Barnett Shale

Study links Azle earthquakes to drilling activity

Wastewater injection wells, along with brine production, are the most likely cause of the recent earthquakes in the Azle and Reno areasa published scientific study states. Michael Grizzle of Springtown stands at the door to greet attendants of public meeting in Azle to discuss recent area earthquakes on  January 13, 2014.
Wastewater injection wells, along with brine production, are the most likely cause of the recent earthquakes in the Azle and Reno areasa published scientific study states. Michael Grizzle of Springtown stands at the door to greet attendants of public meeting in Azle to discuss recent area earthquakes on January 13, 2014. Star-Telegram archives

Wastewater injection wells and other processes tied to oil and natural gas drilling are the most likely cause of a rash of earthquakes that hit in the Azle and Reno area northwest of Fort Worth, according to a newly published scientific study.

An article published Tuesday in the science journal Nature Communications states that the 27 earthquakes in that region from November 2013 to January 2014 were in an area where no earthquakes had been reported or felt for 150 years, leading them to link the quakes to nearby wells.

Matthew Hornbach, an associate professor of geophyics at Southern Methodist University and one of the lead authors, said their computer model found that high rates of wastewater flowing into the earth as part of the hydraulic fracturing process put strain onto fault lines.

“When we ran the model over a 10-year period through a wide range of parameters, it predicted pressure changes significant enough to trigger earthquakes on faults that are already stressed,” Hornbach said. “Can we say with 100 percent certainty that these are caused by oil and gas activities? No. But if you look at the breadth of the data available, we’d argue that the most likely cause is oil and gas activity.”

Analysis of lake levels and groundwater variations caused by the recent drought also shows that there were no significant natural “stress changers” associated with the earthquake activity along the faults in the area before or during the earthquake activity, the authors state.

“We can rule out stress changes included by local water table changes,” said Heather DeShon, an SMU professor of geophysics and the other lead author. “While some uncertainties remain, it is unlikely that natural increases to tectonic stresses led to these events.”

The study, written by researchers from SMU, the University of Texas at Austin and the U.S. Geological Survey, is the latest in a string of academic studies that point to a possible link between oil and natural gas drilling and the uptick in seismic activity.

Azle Mayor Alan Brundrett was not the least bit surprised by the study’s findings. His residents complained for years that the state was not doing enough to study the possible link between hydraulic fracturing, or fracking, and the earthquakes.

“It is what we expected all along but it’s nice to get confirmation,” Brundrett said. “Science can’t ever say anything is 100 percent for sure, but they have said that is the only likely cause.”

Texas Railroad Commission Executive Director Milton Riser sent a letter to Hornbach on Tuesday saying the agency “takes very seriously the issue of seismicity” and invited the authors to brief the commission “so they will better understand their conclusions.” The agency regulates the oil and gas industry.

But Craig Pearson, the state seismologist hired by the agency last year in response to citizen concerns after the Azle earthquakes, said during an afternoon conference call with reporters that he had just read the report and wanted to know more about its methodology before acknowledging any link between drilling and earthquakes.

“It raises a lot of questions,” said Pearson. “We’re still asking those questions and are looking forward to a dialogue with the authors.”

Steve Everley, spokesman for Energy In Depth, an oil and gas industry group, said the SMU paper “is a valuable contribution, and the model is a giant leap forward in terms of understanding the issue.”

But he also said the study raises a number of interesting questions, particularly that there are tens of thousands of injection wells that do not induce quakes, or at least ones that were not felt. He also pointed out that the industry worked with the researchers as they prepared the report.

“But as any scientist will likely admit, a model is only as good as its inputs, and it will never provide a perfect explanation,” Everley said. “While the SMU report is a great starting point … there are aspects of the research that deserve a closer look.”

Previous studies suggested link

Seismologists already suspected that hydraulic fracturing — which injects water, sand and chemicals deep into rock formations to free oil and gas — can cause small quakes that are rarely strong enough to register on monitoring equipment.

But fracking also generates vast amounts of wastewater, which is then pumped into so-called injection wells, which send the waste thousands of feet underground.

Concerns about the connection between drilling and earthquakes has been heightened lately with a series of earthquakes in Irving, with much of the activity occurring near the old Texas Stadium site. The exact cause of those quakes has not been determined and DeShon didn’t want to speculate.

Other studies have pointed to a link between wells and quakes, especially in Oklahoma and southern Kansas where there have been more than 950 magnitude 2 or higher earthquakes so far this year, according to the U.S. Geological Survey.

This week the Oklahoma Geological Society said it’s “very likely” that most of the state’s recent earthquakes have been triggered by the subsurface injection of wastewater from oil and natural gas operations. The society investigated dozens of earthquakes in central and north-central Oklahoma.

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 DFW 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 concluding that “injection-triggered earthquakes are more common than is generally recognized.”

Both studies also said it was “plausible” that injection wells triggered the quakes. Frohlich is one of the authors of the article in Nature Communications. DeShon said in 2014 that they hoped additional study would allow them to be more “precise.”

Last year, the Railroad Commission approved rules requiring drillers to provide additional information before sinking injection wells in areas where there has been seismic activity. Drillers seeking a permit are now required to provide information on the history of seismic events.

The rules also allow the commission to suspend or terminate a permit if seismic activity occurs near an injection well.

Not stacking the deck

The researchers deployed five temporary seismic stations in the Reno-Azle area in mid-December 2013 and added another 12 stations in January 2014. During that time, the geological survey reported 27 earthquakes, including two magnitude 3.6 events that were widely felt.

In an area where the seismology team identified two intersecting faults, they used a sophisticated 3D model to measure the changing fluid pressure within a rock formation. The researchers then used that model to estimate stress changes induced in the area by two wastewater injection wells and the more than 70 wells that remove natural gas and significant amounts of salty water, or brine.

The two wells are owned by XTO Energy, a subsidiary of Exxon Mobil, and EnerVest Operating, state officials said.

Hornbach said that they were somewhat surprised by the results.

“Yeah, in some sense, absolutely. Particularly, I was surprised by the complexity of the situation. There’s a lot of things going on,” he said.

Hornbach said the reduced number of quakes might be because of the slowdown in drilling activity, but that they really don’t know why. He said they have noticed that there have been lower injection rates, but that that is “only one side of the equation.”

When asked about the methodology used by the researchers, Hornbach admitted there is a “tremendous amount of uncertainty” with the models. “We’ve drawn these conclusions with limited, but very valuable, data. And we used a wide array of [methods]. We’re not trying to load the deck here, but we’re trying to get the best estimate.”

When asked if there is potential for catastrophic earthquakes caused by the drilling, DeShon said that the occurrence of any earthquake increases the chance for more.

“The faults that we’ve looked at so far are not long enough … don’t have the fault lengths for a truly catastrophic earthquake. That doesn’t mean these faults we know of don’t have the potential to cause an earthquake of a 4 or 5 [magnitude] that could cause damage in an area like North Texas, which has not been built to seismic safety engineering standards.,” she said.

This report includes material from The Associated Press and the Star-Telegram archives.

Max B. Baker, 817-390-7714

Twitter: @MaxbakerBB

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