BAYOU CORNE — More than three weeks since crews began driving well casings into the ground to remove natural gas trapped under the Bayou Corne area, state officials and their contractors have not struck paydirt and, as a result, face uncertainty about just how much gas is underground, authorities said.
Removal of the gas is one step, parish officials have said, that could ultimately lift the evacuation order clamped on 150 homes in the Bayou Corne community shortly after a giant sinkhole emerged in early August.
At high enough pressures, the fear is that the colorless and odorless gas could escape from an aquifer through solid earth, collect unseen and present an explosive risk, though those kinds of levels are not suspected, parish and state officials have said.
The area has been evacuated for nearly three months since the sinkhole, now spreading across 5.5 acres at the surface, was found in the swamps east of the community on Aug. 3.
The large underground salt formation adjacent to the sinkhole was pushed up vertically from ancient sea beds and, for decades, industry has used the dome for brine production.
The perimeter has also been the focus of intensive oil and gas exploration.
Hollowed from the solid salt formation, caverns left by brine production are often used later for storage of natural gas, butane and other hydrocarbons.
Brine is used for several industrial processes.
The failure of a nearby Texas Brine Co. salt cavern inside but on the edge of the Napoleonville Dome is believed to be the cause of the sinkhole as well as crude oil releases and the natural gas underground and in the sinkhole and bubbling up in area bayous.
While the wells designed to remove the underground gas, called vent wells, have pierced the top of the aquifer believed to hold the gas and have found such gas, openings inside the well casings are clogged with a silty clay, officials said.
These openings — thumb-sized holes called perforations — are meant to provide the necessary connection between the gas-bearing Mississippi River Alluvial Aquifer and the interior of the wells.
The blocked perforations aren’t allowing enough gas from the aquifer inside the vent wells to enable the gas to reach the surface, officials said.
Shaw Environmental and Infrastructure geologist Gary Hecox laid out the problem and what it means for Bayou Corne residents last week during a meeting in Pierre Part.
“One of the problems doing this stuff real time — which this is real time — sometimes you reach a conclusion that subsequent data says, ‘No, you were wrong,’” Hecox told the crowd of more than 100 people.
He said that some days earlier, officials found they did not have high gas pressures in three Shaw vent wells and believed that was a good thing because that meant there was not a lot gas pressure underground.
“I am here tonight to tell you I was wrong on that because what we found subsequent to that, all the perforations in those wells are clogged up, so we were reading low pressure, but it didn’t mean anything because the perforations were clogged up,” Hecox said.
“So ... that’s one of things that was not correct.”
Hecox said he could not say how high the pressure is until there are wells with open perforations in contact with the underground gas.
Three vent wells constructed under the supervision of Shaw and a fourth finished Friday by Texas Brine have run into this problem, parish, state and company officials said Friday.
Patrick Courreges, Louisiana Department of Natural Resources spokesman, said well logging equipment had indicated gas was in the aquifer around the Texas Brine well, but the gas did not flow into the well after the perforations were made.
Shaw is working on a fourth vent well near where an earlier surface probe hit gas south of La 70 South and also has plans to try to unclog the existing wells, Hecox has said.
Despite the questions about the extent of the gas, Shaw scientists have been able to determine that some of the natural gas bubble sites in area waterways are from natural decomposition and not underground formations, including some found at more remote sites nearer to Pierre Part, a Shaw map shows.
Other bubble sites around the sinkhole are from underground formations while a few sites involve a mix of gas from formations and natural decomposition.
Shaw has also set up 18 Geoprobes, another shallow-duty type of well, around the vent wells and in the community to test for gas pressure in the upper strata. The first round of tests from probes are pending and at least six more probes are planned by Texas Brine.
In addition, Shaw is now making weekly, 30-second videos of the bubble sites in area waterways to get a better sense of what is happening with the bubbling as time elapses.
Sufficiently high pressures of underground gas can present a risk and have prompted evacuations in the area around the Napoleonville Dome in the past.
In nearby Grand Bayou, east of the sinkhole, natural gas was released in late 2003 into the same aquifer from a Gulf South Pipeline Co. cavern.
Fears of unseen gas collection led to a 50-day evacuation of 28 people and the installation of 36 vent wells that removed 375 million cubic feet of gas.
The gas involved in the Grand Bayou incident came from a failed well tied to storage caverns holding gas under high pressure.
Scientists think the gas under Bayou Corne stems from gas occurring naturally in underground pockets along the salt dome flank that the Texas Brine cavern failure unleashed into the aquifer, the sinkhole, area waterways and the cavern itself.
In late August, aerial testing of the Bayou Corne and Grand Bayou areas showed that gas was not coming directly out of solid land, according to a report.
“While the pressures of the natural gas accumulations in the aquifer near the observation/vent wells have not yet been directly measured, they are believed to be much lower than the pressures observed during the 2003 Gulf South facility natural gas event,” Courreges said in an email.
But the vent wells used to remove gas in Grand Bayou in 2004 did run into a similar problem of silting in, or clogging, as gas pressures diminished, according to regulatory filings with the Louisiana Department of Environmental Quality. Regulators agreed to halt venting once the gas was reduced to a minimum level due to these problems.
Courreges said the problem with the vent wells this time appears to be not only the silty nature of the top of the aquifer but also the low pressure of the natural gas. The aquifer’s tendency to clog makes it a poor medium for producing gas.
“However, the observation/vent wells must target the top of the aquifer, because that is where the natural gas collects,” he said.