A study published in the journal Seismological Research Letters suggests that horizontal hydraulic fracturing (fracking) in Ohio's Utica Shale triggered a series of 400 small earthquakes in Harrison County over a two-month period last year.
The study comes after a series of seismic events in Northeast Ohio and several academic and regulatory reports that linked their causes to well stimulation or underground injection wells. But the latest report is of the first known instance of seismicity in the area related to drilling operations, according to its authors. Harrison County is in southeast Ohio and is home to a hotbed of unconventional development, where more than 280 horizontal Utica well permits have been issued, according to state data. It is second only to Carroll County's 438 horizontal well permits.
"Hydraulic fracturing has the potential to trigger earthquakes, and in this case, small ones that could not be felt; however, the earthquakes were three orders of magnitude larger than normally expected," said Paul Friberg, a seismologist at Instrumental Software Technologies Inc. and a co-author of the study.
The earthquakes occurred between October and December 2013 near the town of Uhrichsville, OH. Ten of the 400 quakes recorded were considered "positive magnitude," which ranged from 1.7-2.2 on the Richter Scale, according to the study. The earthquakes revealed a fault line at two miles deep directly below three horizontal wells in the area.
Up to 190 of the earthquakes, the study said, took place hours after hydraulic fracturing began on one of the wells.
The authors said that as unconventional drilling increases around the country, more seismic monitoring will be needed to help map unknown fault lines, which are are sometimes not revealed until seismic activity takes place.
The Ohio Department of Natural Resources (ODNR), which oversees the state's oil and gas industry, established permitting conditions in April for fracking operations within three miles of a known fault line or previous seismic activity (see Shale Daily, April 11). Those conditions require an operator that has plans for a well near such areas to install seismic monitoring devices. The agency set forth the rules after a series of earthquakes in March that occurred just miles southeast of Youngstown, OH, near a Hilcorp Energy Site (see Shale Daily, March 11).
ODNR tied those earthquakes, one of which was a 3.0-magnitude that the public felt, to stimulation operations at one of Hilcorp's wells (see Shale Daily, March 12). The state also finished installing more than 20 seismic monitoring devices throughout eastern Ohio over the summer, following up on a plan to monitor Class II underground injection wells after ODNR linked one in Youngstown to a 4.0-magnitude earthquake on New Year's Eve 2011 (see Shale Daily, May 22; Jan. 2, 2012). That well was later linked to more than 100 small earthquakes by researchers at Columbia University's Lamont-Doherty Earth Observatory (see Shale Daily, Sept. 6, 2013).
The latest seismic activity linked to operations in Harrison County, the study said, varied and corresponded with fracturing at those wells. The authors said the timing of the earthquakes, along with their clustering near the wells and waveform suggested the source was human-induced stimulation.
"The fracturing likely triggered slip on a pre-existing fault, though one that is located below the formation expected to confine the fracturing," the authors said.
The study is one of the first of its kind to examine the link between fracking and earthquakes in Ohio. Others have been released in the U.S. and Canada over the years suggesting well stimulation can cause minor earthquakes (see Shale Daily, April 11, 2013; Jan. 18, 2013; June 18, 2012; Oct. 11, 2011).