A new study assesses potential correlations between small-magnitude seismic events with fluid extraction and injection in the area of the Eagle Ford Shale of south-central Texas. The article –titled Two-year survey of earthquakes and injection/production wells in the Eagle Ford Shale, Texas, prior to the MW4.8 20 October 2011 earthquake – was published online this week by Earth and Planetary Science Letters, an Elsevier publication. Its lead author is Cliff Frohlich, Associate Director of the Institute for Geophysics, University of Texas at Austin.
The article examines 62 “probable earthquakes” between November 2009 and September 2011 occurring at 14 foci in the study area. According to the study, eight of the foci were located near wells that were “extracting recently increased volumes of oil and/or water,” two were located near wells that were “injecting recently increased volumes of water,” and four were not located near “wells reporting significant injection/extraction increases.” The study also states that the “MW4.8 20 October 2011 Flashing earthquake,” described as “the largest historically reported earthquake in south-central Texas,” “coincide[d] with a significant increase in oil/water extraction volumes at wells within the . . . region.” The study describes this scenario as being similar to seismic events at Flashing in 1973 and 1983. The study concludes that “[t]hese associations between seismic activity and increases in injection/production volumes imply that many of the Eagle Ford earthquakes were triggered/induced.” The study was “unable to identify a critical monthly rate,” and noted that “there are numerous high-volume production and injection wells with no nearby seismicity.”
Cliff Frohlich also authored a similar study in August 2012 that surveyed seismic activity in the area of the Barnett Shale of northeast Texas. That study, which was published in the Proceedings of the National Academy of Sciences, reported a correlation between seismic events and injection activity, but not with extraction activity. The recent study hypothesizes that “[t]here are geological and historical differences between the Barnett and Eagle Ford that may explain the differences in their induced seismicity.”
The issue of triggered/induced seismicity is being litigated in several related lawsuits pending before Judge J. Leon Holmes in the U.S. District Court for the Eastern District of Arkansas. The plaintiffs, who began filing these lawsuits in 2011, are residents of central Arkansas who allege that their properties were damage by seismic events in 2010 and 2011, which they attribute to deep well injection of wastewater from hydraulic fracturing. The defendants are BHP Billiton Petroleum (Fayetteville) LLC and Chesapeake Operating Inc. One of these cases settled on August 28, 2013, on confidential terms. It had been scheduled for trial on March 24, 2014. Trials for the remaining cases are scheduled for late 2014 and early 2015.