Although the number of earthquakes in Oklahoma measuring 3.0-magnitude or greater declined for the third straight year, the state’s seismologist said researchers expect such quakes, most of which are attributed to wastewater injection, will continue for years, perhaps a decade or longer.

According to data from the Oklahoma Geological Survey (OGS), the number of temblors measuring at least 3.0-magnitude peaked at 901 in 2015, but it declined to 619 in 2016 and 302 in 2017. Last year, there were 191 such earthquakes in the state, nearly half of which occurred in four of the 77 counties: Grant (27), Garfield (22), Logan (22) and Kingfisher (20).

Despite the 78.8% decline in seismicity between 2015 and 2018, OGS’s Jake Walter, lead seismologist, said the recent totals are a far cry from a pre-2008 rate of just a few such quakes per year.

“Several hundred is still significantly higher,” Walter told NGI’s Shale Daily. “And when you look at some of the estimates or some of the modeling that’s been conducted related to continued earthquake activity, we don’t fall back to that background rate anytime soon.”

Regulators with the Oklahoma Corporation Commission’s (OCC) Oil and Gas Conservation Division (OGCD) have been attempting to mitigate induced seismic activity since 2015. The agencies have focused on wastewater injection wells targeting the Arbuckle formation, especially the Mississippian Lime and the Hunton Dewatering play, within a 15,000-square mile area of interest in the state.

“Earthquakes make other earthquakes,” Walter said. “There’s the obvious main shock-aftershock connection, of earthquakes immediately following any kind of main shock with smaller earthquakes. Sometimes those aftershocks are larger. So you have this cascading sequence of activity, and what we’re finding is that these sequences of activity continue for several years.”

For example, Walter said researchers are still observing earthquakes in the area of Pawnee, site of the largest recorded earthquake in state history. A 5.8-magnitude temblor struck the area on Sept. 3, 2016.

“This is an area that no longer has very much Arbuckle deep wastewater injection in the area,” Walter said. “There just seems to be these continuous, long-lived sequences of activity. We don’t necessarily understand why they’re so long-lived and propagate for several years. But we have some historical footing.”

That footing includes analysis of what transpired at the Rocky Mountain Arsenal, a former U.S. Army facility in Colorado, in the 1960s. A single injection well was drilled to a depth of 12,045 feet at the arsenal in 1961, and through 1966 the well was used to dispose of about 165 million gallons of liquid waste from chemical manufacturing operations by the Army. But use of the well was discontinued in 1966 after injections triggered a series of earthquakes.

“The earthquakes continued for a decade after that,” Walter said. “That was one single isolated well. Meanwhile, Arbuckle wastewater injection in Oklahoma continues. We don’t necessarily know what’s going to happen long term, other than there is a reasonable expectation that you’re going to continue to see frequent shaking from these small earthquakes.”

Fracking Triggering Quakes, Too

Walter added that while more than 90% of the 3.0-magnitude or greater earthquakes could be attributed to wastewater injection, some of the remainder were the result of hydraulic fracturing (fracking) operations, including in parts of the Sooner Trend of the Anadarko Basin, mostly in Canadian and Kingfisher counties, aka the STACK, and the South Central Oklahoma Oil Province, aka the SCOOP.

“Earthquakes from wastewater injection have declined significantly throughout the state, but we’ve seen a slight uptick in the number of earthquakes associated with fracking, in areas like the SCOOP/STACK and other areas of Oklahoma,” Walter said.

According to Walter, the researchers have been “better equipped” to associate fracking with earthquakes since December 2016, when the OCC, OGCD and OGS developed seismicity guidelines for companies operating in the Midcontinent, including the SCOOP and STACK. The guidelines called for three courses of action, depending upon the magnitude of the earthquake, for wells conducting fracking operations that are also within two kilometers (1.25 miles) of seismic activity.

“Rather than having to pore through the regulatory records several months after the fact, they have that ‘up on their dashboard’ and they’re able to make decisions accordingly,” Walter said. “What we’ve observed is that some of these earthquakes have been associated with fracking.

“That’s something that’s consistent across the last couple of years. It’s not that we’ve noticed an uptick in the ones that are associated with fracking, just that some of the earthquakes within the statewide counts are not necessarily just wastewater injection induced, but also associated with an ongoing hydraulic frack.”

One such incident occurred in November, when regulators ordered Roan Resources Inc. to halt completion operations at the Victory Slide well in Grady County after the region was struck by a 3.4-magnitude earthquake.

OCC spokesman Matt Skinner told NGI’s Shale Daily that disposal well information for 2018, as well as injection volumes, won’t be tabulated until April. He said the information would be released possibly as early as May, following audits by the OCC. No other regulations are imminent.

Last April, the U.S. Geological Survey reported that while small earthquakes linked to oil and gas activity in Oklahoma has eased, the risks from seismic events remain high.