After a 3.2 magnitude earthquake hit Saturday evening near the northeastern Colorado town of Greeley, questions about its connection to oil and gas development started popping up on social media and in the blogosphere, with anti-fracking activists trying to make a link between the two.
— Gary Wockner (@GaryWockner) June 1, 2014
While it is possible for earthquakes to be linked to energy development, calling the temblor a "frack quake" is probably taking the connection a little too far.
As U.S. Geological Survey seismologist Justin Rubinstein explained, the process of hydraulic fracturing does not cause earthquakes -- at least the kind people feel. The earthquake connection comes from a different part of the process, a disposal technique called wastewater injection, Rubinstein said.
As oil and gas comes out of drilled wells, a lot of water does too. That water often carries nasty chemicals from deep inside the earth, including arsenic and heavy metals, as well as the chemicals used in hydraulic fracturing. In order to get rid of that toxic water, companies often drill special wells, called injection wells, to dispose of it. They then inject the dirty water into those wells, deep in the ground. Injection wells are also used outside of the energy industry; the Bureau of Reclamation injects saltwater into the ground in part of Western Colorado, which has also led to earthquakes in the past.
"Most of the time, this is no big deal. The water goes in there; we never hear from it again," said Rubinstein.
But every once in a while, it leads to earthquakes. In recent years, the number of magnitude 3 or greater earthquakes has increased significantly. The U.S. Geological Survey is on a mission to understand why, and one of its prime suspects is injection wells. When earthquakes are caused by humans it is referred to as "induced seismicity," and the increase in earthquakes has made this a hot research area right now, seismologists say.
Over the past few years, injection wells have been linked to earthquakes in many states, including Colorado, Oklahoma, and Texas. While scientists are making strides in linking the quakes to wells, they have many unanswered questions. They don't know why some injection wells are linked with earthquakes and others, nearby, are not. They also don't know if the depth or the pressure of the injection affects the likelihood of an earthquake.
Ideally, if researchers knew what led some injection wells to cause earthquakes, it could help companies injecting the waste plan how to avoid causing them.
In Greeley, according to online data from the Colorado Oil and Gas Conservation Commission, there are two active injection wells within about a mile and a half of the earthquake's epicenter, and another one about 10 miles away. Injection wells have been linked with earthquakes as far as 15 miles away, although most researchers have not found linkages at distances further than 10 miles from an injection well. Overall, Weld County has 28 active wells injecting water from oil and gas operations, and there are 334 statewide.
Matt Lepore, COGCC director, pointed out that the three past instances of earthquakes in the state he knew were caused by injection wells all came from wells almost adjacent to the earthquake, with injection depths akin to that of the earthquake epicenter. The May 31 earthquake’s epicenter was about 25,000 feet below ground. The injection well nearest the quake has a depth of 8,700 feet; the second nearest was 10,700 feet deep.
Injection wells are also individually permitted with a maximum pressure, which is calculated to be less than the pressure it takes to cause a fracture in the area of the well, Lepore said. The COGCC sets a maximum limit of water each injection well can accept, although some older wells may have been drilled before the regulations setting maximum limits went into effect.
The Commission is also part of a multistate working group of regulators, industry, non-governmental organizations and others to explore the potential relationship between injection wells, other oil and gas activity, and earthquakes they may cause, Lepore added.
Human-caused Quakes Not Unique To The Age Of Fracking
Some of the earliest understanding that humans could cause earthquakes came from Colorado. In the 1960s, an injection of wastewater in the Rocky Mountain Arsenal led to the most damaging earthquake in Colorado's history, a magnitude 4.8 that caused over a million dollars in damages.
This earthquake is believed to have been triggered by the deep injection of liquid waste into a borehole at the arsenal. It was followed by an earthquake of magnitude 4.5 three months later in November 1967.
"That was perhaps the first known example of how we could cause earthquakes by injecting fluids," said Paul Morgan, a senior geologist with the Colorado Geological Survey.
So while geologists have known for over 50 years that injection can cause earthquakes, what's surprising about the recent quakes is that the injection is of a different type. Past human-triggered seismic events, like the one at the Arsenal in 1967, came from fluids being injected at high pressures into what Morgan called "crystalline" rocks, like granite. Now, fluids are being injected at lower pressures, or sometimes even without added pressurization, into sedimentary rocks. That this has resulted in earthquakes as big or bigger than a magnitude 3 on the Richter scale is somewhat unexpected, said Morgan.
To date, the largest earthquake known to be linked to wastewater injection was in 2011 in Prague, Okla., a magnitude 5.6. The second largest also occurred in 2011, in southern Colorado's Raton Basin, a 5.3.
The Greeley earthquake was unusual; while Western Colorado is no stranger to small quakes, this quake breaks a 45-year earthquake free streak in this part of the state; a magnitude 4.2 hit east of Greeley in 1969.
Seismologists Eager To Learn More
Researcher Richard Aster of Colorado State University is rushing to put out a network of seismographs, which measure earthquakes, in the area.
Aster is also interested in learning more about how wastewater injection may be connected to earthquakes.
"In rare cases injection wells can cause earthquakes. And that's a phenomenon that we need to understand better," said Aster.
If Aster can organize a network of seismographs, he'll be able to learn a lot more about any aftershocks that may follow this earthquake. He'll know a lot more about the quakes' location, depth, size and the physics behind them.
Whether or not the quake was caused by wastewater injection, if the seismographs get installed, the activity -- or lack thereof -- they measure could help scientists learn more know about why some injection wells cause quakes, and most do not.