Every day across the United States, about two billion gallons of fossil-fuel-industry wastewater is forced through underground tubes, descending into the porous rock, filling the holes and gaps with brine and chemicals, the result of extracting oil and gas from the ground.
In the United States, injection well activity is regulated by the EPA and state governments under the Safe Drinking Water Act (SDWA). The EPA has issued Underground Injection Control (UIC) regulations in order to protect drinking water sources.
And while energy companies are following EPA regulations to get the wastewater out of sight, out of drinking water and out of harm’s way, the practice is causing earthquakes, some as far as 15 miles away from the wells.
In a new study in the journal Science, scientists from UC Santa Cruz compiled and analyzed data from around the world for earthquakes clearly associated with injection wells. They describe for the first time how earthquakes can be triggered so far away from the wells themselves, putting into question the use of injection wells as a means of disposing of fracking fluids.
Injection wells and associated earthquakes
A July 2013 study by U.S. Geological Survey scientist William Ellsworth links earthquakes to wastewater injection sites. In the four years from 2010-2013, the number of earthquakes of magnitude 3.0 or greater in the central and eastern United States increased dramatically.
Results of ongoing multi-year research on induced earthquakes by the United States Geological Survey (USGS) published in 2015 suggested that most of the significant earthquakes in Oklahoma, such as the 1952 magnitude 5.5 El Reno earthquake may have been induced by deep injection of wastewater by the oil industry.
In this latest study, the researchers found that a single injection well can cause earthquakes at distances more than 6 miles (10 kilometers) from the well. And they also found that injecting fluids into sedimentary rock can cause larger, more distant earthquakes than injecting into the underlying basement rock.
“This is problematic since the current advice is to preferentially inject into the sedimentary sequence as a theoretically safer alternative to the basement rock,” said Emily Brodsky, professor of Earth and planetary sciences at UC Santa Cruz, according to Phys.org.
However, Postdoctoral researcher Thomas Goebel says the major issue is the spatial footprint of induced seismicity around the injection well. “It’s not that the basement rock is safe because there is still the possibility of encountering a fault in the basement rock that can cause a large earthquake, but the probability is reduced because the spatial footprint is smaller,” he explains.
In the study, published August 31, Goebel and Brodsky describe two distinct patterns of induced seismicity that have different physical mechanisms that act on sedimentary and basement rock.
In the first pattern, associated with injection into basement rock, earthquakes tend to occur in a cluster around the well site, with a steep decline in earthquakes the further away from the well you move. With the second pattern, associated with sedimentary rock, induced earthquakes decline gradually with distance from the well and occur at much greater distances.
It has always been thought that the physical mechanism that causes injection wells to induce earthquakes was caused by increased fluid pressure in the pores of the rock, causing faults to slip more easily. This mechanism can account for the spatial pattern of seismicity seen with injection into basement rock, Goebel said.
But Goebel says there seems to be a different mechanism working when wastewater is injected into sedimentary rock. He suggests it is due to efficient “poroelastic coupling,” which controls the ability of the rock to transmit fluid stresses into the solid rock matrix.
“When you inject water into the ground, it pushes on the surrounding rock and creates elastic stress in the rock, which can put pressure on faults at a distance without putting water into those faults. So if poroelasticity is dominant, you end up with a larger footprint because it’s loading neighboring faults beyond the area of increased pore pressure,” Brodsky said.
According to the study, “Far-reaching spatial effects during injection may increase event magnitudes and seismic hazard beyond expectations based on purely pressure-driven seismicity.” And according to the researchers, this is why Oklahoma has seen a dramatic surge in earthquakes since 2010, to the extent that there are now more earthquakes each year in Oklahoma than in California.
“In Oklahoma, they are injecting into the high-porosity sedimentary unit above the basement, but these elastic stresses can be transmitted over a large distance, so you could activate a large basement fault at a distance of 10 kilometers,” Goebel said. “That may be what we’re seeing in places like Oklahoma.”
Will this study make it up to regulatory authorities? Hard to tell, but it is convincing, especially to those people living in Oklahoma.
