not have happened earlier.

BP has pointed out that the concept — essentially, putting a new blowout preventer atop the existing one that failed when the Deepwater Horizon drill rig exploded on April 20, killing 11 workers — had been in the works since shortly after the disaster occurred. They have said, and diaries and other documents tend to bear out, that ideas were worked on in parallel, with those that were easier to accomplish and had a greater chance of succeeding being tried first.

In a discussion with a reporter in mid-May, Kent Wells, a senior BP vice president in charge of the subsea work, and others described in broad terms an option to install a second preventer if the top kill, in which heavy drilling mud was to be pumped into the well to stop oil and gas from coming up, did not work.