Geologist explains how breach in cavern wall allowed ‘frack out’
PIERRE PART — Shaw Environmental and Infrastructure scientists offered residents Tuesday the most detailed explanation yet about how they believe a Texas Brine Co. salt cavern failed and caused the formation of a sinkhole this summer in northern Assumption Parish.
Shaw geologist Gary Hecox explained to more than 100 residents during a community meeting in St. Joseph the Worker Catholic Church hall in Pierre Part how a breach in the side wall of the cavern inside the Napoleonville Dome allowed sediments into the cavern.
The solid salt dome is a 1-by-3-mile salt deposit formed from the evaporation of ancient seas in areas now covered by thousands of feet of sediment. The sinkhole’s emergence prompted an evacuation order of 150 homes Aug. 3 that remains in place.
Enough earth flowed into the formerly plugged and abandoned cavern to squeeze the brine inside the cavern and raise its pressure so that a “frack out” of the cavern occurred, extending to the surface.
Hecox said a frack out happens when hydraulic pressure is raised enough to cause cracks that serve as channels to release the pressure.
The frack out brought up brine, oil and natural gas from surrounding natural formations along the salt dome’s edge to the overlying water aquifer and to the surface.
The movement of the compacted earth into the cavern created instability that led to the sinkhole, he said.
When pressed by Parish Police Jury President Martin “Marty” Triche and some residents, Hecox also addressed concerns that the volume of the sinkhole is far less than the volume of material believed to have filled the cavern.
The sinkhole is a brine-filled geological feature about 550,000 cubic yards in size. The failed cavern in the salt dome has about three-fourths of its volume filled with 3.3 million cubic yards of material, according to the latest estimate from Shaw.
The discrepancy has led to fears that additional subsidence events or sinkholes could occur away from the existing sinkhole between Bayou Corne and Grand Bayou and south of La. 70 South.
Hecox presented a scenario taking into account all the unaccounted-for material that demonstrated the present sinkhole would grow to 1,500 feet in diameter and remain far from any residents. The sinkhole is 550 feet across.
Hecox said the weakening areas of earth around the sinkhole’s rim are moving west and conform with this theory.
He also said survey elevations taken of La. 70 South that show it has not subsided support this conclusion.
However, he also told residents that scientists could not know what was actually happening in the collapse zone until seismic surveying work is done.
Louisiana Office of Conservation Commissioner James Welsh has ordered Texas Brine to conduct that work.
When pressed by residents, Hecox said the standard he is using to determine whether it is safe for evacuated residents to return to Bayou Corne is whether he would tell his children and grandchildren to go back to Bayou Corne.
He said he would not tell them that at the present time.
“Right now, the evacuation order is appropriate,” Hecox said.
In other developments, Texas Brine officials provided updated and increased totals on the amount of crude oil removed from the cavern and the sinkhole’s surface since an observational well pierced the cavern weeks ago.
Mark Cartwright, a Texas Brine vice president, said about 2,800 barrels of crude oil were removed from the cavern and another 600 barrels skimmed from the surface of the sinkhole.
Cartwright added that it appears all the recoverable oil had been removed from the cavern at this point.
In previous statements, Sonny Cranch, spokesman for the Houston company, has said the observational well drilled into the company cavern has been shut in, but would be opened periodically this week to bleed off any oil left inside the well bore and to see if additional crude oil enters the cavern.
The marketable crude was hauled from the site under a reclamation agreement with PSC Industrial Outsourcing Inc., a DNR-approved contractor.
Proceeds from sale of the oil go to the original owner of the property where the cavern is located, Cranch has said. Texas Brine leases the site from Occidental Chemical Corp.