'Virginia Tech scientists have found that in regions where oilfield wastewater disposal is widespread — and where injected water has a higher density than deep naturally occurring fluids — earthquakes are getting deeper at the same rate as the wastewater sinks.
Perhaps more critically, the research team of geoscientists found that the percentage of high-magnitude earthquakes increases with depth, and may create — although fewer in number — greater magnitude earthquakes years after injection rates decline or stop altogether.
The study, led by Ryan M. Pollyea in the Virginia Tech College of Science’s Department of Geosciences, was published July 16 in Nature Communications. It shows that in areas such as Oklahoma and southern Kansas there is evidence that oilfield wastewater injected underground into the Arbuckle formation has a much higher density than natural fluids occurring within the deeper seismogenic zone faults.
The problem: The wastewater sinks and increases fluid pressure deep underground when it has a higher density than fluids already there naturally. Pressure changes so deep — at depths up to 5 miles or greater — can cause more high-magnitude earthquakes even though the overall number of earthquakes is decreasing.
“Earthquakes are now common in the central United States where the number of magnitude-3 or greater earthquakes increased from about 19 per year before 2008 to more than 400 per year since,” said Pollyea, an assistant professor of geosciences and director of the Computational Geofluids Laboratory at Virginia Tech. (Pollyea adds that the overall earthquake rate per year has been declining since 2016.)
“In many cases, these earthquakes occur when oilfield wastewater is disposed of by pumping it into deep geologic formations,” Pollyea added. “As wastewater is injected deep underground, fluid pressure builds up and migrates away from injection wells. This destabilizes faults and causes ‘injection-induced’ earthquakes, such as the damaging 5.8-magnitude earthquake that struck Pawnee, Oklahoma, in 2016.”'
Read more: Oilfield Wastewater Disposed Underground Induces Stronger Earthquakes
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14 July 2019
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