What’s causing the Azle earthquakes? SMU researchers say that wastewater injection and saltwater extraction from natural gas wells is the most likely cause.
A study published Tuesday in the journal Nature Communications says researchers from Southern Methodist University and the U.S. Geological Survey monitored the shaking from nearly 30 small quakes west of Fort Worth from November 2013 to January 2014. The area hadn't had any recorded quakes in 150 years.
The scientists say the shaking decreased when the volume of injections did. They have concluded that removing saltwater from the wells and injecting that wastewater back underground is "the most likely cause" for the swarm of quakes.
Other studies have made a connection between wastewater injections and a spike in earthquakes in Oklahoma and southern Kansas.
The state's official seismologist has no plans for immediate action following the report. Craig Pearson said at a news conference Tuesday that he wouldn't recommend that the Railroad Commission, the state's oil and gas regulator, suspend activity at two wells the report's authors identify as the source of the temblors in Azle.
Pearson, who previously studied induced seismicity at the Los Alamos National Laboratory, was hired last March after residents from Azle and neighboring Reno traveled to Austin to ask the Railroad Commission to halt oil and gas operations.
Pearson said he will review the SMU report and likely offer his opinion next month.
'We haven't seen earthquakes in this area in 150 years'
KERA's Courtney Collins attended a press conference featuring SMU researchers, and filed this report:
When the earthquake research started, SMU scientists zoomed in on three potential causes: oil and gas drilling; changes in the groundwater (translation: drought) or natural earthquakes. Seismologist Matthew Hornbach says that was the easiest to all but rule out.
“We haven’t seen earthquakes in this area in 150 years of European settlement if you will,” he said. “So it’s surprising that they’re starting now.”
Drought was almost as easy to dismiss because the quakes in question began far too deep. So a more likely scenario? Earthquakes caused by man.
“The most probable cause is oil and gas activities in the Azle, Reno area,” Hornbach said. “Primarily wastewater injection as well as brine removal during gas production.”
At a news conference Tuesday, Hornbach and his colleague Heather DeShon would not talk about drilling regulations or anything related to policy. DeShon did say a swarm of earthquakes, even small ones, is enough to get scientists to stand up and take notice because something bigger could be en route.
But she wouldn’t expect to see a catastrophic earthquake in North Texas.
“The faults that have generated earthquakes so far have not had the lengths that would be associated with a catastrophic earthquake,” she said. “But that doesn’t mean a mid-magnitude earthquake wouldn’t cause damage in the Dallas-Fort Worth area.”
That’s because, unlike California, North Texas isn’t built to sustain them.
The most recent cluster of Irving-area earthquakes wasn’t part of this study. SMU is also looking into the quakes that rocked the area near where the old Texas Stadium stood in Irving. When it comes to injecting and extracting water 8,000 feet below the surface of the earth, you can never have too much data.
KERA earthquake coverage
KERA has been covering the North Texas earthquakes over the past couple of years. Catch up on the stories.
Last summer, KERA held a community forum in Azle to discuss the earthquake swarm. Here's a recap of that event, as well as other earthquake stories.
More details on the SMU study
An SMU-led seismology team finds that high volumes of wastewater injection combined with saltwater (brine) extraction from natural gas wells is the most likely cause of earthquakes occurring near Azle, Texas, from late 2013 through spring 2014.
In an area where the seismology team identified two intersecting faults, they developed a sophisticated 3D model to assess the changing fluid pressure within a rock formation in the affected area. They used the model to estimate stress changes induced in the area by two wastewater injection wells and the more than 70 production wells that remove both natural gas and significant volumes of salty water known as brine.
Conclusions from the modeling study integrate a broad-range of estimates for uncertain subsurface conditions. Ultimately, better information on fluid volumes, flow parameters, and subsurface pressures in the region will provide more accurate estimates of the fluid pressure along this fault.
“The model shows that a pressure differential develops along one of the faults as a combined result of high fluid injection rates to the west and high water removal rates to the east,” said Matthew Hornbach, SMU associate professor of geophysics. “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.” Model-predicted stress changes on the fault were typically tens to thousands of times larger than stress changes associated with water level fluctuations caused by the recent Texas drought.
“What we refer to as induced seismicity – earthquakes caused by something other than strictly natural forces – is often associated with subsurface pressure changes,” said Heather DeShon, SMU associate professor of geophysics. “We can rule out stress changes induced by local water table changes. While some uncertainties remain, it is unlikely that natural increases to tectonic stresses led to these events.”
DeShon explained that some ancient faults in the region are more susceptible to movement – “near critically stressed” – due to their orientation and direction. “In other words, surprisingly small changes in stress can reactivate certain faults in the region and cause earthquakes,” DeShon said.
The study, “Causal Factors for Seismicity near Azle, Texas,” has been published in the journal Nature Communications. The study was produced by a team of scientists from SMU’s Department of Earth Sciences in Dedman College of Humanities and Sciences, the U.S. Geological Survey, the University of Texas Institute for Geophysics and the University of Texas Department of Petroleum and Geosystems Engineering. SMU scientists Hornbach and DeShon are the lead authors.
SMU seismologists have been studying earthquakes in North Texas since 2008, when the first series of felt tremors hit near DFW International Airport between Oct. 30, 2008, and May 16, 2009. Next came a series of quakes in Cleburne between June 2009 and June 2010, and this third series in the Azle-Reno area northwest of Fort Worth occurred between November 2013 and January 2014. The SMU team also is studying an ongoing series of earthquakes in the Irving-Dallas area that began in April 2014.
In both the DFW sequence and the Cleburne sequence, the operation of injection wells used in the disposal of natural gas production fluids was listed as a possible cause of the seismicity. The introduction of fluid pressure modeling of both industry activity and water table fluctuations in the Azle study represents the first of its kind, and has allowed the SMU team to move beyond assessment of possible causes to the most likely cause identified in this report.
Prior to the DFW Airport earthquakes in 2008, an earthquake large enough to be felt had not been reported in the Fort Worth Basin since 1950. The North Texas earthquakes of the last seven years have all occurred in areas developed for natural gas extraction from a geologic formation known as the Barnett Shale. The Texas Railroad Commission reports that production in the Barnett Shale grew exponentially from 216 million cubic feet a day in 2000, to 4.4 billion cubic feet a day in 2008, to a peak of 5.74 billion cubic feet of gas a day in 2012.
While the SMU Azle study adds to the growing body of evidence connecting some injection wells and, to a lesser extent, some oil and gas production to induced earthquakes, SMU’s team notes that there are many thousands of injection and/or production wells that are not associated with earthquakes.
The area of study addressed in the report is in the Newark East Gas Field (NEGF), north and east of Azle. In this field, hydraulic fracturing is applied to loosen and extract gas trapped in the Barnett Shale, a sedimentary rock formation formed approximately 350 million years ago. The report explains that along with natural gas, production wells in the Azle area of the NEGF can also bring to the surface significant volumes of water from the highly permeable Ellenburger Formation – both naturally occurring brine as well as fluids that were introduced during the fracking process.
Subsurface fluid pressures are known to play a key role in causing seismicity. A primer produced by the U.S. Department of Energy explains the interplay of fluids and faults.