Geophysicists are increasingly certain that expanding production of shale gas is responsible for a spate of minor earthquakes that have upset some communities and prompted authorities in Arkansas, Ohio, Oklahoma, and the U.K. to shut down some natural-gas operations. The question now, say the experts, is whether the underground operations causing the trouble should be scaled back or more closely monitored to minimize future quakes—and whether the relatively small quakes may yet have the potential to trigger truly destructive ones.
At least one shale gas producer is already talking change: U.K.-based Cuadrilla Resources, whose first project set off quakes near Blackpool last year.
Shale gas operations generate microseismicity in two ways. One is through hydraulic fracturing, or “fracking,” the underground blasts of water, sand, and chemicals used to release the natural gas trapped within shale deposits. Fracking is how Cuadrilla caused a quake that measured 2.3 on the Richter scale last April, according to an analysis by the firm’s geophysical consultants.
Similarly, a fracking operation that injected 2.4 million gallons of fluid into an Oklahoma well over six days last January is a likely cause of the 43 earthquakes that followed, according to a state geologist’s report. The 1.0 to 2.8 magnitude quakes began on the second day of injection, and most were centered within 3.5 kilometers of the well. These small quakes were felt on the surface and disturbed nearby residents, but they caused no structural damage.
A second source of shaking from shale gas operations is common to many oil and gas fields: the subsurface disposal of wastewater and of naturally occurring brines that surface with the desired hydrocarbons. Deep-injection disposal wells were probably behind a string of quakes in Arkansas that began in 2010, as well as more recent tremors around Youngstown, Ohio, that culminated in a magnitude 4.0 shake this New Year’s Eve. “There’s no doubt that those Youngstown earthquakes are directly associated with the disposal well there,” says Arthur McGarr, a geophysicist and induced-seismicity expert with the U.S. Geological Survey.