Studies Link Quakes to Oil and Gas Wastewater Disposal

Two studies out today point to oil and gas activity, specifically deep injection of wastewater, as the cause of a surge in earthquakes in Oklahoma and the central United States.

To slow or stop the earthquakes, the studies say, oil and gas producers will need to cut the volume of waste fluid they're injecting into wells.

But the studies differ on where to focus those efforts. One from the University of Colorado and the U.S. Geological Survey, published in Science, suggests regulators look at high-volume disposal wells. The other study, from Stanford University, looking at Oklahoma, puts the focus on cumulative injection across broad areas of earthquake activity.

[+] This graphic from Stanford University shows how wastewater from oil and gas production gets injected underground. This image shows the Arbuckle Formation, which in Oklahoma sits directly above crystalline basement rocks. Rising pore pressure in the Arbuckle Formation can penetrate already-stressed basement faults and trigger earthquakes. Graphic by Steven Than, courtesy of Stanford University.

"You just can't put an arbitrary limit on how much any individual well should be injecting," said Stanford geophysicist Mark Zoback, whose study was published in Science Advances. The cause of the quakes, he found, "was this cumulative process from so many wells over such a large area injecting such large volumes in the past five to 10 years."

Seismologists have been saying for several years that disposal of oil and gas waste causes earthquakes, though not all disposal wells trigger quakes. Today's studies go deeper by digging into the injection data kept by states and comparing them broadly with earthquake locations.

The Colorado study by doctoral candidate Matthew Weingarten looked at the correlation across the central and eastern United States between earthquakes and wastewater disposal wells, sometimes called saltwater disposal wells or SWD wells. It found that earthquakes are more likely near wells that inject more than 300,000 barrels (12.6 million gallons) a month.

"High-rate SWD wells are nearly twice as likely as low-rate wells to be near an earthquake," the study says. But that's not the only factor. There are high-rate injection wells in areas such as North Dakota not associated with earthquakes.

The Stanford study focused on Oklahoma, where the number of quakes has skyrocketed since 2009. Last year, the state had 585 quakes of magnitude 3 or greater, compared with an average of about two a year before 2009. This year, it's on track for more than 600.

The study notes that the volume of wastewater getting disposed of in Oklahoma has doubled since 1997. But in one of the earthquake-prone areas of the state, around the Mississippi Lime play, it has risen tenfold.

"We really think the important concept is how much water has been injected in those areas," Zoback said.

The Stanford paper also points out that the earthquakes are not linked to hydraulic fracturing. They're not really even linked to disposal of wastewater from fracking (often called "flowback").

"Hydraulic fracturing flow-back water comprises an extremely small fraction of the injection into the SWD wells," the study states.

The paper also looks at whether the state's largest recorded earthquake, a magnitude-5.7 event in November 2011 near Prague, was caused by injection from oil production. It is something of a consensus among seismologists outside Oklahoma that the quake was linked to injection. But the Stanford study didn't back that up.

"We can't rule it out," Zoback said. But "we're sort of agnostic on whether it was an induced event."

Overall, though, the studies agree that the sharp increase in quakes in Oklahoma and the central United States is related to oil and gas wastewater disposal.

"This population of earthquakes," Weingarten said, "is associated with this population of wells."

The Stanford study's findings were a major contributing factor in the development of a position statement by the Oklahoma Geological Survey that most of the increase in the state's quakes is "very likely" the result of wastewater disposal from oil and gas production.

"The Stanford scientists' findings were carefully considered before we issued the statement, and contributed to the scientific credibility of the statement," said OGS seismologist Austin Holland, who was not involved in the study.

"We've been waiting for exactly this type of study," said Michael Teague, Oklahoma's secretary of energy and environment. "These findings help us understand the case better so that we can evaluate options that we can take to go forward in finding ways to reduce the quakes."

Mike Soraghan

Republished from EnergyWire with permission. EnergyWire covers the politics and business of unconventional energy. Click here for a free trial

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