August 29, 2016 by Canadian Underwriter
Actions taken by drillers and regulators can lessen risk in the case of earthquakes likely caused by the injection of industrial wastewater deep underground, according to new research from the U.S. Geological Survey (USGS) and the University of Colorado.
While the earthquake that rumbled below Colorado’s eastern plains on May 31, 2014, did no major damage, its occurrence surprised both Greeley residents and local seismologists. “To some Greeley residents, the magnitude 3.2 earthquake felt like a large truck hitting the house,” USGS said in a press release last week.
The earthquake happened in an area that had seen no seismic activity in at least four decades, “centered in the heart of oil and gas country in Weld County, where drillers sometimes disposed of wastewater deep underground – an activity no known to sometimes trigger earthquakes,” according to the analysis. The research, titled Rapid Response, Monitoring, and Mitigation of Induced Seismicity near Greeley, Colorado and published in the July/August issue of Seismological Research Letters, determined that the earthquake was “likely caused by the injection of industrial wastewater deep underground – and, the team concluded, quick action taken by scientists, regulators and industry may have reduced the risk of larger quakes in the area.”
In the first week of June after the earthquake, lead study author Will Yeck (then finishing up his Ph.D. in geophysics at the University of Colorado Boulder) and fellow graduate students Jenny Nakai and Matthew Weingarten deployed six seismometers in an array around the earthquake’s epicenter to monitor further seismic activity. As data flowed in, they analyzed it in detail to pinpoint the locations and the timing of smaller earthquakes following that first one. The geophysicists communicated their findings with state oil and gas regulators and wastewater disposal company staff, and helped those staff learn to read and understand real-time seismic data themselves, the release explained.
“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,” USGS reported. The well had been pumping an average of 250,000 barrels per month since August 2013, more than a mile deep.
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. Nakai, also a co-author of the new study, reported that “we could see fluctuations in seismic activity as the well was shut down and cemented.”
Seismicity dropped, the team found. Following mitigation, between Aug. 13, 2014, and Dec. 29, 2015, no earthquakes larger than magnitude 1.5 occurred near Greeley. “This study demonstrates that a detailed and rapid characterization of a seismic sequence in space and time relative to disposal, combined with collaboration and communication between scientists, regulators, and industry, can lead to objective and actionable mitigation efforts that potentially reduced the rate of earthquakes and the possible generation of larger earthquakes,” the study said.
The research team also used data from more distant seismometers, deployed well before the 2014 earthquake, to detect past seismic activity in the area. They found the Greeley earthquake sequence began roughly four months after the initiation of high-rate wastewater injection in 2013. Their analysis suggested that the biggest observed earthquakes in the area were getting bigger over time, an observation made at other injection-induced earthquake locations as well.
State regulators with the Colorado Oil and Gas Conservation Commission modified requirements as a result of the seismology team’s findings, Yeck and his colleagues reported in the paper. Regulators began requiring seismic monitoring at recently permitted commercial disposal wells pumping more than 10,000 barrels per day.
Greeley-area seismicity continues to be monitored both by the University of Colorado Boulder team and by an independent contractor.