Here is some more about the new cap and pressure tests.
If the tests on the well show the pressure rising and holding — an indication that the well is intact, with no significant damage to the casing pipe that runs the length of the well bore to 13,000 feet below the seafloor — BP, working with government scientists, could decide to leave the valves closed, effectively shutting off the well like a cap on a soda bottle.See also this diagram for a better idea of how the system is supposed to work.
“The best-case scenario is that pressures rise to the point we anticipate they would,” Mr. Suttles said at a briefing. “We’d likely be able to keep the well shut in.”
On the other hand, the tests could show pressures that are lower than expected, Mr. Suttles said, an indication that the well is damaged. That could mean that oil and gas are leaking into the surrounding rock.
In that case, keeping the cap closed could damage the well further. The valves would have to be reopened, he said, and oil would start escaping from the well again, although much of it, and perhaps eventually all, would be funneled through pipes to surface ships.
A technician with knowledge of the operation said that it was unlikely that the well would be left shut beyond the test period, given the risk that the pressure could eventually cause problems within the well and given that with the new cap BP should soon be able to collect all the oil.
If the valves need to be reopened, there are ships on standby to resume collecting some of the leaking oil. BP was already collecting about 25,000 barrels per day from the well and has additional ships ready to bring that total up to 60,000 barrels. (Oddly, The New York Times refers to this figure as "the current high-end estimate" for the oil flowing from the well even though there are credible estimates a good deal higher than that, probably reported in past Times articles.) Given the way past operations have proceeded, I am not optimistic that BP will be able to keep those valves closed for long beyond the test period.