New research shows that even fracking using liquid carbon dioxide can cause small-scale earthquakes, a phenomenon that could not previously be clearly attributed to the fracking process. While CO2 fracking is good for the environment by sequestering carbon, both CO2 and water-based fracking can cause these tremors and potentially trigger more damaging earthquakes.
New research confirms that fracking is responsible for previously unexplained slow, small earthquakes, or tremors. The process that creates tremors is the same process that creates large, damaging earthquakes.
Fracking involves forcibly injecting liquids below the surface of the earth to extract oil and natural gas. While this method typically uses wastewater, this particular investigation looked at the results when liquid carbon dioxide was used. This method drives carbon deep underground, preventing it from trapping atmospheric heat.
It is estimated that CO2 fracking could save as much carbon as 1 billion solar panels per year. Fracking with liquid carbon dioxide is better for the environment than using wastewater because wastewater cannot keep the carbon out of the atmosphere.
"Because this study examines a process that sequesters carbon underground, it could have positive implications for sustainability and climate science," said Abhijit Ghosh, associate professor of geophysics at UC Riverside and co-author of the study in the journal Science.
However, because carbon dioxide is liquid, Ghosh said the study's results would almost certainly apply to fracking with water, both of which have the potential to cause earthquakes.
On a seismometer, ordinary earthquakes and tremors appear differently. Large earthquakes cause violent jolts of high-amplitude pulses. Tremors, on the other hand, are gentler and smaller in amplitude, rising slowly above the background noise and then falling slowly.
"We're excited to now be able to use these tremors to track the movement of fracking fluids and monitor fault movement as a result of fluid injection," Ghosh said.
Previously, seismologists had disputed the source of the quake. Some papers suggest that the tremor signals come from large earthquakes occurring thousands of miles away, while others suggest that the tremor signals may be noise generated by human activity, such as the movement of trains or industrial machinery.
"Seismometers are not smart. You can drive a truck nearby or give it a kick and it will register the vibrations," Ghosh said. "That's why for a while we couldn't tell whether these signals were related to fluid injection."
To determine the source of the signal, the researchers used seismometers installed around a fracking site in Wellington, Kansas. The data covers the entire six-month frac injection period, as well as one month before injection and one month after injection.
After subtracting the background noise, the team found that the remaining signal was generated underground and only appeared when fluid was injected. "We detected no tremor before or after the injection, suggesting that the tremor was related to the injection," Ghosh said.
Fracking has long been known to produce larger earthquakes. One way to prevent faults from sliding underground and creating earthquakes or tremors is to stop fracking. Because this is unlikely, Ghosh said these activities must be monitored to understand how the rock is deforming and to track its movement after fluid injection.
The oil and gas industry can now conduct modeling experiments to help companies determine fluid injection pressures that should not be exceeded. Staying within these limits helps ensure that fluids do not migrate toward large faults underground, triggering damaging seismic activity. However, not all faults will be mapped.
"We can only build this kind of experimental model if we know that faults exist. It's possible that there are faults that we don't know about, in which case we can't predict what will happen," Ghosh said.