* BP says "top kill" could happen later than Wednesday
* Scientists testing its viability
By Kristen Hays
HOUSTON, May 25 (Reuters) - BP Plc (BP.L) will begin a process to plug a leaking undersea oil well on Wednesday at the earliest, but it could be delayed or even abandoned if tests show it would not work, a company executive said on Tuesday.
"In terms of when the actual kill might go forward, the earliest would be tomorrow and it could extend on from there," BP senior vice president Kent Wells told reporters on a conference call, referring to the "top kill" procedure.
Under intense pressure from the Obama administration to plug the five-week-old gushing leak in the Gulf of Mexico, BP sought to manage expectations of its latest effort.
The company has failed to plug or completely corral the leak that burst after a rig drilling the well a mile beneath the water's surface exploded and sank, killing 11 workers.
President Barack Obama and U.S. Interior Secretary Ken Salazar have publicly scolded BP for a breakdown of responsibility and missing deadlines in sealing the well.
BP officials had said the top kill, which involves injecting heavy drilling fluids twice as dense as water into the well to stop the oil flow, would begin last Sunday at the earliest.
They subsequently pushed its start to Tuesday, then Wednesday, and Wells said it might start later as scientists finish tests to gauge its chances of success.
"In terms of timing, the pace at which we're doing this -- subsea construction -- we usually spend months to do what we've done in days and weeks," Wells said. "We have to be careful in terms of setting expectations."
MAY ABANDON KILL ATTEMPT
Wells said the tests may prompt BP to abandon the top kill altogether if scientists determine it can't be done safely or will worsen the leak.
"What we learn during this diagnostic phase will be crucial to us," he said.
Russell Hoshman, a petroleum engineer with the Interior Department's Minerals Management Service, said the agency is reviewing procedures to ensure they are technically sound so as to "not make this situation worse."
Wells said the 12- to 24-hour diagnostic phase would take place "over the next day or so." If given the go-ahead, the top kill could take half a day to two days to show results, he said.
The top kill involves injecting drilling fluids, which are heavier than oil, into the failed five-story blowout preventer at the seabed, at the rate of 50 barrels (2,100 gallons) per minute. The tests are supposed to show which of the five points of entry into the blowout preventer can be used.
The biggest risk in the procedure is that the upward pressure of the oil and gas rushing from the well would overcome the downward pressure from the mud and blow it out the top of the blowout preventer, BP executives have said.
Wells said some oil could get past the fluids and escape, but the concept is to pump them fast enough to overcome the oil and kill the well.
"JUNK SHOT" OPTION
If the fluids aren't enough, BP could employ a "junk shot," or pump solid materials like shredded rubber golf balls as a "bridging agent" to slow the oil flow and allow more fluids down the well.
If those options don't work, BP can remove the bent pipe coming out of a piece of equipment on top of the blowout preventer and place a containment dome with a seal on top of it to corral the oil. The oil would be transported by pipe to a drilling ship at the surface.
BP tried such a containment dome over the leak before. Too much seawater inside mixed with natural gas coming from the leak and formed ice, known as hydrates, which blocked oil from flowing to the drillship. Wells said the seal should reduce seawater to cut potential for hydrates to form. (Reporting by Kristen Hays and Chris Baltimore; Editing by David Storey)