Expert: Deep gas sources feeding Assumption aquifer

A state seismic expert has found two likely existing “deep” sources continuing to feed methane gas under the Bayou Corne and Grand Bayou communities and more than 90 bubbles sites in the area of northern Assumption Parish.

But officials working on the removal of the gas, a suspected byproduct of the formation of the Bayou Corne-area sinkhole last year, said Tuesday they are focused on cutting off or removing gas in two shallower layers first while they work on how to eliminate these two deeper sources.

During the parish community meeting where the new seismic results were revealed, some residents complained that response officials had “the cart before the horse” by not going after the deep sources first.

Bayou Corne businessman Dennis Landry asked whether drilling to remove the gas from the deep sources was still a viable option as had been discussed at a previous meeting.

Gary Hecox, CB&I hydrogeologist and leading scientist on the sinkhole response, said drilling for the sources is certainly viable.

“The problem you’ve got, is even if we start drilling tomorrow and relieve that gas, there is so much gas in the aquifer you probably would not even notice it,” he said during the presentation to 40 to 50 residents.

Hecox added it may take years for an effect to be seen. He has said the gas in the aquifer is much greater than the 45 million cubic feet once estimated and is collected heavily under the Sportsmans Drive community south of La. 70 in Bayou Corne.

Rising gas unleashed by the failure of a Texas Brine Co. salt dome cavern last year has presented a continuing risk of explosion for the communities that once held about 350 people and remain under a more than year-old evacuation order. The cavern failure is also suspected of causing the 25-acre Bayou Corne-area sinkhole.

Including the deep sources, the methane is essentially in three broad layers under the Bayou Corne and Grand Bayou communities. It is suspected of rising from the deep sources and collecting at the tops of two shallow rock layers like bubbles in a large aquifer at 90 to 125 feet deep and in shallow sands at 25 to 30 feet deep.

Last week, scientists announced plans to use a vacuum system to suck out gas and water from the aquifer and from the shallow sand to keep gas from getting any closer to homes above.

Hecox said Tuesday that officials are still digesting a report on the deep sources and still trying to determine what to do with the help of LSU associate professor John Rogers Smith, who is a petroleum engineer.

Don Marlin, whom the Louisiana Office of Conservation hired to synthesize reams of data collected around and under the Bayou Corne-area sinkhole, said Tuesday the two likely deep sources held a combined 20 million cubic feet of methane under the sinkhole when the seismic data was collected in March.

One was three acres in size at 1,000 feet and another was 10 acres in size at 1,450 feet deep. Both are outside but next to the salt dome and next to a zone of broken rock that tracks along the edge of the Napoleonville Dome.

The hourglass-shaped zone, which Marlin called a “hydraulic highway” and formed as a result of the Texas Brine cavern failure, provides the connection for the gas to rise to the surface and for rock to flow down from the sinkhole into the cavern.

“Where do these (sources) fit in the overall picture? Let’s go to the next step,” Marlin asked.

“The gases that I think are feeding the aquifer and the gases that I think are feeding the bubbling at surface are dominantly coming from three areas,” he said.

Marlin then raised the possibility that in addition to the two deep sources, other small gas reservoirs had existed before the sinkhole but they are no longer visible because of the zone of broken rock.

He also suggested the dissolution of salt from the salt dome and its caprock could be releasing gas naturally trapped in the crystallized salt.

Part of Marlin’s analysis relied upon three-dimensional seismic data collected by Texas Brine in March but he built upon it with more advanced computer processing and other information.

His findings contradict Texas Brine’s earlier interpretation of the data, which suggested the gas in the aquifer was no longer being fed by deep sources.