Well design may be hampering spill efforts
Reduction in thickness of casing raises concern about well’s integrity
By TOM FOWLER and ERIC NALDER
HOUSTON CHRONICLE
June 21, 2010, 10:59PM
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Kari Gooodnough Bloomberg
Seaweed covered in oil from the Macondo well sits on a public beach in Gulf Shores, Ala., earlier this month.
Documents released by congressional investigators show that modifications to the well design BP made last year included a reduction in the thickness of a section of the casing — steel piping in the wellbore
The modification included a slight reduction in the specified thickness for the wall of a 16-inch-diameter section of pipe toward the bottom of the well, according to a May 14, 2009, document.
Retired Coast Guard Adm. Thad Allen, commander of the response to the blowout and oil spill, has confirmed reports that concern about the strength of the casing led officials to stop efforts last month to plug the well from the top by injecting drilling mud and cement in a procedure called a top kill.
Another proposed spill control method, placing a blowout preventer on top of the one that failed in the original April 20 blowout, also was abandoned over concerns about well integrity. A blowout preventer is a system of shears and valves installed as a last line of defense against loss of well control.
The condition of the well also limits how much oil and gas can flow into containment systems now being used successfully to capture some of the flow. Even if a vessel could capture all the hydrocarbons gushing from the well, some would have to be released to keep well pressure under control.
Marvin Odum, president of Houston-based Shell Oil, the U.S. arm of Royal Dutch Shell, told the Houston Chronicle last week that the integrity of the well casing is a major concern. Odum and others from the industry regularly sit in on high-level meetings with BP and government officials about the spill.
If the well casing burst it could send oil and gas streaming through the strata to appear elsewhere on the sea floor, or create a crater underneath the wellhead - a device placed at the top of the well where the casing meets the seafloor - that would destabilize it and the blowout preventer.
The steel casing used in oil wells is strong, said Gene Beck, petroleum engineering professor at Texas A&M, but pressures deep in a well are powerful enough to split strong steel pipe or "crush it like a beer can."
The strength and thickness of casing walls are key decisions in well design, he said. If the BP well's casing wasn't strong enough, it may already be split or could split during a containment effort.
BP spokesman Toby Odone said the decision to reduce the pipe thickness was made after careful review. The company said it doesn't know the condition of the well casing and has no way of inspecting it.
BP is drilling two relief wells to intercept the Macondo well near the reservoir and plug it with cement. A rupture in the Macondo well casing probably wouldn't affect that effort, said Donald Van Nieuwenhuise, director of geoscience programs at the University of Houston.
"When they start the bottom kill the cement will try to follow oil wherever it's escaping, so it would actually hide a lot of sins in the well bore," Van Nieuwenhuise said.
So far there are no signs that the section of the pipe below the sea floor is leaking.
The blowout preventer has been listing slightly since the accident, but officials believe that may have happened when the Deepwater Horizon sank while still attached to the well via a pipe called a riser.
A report from the National Oceanic and Atmospheric Administration on Monday noted research vessels found natural gas seeping from the sea floor several miles away from the well. Those appear to be pre-existing seeps that occur naturally, a NOAA spokeswoman said, and unrelated to the spill.
But the longer the well flows uncontrolled the more likely it is that the well casing could be damaged or the blowout preventer damaged further. Sand and other debris that flows through the pipes at high velocity can wear through metal over time, said Van Nieuwenhuise.
The chances of the well eroding from underneath and the blowout preventer tipping may seem unlikely.
"But everything about this well has been unlikely," said David Pursell, an analyst with Tudor Pickering Holt & Co.
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National Oceanic and Atmospheric Administration research ship Thomas Jefferson returned to Galveston on June 11 from an eight-day research mission in which its crew collected water samples for chemical analysis and tested the the use of acoustic and flourometric scanning to help find pockets of subsurface oil.
The findings from the researchers include:
• Observations of "high fluorescence" and reduced dissolved oxygen anomalies at around 1100 meters depth, 7.5 nautical miles southwest of the wellhead. Laboratory analysis of water samples from this area is underway to help determine if this is an indication of sub surface oil.
• A subtle "acoustic anomaly" in the same vicinity. It's not yet clear what that means, but NOAA says additional analysis of the data from both NOAA Ship Thomas Jefferson and NOAA Ship Gordon Gunter will be needed to make further conclusions. Additional field work is also planned to test this method of using acoustic data to locate underwater oil.
• A device called "The Moving Vessel Profiler" -- which allows data to be collected throughout the water column while a ship is underway -- was equipped with a special fluorometer tuned to crude oil and was used to collect flourometric data from the surface down to about 100 meters deep in shallower water from Mobile, Ala. to Port Fourchon, La. The samples were taken while the boat was underway, with the instrument moving from the surface to the bottom and back to the surface approximately every 1.5 miles.
• Scientists observed several seeps of what appears to be natural gas in an area of known gas seepage, located to the southwest of the spill site.
These natural gas seeps are described as naturally occurring in the NOAA report, but it's such data that appears to be behind the concerns some observers have that the well is damaged below the mud line and oil and gas is seeping into the surrounding strata and finding its way to the surface.
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