Findings can help oil and gas operators minimize seismic risks
For nine months, oil and gas companies pumped 250,000 barrels of industrial wastewater deep underground in the fossil fuel sacrifice zone around Greeley, Colorado — and then, the Earth burped.
On the last day of May, 2014, the wastewater triggered a magnitude 3.2 earthquake that for some area residents felt like a truck hitting their house. The quake was the first in the area in about 40 years, fitting a regional pattern of earthquakes linked with fracking.
Now, a new study by scientists with the U.S. Geological Survey and the University of Colorado shows it may be possible to lessen the risk of frack-quakes by controlling the amount of wastewater being pumped into the ground, and by carefully monitoring seismic activity in fossil fuel development areas.
“We were surprised to observe an earthquake right in our backyard, and we knew we needed to know more, so we quickly mobilized seismic monitoring equipment,” said Will Yeck, lead author of the study that appeared in the journal Seismological Research Letters.
The quake was felt in Boulder, where researchers located its center in the heart of oil and gas country in Weld County, where drillers sometimes dispose of wastewater deep underground — an activity now known to sometimes trigger earthquakes.
CIRES geophysicist Anne Sheehan and other researchers deployed six seismometers around the earthquake’s epicenter to pinpoint further seismic activity. The scientists shared their findings with state oil and gas regulators and pump operators, helping them understand the real-time seismic data. It quickly became clear that the earthquakes were centered under one specific well: the wastewater disposal well closest to the Greeley earthquake epicenter which happened to be the highest‐rate injection well in northeast Colorado in 2013, according to data compiled by the state.
The real-time tracking enabled the researchers to observe fluctuations in seismic activity as the well was shut down and cemented, said Jenny Nakai, a co-author of the new study and a graduate student in geophysics at CU Boulder. “We could see fluctuations in seismic activity as the well was shut down and cemented.”
Injection stopped on June 24 for a month, and the company that drilled the disposal well took two actions to reduce seismicity: They reduced injection rates and used cement to plug the bottom of the well, impeding fluid interaction with deeper, subsurface faults. Injection resumed a month later at reduced rates, starting at just 5,000 barrels a day mid-July. The injection rates were slowly increased over time.
The team also incorporated historical data from other seismometers in the region, finding that the Greeley earthquake sequence started about four months after the initiation of high-rate wastewater injection in 2013. The quakes grew stronger over time, in a pattern that’s been observed at other injection induced earthquake locations as well.
State regulators with the Colorado Oil and Gas Conservation Commission used the findings to change the permitting process, requiring seismic monitoring at recently permitted commercial disposal wells pumping more than 10,000 barrels per day.
Authors of “Rapid Response, Monitoring, and Mitigation of Induced Seismicity near Greeley, Colorado” in Seismological Research Letters are William Yeck and Harley Benz (U.S. Geological Survey), Anne Sheehan and Jenny Nakai (CIRES and University of Colorado Boulder), and Matthew Weingarten (Stanford University).