The good news for Oklahoma is that the number of earthquakes stronger than magnitude 2.7 that hit the state last year fell by more than a third to 2,500, compared with 4,000 in 2015.
The bad news is the 2016 total is still astronomical compared to the two earthquakes the state experienced annually between 1980 and 2000.
A key question for Oklahoma regulators and for oil and gas drillers there is what caused last year’s decline: Was it mainly the result of state efforts to restrict drillers’ wastewater injections, which the US Geological Survey has linked to the increase in seismicity? Was it the slowdown in drilling activity from low oil prices? Or was it a combination of the two?
That answer could become clearer as drilling picks back up as expected this year.
One positive sign for oil and gas drillers in Oklahoma is that the promising SCOOP and STACK plays generate much less produced water than the Mississippi Lime, leaving producers with less wastewater to dispose underground.
But even if the Oklahoma Corporation Commission’s action to restrict wastewater injections by as much as 40% is the main factor behind the lower earthquake rate, USGS geophysicist Rob Williams still sees a reason for caution ahead.
He said the vast amounts of wastewater already pumped into the deep Arbuckle formation could still affect underground pressure and trigger damaging earthquakes.
“They’ve injected billions of barrels of water in that region in Oklahoma and southern Kansas over the past few years, and the lingering effects of those changes in the stress conditions may last several years,” Williams said in an interview this week. “The chances of having earthquakes is going to be high for a while. We can’t rule out a damaging earthquake in that region, even if there are severe restrictions on injection.
USGS said in a study this week that Oklahoma’s efforts to restrict wastewater injections appear to be curbing earthquake activity linked to oil and gas drilling, but large areas of the state — including key oil storage hub Cushing — remain at risk of damaging seismicity in 2017.
The agency’s 2017 earthquake hazard map shifted the highest-risk zone in Oklahoma to the Cushing-Pawnee area based on two strong earthquakes that shook that area last year: a magnitude 5.8 tremor near Pawnee on September 3, the strongest in state history, and a magnitude 5.0 near Cushing on November 7.
The November earthquake near Cushing didn’t damage any pipelines or oil storage facilities, but it was a wake-up call for the industry to check emergency procedures.
The Cushing-Pawnee area has a 10%-12% chance of a damaging earthquake this year, up from 5%-10% last year, USGS said in its 2017 hazard map.
The 2017 hazard map eliminated Dallas from the risk zone after giving it a 1%-5% chance of a damaging quake in 2016, but it added a small area in the Permian Basin with a 1%-2% chance in 2017.
About 3.5 million people live and work in areas with significant potential for damaging shaking from induced seismicity in 2017 — most of them in Oklahoma and southern Kansas, USGS said. That’s down from 7 million last year.
Oklahoma officials were encouraged by the USGS study but said they had more work to do to keep seismicity down.
“We’re going to continue to monitor it and see how things go,” Tim Baker, director of Oklahoma’s Oil and Gas Conservation Division, said during a news conference. “Obviously if we start to see an increase in earthquake activity and increase in volume, we will be ready to do something about it. But hopefully we’ll keep seeing trends that we’ve been seeing for the last few months.”