The Oklahoma Corporation Commission issued a directive to further restrict injection rates into the Arbuckle formation in north-central Oklahoma as parts of the state keep shaking.
A release issued by the commission Wednesday said the agency directed operators of three wells within 3 miles of two magnitude 4 or greater earthquakes that happened north of Crescent earlier this month to close those wells. The operator of a fourth well that hasn’t injected any saltwater in at least the past 30 days also was directed to keep that well closed.
May’s earthquakes, a magnitude 4 north of Crescent on May 13 and a magnitude 4.1 in the same area the following day, brought the number of earthquakes with a magnitude of 4 or greater in the state so far this year to seven, which is one more than Oklahoma experienced during all of 2017, officials have said.
State seismologists said this month they continue to conduct research into the ongoing activity.
“What we are observing is there are these little clusters that keep popping off in north-central Oklahoma, about 30 minutes off Interstate 35 in either direction,” said Jake Walter, a seismologist at the Oklahoma Geological Survey.
“As far as we can tell, wastewater injection is not up significantly, although in the particular case (of the Crescent earthquakes), there was some increase in injection … although nothing approaching what they had been injecting in 2015.
“So when you are talking about causation, it is a lot of maybes,” Walter said.
“It likely is related to wastewater injection. There probably is some aspect that the renewed activity is continued seismicity on these faults that were reactivated by past injection.
“We just don’t know exactly what thresholds are required to reawaken some of those fault activities,” Walter said.
Walter said the Oklahoma Coordinating Council on Seismicity, which consists of seismologists, geologists, industry representatives and corporation commission regulators, continues to consider multiple approaches to handling the issue.
In some cases, such as the directive announced Wednesday, an appropriate response might be directing operators in a specific area to reduce injections in an attempt to mitigate renewed earthquake activity.
But Walter said the council also is contemplating whether to require additional reductions across the entire area of interest, which covers much of north central and northwest Oklahoma.
Getting back to ‘normal’
Walter said he has heard Oklahomans generally were pleased with progress the council and regulators made in reducing seismic activity within the state during the past year.
Excluding earthquakes that are magnitude 4 or stronger, the numbers of observed earthquakes in Oklahoma continues to decline.
The overall number of earthquakes magnitude 2.7 or greater this year, for example, remains significantly lower than in previous years.
“But in 2017, we still had more felt earthquakes than California did, and that’s not accounting for the relative differences in the sizes of the states,” Walter said.
“We are not anywhere close to getting back to ‘normal,’ whatever that is.”
As for the wells directed to be shut in by this latest directive, Oklahoma Corporation Commission officials said Wednesday that Stephens Energy Group and White Star Petroleum each operated two of the affected wells.
Additionally, the commission directed operators of another 21 disposal wells operating within 10 miles of the recent stronger earthquakes to take steps to reduce or limit average daily injection rates into the Arbuckle formation.
Officials said operators of nine wells that had an average daily injection rate of 500 barrels or greater are required by the directive to reduce that average by 20 percent. They said the remaining well operators are required by the directive to limit their average daily injection rates at each well to 500 barrels.
Officials said they expect those actions will reduce disposal into the Arbuckle formation in that part of north-central Oklahoma by 2,848 barrels per day.
This month’s directive follows another issued in April, when the Oklahoma Corporation Commission directed the operators to close one saltwater disposal well south of Hennessey and to limit ongoing injections at another 15 wells near Enid.
Those actions aimed to further moderate earthquake issues in those parts of the state, including some in early March that were magnitude 4 or stronger.