EDITORS NOTE: THE SPILL IN THE GULF OF MEXICO BACK IN 1979 ALSO FOUND THAT RELIEF WELLS WERE THE ONLY WAY TO SPOT THE GUSHER. SEE PREVIOUS POST YESTERDAY.
* Relief wells ahead of schedule, but date still August
* Experts say proven method to kill leak, but still risky
By Kristen Hays
HOUSTON, June 18 (Reuters) -
A pair of relief wells snaking their way beneath the Gulf of Mexico are energy giant BP Plc.'s (BP.L) (BP.N) last and best hope for choking off its blown-out Macondo well in the near-term.
But after drilling through more than 10,000 feet (3,050 metres) of rock layers a mile (1.6 km) beneath the ocean surface, BP engineers in coming weeks face the most challenging part of the drilling assignment: hitting a target that is the size of a large dinner plate with the drill bit.
The drill is tantalizingly close to the side of the ruptured well -- about 200 feet (61 metres) away -- but U.S. officials say they don't expect the relief wells to be completed before August even though drilling is ahead of schedule, BP and Coast Guard officials said.
"The next section of the hole is more challenging," Kent Wells, BP's senior vice president of exploration and production, told reporters on Friday.
Nansen Saleri, former head of reservoir management for Saudi Aramco, say BP's relief wells will eventually succeed in killing the well, which has spewed the equivalent of over eight times the oil spilled in the 1989 Exxon Valdez disaster.
"It's only a matter of time. They are going to get it under control and they are going to stop the flow completely," said Saleri, now chief executive of Quantum Reservoir Impact in Houston.
Once the relief wells tap into the stricken Macondo well and begin pumping heavy drilling fluid, or "mud," it
will be a matter of days before the flow will decrease substantially, and then stop, Saleri said.
At that point, BP can pump in cement to permanently cap the well from the bottom.
Relief wells are a proven industry standard to kill blowouts. Nine months after the Pemex-operated PEMX.UL Ixtoc well blew out in 1979, a pair of relief wells plugged the leak. That well spewed 3.2 million barrels of oil in the interim.
But relief wells are just as complex and difficult to drill as an original well, said John Smith, associate professor of petroleum engineering at Louisiana State University.
He said BP had complications with the Macondo well, which took longer to drill than expected.
BP's experience with the blown-out well gives the company an advantage over drilling into strata for the first time, but the paths of the relief wells need to be more precisely controlled, he said.
"There are risks as well," Smith said.
Sometimes a relief well misses its target on the first try. Last August an oil well in shallow waters off the coast of Australia blew out at the Mantara field operated by Thailand's PTT Exploration and Production PTTP.BK. The leak spewed for two months until a relief well came close enough to plug it -- on the fifth pass.
Ideally, the relief wells will connect with the blown-out well, the experts said. But it can still do the job if it gets close enough, said Jamal Azar, co-author of "Drilling Engineering" and professor emeritus at the University of Tulsa.
Companies keep detailed data pinpointing the location of wells drilled, so BP would know with near certainty where to aim the relief wells, he said.
BP tried to smother the leak last month with a "top kill," a similar approach that involved pumping thousands of barrels of drilling mud into the failed blowout preventer and down the well. But the mud could not overcome the flow from the top.
If the relief wells intersect the ruptured well at or close to the bottom, drilling mud can attack the leak at its core, Saleri said.
"When you're pumping from the top, you may not get to the bottom," Saleri said. "Here, you start at the bottom. At one point they will pump from the top as well, but first they need to degrade the flow capacity of the reservoir." (Reporting by Kristen Hays; Editing by Xavier Briand)