The sharp increase in seismic events that hit central Oklahoma over the last five years was caused by a small number of wastewater injection wells with high disposal rates, according to researchers from three universities and the U.S. Geological Survey (USGS). They said four wells may have caused up to 20% of the earthquakes between 2008 and 2013 in the central United States, including Oklahoma.

The researchers said seismicity and hydrogeological models show “distant fluid migration from high-rate disposal wells in Oklahoma is likely responsible for the largest swarm [of earthquakes].

“Earthquake hypocenters occur within disposal formations and upper-basement, between 2-5 kilometers [1.2-3.1 miles] depth. The modeled fluid pressure perturbation propagates throughout the same depth range and tracks earthquakes to distances of 35 kilometers [21.7 miles], with a triggering threshold of about 0.07 MPa [megapascals].”

The researchers are Katie Keranen of the Department of Earth and Atmospheric Sciences at Cornell University; Matthew Weingarten and Shemin Ge of the Department of Geological Sciences at the University of Colorado; Geoffrey Abers of the Lamont-Doherty Earth Observatory at Columbia University; and Barbara Bekins of the USGS. Their findings were published online this month in Science Express, a publication of the American Association for the Advancement of Science.

According to the researchers, a single swarm of earthquakes that began in 2008 near Jones, OK, accounted for 20% of the seismicity in the region. They asserted that a 5.7-magnitude earthquake near Prague, OK, in 2011 “was likely induced by wastewater injection,” (see Shale Daily, Oct. 25, 2013).

The researchers said models showed the Jones swarm began within 20 kilometers (12.4 miles) of the highest-rate disposal wells in Oklahoma, between two regions of fluid injection. They said four of the highest-rate wells — located southwest of Jones, in southeast Oklahoma City — collectively dispose of more than 4 million bbl of wastewater a month, and are injecting at a depth of 2.2-3.5 kilometers (1.4-2.2 miles), into the Cambrian-Ordovician Arbuckle Group, a dolomitized carbonate. One disposal well reached the Precambrian basement.

A hydrogeological model used by the researchers predicted “a region of high fluid pressure perturbation spreading radially eastward” from the four wells in southeast Oklahoma City. The model used reported injection data from 89 wells located within 50 kilometers (31.1 miles) of Jones.

“Our results indicate that over 85% of the pore pressure perturbation is contributed by the four high-rate southeast Oklahoma City wells,” the researchers said. “The 85 wells to the northeast contribute up to about 15% additional pore pressure change at the center of the Jones swarm by the end of 2012, and may contribute to the triggering of earthquakes, particularly outside the region affected by the southeast Oklahoma City wells…

“We view the expanding Jones earthquake swarm as a response to regionally increased pore pressure from fluids injected at the southeast Oklahoma City wells. As the pressure perturbation expanded and encountered faults at various orientations, critically stressed optimally oriented faults are expected to rupture first.”

The researchers said that although seven earthquakes were recorded between 2006 and 2009 near the base of the four southeast Oklahoma City injection wells, the main swarm of earthquakes during that time frame began about 15 kilometers (9.3 miles) to the northeast, despite high modeled pressure near the wells.

“Earthquakes in 2009 primarily occurred, within location uncertainty, near injection wells or on the nearest known faults to the northeast of the wells,” the researchers said. “Focal mechanisms near the swarm onset indicate fault planes at orientations favorable to failure.”

According to the researchers, faults running subparallel to the Nemaha Fault, which runs in a north-northwest to south-southeast direction, are not inclined to fail and would require much more pressure to do so. Still, they cautioned that a 50-kilometer (31.1-mile) stretch of the Nemaha is capable of hosting a 7.0-magnitude earthquake, which could run for hundreds of kilometers.

“The increasing proximity of the earthquake swarm to the Nemaha Fault presents a potential hazard for the Oklahoma City metropolitan area,” they said.

Last May, the USGS and the Oklahoma Geological Survey issued a joint report that said 145 earthquakes of 3.0-magnitude or greater had occurred in the state so far in 2014 (see Shale Daily, May 6).