While earlier research in Texas' Barnett Shale linked drilling waste injection into underground storage wells with increased seismic activity, new work in the Eagle Ford by the same geologist suggests that drawing liquids out of the ground can induce earthquakes there.

"...[T]he Eagle Ford geographic region, with seismic activity associated both with extraction and injection, appears to be more complex than the Barnett Shale of northeast Texas, where a similar survey found possible correlations only with fluid injection [see Shale DailyAug. 8, 2012]," wrote Cliff Frohlich of the University of Texas at Austin Institute for Geophysics in a paper being published in Earth and Planetary Science Letters.

In the latest study, Frohlich and Michael Brunt, a science teacher in the Eagle Pass, TX, school district, surveyed "small magnitude seismic events" and correlated them with fluid extraction and injection activities in the Eagle Ford region. They identified and located 62 probable earthquakes, including 58 that had not been reported by the U.S. Geological Survey.

The 62 probable quakes occurred singly or in clusters at 14 foci. Two of the foci were near wells injecting recently increased volumes of water; eight were near wells extracting recently increased volumes of oil and/or water; and four were not near wells reporting significant injection or extraction increases.

In the Eagle Ford, the "majority of small earthquakes may be triggered/induced by human activity, they are more often associated with fluid extraction than with injection," the paper said.

Frohlich and Brunt also investigated the Oct. 20, 2011 earthquake in Fashing, TX, the largest reported in south-central Texas. While there was no high-volume fluid injection taking place in the region at the time, there was "a significant increase in oil/water extraction volumes" in the region. This was also true of previous earthquakes that were felt at Fashing in 1973 and 1983, the researchers said.

In the Eagle Ford, the associations between seismic activity and increases in volumes of water/oil extracted/produced imply that many of the quakes in the region were triggered or induced, the paper said.

"Of course, injection/production activity is nearly ubiquitous throughout much of the Eagle Ford, and in many areas this activity increased markedly in 2010. Thus it is possible that earthquakes of natural origin may occur coincidentally near active wells," the paper said. "However, the observation that most earthquakes identified in this study occurred during the second year of the survey, when regional injection/production rates were generally higher, favors an induced/triggered origin."

The research found that in the Eagle Ford the relationship between seismicity and injection/extraction is more complex than in the Barnett.

"In the Barnett, [the]...two-year survey found that seismic activity was clustered near injection wells, and these were wells having monthly injection rates exceeding...24,000 cubic meters per month," the paper said. "In the Eagle Ford, our survey finds that seismicity is associated with increases of both injection and extraction, and we were unable to identify a critical monthly rate. In both the Barnett and Eagle Ford, there are numerous high-volume production and injection wells with no nearby seismicity."

 A significant amount of research into hydraulic fracturing and injection wells and their relationship, or non-relationship, to seismic activity has been done in the United States and abroad (see Shale DailyJuly 15April 11;Jan. 18). In Arkansas, landowners have sued to stop wastewater injection wells (see Shale DailyOct. 9, 2012). In 2011, the Arkansas Oil and Gas Commission established an injection well moratorium area following earthquake swarms in the region (see Shale DailyJuly 29, 2011).