A new report published by the Seismological Society of America links activities associated with shale gas exploration – such as fracking and deep well injections – to earthquakes, sometimes occurring as far as tens of kilometres from the wellbore.
Oil and gas development activities, including underground disposal of wastewater and hydraulic fracturing, may induce earthquakes by changing the level of stress on existing faults causing them to fail. Earthquakes from wastewater disposal may be triggered at tens of kilometres from the wellbore, which is a greater range than previously thought.
The number of earthquakes within central and eastern United States has increased dramatically over the past few years. This coincides with increased hydraulic fracturing of horizontally drilled wells, and the injection of wastewater in deep disposal wells in many locations, including Colorado, Oklahoma, Texas, Arkansas and Ohio. According to the U.S. Geological Survey (USGS), an average rate of 100 earthquakes per year above a magnitude 3.0 occurred in the three years from 2010-2012, compared with an average rate of 21 events per year observed from 1967-2000.
“Induced seismicity complicates the seismic hazard equation,” said Gail Atkinson, professor of earth sciences at Western University in Ontario Canada. Her research details how a new source of seismicity, such as an injection disposal well, can fundamentally alter the potential seismic hazard in an area.
Atkinson suggests that induced seismicity from shale gas operations can increase hazard to critical structures, such as dams, nuclear power plants and other major facilities. This is because these structures were originally designed for regions of low to moderate seismic activity and new, induced, seismic pressure may have a compounding effect, causing fault to occur.
A new study of the Jones earthquake swarm, occurring near Oklahoma City since 2008, demonstrates that a small cluster of high-volume injection wells triggered earthquakes up to 50 kilometres away. Both increasing pore pressure and the number of earthquakes were observed migrating away from the injection wells.
Katie Keranen, assistant professor of geophysics at Cornell University, who led the study of the Jones earthquake swarm, said “Our results, using seismology and hydrogeology, show a strong link between a small number of wells and earthquakes migrating up to 50 kilometres away”.
While there are relatively few wells linked to increased seismicity, seismologists seek to anticipate when activity might trigger earthquakes and at what magnitude.
Art McGarr, a geophysicist with USGS, investigates noticeable earthquakes caused by fluid injections. The injection activities considered in McGarr’s study include underground disposal of wastewater, development of enhanced geothermal systems and hydraulic fracturing. Of the three, wastwater disposal is the most likely to induce earthquakes that are large enough to be felt or capable of causing damage.
“From the results of this study, the total volume of injected fluid seems to be the factor that limits the magnitude, whereas the injection rate controls the frequency of earthquake occurrence,” said McGarr.
Despite the increasing seismicity in the central and eastern US, induced earthquakes are presently excluded from USGS estimates of earthquake hazard. Justin Rubinstein, geophysicist with USGS, believes that this approach is limited. “In some sense, from a hazard perspective, it doesn’t matter whether the earthquakes are natural or induced. An increase in earthquake rate implies that the probability of a larger earthquake has also risen,” said Rubinstein.
But what’s the likelihood of induced seismicity from any specific well?
“We can’t answer the question at this time,” said Atkinson. The problem is complex and the studies of induced seismicity are still at an early stage.
“There is a real dearth of regulations,” said Atkinson. “We need a clear understanding of the likely induced seismicity in response to new activity. And who is the onus on to identify the likely seismic hazard?”
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