BAYOU CORNE — A nighttime tremor last week did not have “any discernible effect” on pressure inside a failed Texas Brine Co. salt cavern or natural gas bubble sites in and around a sinkhole believed to have been caused by the damaged cavern, Assumption Parish officials reported.
In a blog post Monday, parish officials detailed an analysis from Shaw Environmental and Infrastructure geologist Gary Hecox about the cavern after the tremor east of the Bayou Corne community.
He reported that Shaw officials reviewed pressure readings inside the cavern and bubble sites following the tremor at 9:06 p.m. Oct. 23.
“The seismic event did not have any discernible effect on cavern pressure or sinkhole bubble activity,” Hecox said. “No new bubble sites have been observed since this event occurred and activity at existing bubble sites has not visibly changed.”
Hecox and other Shaw scientists are working with the Louisiana Department of Natural Resources to investigate the 5.5-acre sinkhole, the nearby failed cavern and the related consequences, including natural gas trapped in a portion of the Mississippi River Alluvial Aquifer under the area.
The 449-foot-deep sinkhole was found early Aug. 3 in swamps between Bayou Corne and Grand Bayou south of La. 70 South on property leased by Texas Brine. The discovery led to an evacuation that same evening of 150 area homes. The evacuation order remains in effect nearly three months later.
University of Memphis researcher Stephen Horton, who is working with the U.S. Geological Survey to track the tremors, has said the tremor Oct. 23 occurred about 1,600 feet underground on the northwest corner of the Napoleonville Dome. Two more back-to-back tremors occurred Wednesday night in the same area.
Though researchers have had a hard time determining the precise depth of the tremors in the area, the estimated location of the Oct. 23 tremor is at a point inside the salt dome but above the cavern.
Beginning at 3,400 feet, the cavern was carved from inside the dome along the large salt deposit’s western edge, in the same general area as the tremors.
In other developments, Texas Brine cleanup contractors had removed 825 barrels of crude oil and 99 large, portable containers of hydrocarbon-contaminated vegetative debris from the surface of the sinkhole through Friday, a state Homeland Security situation summary says.
A breach in the cavern’s wall deep underground released oil and gas from natural underground pockets along the flank of the salt dome into the cavern and toward the surface, Hecox and other scientist believe.
John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness, said a north wind has aided workers with oil skimming in the past week or so, pushing crude to the south bank.
“The north wind is in their favor where it (the oil) all collects where the skimmers are, so they have made some good strides in getting it done,” he said.
Boudreaux said the majority of the surface cleanup is done.
The cleanup has also revealed pre-existing bubble sites inside the sinkhole, including the discovery of a large concentration in the center about a week ago.
“They became noticeable once you started getting some vegetation off,” Boudreaux said.
The gathering of oil to one side of the sinkhole has provided some indication that at least the oil could be from a finite source that would not be constantly feeding the sinkhole.
Mark Cartwright, a Texas Brine company president, told residents last week in Pierre Part that workers were able to divide the sinkhole in half with oil retardant boom.
The side where oil was no longer located showed no re-accumulation of oil, Cartwright said.
Also, he pointed out a positive benefit, noted by Hecox last week, of gas bubbling in the sinkhole, as officials work on clogged wells meant to remove the natural gas.
“We believe the sinkhole forms a pretty good relief well, just by itself,” Cartwright told residents.
Along with 3.3 million cubic yards of exterior sediments, 700,000 cubic yards of brine and a sludge mixture of brine and sediment are believed to remain in the cavern.