Gas trapped near sinkhole

(DAVID J. MITCHELL)   An as-yet undetermined amount of natural gas is trapped in the aquifer underneath the Bayou Corne community, state and parish officials have said.

The area has been rattled by earth tremors, has waterways with gas bubbling to the surface, and is in the vicinity of a 4-acre sinkhole south of La. 70 that has grown larger since its emergence Aug. 3. Bayou Corne’s 150 households have been evacuated since the sinkhole appeared just off the edge of the Napoleonville Dome, a 1-mile-by-3-mile underground salt deposit.

The Mississippi River Alluvial Aquifer is a broad underground belt that tracks the river and flows beneath dozens of Louisiana parishes, but is not a continuously linked formation of ground water.

The aquifer under Assumption Parish is not used as its major water source — Bayou Lafourche is — but some parish officials say they are concerned the odorless, colorless gas underground could accumulate unseen to explosive concentrations if left unchecked.

John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness, said there are concerns the gas can build up pressure under the clay layer that lies above the aquifer.

Once the pressure in the aquifer reaches a pressure greater than 75 to 85 pounds per square inch, the clay layer might not be able to hold back the accumulated gas, according to Boudreaux and geologists.

“And then it could be a problem because you do not know where the weak point would be until after it has already done its thing,” Boudreaux said.

He said contractors started driving casing Saturday for three vent wells in Bayou Corne to get a better idea about what is happening underground, where the gas is located, so the gas can be vented off.

Work also started Friday on installing geoprobes, a type of underground boring device, to monitor gas levels, he said.

In 2001, in Hutchinson, Kan., and in late 2003, in Assumption Parish’s Grand Bayou community next door to Bayou Corne, natural gas also became trapped underground.

The gas prompted evacuations and, in the case of Hutchinson, caused a major downtown explosion, fires elsewhere and two fatalities, according to a Kansas Geological Survey report and Louisiana regulatory files.

Though cautionary examples of what the worst-case scenario has brought, Louisiana Department of Natural Resources officials said the events in Kansas and Grand Bayou are not analogous to what is happening underneath the Bayou Corne community.

DNR spokesman Patrick Courreges said that in Hutchinson and Grand Bayou, the gas was at much higher pressures underground and at the surface than in the Bayou Corne community.

“Those were not slow, steady bubbling events,” Courreges said in an email.

Around the Bayou Corne community, gas had bubbled up from bayous and land-based water wells for two months before the sinkhole appeared.

“Though it is still too soon to draw final conclusions on the source of the gas occurring in the Bayou Corne area,” Courreges said, “it can certainly be observed that it has shown neither flammable concentrations or pressure.”

Various and repeated aerial-, land- and water-based tests for months, in fact, have also not yet found risk of explosive concentrations of the gas, parish and state officials have said.

In late 2003, casing in a well tied to a underground salt storage cavern then leased by then Gulf South Pipeline Co. LP in the Napoleonville Dome leaked gas into the subsurface over the dome. Twenty-eight residents were evacuated on Christmas Day and kept out of their homes for 50 days.

Enough gas was vented off to allow residents to return home without major incident.

Two years earlier, in Hutchinson, Kan., the worst that could happen did.

Hutchinson Fire Chief Kim Forbes said residents saw the roof of the former Decor Party Store do a summersault in the air from the initial gas explosion.

The blast, on Jan. 17, 2001, stemmed from an old uncapped brine well forgotten inside the downtown store that once was part of a turn-of-the-20th century spa.

Seven miles away, propane had escaped, unknown, through a failed well casing tied to the Yaggy field salt storage cavern belonging to Oneok Inc., of Tulsa, Okla.

The propane moved through the underground Milan Limestone formation, which tilted upward from the cavern toward Hutchinson, a city of almost 41,000 then, until the gas found the old well.

An overhead furnace in the party store provided the ignition source, Forbes said. The explosion set fire to neighboring Woody’s Furniture Store, also part of the old spa.

The gas continued to move under the town, finding other old wells in an area once heavily solution-mined for salt, sparking a trailer fire that killed two people the next day and shooting geysers of water 100 feet high.

Meanwhile, DNR and Texas Brine Co. officials have spent weeks drilling into and now plumbing the depths of an underground Texas Brine salt cavern 200 feet to the southeast of the slurry hole near Bayou Corne.

They are trying to see if the cavern inside the Napoleonville Dome is the source of the 475-foot-wide sinkhole.

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