AUSTIN, Texas -- The earthquakes that have struck near Azle and Irving, Texas, are naturally occurring and not related to oil and gas development, scientists for XTO Energy Inc. told state regulators.
XTO, a subsidiary of Exxon Mobil Corp., mounted a full-throated defense of its operations during a hearing called by the Texas Railroad Commission here yesterday, saying there have been earthquakes in the Fort Worth Basin, which lies Azle and Irving, for 600 million years.
"It looks to me like we're in a period of natural tectonism," Andree Griffin, XTO's vice president for geology and geophysics, testified.
It's the first time the Railroad Commission, which regulates oil and gas but not railroads, has held a legal proceeding against an oil company over the possibility of earthquakes. A second hearing is scheduled for next week involving EnerVest Operating Co. The commission adopted rules in the fall that allow it to shut down wells that cause earthquakes.
More than 20 earthquakes struck the area around Azle in 2013 and 2014. A peer-reviewed research paper by geologists at Southern Methodist University said injection wells run by XTO and EnerVest were the most likely cause of the events (GreenWire, April 21).
The area is part of the Barnett Shale gas field, where producers generally use a high-pressure mix of water and chemicals to break up underground rocks and release oil and gas.
The injection wells are drilled into an aquifer called the Ellenberger formation and store wastewater from natural gas production. The XTO and EnerVest wells are located near two underground faults. The SMU paper concluded that injecting fluid into one side of the faults, combined with water production from gas wells on other side of the faults, created enough underground pressure to cause the earthquakes.
The researchers used seismic data provided by the oil industry to rule out natural causes such as the drought or changes in the level of nearby lakes.
Fracking, the process of breaking up rock formations with water, sand and chemicals, hasn't been linked to earthquakes in Texas. But the process creates a disposal problem, since large amounts of the fluid return to the surface.
Scientists have known since the 1960s that injecting fluid near faults can trigger earthquakes. Peer-reviewed studies have linked injection to earthquakes in Oklahoma, Ohio, Colorado and other parts of Texas.
The SMU researchers didn't testify, although the hearing examiner for the Railroad Commission took official notice of the team's paper.
The university researchers said in a statement that they don't want to comment on regulatory policy or on research that hasn't been peer-reviewed.
"We remain confident in the conclusions presented in our peer-reviewed publication, which was based on multiple lines of evidence," the statement said.
Since the Azle tremors ended, other earthquakes have struck in Irving and Venus, Texas. All three communities lie over the Barnett Shale field.
The Railroad Commission has ordered tests on five injection wells near the site of the Venus quakes, but the Irving quakes were about 8 miles from the nearest injection well.
XTO said all the earthquake clusters in the area started in the granite "basement," which is several thousand feet below the Ellenberger formation. XTO's tests showed there was no pressure change in the Ellenberger formation.
Griffin said that indicates that all the earthquakes in the area -- including those in Irving and others in 1950 and 1985 -- have been caused by natural tectonic activity along the deep faults.
EnverVest made a similar argument during a June 5 meeting with the Railroad Commission, saying the earthquakes began thousands of feet below the bottom of its disposal well (EnergyWire, June 8).
Tim Tyrrell, a geophysicist for XTO, testified yesterday that oil and gas activity can cause earthquakes, but not the ones in Azle and Irving.
Under cross-examination, Tyrrell said it's possible for a pressure change in the Ellenberger formation to trigger an earthquake in the basement rock. He also said injecting fluid could cause a pressure change on the faults, which are located 1.5 miles and 2 miles, respectively, from XTO's injection well.
Railroad Commission attorney David Cooney also pointed out during questioning that the Azle earthquakes started around the same time as a spike in disposal volumes and pressure at XTO's well and ended around the same time the well was closed for repairs.
XTO said the well was closed for routine work.
It could take months to get a final decision. The Railroad Commission's hearing examiner will issue a proposed ruling, and both the commission's staff attorneys and XTO will be able to suggest changes, commission spokesman Rich Parsons said.
The final recommendation will go to the three Railroad Commission members, who are elected statewide and act as judges in contested cases.