The sidewall of a Texas Brine Co. LLC salt cavern inside the Napoleonville Dome was dissolved through the dome’s outer edge, bringing the cavern into contact with surrounding sediment layers outside the dome before the cavern failed, an LSU geologist said Friday.
The assessment by Jeffrey Nunn, Ernest and Alice Neal professor of geology and geophysics and Pereboom professor of science, was a step beyond what other officials and scientists working on the sinkhole response have presented in public meetings and appeared to more closely lay the blame with Texas Brine, which mined the cavern by dissolving the salt.
Nunn spoke about factors leading to the sinkhole Friday during a luncheon talk at Mike Anderson’s restaurant to the Baton Rouge Geological Society.
Other scientists have described the Texas Brine cavern’s sidewall as being close to the edge of the salt dome before the salt wall collapsed thousands of feet underground and allowed 3.3 million cubic yards of earth into the cavern. The failure of the cavern, which is inside but near the salt dome’s western edge, is believed to be the cause of a large sinkhole nearby and methane and crude oil releases in the area.
The sinkhole’s discovery on Aug. 3 between the Grand Bayou and Bayou Corne communities and south of La. 70 South prompted the evacuation of 150 homes in those two communities. The evacuation remains in effect.
Nunn, who has been speaking with a group of scientists working closely on the sinkhole, pointed to 3-D seismic imagery of the salt dome from 2007 to make his case.
“What this indicates is that the bottom part of this abandoned cavern completely dissolved away the salt and the cavern was in direct contact with whatever formation is in the area,” Nunn said.
The seismic data indicates that the western side of the dome has an overhang, or bulge on its upper side, where the salt edge goes out but then curves back inward and possibly intersects with the Texas Brine cavity, exposing it to outer sediments.
One suspicion of scientists working on the sinkhole has been that the overhang, which is above the Texas Brine cavern, collapsed as part of the cavern failure.
Nunn told geologists Friday that one of the scientists’ worst-case fears is that the salt dome could continue to break up from its western edge and threaten other underground caverns.
“We don’t know. That’s a worst case scenario. That’s simply an expressed concern,” he said in a later interview.
Describing what scientists fear, Nunn added: “ ‘OK, you know, this one cavern has collapsed, so is that going to have any impact on the adjacent caverns or not?’ ”
He noted that testing by operators on the dome have showed those other caverns have integrity. He said those tests probably settle those concerns in the short term, adding the fear is probably unlikely but more study is needed to be certain.
The Texas Brine cavern, like many others like it across the Gulf Coast, was hollowed out of a salt dome for brine production with what is called solution mining.
The process uses water to dissolve the salt and create a cavity. The remaining salt around the cavity creates walls, a floor and a roof that provide structural support.
The dome, a solid salt deposit that Nunn described Friday as an underground Mt. Everest, was thrust up over geologic time through overlying sediments.
The cavern had been plugged and abandoned by Texas Brine in June 2011 after an effort to test upper salt strata for mining, company officials have said, led to a compromise of the cavern’s surface access well.
Texas Brine officials disputed Nunn’s explanation of the cavern failure Friday, based on the company’s history with the brine cavern.
“We can state that during the 25-plus years of the life of this cavern, there was never any indication of a loss of integrity of the cavern,” Sonny Cranch, Texas Brine spokesman, said.
He added that Texas Brine never detected from the brine that the company was removing from the cavern that the company had gone through the salt face.
In a later email response to questions Friday, Patrick Courreges, spokesman for the Louisiana Department of Natural Resources, said officials are still studying the Texas Brine cavern failure.
“Analysis of the circumstances surrounding the failure of the Texas Brine cavern sidewall have not yet shown conclusively whether the company mined through the side of the salt dome,” Courreges, who attended Nunn’s talk, wrote.
“This analysis is continuing by experts involved, and information will be provided as it is learned.”