Monday, April 1, 2013

1031. Study Links 2011 Quake to Technique at Oil Wells

By Henry Fountain, The New York Times, March 28, 2013
Damage from 2011 earthquake in central Oklahoma

A damaging earthquake in central Oklahoma two years ago most likely resulted from the pumping of wastewater from oil production into deep wells, scientists say.

The magnitude 5.7 quake, which destroyed more than a dozen homes and injured two people, was one in a series that occurred in November 2011 in an oil-producing area near Prague, Okla. The researchers said the quakes occurred near wells where wastewater had been injected into porous rock for two decades.
“The link is pretty compelling,” said Heather M. Savage, a geophysicist at the Lamont-Doherty Earth Observatory, part of Columbia University, and an author of a paper on the quake published online this week by the journal Geology. “The aftershocks show that the first fault that ruptured comes very close to one of the active wells.” The first quake then touched off the others, including the largest one, the researchers said.
The findings are the latest to link earthquakes to underground disposal of wastewater from oil and gas production. Most of those quakes have been minor, causing little or no damage; the 5.7 quake is the largest in the United States to be connected to disposal wells.
The National Academy of Sciences has called for more research into links between quakes and well activities.
Last year, a well in Youngstown, Ohio, that was used to dispose of waste fluids from the production method known as hydraulic fracturing, or fracking, was shut down after scientists showed a link to a series of small earthquakes in the area. The Oklahoma oil wells used more conventional production techniques, said the new study’s lead author, Katie M. Keranen, a seismologist at the University of Oklahoma.
The Oklahoma Geological Survey, a state agency whose mandate includes promoting “the wise use of Oklahoma’s natural resources,” took issue with the findings. In a statement, it noted that earthquakes had occurred regularly in the state, including some of magnitude 4.0, and added, “The interpretation that best fits the current data is that the Prague earthquake sequence was the result of natural causes.”
Pumping of wastewater continues at the wells in the area, and minor quakes still occur, but the authors of the new paper said it was not possible to say whether those smaller quakes were related to the disposal wells.
The researchers said that although the wastewater injection at the wells started in the early 1990s, data showed that injection pressures were increased beginning about a decade ago. Dr. Keranen said that was an indication that pressure down in the rock was rising when it became filled with water. The pressure would have reduced stress on the fault, causing it to slip.
But Steve Horton, a researcher at the University of Memphis who studied a series of quakes in Arkansas in 2010 and 2011 that were linked to disposal wells, said that in most cases the time between the start of wastewater disposal and the occurrence of earthquakes was much shorter.
“Even if the earthquakes just started five years ago, that would still be quite a long time,” he said.
That is why the researchers could not say definitively that the disposal led to the quakes, Dr. Horton said, adding, “What they said is as much as they could say, given the data.”

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