Small earthquakes in South Texas' burgeoning Eagle Ford Shale probably result from producing lots of oil and associated water, rather than from hydraulic fracturing, says a new study by Cliff Frohlich, associatedirector of the Institute for Geophysics at the University of Texas at Austin. That's in contrast to a study last year by Frohlich and colleagues at the institute that found that earthquakes in the Barnett Shale likely were caused by injecting waste water deep underground. Much of that waste water is fluid that flows back to the surface after being used for hydraulic fracturing.
"Although there is a considerable amount of hydraulic fracturing activity in the Eagle Ford, we don't see a strong signal associated with that and earthquakes," Frohlich said in a prepared release. The study identified 62 "probable" earthquakes at 14 sites in the Eagle Ford, which runs south and east of San Antonio, between November 2009 and September 2011. Eight of the sites were near newly active producing wells, while two were near injection wells where volumes had recently increased. The other four sites weren't close to either. The study said the largest quake, a magnitude 4.8, was near the town of Fashing on Oct. 20, 2011. That's a relatively large quake for South Texas, the study said, but Fashing also experienced a magnitude 3.4 quake in 1983 and a 3.2 quake in 1973. Frohlich said Texas has a long history of petroleum production and but not of destructive earthquakes, so "this is a phenomenon that we need to understand, but it's not appropriate to say it's vastly dangerous." The study will appear in the online edition of the journal Earth and Planetary Science Letters.
-- Jim Fuquay