A scientist monitoring earthquakes near a Weld County wastewater injection well said the tremors have decreased significantly in both number and intensity.
"Seismicity has been very low," since the well – which had been shut down for 20 days – was reopened, said Anne Sheehan, a geophysics professor at the University of Colorado Boulder.
After a magnitude 3.2 earthquake hit Greeley in late May, followed by subsequent tremors, regulators suspected the well, which accepts wastewater from oil and gas drilling, may have caused the quakes. Following a magnitude 2.6 quake June 23, regulators ordered it shut down while they investigated the link.
Prior to the shut down, the area was experiencing a lot of small earthquakes – as many as 60 on June 22, 2014, the day before the second biggest quake, said Sheehan.
"There were a lot of earthquakes. And it seemed kind of unusual. So that was a bit alarming. And then the next day there was a magnitude 2.6 earthquake," she said.
During the well shutdown, Sheehan said the six instruments set out to monitor the quakes recorded a declining amount of tremors. The quakes that are now being recorded are very tiny.
"In the last week or so it's also been been less than 10 a day. And their magnitudes are all quite small, they are all less than a magnitude 0.5," she said.
Before the well was allowed to reopen, the company that owns it, NGL Water Solutions DJ LLC, plugged the bottom, aiming to eliminate the escape of fluids that might be causing earthquakes.
The company is also injecting at lower pressures and volumes than before the quake. Sheehan said the seismographs will continue to monitor the well as injection pressures are gradually increased, with the goal of determining which pressures and volumes are safe and will not cause earthquakes.
The researchers have also been able to look back at past seismic records and see earthquakes at the site that occurred as early as November 2013, but were too small for most people to notice. The well opened in September 2013, said Sheehan – another piece of circumstantial evidence linking injection at the site with quakes.
A final verdict linking the two will not be possible without additional analysis and research that goes through the peer-review process, she added.
