It turns out disposing of hydraulic fracturing wastewater may not be to blame for the earthquakes in Oklahoma after all. A new study conducted by seismologists from Stanford University confirms the widely held belief that injecting large volumes of fluid into underground disposal wells is likely responsible for most of the recent quakes in Oklahoma. The study also found the source of the vast majority of this fluid is unrelated to hydraulic fracturing.
No state has experienced a more significant increase in earthquakes than Oklahoma, which has had 585 of the 688 recent earthquakes of magnitude three or larger in the Midwest, with the increase beginning largely in 2009. The increase coincides with an increase in wastewater disposal into the Arbuckle Formation—a 7,000-foot-deep sedimentary formation under Oklahoma—from about 3.36 billion gallons per month in 1997 to about 7.6 billion gallons per month in 2013.
Anti-fracking groups have been quick to point fingers at hydraulic fracturing, claiming the technique has greatly increased the volume of wastewater in need of disposal into injection wells, which in turn is causing the increase earthquakes.
Fracking does generate wastewater, approximately 800,000 to 1,000,000 gallons per well. However, the Stanford researchers found hydraulic fracturing wastewater comprised less than 10 percent of the wastewater generated from oil and natural gas production in the three areas of Oklahoma with the highest rates of seismicity.
About 90 to 95 percent of the wastewater injected into underground disposal wells in these areas of Oklahoma is classified as “produced water,” a naturally salty type of water that is often present along with the hydrocarbons in oil reservoirs. This water is coproduced when the oil is pumped to the surface, hence the term “produced water.”
Although fracking generates approximately 800,000 to 1 million gallons per well on average, the amount of wastewater generated by all of the hydraulically fractured wells in Oklahoma constitutes a tiny fraction of the 7.6 billion gallons of wastewater generated on average each month in the state. In fact, the U.S. Geologic Survey (USGS) reports much of the wastewater is produced in rock formations that did not experience any hydraulic fracturing. Much of this wastewater is “produced water” associated with oil production in the Mississippi Lime formation, a rock formation that contains a high ratio of produced water to hydrocarbons.
Wastewater disposal has undoubtedly resulted in incidents of induced seismicity, but USGS reports it is rare for a wastewater disposal well to induce these types of quakes. Of the 35,000 wells used to dispose of oil and natural gas wastewater, USGS reports only a few dozen are known to have triggered tremors large enough to be felt at the surface, meaning the vast majority of these wells are aseismic, causing no earthquakes.
The Stanford study is important because it not only gives us a more in-depth look at how injection wells can induce seismicity and demonstrates where the fluid is being injected from; it also gives us insight into how we can prevent future seismic events.
By carefully monitoring injection well pressures, oil and gas producers in Oklahoma and Texas can avoid over-pressurizing formations, such as the Arbuckle Formation, which may be more likely to cause induced seismicity. Producers may also inject produced water back into formations such as the Mississippi Lime, which is unlikely to change the formation pressure because that’s where the water originated. This strategy could greatly reduce the risk of tremors.
There is no question there is still work to do to prevent earthquakes, but it appears fracking and disposal of fracking flow-back water may not be the culprit for Oklahoma’s earthquakes after all.