A report by the group Proceedings of the National Academy of Sciences of the United States asserts that there is a link between dozens of small earthquakes in the Barnett Shale of Texas and wastewater injection wells that support natural gas drilling.
Cliff Frohlich, a geophysics professor at the University of Texas who authored the report, said he analyzed data taken from temporary seismographs that were deployed within a 70-kilometer grid in the Barnett from November 2009 to September 2011 where he was able to identify 67 earthquakes, more than eight times the number of quakes reported by the National Earthquake Information Center.
"All 24 of the most reliably located epicenters occurred in eight groups within 3.2 kilometers of one or more injection wells," Frohlich said. "This suggests injection-triggered earthquakes are more common than is generally recognized."
Frohlich added that all of the wastewater wells nearest the earthquake epicenters had reported maximum monthly injection rates in excess of 150,000 barrels per month since October 2006. But the researcher added that while nine of 27 wastewater wells in Johnson County, TX, were near earthquake epicenters, there were no reported earthquakes near wastewater wells with similar injection rates.
"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 said. "Testing this hypothesis would require identifying geographic regions where there is interpreted subsurface structure information available to determine whether there are faults near seismically active and seismically quiescent injection wells."
In mid-July, Johnson County, TX, reported about 10 small earthquakes within a one-month period. The quakes caused no damage or injuries. Regulators with the Railroad Commission of Texas asserted that two earthquakes in May near Timpson, TX, in the eastern part of the state, were unlikely to have been caused by natural gas drilling or wastewater injection wells (see NGI, May 28). But researchers at the U.S. Geological Survey (USGS) have cited drilling-related injection wells as the reason for a "remarkable" uptick in seismic activity (see NGI, April 2).
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