But problems with drilling a well nearby can exacerbate the seeps and cause greater flow of oil, which can be hard to control, said George Hirasaki, a Rice University engineering professor who was involved in the Bay Marchand oil containment effort for Shell off Louisiana in the 1970s.
"Anytime there is movement of fluids, even if it didn't go to the surface of the well, the internal flow could result in the fluid going somewhere else," Hirasaki said. "It could move laterally at the same depth or increase the flow rate of natural seeps that are connecting to the surface."
Investigators will want to look at whether the weight of the mud being used during the drilling and abandonment operations was sufficient to contain the pressure inside the well, and they will also want to see whether drilling too deep caused problems in a geopressure zone beneath the seafloor, experts said.
Ed Overton, a Louisiana State University environmental sciences professor, said that to truly control the leak could be difficult.
"If you have this stuff oozing up through the ground you don't have a mechanism for control," Overton said. "If something started that to leak, that would worry me a lot more than a leak around the well. You'd have to drill a relief well and intercept that ooze."
People familiar with last year's BP oil spill off Louisiana know about relief wells.
BP spent four months drilling a relief well that it used to pump cement under the area that was spewing oil into the Gulf of Mexico, and sealed the leak permanently.
Experts said that while there are many physical differences between the BP spill and the Chevron spill, the main common thread is the slow flow of information and different explanations for what happened and the severity of what happened.
"There's a pretty long track record of all the people involved in spills underestimating at least initially the size of the spills," Overton said. "I would suspect they literally don't know, so they are trying to figure out."
The Chevron leak is smaller than those Brazil has seen in the past.
In 2000, crude spewed from a broken pipeline at the Reduc refinery in Rio de Janeiro's scenic Guanabara Bay, spewing at least 344,400 gallons into the water. Just a few months later, more than 1 million gallons of crude burst from a pipeline state-controlled oil company Petrobras into a river in southern Brazil.
Brazil's worst oil disaster was in 1975, when an oil tanker from Iraq dumped more than 8 million gallons of crude into the bay and caused Rio's famous beaches to be closed for nearly three weeks.
Associated Press writer Bradley Brooks reported this story in Rio de Janeiro and Harry Weber reported from Atlanta, Georgia.
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