A University of Texas study has found that seismic activity in the Barnett Shale occurred far more frequently than previously reported and that most took place near high-volume injection wells.
The study by Cliff Frohlich, senior research scientist at UT's Institute for Geophysics, was released today and is being published in Proceedings of the National Academy of Sciences.Frohlich's report doesn't find any link between hydraulic fracturing (fracking) and earthquakes. But it is the latest study to raise questions about whether injection wells, where wastewater from fracking and production is pumped back into the ground, cause increased seismic activity.A June report by the National Research Council also said there were higher earthquake risks near injection wells.The UT study reviewed data from temporary seismographs placed in the Barnett Shale under the USArray program between November 2009 and September 2011."I analyzed these data and located 67 earthquakes, more than eight times as many as reported by the National Earthquake Information Center," Frohlich said in his report.Most of the epicenters were "within 3.2 km of one or more injection wells," Frohlich said. "These included wells near Dallas-Fort Worth and Cleburne, where earthquakes near injection wells were reported by the media in 2008 and 2009, as well as wells in six other locations, including several where no earthquakes have been reported previously. This suggests injection-triggered earthquakes are more common than is generally recognized."The injection wells nearest to the earthquakes had maximum monthly injection rates exceeding 150,000 barrels of water per month. Frohlich said.But there wasn't always a correlation.While 9 of 27 such wells in Johnson County were near earthquakes, no earthquakes occurred near other wells with similar volumes."A plausible hypothesis to explain these observations is that injection only triggers earthquakes if injected fluids reach and relieve friction on a suitably oriented, nearby fault that is experiencing regional tectonic stress," said Frohlich, who said this theory would need more study to be proven.Frohlich also noted that some areas of the Barnett Shale, such as Wise County, have had few quakes despite numerous injection wellsEd Ireland, director of the Barnett Shale Energy Education Council, agreed that any correlation between injection wells and seismic activity would need more study, noting that there are more than 50,000 injection wells statewide and most have had no problems reported."I would think a lot of the research would be looking at the flow rates at different injection wells," Ireland said. "There's not necessarily a connection with all wells but there's possibly a connection with specific wells."Frohlich has said he would also like to study seismic activity in the Eagle Ford Shale in South Texas as a basis for comparison.To Libby Willis, president of the Fort Worth League of Neighborhoods, said the latest study shows that industry doesn't know the full impact of injection wells. She took part in a series of town hall meetings as the city decided whether to lift a moratorium on injection wells. The City Council members voted in April to ban the wells."It just shows you how much is unknown," Willis said. "You can't look at this in isolation. We don't have the full picture but we are starting to get more scientific information like this report."This summer, Johnson County has received more scrutiny as small quakes have continued to occur, the most recent taking place on July 27 when a 2.2 quake took place near Cleburne.Frohlich found no indication in his study that the increased seismic activity would lead to quakes of greater magnitude.Bill Hanna, 817-390-7698Twitter: @fwhanna
UT study maps earthquakes and injection wells in the Barnett Shale (pdf)
Another North Texas quake raises more questions about drilling
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