Oklahoma’s earthquake rate likely is affected by saltwater disposal at distances beyond the ranges addressed by emergency volume reductions issued by state regulators, according to a recent university study.
A Virginia Tech team found a correlation between saltwater disposal and earthquake occurrences to a length of 125 kilometers, or about 78 miles.
Local actions in response to sizable quakes near Prague and Cushing in late 2016 saw the Oklahoma Corporation Commission targeting wells in approximately 15- and 22-mile radius limits.
The four-page paper suggests that emergency restrictions in a greater area around an epicenter or swarm may increase the effectiveness of the mitigation effort.
State seismologist Jake Walter expressed skepticism for a regional effect at a distance as far as 125 kilometers, saying he has seen empirical evidence to support an effect only as far as 20 kilometers away, or about 12½ miles.
Matt Skinner, Corporation Commission spokesman, said regulators keep an “open mind” in each situation and stressed that there is no “one size fits all” in their actions. He noted that in 2016 there were two proactive large-scale volume reductions that encompassed an area of interest of about 15,000 square miles total.
Additionally, in February state regulators imposed volume caps on the Arbuckle formation disposal wells in the area of interest to try to prevent sudden or steep climbs. However, it’s uncertain how much volumes could increase under the caps should oil and natural gas prices rise.
The Virginia Tech researchers analyzed deep saltwater injection volumes and seismicity rates in the earthquake-prone swath of the state from 2014 through 2016. The study was published online Jan. 4 by The Geological Society of America ahead of its inclusion in January’s Geology journal.
Ryan Pollyea, a hydrogeologist, is the study’s lead author. He said the research shows that Arbuckle saltwater injection and earthquakes are correlated “on scales that are quite large.”
Pollyea emphasized that the study doesn’t connect a single injection well with a lone fault. Rather, the study takes a macro approach to the “pressure fronts” of hundreds of wells interacting to “create a regional pressure system,” he said.
The study demonstrates that Oklahoma’s complex geology “appears to permit” the relationship between saltwater volumes and quakes “over a very large area,” he said.
“I think that when we talk about those emergency measures and the radii being drawn around quake swarms, I do think this study suggests a larger circle should be drawn in the wake of larger earthquakes,” Pollyea said. He said it’s open to question just how much bigger the radius should be, adding, “I certainly believe it’s larger than 24 kilometers,” or almost 15 miles.
Walter said he was “skeptical that there’s a regional effect” as broad as nearly 78 miles. He said he thinks it’s more likely an effect of oil and gas companies using the same industrial practices at the same time across a wide region.
“That would suggest that if you have a monster well you would see earthquake causation across distances that just aren’t empirically known to exist,” Walter said of the Virginia Tech research.
In Oklahoma, Walter said he is aware of wastewater injection causing seismicity only up to about 12 miles away.
As an example, he cited the magnitude 5.1 quake near Fairview in Major County in February 2016. There were “very high volume” disposal wells in Alfalfa and Grant counties causing that quake and swarms in that area from a distance of about 12 miles.
But Walter did note that there is newer quake activity that has spread farther outward from those wells to the western corner of Woods County, as well as in Harper County. It could be an emerging trend but is uncertain, he said.
“These are getting further and further away from some of the high-volume Arbuckle injection wells,” Walter said. “So it may just be an emerging issue that past injection will still induce earthquakes as the pore pressure fronts (spread) and interact with pre-existing faults that are critically stressed.”