BP is a few hours away from beginning an integrity test of its leaking Macondo well Tuesday afternoon that will determine whether it's still feasible to shut off the flow of oil into the Gulf of Mexico from the top of the well, or if all hopes for stopping the gusher should continue to be pinned on the ongoing relief-well effort.

The integrity test involves a capping stack that was placed atop the leaking well riser Monday by robotic subs. The stack has three rams inside that, when activated, are supposed to block oil from flowing through the stack into the Gulf. For the duration of the test, which will last a minimum of six hours and could extend up to 48 hours, the three-ram capping stack will be closed, and all efforts to collect oil via the lower marine riser package (LMRP) cap installed June 3 and the Helix Producer containment system activated Monday will be stopped temporarily to isolate the pressure to the main well.

The test is a complicated procedure that involves, in essence, choking off the leaking well with the cap and measuring the resulting pressure of the oil coming up through the well, Kent Wells, a BP senior vice president for exploration and production, said Tuesday during a press briefing. BP will receive pressure readings every 12 seconds from sensors at the well. If the oil pressure is too low, it's a safe bet that pressure is being relieved elsewhere through another leak or leaks. "If the pressure's not building," Wells said, "that would raise questions, and we would choose to open [the cap] at some point."

The higher the pressure, the better, because that indicates the leaking well still has a high level of integrity and is not leaking further down below the Gulf's seafloor. Despite the relief-well efforts, choking off the top of the leaking well remains the fastest option for stopping the oil pouring into the Gulf. The best-case scenario is that the integrity test confirms that the new cap on the well can close off the leak a few weeks in advance of the first relief well, which would continue to be drilled and would be used to permanently kill the well.

Drilling of the first relief well continues, according to Wells, who said that it has reached a depth of 5.4 kilometers below the Gulf's surface. BP hopes by Sunday to set the final drill casing (to stabilize the borehole) and perform another round of sensing to ensure the relief well is still en route to intercept the leaking well. The final 30 to 45 meters of drilling should be completed by the end of the month. Once BP is able to tap into the damaged well (this is not an easy thing to do and may take several attempts), the drill crew will begin pumping in mud to kill the well, a procedure that could take a couple of weeks. BP has halted drilling of the second relief well so that it doesn't interfere with the first relief-well efforts, Wells said.

The integrity test came as the federal government issued a new moratorium on Gulf drilling that will extend until November 30 and affects the same drill rigs as before, although it is based on types of drilling technologies rather than on water depths, as the old one was.

Image of capping stack being lowered into the Gulf © BP p.l.c.