As hydrofracturing, also known as fracking, has become more widespread, it has also grown more controversial.
Beyond groundwater pollution and environmental contamination, the multi-step process of extracting natural gas from the Earth by injecting liquids deep underground has raised concerns that parts of the process may be triggering earthquakes.
In fact, found a new study, small earthquakes are more common than expected near wells that are used for injecting waste fluids from fracking, as well as from other processes, including oil extraction and geothermal energy production.
It’s not the drilling into the earth or the fracturing of rock that causes the quakes, emphasized lead author Cliff Frohlich, a geophysicist and seismologist at the University of Texas at Austin. Instead, it appears to be the disposal of large amounts of fluids by injection deep underground that can trigger certain types of faults.
The vast majority of triggered quakes are too small to cause any damage. By better understanding how and why certain processes shake the Earth, though, scientists can gain a better understanding of how earthquakes work. And industries may be better able to choose sites for disposal wells.
“Studies have been coming out in the last five years, and most carry the idea that yeah, humans can cause earthquakes, but it’s a relatively unusual phenomenon,” Froholich said. “Studies like mine suggest that humans may be causing a lot more small earthquakes than we were aware of.”
“I’m not an alarmist,” he added. “These, for the most part, are not harmful. We need to study them more, but there’s nothing in my study that shows there’s great danger.”
Human activities have been triggering earthquakes for decades. One of the earliest known examples came in the 1930s, when the Hoover Dam filled Lake Mead, exerting great pressure on the earth, and inducing a series of quakes.
Since the 1960s, seismologists have been recording earthquakes that happen after liquids are injected into the ground, which happens during a variety of processes. Fluids, for example, are often used to increase pressure and push oil and gas from deep underground.
In the last few years, several high-profile earthquakes from Ohio to Texas have occurred near wells where companies dispose of liquid fracking wastes by injecting them into the ground. By highlighting the link between injections and quakes, Frohlich said, the media often frightens the public.
It would be like living in a country where there were no cars and hearing about cars only when there was a big crash, he said. It would be easy to think that getting into a car inevitably leads to a wreck.
To add context and perspective to the issue, Frohlich turned to an array of seismometers that collect continuous underground data throughout the Barnett Shale, a huge and highly tapped natural gas field in north central Texas.
Looking at data gathered over a two-year period, from the fall of 2009 to the fall of 2011, Frohlich detected 67 earthquakes of magnitude 1.5 or larger in the region. The total was eight times higher than the number of quakes reported in the area by the United States Geological Survey over the same period, mostly because the USGS doesn’t routinely monitor earthquakes below a magnitude of three.
Frohlich’s study area contained hundreds of wastewater injection wells. Virtually all of the earthquakes he detected were clustered within 3.5 kilometers (2.2 miles) of wells that took in large amounts of fluids, he reported today in the journal Proceedings of the National Academy of Sciences. High-volume injection wells receive more than 1,500 barrels – or 24,000 cubic meters (31,390 cubic yards) – of water each month.
On the flip side, there were more than 100 high-volume wells in the area that were not associated with any earthquakes during the same two-year period. Frohlich suspect that wells must be located near locked faults, which are stuck because of friction between two plates but can give way with enough strain. The ground must also be permeable enough and faults must also be oriented in the right direction to give way.
“Usually, we respond to these induced quakes associated with wastewater disposal when they happen, when they’re reported in newspapers, and when people living nearby get concerned,” said Art McGarr, a seismologist with the USGS in Menlo Park, Calif. “This is one of the few systematic studies of an area where there are many wells that are injecting large amounts of wastewater deep underground.”
There are more than 40,000 wastewater disposal wells across the United States and more coming online every day, McGarr said. Understanding why some of them sometimes trigger quakes will be useful information – for industries, regulators, and scientists. Very occasionally, an induced quake causes damage to buildings and other structures.
“From what he presents in this paper, it looks like there’s a glimmer of hope that the oil and gas industry might use this sort of data along with mapping of underground formations to predict in advance whether or not an injection well might…encourage a slip,” McGarr said.
“Any time you study manmade earthquakes and you know what caused the earthquake, what the forces were, and what man did to the earth to cause the earthquake, it gives you a lot of information about how the earthquake works,” he added. “We’ve put a lot of effort into studying these types of triggered earthquakes, and they’ve given us some valuable insights.”