RELIEF WELL DIAGRAM: The relief wells are drilled straight down into the sea bottom, parallel to the Macondo 252 well bore (in red) for several thousand meters and then angled to intersect the original wellbore at about 4,000 meters below the seafloor. It is expected to take another two months for BP to reach this depth, which is just above the oil reservoir, a subsurface pool of crude and natural gas sandwiched between layers of rock. Image: COURTESY OF BP
BP's efforts to drill relief wells are generally viewed as the company's best, and perhaps only, chance to plug the Macondo 252 well gushing thousands of barrels of oil and natural gas into the Gulf of Mexico each day for the past month and a half. With the first relief well nearly two months away, however, BP has wiped a bit of sweat from its collective brow as the lower marine riser package (LMRP) cap installed June 3 has begun collecting an increasing amount of oil each day, according to the company.
This is a precursor to the two relief wells that BP is drilling on either side of its leaking well. The relief wells are drilled straight down into the sea bottom, parallel to the blown Macondo well for several thousand meters and then angled to intersect the original well bore at about 5,500 meters beneath the Gulf's surface (4,000 meters below the seafloor). It is expected to take another two months for BP to reach this depth, which is just above the oil reservoir, a subsurface pool of crude and natural gas sandwiched between layers of rock. Once the relief wells are drilled, BP can use them to pump heavy fluid and then cement into the main well, clogging the flow of oil near its source.
"When drilling a relief well, you want to get as deep as possible so that you can seal the well close to the [oil] reservoir," says Roger Anderson, an oil geophysicist and a professor at Columbia University's Lamont–Doherty Earth Observatory in Palisades, N.Y. As an oil well is drilled casings are cemented in place at different depths to reinforce the borehole. If a well is sealed too high, any leaks between well casings beneath the seal could still leak oil out into the surrounding rock and up to the seafloor.
Drilling a deep relief well creates a good news–bad news situation. The good news is that the pipe casing closest to the reservoir is narrower in diameter than it is near the top of the well, so it takes less heavy fluid to close the opening. This also means, however, that in addition to taking longer to reach the desired spot, the relief well drillers are trying to hit a much smaller target the deeper they drill.
Still, a relief well is generally considered a safe bet to stop a leaky well. Anderson says relief wells have a 95 percent chance of working. The fact that BP is drilling two relief wells gives the effort a 98 percent chance of success, he adds.
In the meantime, BP's 1,600-kilogram LMRP "top hat" cap—four meters long and 1.2 meters in diameter—has enabled workers to collect more than 57,000 barrels of oil total through June 8, according to Reuters. This is, of course, a drop in the bucket—the wellhead has been spewing as much as 19,000 barrels per day since the Deepwater Horizon rig sunk on April 22. (One U.S. petroleum barrel contains about 160 liters of oil.)
At this time the LMRP cap's riser pipe is emptying directly into a drill ship at the surface. The arrival of hurricane season on June 1 has forced BP to develop a more flexible oil collection system, Kent Wells, a BP senior vice president for exploration and production, said Monday at a press briefing. By the end of next week, BP is hoping to use the hoses and multi–valve manifold connector that it had deployed for the failed "top kill" operation to take oil and gas from the Deepwater Horizon's idle blowout preventer through a separate riser to the Q4000 multipurpose floating rig on the surface not far from the drill ship, Wells said.
If that is successful—and everything thus far has been a big "if"—BP wants to create a more durable LMRP containment cap system by directing the oil and gas to a new free-floating riser ending about 90 meters below sea level (a flexible hose will be used to connect it to vessels on the surface). This option is designed to permit more effective disconnection and reconnection of the riser if a storm blows in and the oil collection ships need to head back to port, Wells said.
Such contingency plans are far from a luxury for BP; the U.S. government has begun to demand better planning from the oil company. U.S. Coast Guard Rear Adm. James A. Watson, the federal on-scene coordinator for the cleanup and containment efforts, Tuesday gave BP 72 hours to build redundancies and fail-safes into its efforts to collect oil and gas from the Macondo 252 well now that the top hat containment system has begun to capture and recover some of the oil escaping from the wellhead. "For example, if multiple oil recovery vessels are employed for collection/recovery efforts, redundancies must ensure that the failure of a vessel(s) does not reduce the capacity of the system for continuous recovery of oil," Watson wrote in his June 8 letter (pdf) to Doug Suttles, BP's chief operating officer for exploration and production. "There should be no interruptions of the recovery effort while awaiting another recovery vessel to arrive on scene."