Cavity near sinkhole almost filled

New tests of a failed salt dome cavern in Assumption Parish show that 97 percent of the 20-million-barrel subsurface cavity is filled with rock and other underground material, Texas Brine Co. officials said Friday.

Scientists studying the Texas Brine cavern inside the Napoleonville Dome believe that once the cavern is completely filled, a large, sometimes rumbling sinkhole near the Bayou Corne community will begin to calm down.

Texas Brine took the newest cavern measurements Tuesday following an active period of tremors in March and after a key access well used for testing was cleared of a salt blockage.

The Texas Brine cavern’s sidewall collapsed at a depth of more than 5,000 feet after the cavern was mined too close to the salt dome’s outer face.

This allowed millions of cubic yards of material from outside the salt dome to enter the cavern near its bottom and rise up inside. The sinkhole emerged in swampland as a result, scientists believe, between Aug. 2 and 3.

About 350 people in the nearby Bayou Corne and Grand Bayou communities have been under an evacuation order since then.

“We expect that the sediment within the cavern, as well as the sediment just outside of the face of the dome, will continue to compact,” Sonny Cranch, Texas Brine spokesman, said in an email Friday.

“As that progresses, there should be a gradual reduction in the seismic activity that’s been recorded beneath the sinkhole in recent weeks.”

Tremors actually preceded the sinkhole’s appearance for at least a few months, scientists have said. But tremors then continued with periodic upswings in frequency sometimes followed with sinkhole burps of gas, crude oil and debris as well as edge collapses that have increased the 160-foot-deep hole’s surface area to about 13 acres.

Patrick Courreges, spokesman for the Louisiana Department of Natural Resources, said Texas Brine’s latest results pointing to continued filling of the cavern seem to mesh with scientific models.

“That is what we expected,” he said.

A panel of experts appointed last month by DNR Secretary Stephen Chustz is working to determine when it will be safe for evacuated residents to return home.

Among its charges, the group will look not only at the long-term stability of the western side of the salt dome where the sinkhole is located but also when gas levels under area communities are safe enough to permit resettlement.

The discovery of methane in an aquifer and even shallower strata under Bayou Corne and Grand Bayou has helped keep the evacuation order in place, along with stability questions related to the Texas Brine cavern and the sinkhole.

Texas Brine’s new depth measurements show that materials pushing upward inside the cavern rose 324 feet between Jan. 31 and Tuesday, according to revised figures provided by company officials. The top of the cavern remains a void to a depth of 123 feet, the figures say.

In measurements taken in 2011, before the sidewall failure, the then-brine-filled cavern’s top was at 3,397 feet and its bottom was at 5,655 feet, stretching 2,258 feet from cavern roof to cavern floor, according to revised figures Texas Brine officials isued Friday night.

A sonar survey of the somewhat conically shaped cavern’s remaining open space estimated that 97 percent of the cavern is now filled, Texas Brine officials said.

Texas Brine officials said the material filling the cavern is most likely dirt, rock and other sediments from just outside the salt dome’s face.

Houston-based Texas Brine is working to vent the gas under the nearby communities, so far burning a total of 12.36 million cubic feet as of Monday, Cranch said.

Under pressure from Gov. Bobby Jindal and facing several lawsuits, the company is also working on buyouts of residents who want to move away.