Minor earthquakes have occurred far more frequently in the Barnett Shale than previously reported, and most took place near high-volume injection wells, according to a University of Texas study of local seismic activity.
The study, released Monday by Cliff Frohlich, senior research scientist at UT's Institute for Geophysics, is the latest academic effort to determine whether any aspect of natural gas drilling causes earthquakes.Frohlich's report doesn't find any direct link between hydraulic fracturing and earthquakes. But it adds to concerns that injection wells, where wastewater from fracking and production is pumped back into the ground, can cause increased seismic activity.A June report by the National Research Council also said earthquake risks are higher near injection wells.The UT study reviewed data from temporary seismographs placed in the Barnett Shale under the USArray program from November 2009 to 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, to be published in Proceedings of the National Academy of Sciences.Most of the epicenters were within two miles of one or more injection wells, Frohlich reported. "These included wells near Dallas-Fort Worth and Cleburne," he wrote, "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 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 earthquakes occurred near nine of 27 high-volume injection wells in Johnson County, none occurred near other wells with similar volumes. Frohlich noted that some areas of the Barnett Shale, such as Wise County, have had few quakes despite the presence of many injection wells."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," Frohlich wrote, adding that the theory would need more study to be proved.Ed 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 in Texas and that 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."Johnson County Judge Roger Harmon said he hadn't read the UT study but has become well-versed in the topic since the ground started shaking around Cleburne several years ago."I think most of it can be attributed to these injection wells," Harmon said.Harmon, who stressed that he is not a geologist, called on the Texas Railroad Commission to step up monitoring of the volume of wastewater injected into the wells. Based on other studies he has read, Harmon said, he believes most problems can be mitigated by controlling the wastewater volume and conducting more studies of fault lines surrounding well sites.Frohlich has said he would like to study seismic activity in the Eagle Ford Shale in South Texas for comparison.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 town hall meetings as the city decided whether to lift a moratorium on injection wells. 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 continued, most recently, a magnitude 2.2 quake on July 27 near Cleburne.Frohlich found no indication that increased seismic activity would lead to bigger quakes.Bill Hanna, 817-390-7698Twitter: @fwhannaHave more to add? News tip? Tell us
