BY DAVID J. MITCHELL
River Parishes bureau
October 22, 2012
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The math isn’t adding up in Assumption Parish, and some parish officials are worried about what might be happening underground near a Bayou Corne area sinkhole.
State Department of Natural Resources scientists said they think a subterranean Texas Brine Co. salt cavern failed, allowing earth to partially fill the 2,250-foot-long vault and ultimately cause the sinkhole to form about 200 feet to the northwest of the cavern’s top.
Parish officials say the volume of the brine-filled sinkhole is much smaller than the amount of displaced earth now in the cavern, prompting worries about other unknown subterranean voids or gaps left in the area by the shifted sediments that could lead to further disturbance at the surface.
With this uncertainty as well as other factors — such as the natural gas still trapped underground — officials said the end date of the evacuation of the Bayou Corne and Grand Bayou communities remains unclear.
“Until we figure out what is going on below the surface, it would be very negligent on our part to ask these people to come back into their homes,” Assumption Parish Sheriff Mike Waguespack said.
The evacuation has been in place since the evening of Aug. 3, hours after the sinkhole was found south of La. 70 South on Texas Brine’s leased property.
The evacuation affects about 150 homes.
The Texas Brine cavern was carved from the Napoleonville Dome after years of solution mining and is near the salt dome’s western edge. The dome is a 1-by-3-mile solid salt deposit that has been used for brine production, hydrocarbon storage and oil and gas exploration for many years.
In the Oct. 11 order from state Commissioner of Conservation James Welsh laying out the current theory on the failure cavern and the sinkhole’s formation, Welsh ordered Texas Brine to take a variety of steps to test and monitor the cavern and sinkhole.
One of those involves doing three-dimensional seismic surveying in the area around the sinkhole to assess just the kind of questions the parish officials are asking.
The order, which lays out a series of deadlines for the various tests and monitoring, requires that Texas Brine submit a plan for the seismic work next month.
Sonny Cranch, Texas Brine spokesman, reiterated Friday that the company would comply with the order and said the plan to collect the geophysical data is in the developmental stage.
“We expect to submit our plan on or before Nov. 13, the date specified in the order,” he said.
Other developments also came to light in recent days, company, parish and state officials said:
- Texas Brine removed 1,320 barrels of oil from its failed cavern between Tuesday afternoon and Thursday evening and replaced the oil with an equal amount of brine for cavern stability. The company was testing Friday where the oil and brine are located inside the cavern.
- The company plans to finish a mandated vent well on its property to remove natural gas from the underlying aquifer early this week while scientists working for DNR are trying to get gas to flow from two finished vent wells north of La. 70.
- In the past three weeks, Waguespack and the Assumption Parish Police Jury have sent Texas Brine bills for parish expenses in responding to the sinkhole emergency, totaling $125,000 and $196,000, respectively.
Encompassing more than 4 acres on its surface and 449 feet deep, the funnel-shaped sinkhole has a volume of 550,000 cubic yards, according to the latest estimates from Texas Brine.
But an estimated 3.3 million cubic yards of sediment along with oil and gas flowed into the formerly brine-filled salt cavern after a breach along the cavity’s lower edge.
Scientists have said they believe it is this movement of material into the cavern that released pressure in the compacted earth underground along the edge of the salt dome and caused the sinkhole to surface in the forest swamps on Texas Brine’s leased property.
According to the accounting of Waguespack and Assumption Parish Homeland Security Director John Boudreaux, though, some 2.7 million cubic yards of displaced material — now inside the Texas Brine cavern — are not reflected in the brine-filled space of the sinkhole.
Boudreaux and Waguespack said it is their worry that the possible voids or gaps underground left by the 2.7 million cubic yards of earth that shifted could lead to sinkholes appearing in other locations.
In early August, DNR scientists presented a worst-case scenario for the sinkhole.
Under that model, which was based on a complete collapse of the Texas Brine cavern and a displacement of its entire volume, scientists estimated the existing sinkhole would reach 1,400 feet in diameter and still be far from any residences.
The current failure filled about three-fourths of the cavern’s space with earth. The sinkhole is roughly 550 feet across and has had occasional edge collapses that incrementally increased its surface dimensions, according to figures provided by the parish, DNR and Texas Brine.
Boudreaux said the assumption all along has been that the existing sinkhole would continue to grow, but last week he asked who is to say whether another sinkhole might form somewhere else.
DNR spokesman Patrick Courreges said none of the scientists and officials have seen any indications of that happening, but they are still being cautious until they fully understand the mechanics of the incident.
Assumption Parish Police Jury President Marty Triche said parish officials have to rely on scientific findings to make decisions about the evacuation order and will err on the side of caution and not put people in harm’s way.
He said the evacuation order remains a work in progress as officials consider the status of the vent wells aimed at removing natural gas from underground and find answers about whether any voids are actually underground.
“Obviously, it has been a long time, and still there is no definitive answer yet. The scary part is at what point will we get a definitive answer,” he said. “I am not sure we will ever get that.”