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.
Fracking and disposal wells create quakes that can be felt at the surface when shock waves or fluids release strain on a preëxisting fault. For example, high-pressure fluid can squeeze into and push apart a planar fault, freeing adjacent rock formations to slide past one another.
Such induced fault slips probably occurred at Youngstown, says Thomas Stewart, executive vice president of the Ohio Oil and Gas Association. But Stewart says induced quakes are rare events because well operators deliberately avoid drilling near known faults. Ohio’s other 180 oil and gas wastewater wells have prompted few complaints, he notes. He adds that the Youngstown shakes hurt no one other than local gas producer D&L Energy, whose well was shut down by state regulators. “This guy’s probably going to lose a $3 million to $4 million investment,” says Stewart.
Cuadrilla Resources’ geomechanical consultants also downplay the risk that its operations could induce damaging quakes greater than magnitude 3.0. Nevertheless, their report, authored by senior researchers at German geophysical consultancy Q-con and Dutch consultancy StrataGen Delft, recommends that Cuadrilla initiate fracking operations with less fluid than it employed at Blackpool. In addition, they call for underground seismometers to identify any problems early. Cuadrilla says it plans to implement the proposals.
McGarr at USGS says an early warning system is a good idea, and one in keeping with the seismic risk assessment protocol for well-blasting operations employed by geothermal-energy producers. He is less sanguine, however, about estimates of the maximum severity that earthquakes triggered by fracking and injection wells can reach, saying this question needs more science. That means the risk of anthropogenically inducing large, deadly quakes cannot be ruled out.