A few years ago, the number of earthquakes in the eastern and central United States mysteriously began to climb. Before 2009, the background rate for earthquakes in this area was an average of 21 magnitude 3 or larger (M3+) events per year. From 2009-2013, that rate jumped to 99 M3+ per year. The vast majority of the increase across the central and eastern part of the country occurred in Oklahoma, where an astounding 585 M3+ earthquakes took place in 2014. Most of these quakes, although large enough to be felt, have been harmless, but the 2011 M5.6 Prague, Oklahoma, earthquake damaged several buildings. Scientists studying this problem warn that if this trend continues, larger and more damaging earthquakes are possible.
So what’s causing the spike? Many possible culprits have been blamed. Is it “fracking?” Or is it reinjection of flowback water—fracking fluid that returns to the surface? Can we blame enhanced oil recovery (EOR), a production method in which fluid is pumped into the subsurface to drive out oil? Maybe the quakes happen because in Oklahoma companies reinject vast volumes of water produced along with the state’s oil? Or is this all simply part of the natural earthquake cycle?
Unsurprisingly, some environmentalists have been quick to blame “fracking” and EOR. Equally unsurprisingly, the Oklahoma Oil and Gas Association and other industry groups argued that it was far too early to know the cause.
Despite the initial confusion, a flurry of scientific research has recently produced a clearer picture of the true culprit behind most of the extra seismicity. The Oklahoma government has acknowledged that most of the spike in seismicity is caused by reinjection back into deep formations of water coproduced with oil. This “produced” water is brought to the surface during conventional oil production (not during hydraulic fracturing operations). Having spent its recent geologic lifetime in contact with oil, heavy metals, and other chemical undesirables, this produced water usually emerges from the ground as toxic brine. Often, this wastewater is pumped back into saline aquifers through thousands of wells (loosely) regulated by the EPA as Class II Underground Injection Control (UIC) wells.
In a paper published recently in Science Advances, Rall Walsh and Mark D. Zoback, two geophysicists at Stanford University, not only demonstrate a strong correlation between this wastewater injection and the rise in seismicity, but they also offer a physical mechanism linking injection to the earthquakes (full disclosure: Mark Zoback is my graduate advisor). By totaling the monthly injected volumes in several study areas and comparing this with the number and magnitude of earthquakes in the same areas, these authors show that the areas that experienced the largest increases in saltwater disposal also experienced the greatest number of earthquakes.
Importantly, earthquake activity doesn’t always coincide exactly in time or space with injection increases at specific wells. This is probably because most produced water is, in the seismically active parts of Oklahoma, injected into a geologic unit called the Arbuckle Group. The Arbuckle is a sedimentary layer that sits just above the crystalline basement, which hosts pre-existing faults large enough to produce significant earthquakes. Thus, it is not surprising that earthquakes might happen many miles away from wells injecting large volumes, or that the earthquakes could take place even after originally high injection rates decrease. It takes time for elevated fluid pressures to find their way to faults large enough to host earthquakes that can be felt on the surface.
Walsh and Zoback also offer a solution to the earthquake problem. Instead of injecting the produced water down into the Arbuckle, why not pump it into shallower geologic units that aren’t directly above the crystalline basement? Not only would this reduce the chance of triggering slip on a large, already stressed fault, but operators could potentially inject these fluids into producing oil reservoirs. Rather than being a liability, this wastewater could be used to produce oil via EOR. Notably, Walsh and Zoback show that injection for EOR appears to be mostly blameless for the Oklahoma earthquakes. Although this solution would probably resolve much of the seismicity problem, it could also require expensive infrastructure. Nevertheless, as NPR reports, over 100 injection wells in Oklahoma have already been modified to inject into shallower strata.
Interestingly, this new paper by Walsh and Zoback also offers a case study in how the news media and prominent blogs cover contentious scientific issues. Although science reporting often attracts scorn for allegedly being misleading, simplistic, or plain wrong, traditional media reporting on the Walsh and Zoback study has generally been accurate. Sources like the Associated Press, VICE News, and NPR’s Morning Edition have all correctly reported this study’s findings that disposal of coproduced saltwater—and neither fracking nor EOR—is causing the surge in Oklahoma’s earthquakes.
However, the rather unambiguous findings of this new study provide an especially good opportunity to witness the tactics that nontraditional sources at the fringes employ to spin science to match their interests. On one end of the spectrum, the industry advocacy website Energy In Depth published a post casting doubt on the study’s scientific findings. Their post employs tactics such as (incorrectly) implying that high volumes of produced water in past decades can be conflated with (unknown) volumes of injection specifically into the Arbuckle at that time, regardless of the clarity of the present-day link between Arbuckle injection and earthquakes (which they appear not to dispute anyway).
Meanwhile, the major environmental blog Inside Climate News published an astonishingly misleading post that correctly acknowledges that produced water is causing the rise in earthquakes, but also declares—falsely—that the Walsh and Zoback study confirms their belief that the “earthquakes are caused by [the] fracking boom.”
These posts from both political extremes bookend generally accurate portrayals of the new study between contradictory and misleading information. Such tactics lead readers toward false assumptions while simultaneously shielding the blogs from accusations of distributing patently false information.
Any scientific knowledge—even inconvenient or unexpected news—is beneficial to society because information provides us with greater understanding of the natural processes that might be leveraged to find solutions to pressing problems, such as man-made earthquakes. Indeed, the Walsh and Zoback study is good news both for industry and for those concerned about Oklahoma’s earthquakes: it demonstrates clearly that petroleum production itself is not to blame for most of the seismicity, and it reveals a relatively painless remedy.
Image courtesy of Wikimedia Commons.