The U.S. Geological Survey (USGS) turned to Sandia National Laboratories for help when the earth opened up in early August near Bayou Corne, La.
Sandia’s David Borns is providing technical evaluations in weekly
teleconferences about possible causes and remedies for a 300-foot-wide sinkhole
“We try to be of support to adding expertise to federal and local
governments when they’re faced with understanding technical issues that impact
their resources,” says Borns, a geotechnology and engineering manager.
Authorities have been trying to determine whether the sinkhole was
caused by the collapse of an abandoned brine mining cavern along the margin of
the Napoleonville Salt Dome, or by something else. The operator of that cavern
has drilled a borehole into the cavern at a depth of 3,500 feet to learn
whether the cavern is the cause. The results of the drilling will determine
what the technical evaluation committee recommends, Borns says.
The sinkhole opened up overnight on Aug. 2 off the western edge of
the salt dome near Bayou Corne. It reportedly was originally about 300 feet
deep, but Borns says that only one part was that deep; the rest was about 50 feet
“There were some broad impacts to the area,” he explains. “A
nearby community was evacuated, this big sinkhole formed, and it forced the
closure of a two major natural gas pipelines.”
The USGS, known for its seismic expertise, already had been
keeping an eye on the area because of harmonic tremors that began in June,
along with gas bubbling up at seven different locations in the wetlands of
Bayou Corne and nearby Grand Bayou.
“What they were seeing was some sort of fluid movement through fractures,
which they thought might be the natural gas that was bubbling up in the bayou,”
Authorities first thought the source might be a broken pipeline,
but all pipelines checked out. Then they started exploring whether something
was happening within the caprock or surrounding sediments where natural gas
comes from. The harmonic tremors continued for about six weeks, but stopped
after the sinkhole formed. Since then, only small seismic events continue to be
recorded near the cavern under investigation, Borns says.
The cavern was developed for brining operations, in which
companies dissolve salt to extract chlorine for use as a precursor for
petrochemicals, he notes.
On Aug. 22, the Louisiana Governor’s Office of Homeland Security
and Emergency Preparedness formally asked Energy Secretary Steven Chu for help
from Sandia. The Labs previously worked on cavern collapse and sinkhole
formation problems on Weeks Island, La. Sandia experts are called in once or
twice a year to study similar concerns.
USGS had suggested the state of Louisiana include Sandia on technical
conference calls based on the Labs’ expertise in salt and salt caverns. According
to Borns, Sandia began working on salt formations in the 1970s, when it began
investigating the geomechanical response of salt caverns as a potential medium
for underground nuclear weapons testing.