So far, the test that started the afternoon of July 15 has showed pressure slowly rising, therefore indicating no damage. If that's the case, a new cap installed July 12 on the wellhead could possibly shut in all oil flow if oil-capture vessels at the surface must disconnect and move when a hurricane approaches.
The first of two relief wells remains on target to intercept and permanently plug the leak by mid-August.
Here is an explanation of the test, BP's oil-capture plans and the relief wells:
* The test began July 15 by closing vents and valves on the cap to shut in all oil flow for the first time since April and to gauge well pressure.
* High pressure above 7,500 pounds per square inch would indicate the pipe and cement in the well remain intact after the blowout.
* Lower pressure at or below 6,000 psi would be a sign oil and gas were leaking from sides of the piping and cement holding the well open and could possibly breach the sea floor.
* Pressure had risen to 6,745 psi by early July 17 and was slowly rising by 2 psi an hour.
* Regardless of test results, the cap is part of a larger, four-vessel oil-capture system planned to be in place by late July that can handle up to 80,000 barrels per day.
* On July 12, BP started up a new oil-capture vessel, the Helix Producer, that can collect up to 25,000 barrels a day.
* Another rig -- the Q4000, installed on June 16 -- collects and burns off an average of 8,000 barrels of oil per day.
*The vessels have a combined oil-capture capacity of about 35,000 barrels a day.
* Both were shut down during the pressure test.
* Along with the new cap, the Helix Producer is part of the upcoming four-vessel oil-capture system that is hurricane-ready.
* By the end of July, the Toisa Pisces, a well-testing ship revamped to process up to 25,000 barrels a day, will replace the Q4000. Like the Helix, the Toisa Pisces will be hooked up to the failed blowout preventer atop the well via a hose and pipe.
* Transocean Ltd's Discoverer Enterprise, which had collected crude via the containment cap removed on July 10, and a second drillship, Transocean's Discoverer Clear Leader, will each be connected to the new cap via drillpipes.
* The four vessels will have a combined capacity of up to 80,000 barrels a day and can disconnect quickly to move out of a hurricane's path.
* If the well test shows high pressure and the well is intact, the new cap could shut in the leak while vessels are disconnected for a storm. But if low pressure indicates well damage, the cap could not shut in the leak while vessels are gone and crude will spew unchecked they return and reconnect.
* To prevent it from interfering with the integrity test, BP on July 13 suspended drilling on the first of two relief wells at 12,840 feet (3,904 metres) beneath the seabed on July 13, or 160 feet (58 metres) from the bottom of the blown-out well.
* Drilling was to resume July 17 on the first relief well, which remains on target to intercept the Macondo well far beneath the seabed in late July.
* Plugging the leak could take until mid-August, depending on how deep the relief well must bore into the stricken well and how many times BP must pump in heavy drilling fluid and cement.
* The second relief well, a backup to the first relief well, has bored 10,961 feet (3,341 metres) beneath the seabed by July 12. It suspended drilling to avoid disturbing the first relief well's use of sensors to find the right intercept target. (Reporting by Kristen Hays in Houston; Editing by Eric Walsh)