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April 10, 2013
Wastewater injection linked to large earthquake

Petroleum and natural gas companies may need to provide their own research to respond to a study from a team of scientists that linked the largest earthquake ever recorded in Oklahoma to the geologic injection of wastewater produced by petroleum development.  The study’s authors included scientists from Columbia University’s Lamont-Doherty Earth Observatory, which discussed the findings in a news release. 

650 feet from epicenter

The magnitude 5.7 event that is the subject of the study occurred near Prague, Oklahoma, on November 6, 2011.  Fourteen homes were destroyed, a federal highway buckled, and two people were injured.  Shaking was felt as far away as Milwaukee, 800 miles from the epicenter.  Aftershocks are still being felt in the area, reported Lamont-Doherty. 

The wastewater linked to the Prague quakes was a by-product of oil extracted at one set of oil wells that was pumped into another set of depleted oil wells targeted for waste storage. The study indicates that the existing fault where the rupture occurred is no more than 650 feet from the active injection wells and, perhaps much closer, in the same sedimentary rock formation. 

The event was not unique, states the report.  In the last 4 years, the number of quakes in the middle of the United States jumped 11-fold from the prior 3 decades, the authors of the study estimate.   Scientists are now linking the rising number of quakes in normally calm parts of Arkansas, Texas, Ohio, and Colorado to the recent boom in U.S. energy that has led to the geologic injection of massive amounts of wastewater.

Small volumes trigger events

Geologic injection of many types and varieties of fluids has been associated with oil and gas production for many years.  The risk that such activity may set off earthquakes has been recognized since at least the 1960s, when injection at the Rocky Mountain Arsenal near Denver was suspended after a quake estimated at magnitude 4.8 or greater struck nearby—the largest tied to wastewater disposal until the one near Prague. 

The authors of the study cite new evidence that relatively small amounts of injected fluids can trigger more seismic activity than previously thought; they add, however, that the amount of fluid used in hydraulic fracturing may not be sufficient to cause “substantial shaking.”

Injection continues

According to the researchers, the Prague event and the thousands of recorded aftershocks were unusual because wastewater was being pumped into the abandoned well for 17 years without incident.  The researchers hypothesize that as wastewater replenished compartments once filled with oil, the increasing pressure needed to force the fluid downward caused the existing fault to “jump.” 

“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 with Lamont-Doherty. 

The fault system that slipped in Oklahoma remains under stress, the study’s authors say, yet regulators continue to allow injection into nearby wells.  Ideally, 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 one researcher.  The study authors also recommend subsurface monitoring of fluid pressure for earthquake warning signs.

Naturally occurring?

The Oklahoma Geological Survey has yet to issue an official account of the Prague seismic sequence.  In a statement responding to the paper, survey seismologist Austin Holland conceded that the earthquake sequence could have been triggered by the injections.   But, he added, “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.”

Click here for more information on the research.

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