“The difference between a 1 and 2 is a factor of 30,” he said, “the difference between a 4 and 5 is a factor of 900.”
Ed Ireland, executive director of the Barnett Shale Energy Education Council, believes, like Wickham, that the earthquakes were not caused by the actual drilling for natural gas.
“The level of drilling is low,” he said, “There are only 35 rigs in all of the Barnett Shale, compared with 200 in 2008.”
Ireland said scientists have observed seismic activity around disposal wells but have not proven whether it’s a random correlation or a real connection.
“My understanding is that three things have to be present if there is induced seismic activity,” he said, “There has to be a fault — and there are faults in Parker County — the fault has to be under pressure already, and the disposal well has to be injected at such a pressure that it relieves the strain on a fault.
“In some cases, where there is a question, operators reduce the injection pressure.”
Ireland said Cliff Frohlich, associate director and senior research scientist at the University of Texas Institute for Geophysics, is the expert when it comes to earthquakes and injection wells.
Frohlich, who studied seismographic data collected on the Barnett Shale from 2009-11, thinks the data “suggests that injection-triggered earthquakes are more common than is generally recognized.”
He authored an article entitled “Two-year survey comparing earthquake activity and injection-well locations in the Barnett Shale, Texas,” published in 2012 in the Proceedings of the National Academy of Sciences of the United States of America.
In the article Frohlich hypothesizes that “injection only triggers earthquakes if injected fluids reach and relieve friction on a suitably oriented nearby fault that is experiencing regional tectonic stress.”
His study uncovered 67 earthquakes — eight times as many as have been reported — with magnitudes 1.5 and larger, with “all 24 of the most reliably located epicenters — the spot on the Earth’s surface above an earthquake’s point of origin — occurring in eight groups within 3.2 km of one or more injection wells.”
In Johnson County, while nine of the 27 injection wells were near earthquakes, the other 18 reportedly did not occur “near wells with similar injection rates.”