NEW YORK, New York, March 28, 2013 (ENS) – The rising number of earthquakes in normally calm parts of Arkansas, Texas, Ohio and Colorado are linked to the underground injection of wastewater, finds a new study in the journal “Geology.”
As an example, the study authors point to the magnitude 5.7 earthquake near Prague, Oklahoma on November 6, 2011. The largest quake ever recorded in Oklahoma, this quake is also the largest ever linked to wastewater injection. The quake destroyed 14 homes in the Shawnee-Sparks area, buckled a federal highway and left at least two people injured.
“The earthquake was felt in at least 17 states and caused damage in the epicentral region,” the researchers write. It was felt more than 800 miles away in Milwaukee, Wisconsin.
The wastewater linked to the Prague quake and its aftershocks was a byproduct of oil extraction at one set of oil wells that was pumped into another set of depleted oil wells targeted for wastewater storage.
The recent boom in U.S. energy production has produced wastewater. Before it becomes waste, the water is used in hydrofracking, which cracks open rocks to release natural gas, and also in getting petroleum out of conventional oil wells.
In both cases, the brine and chemical-laced water has to be disposed of, often by injecting it back underground elsewhere, where scientists say it has the potential to trigger earthquakes.
Hydrofracking itself is not implicated in earthquakes as the amount of water used is not enough to produce substantial shaking. It is the underground injection of this wastewater that is of concern.
Since 2009, the number of earthquakes in the middle of the United States jumped 11-fold from the three decades prior, the authors of the Geology study estimate. Last year, a group at the U.S. Geological Survey also attributed a remarkable rise in small- to mid-size quakes in the region to humans.
The magnitude 5.7 quake near Prague was preceded by a 5.0 shock and followed by thousands of aftershocks. What made the swarm unusual is that wastewater had been pumped into abandoned oil wells nearby for 17 years without incident.
In the “Geology” study, researchers hypothesize that as wastewater filled compartments that once held oil, the injection pressure had to be increased.
As pressure built up, a fault, known to geologists as the Wilzetta fault, jumped.
“When you overpressure the fault, you reduce the stress that’s pinning the fault into place and that’s when earthquakes happen,” said study coauthor Heather Savage, a geophysicist at Columbia University’s Lamont-Doherty Earth Observatory.
The amount of wastewater injected into the well was relatively small, yet it triggered a cascading series of tremors that led to the main shock, said co-author Geoffrey Abers, also a seismologist at Lamont-Doherty.
“There’s something important about getting unexpectedly large earthquakes out of small systems that we have discovered here,” Abers said. The observations mean that “the risk of humans inducing large earthquakes from even small injection activities is probably higher” than previously thought, he said.
Hours after the first magnitude 5.0 quake on November 5, 2011, University of Oklahoma seismologist and the study’s lead author Katie Keranen moved quickly to install the first three of several dozen seismographs to record aftershocks.
That night the magnitude 5.7 main shock hit, and Keranen watched as her house began to shake. “It was clearly a significant event,” said Keranen, an assistant professor of geophysics in the School of Geology and Geophysics at the University of Oklahoma. “I gathered more equipment, more students, and headed to the field the next morning to deploy more stations.”
Keranen’s recordings of the quake and aftershocks showed that the first Wilzetta fault rupture was no more than 650 feet from active injection wells and perhaps much closer, in the same sedimentary rocks, the study says. Wellhead records showed that after 13 years of pumping at zero to low pressure, injection pressure rose more than 10-fold from 2001 to 2006.
“Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults,” the study authors state.
The Oklahoma Geological Survey, OGS, has yet to issue an official account of the quake, and wastewater injection at the site continues.
In a March 22 statement, OGS Director and State Geologist Dr. G. Randy Keller, and Holland say that their agency has been working with the Oklahoma Corporation Commission to analyze data related to the 2011 Prague sequence, and recently the U.S. Environmental Protection Agency has participated
in reservoir pressure analysis of the formations into which water is being injected.
The Prague injections were not made into a “fault-bounded block … which would result in increase fluid pressures due to water injection,” conclude Keller and Holland.
Holland said in a statement that the “Geology” study showed the earthquake sequence could have been triggered by the injections. But, he said, “It is still the opinion of those at the Oklahoma Geological Survey that these earthquakes could be naturally occurring.”
“There remain many open questions, and more scientific investigations are underway on this sequence of earthquakes and many others within the state of Oklahoma,” he said.
The risk of setting off earthquakes by injecting fluid underground has been known since the 1960s, when injection at the Rocky Mountain Arsenal near Denver, Colorado was suspended after a quake estimated at magnitude 4.8 struck nearby. It was the largest quake linked to wastewater disposal until the one near Prague.
University of Memphis seismologist Stephen Horton in a 2012 study linked a rise in earthquakes in north-central Arkansas to nearby injection wells.
University of Texas, Austin, seismologist Cliff Frohlich in a 2011 study tied earthquake swarms at the Dallas-Fort Worth Airport to a brine disposal well a third of a mile away.
In Ohio, Lamont-Doherty seismologists Won-Young Kim and John Armbruster traced a series of 2011 earthquakes near Youngstown to a nearby disposal well. That well has since been shut down, and Ohio has tightened its waste-injection rules.
Ideally, wastewater injection should be kept away from known faults, and companies should be required to provide detailed records of how much fluid they are pumping underground and at what pressure, said Keranen.
The study authors recommend sub-surface monitoring of fluid pressure for earthquake warning signs. At a minimum, Abers said, “there should be careful monitoring in regions where you have injection wells and protocols for stopping pumping even when small earthquakes are detected.”
Wastewater injection is not the only way that human activities can touch off earthquakes. Evidence suggests that geothermal drilling, impoundment of water behind dams, enhanced oil recovery, solution salt mining and rock quarrying also can trigger seismic events.
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