Researchers say finding cause of Azle quakes will take time

02/07/2014 6:18 PM

02/07/2014 8:41 PM

It could take a year or longer to nail down whether there is a link between the swarm of small earthquakes around Azle in recent months and nearby wastewater injection wells, researchers at Southern Methodist University said Friday.

The school’s new network of seismic activity sensors in northeast Parker County has detected numerous small earthquakes recently, said lead researcher Heather DeShon, an associate professor of geophysics.

But previous studies of quakes at Dallas/Fort Worth Airport and near Cleburne took one to two years to be published, and the Azle study could follow that timetable, DeShon said.

In 2009, SMU and University of Texas at Austin researchers began investigating small quakes at DFW Airport that occurred from October 2008 to May 2009. They published their study in March 2010. The quakes stopped after Chesapeake Energy in August 2009 shut down one of two injection wells it operated on D/FW property.

UT Austin researchers reviewed seismic data collected in several locations in the Barnett Shale between November 2009 and September 2011. Cliff Frohlich, senior research scientist at UT’s Institute for Geophysics, released his study in August 2012. He concluded that “injection-triggered earthquakes are more common than is generally recognized.”

Both studies also said it was “plausible” that the injection wells triggered the quakes, but DeShon said she hopes the latest study can be more precise.

“We want to get to a point where we can say, ‘This particular well affected this seemingly dead fault’,” she said. “But first we have to gather the data.”

The area around Azle and nearby Reno in northeast Parker County has been shaken by more than 30 small quakes since November. That sparked a community meeting attended by more than 850 area residents and led the Texas Railroad Commission to vote to add a seismologist to its staff.

On Jan. 21 more than 30 residents from the area traveled to Austin to ask the Railroad Commission to consider shutting down injection wells as a precaution. The agency, which regulates the oil and gas industry in the state, said it didn’t have enough information to do that.

Agency officials told the residents that it had inspected 11 injection wells in the area, one of which was being repaired for a pressure problem at the surface. But the others were complying with their permits regulating maximum pressure and volume, they said.

On Friday, Azle Mayor Alan Brundrett did not immediately return a phone call seeking comment on the study update.

12 sensors in place

SMU released some data from five sensors provided by the U.S. Geological Survey and seven additional sensors its researchers have installed. On one day, Jan. 28, the sensors recorded more than a dozen seismic events, but only one was large enough to be reported to the National Earthquake Information Center, DeShon said.

“Quakes are still continuing out there,” she said, and even the smallest detectable events provide data for the study. The more data, the more accurately researchers can pinpoint the quakes, she said.

The existing network of USGS seismic monitors in Texas can locate the epicenter of a quake only to within about 10 kilometers, or roughly 6 miles. With 12 sensors in just a small area, the SMU researchers should be able to locate an epicenter within “a few meters.” That would allow researchers “to see how large the fault’s dimensions are” and ultimately predict when a well might be close enough to a fault to trigger movement.

Earthquake researchers say injection wells, which pump fluids deep underground at high pressure, can trigger quakes by lubricating existing faults. When the liquid seeps into the fault that already has stress on it, it can reduce friction enough to allow the fault to move.

“The vast majority of injection wells in the United States are not associated with earthquakes. The question is why one and not the other?” DeShone said. “This is going to be a unique, well-instrumented earthquake study,” although she said she can’t predict whether it will collect enough data.

“We still have to do the science,” she said.

Researchers are also working with public and private sources of geophysical data that would more accurately release existing faults in the area, she said.

Oil and gas explorers “aren’t going to drill into the Barnett Shale or the Ellenberger [a huge saltwater aquifer below the Barnett] without collecting huge amounts of subsurface data,” she said.

“We’re still working on agreements” to obtain that data, which would become public if it is included in the study, she said.

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