What It Will Take to Keep the Well Capped

Photos: Click to view a timeline of the disastrous oil spill in the Gulf. Ken Cedeno / Corbis

The spill in the gulf seemed to be under control last Friday. Then came troubling news over the weekend that seepage close to the well could spell big trouble. The government has allowed BP to stay the course for now, but the oil giant has been warned to keep a close eye on the containment efforts. So what exactly are they looking for, and was this weekend's emergency a false alarm?

The potential seep was troubling because it indicated that the cap —which has sealed off the well until a relief well can be drilled—could potentially make the spill worse by exacerbating smaller underground leaks of oil that might eventually rise to the surface and gush into the ocean. Ongoing tests at the well head will try to determine whether that's happening, but doing so will not be easy.

Oil leaks naturally from the sea floor every day, making it hard to tell whether any of the reported leaks are coming from the well itself. Dave Rensink, a geologist who worked for the oil industry in the gulf for nearly four decades, says he's not aware of a definitive test to determine whether a well is leaking underneath the ocean floor. As a jury does, the government and BP will have to weigh the evidence. "I don't know of how you could really determine that beyond a reasonable doubt," Rensink says.

A core piece of evidence is the pressure at the well head. After fitting a cap over the gushing oil, BP closed a series of valves to cut off the flow. Pressure gauges at those valves will now indicate whether the cap can be trusted: if the pressure remains stable or rises steadily, BP officials can be reasonably sure that oil is not bleeding out of the well and could keep the cap closed, explains Rensink, who is also president of the American Association of Petroleum Geologists. But if the pressure falls steadily, alarm bells should sound. "If the pressure starts to bleed off, that would be an indication that [the well bore] is not holding," says Rensink. BP has already observed lower-than-expected pressure, but that could also occur because the oil reservoir is draining. BP and the government will have to decide what is causing the lower pressure and whether they can rule out an underground leak. Adm. Thad Allen, the Coast Guard official heading the federal response, has said that even if the current test is successful, BP will probably remove the cap and test it again before securing it permanently.

During those tests, investigators will be searching for another telltale sign of a leak: oil seeping from the sea floor in the area around the wellhead. "If they start to see bubbles and oil coming up, then clearly there's a leak," Rensink says. On Sunday, Allen reminded BP in a letter [PDF] that "monitoring of the seabed is of paramount importance during the test period." The government has already detected a seep "a distance from the well" and "undetermined anomalies" on the sea floor near the well head, Allen wrote. A report from the Associated Press on Sunday indicated methane may have been detected near the well, a possible indicator of seeping oil. And Allen said Monday that small leaks observed coming from the top of the well cap and the base of the blowout preventer are likely not "consequential."

The seepage could be from a natural leak, and Rensink notes that bacteria in the sediments produce methane when they break down organic material that sinks to the bottom. BP and the government will have to weigh this evidence as well. For the moment, the cap remains. "The small seepages we are finding right now do not present, at least at this point, that there is a threat to the well bore," Allen said Monday. "If we thought that was going to happen, we would be taking immediate action."

If there is a leak from the sea floor, the "Unified Command" will have a decision to make as it waits for a relief well that could permanently stop the leak by mid-August, or possibly earlier. Rensink breaks down the decision this way: BP could remove the cap and continue trying to capture as much leaking oil as possible, or it could leave the cap in place, betting that leaks on the sea floor (which could get worse over time) would release less oil than would leak from the uncapped well. But if leaks around the well are too great, capping it may not be a long-term option. Stay tuned as more evidence emerges.