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Wednesday, 27 March 2013

Wastewater injection caused a 5.7 earthquake in the USA

Injection of water into the ground by the oil industry has been considered a threat to land stability by many. It is dismissed by those who benefit from the oil extraction associated with fracking, and the disposal of waste fluids by injection back into the ground.

In the USA a 2011 series of earthquakes (the maximum 5.7) in central Oklahoma have been attributed to the injection of wastewater deep underground. The wastewater was produced as a result of oil extraction in the area:

The water linked to the Prague quakes was a by product of oil extraction at one set of oil wells, and was pumped into another set of depleted oil wells targeted for waste storage.

A media release from a study involving Columbia University and the US Geological Survey has drawn attention to this connection. As the media release explains, the presence of faults near to where these processes occur are at risk of earthquakes. Part of a scientific media release is below. The full release is on the website Science News here.

The magnitude 5.7 quake near Prague (Oklahoma) 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 study, researchers hypothesize that as wastewater replenished compartments once filled with oil, the pressure to keep the fluid going down had to be ratcheted up. As pressure built up, a known 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 study 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," he 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 Nov. 5, 2011, University of Oklahoma seismologist Katie Keranen rushed to install the first three of several dozen seismographs to record aftershocks. That night, on Nov. 6, the magnitude 5.7 main shock hit and Keranen watched as her house began to shake for what she said felt like 20 seconds. "It was clearly a significant event," said Keranen, the Geology study's lead author. "I gathered more equipment, more students, and headed to the field the next morning to deploy more stations."

Keranen's recordings of the magnitude 5.7 quake, and the aftershocks that followed, 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. Further, 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, the study says

The full release is here.

The scientific journal reference is: Katie M. Keranen, Heather M. Savage, Geoffrey A. Abers, and Elizabeth S. Cochran. Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake sequence. Geology, March 26, 2013 DOI: 10.1130/G33909.1


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