BOULDER, Colo. — The uptick in earthquakes in central Oklahoma is most likely being caused by the injection of wastewater from oil and gas operations into layers of underground rock, according to a study by Cornell University that also includes the University of Colorado Boulder.
The study, appearing Thursday in the journal Science, is based on researchers using hydrogeological models to see how much wastewater built up in the pores of rocks.
The researchers determined that under high pressure, fluids can get into faults, prying apart the rocks that allow them to slip past each other more easily, thus causing the earthquakes.
“Deciding whether an earthquake is induced or natural is a very difficult process scientifically,” said Matthew Weingarten, a doctoral student at CU’s Department of Geological Sciences and a co-author of the report. “The classic way of determining the likelihood of an induced event is by looking at the seismological data alone. We took the next step in determining causation.”
The study using hydrogeological modeling gave the researchers better confidence to make the connection between wastewater injections and the quakes.
Before 2008, Oklahoma averaged about two earthquakes a year with a magnitude of 3.0 or greater, the U.S. Geological Survey said. From Jan. 1 to May 2 this year, there have been 145 such quakes. Since 2008, there has been a 40-fold increase in earthquake activity in Oklahoma.
CU geological sciences professor Shemin Ge was a part of the team that looked over the number of earthquakes known as the Jones Swarm, which accounted for one-fifth of all seismic activity in the area.
The researchers then looked at 89 injection wells and the characteristics of the rock layer the fluid was pumped into, creating a map of the pressure known as “pore pressure.” Researchers determined the pore pressure would have been enough to cause the quakes.
“The faults near the injection wells have too much friction to easily fail,” Weingarten said. “But you still have pressure changes far away from those wells and when those pressure changes encounter faults that are closer to failure, you have earthquakes.”
“Induced seismicity is one of the primary challenges we face as shale gas and unconventional hydrocarbon development continues to expand. Adhering to best practices could reduce the risk of inducing seismicity,” Ge said.
“Some best practices during injection operations include avoiding injecting at high rates, avoiding major faults, and keeping a close eye on how pore pressure changes around injection wells. Before permitting, thorough site-specific hydrogeological studies should be conducted to assess the magnitude and extent of pore pressure changes from the injection.”
A separate CU study has been undertaken to determine if recent earthquakes near Greeley are linked to oil and gas drilling. The epicenter of a 3.4-magnitude quake in May was close to wastewater injection wells in the area.
In that study, CU geophysics professor Anne Sheehan and her students added seismographs near the epicenter. There are 28 oil and gas waste disposal wells in Weld County.
“If we find out something useful about whether injection causes earthquakes, it might be something that the industry can use to do a better job of injecting, if that turns out to be a problem,” Sheehan told the Greeley Tribune. “So maybe if they inject at lower volumes or spread it out more, it could be that there are things that we’ll learn that can help inform some sort of best practices.”