Some of the recent surge in earthquake activity in central Oklahoma is likely a result of wastewater disposal at a small number of highly active oil and gas wells in the state, a new study  in the 4 July issue of the journal Science reports.
"Our study demonstrates that a very small number of wastewater disposal wells, injecting at exceedingly high monthly volumes, create substantial anthropogenic seismic hazard," explained Geoff Abers, co-author of the report and professor in the department of earth and atmospheric sciences at Cornell University in New York.
What's particularly surprising is that some of the wells were as far as 22 miles from the earthquakes they triggered. "This is different from many other cited cases of induced seismicity," Abers said. "The earthquakes are much more widespread from the primary wells."
Since 2008, there has been a surge in seismicity in the U.S. midcontinent, including a 40-fold increase in seismicity compared to the previous three decades in Oklahoma alone. Scientists have been exploring whether industrial processes, including the underground fluid injection processes related to conventional or unconventional oil and gas extraction methods like fracking, could be behind the increased activity.
At the end of a drilling job, water that was used to create small fractures in deep rock or to flood a target rock formation so fuel could escape is often injected back into the ground for disposal. It is injected between impermeable layers of rocks to avoid polluting fresh water supplies.
Abers explained how pumping large volumes of it deep into the earth impacts earthquake fault zones. "Continued pumping increases the fluid pressure on earthquake faults," he said, "which decreases the strength of the faults. Under these conditions, even faults that may not have failed for a long time can approach failure conditions quickly."
"Fluid pressure [from wells] reduces the frictional strength [keeping a fault locked together] much like turning on the air jets affects a puck on a tilted air-hockey table," he added. "A puck that might have just stayed there still will start to slide when the air jets turn on."
Previous studies have attempted to evaluate the relationship between earthquake activity and wastewater disposal, but the work of Abers and colleagues takes a different approach, combining seismic analysis with advanced hydrologic modeling. "This is relatively rare," explained Abers. "Many studies just correlate seismicity with pumping records." He says that their approach provides much more confidence in results.
To more rigorously investigate pore pressure's influence on earthquake activity, Abers and colleagues started with publicly available datasets of earthquake occurrence during the Jones swarm, an infamous group of Oklahoma earthquakes magnitude 3 or greater, that began in 2008 within 13 miles of the state's four highest-rate wastewater disposal wells.
These wells are located in southeastern Oklahoma City and produce up to 200 times more water per barrel of oil than conventional production wells.
The researchers compared earthquake activity during the Jones swarm — which caused about 20% of eastern and central U.S. quake activity from 2008 to 2013 — to creeping pressure fronts estimated from pore pressure measurements at 89 nearby injection wells, including the four highly active sites.
Their well analysis predicted a strong pressure front spreading eastward from the four high-rate wells. The migrating Jones swarm, they observed, had a corresponding path of quake activity extending 22 miles.
The work of Abers and his colleagues suggests that the four highly active wells caused most of the pore pressure spread during the Jones swarm, triggering quakes at relatively far distances in some cases. Fault zones that fall within the paths of such pore pressure diffusion are targets for quake activity, the researchers say, especially if they are critically stressed, or poised to rupture.
To provide an early indication of earthquake vulnerability in settings near high-rate disposal wells, the researchers recommend that well operators conduct regular measurements of pressure at a range of distances from well sites. "Both earthquake and subsurface pressure monitoring should be routinely conducted in regions of wastewater disposal," Abers said, "and all measurements should be publicly accessible and undergo some sort of objective quality assurance."
He emphasized that wastewater disposal at oil and gas fields can likely be done in a safe manner if adequate safeguards and monitoring are in place. "We encourage agencies and well operators to find approaches that both provide public safety and consider economic viability of petroleum recovery."
Meanwhile, as small to moderate seismic events continue to dot the American landscape, he and colleagues are looking ahead to their next effort in the field.
"These and other recent examples of induced earthquakes call out for a more proactive series of studies," Abers noted, "where seismic and downhole monitoring begins in advance of pumping activities so that changes due to pumping can be more directly evaluated."