"That has some very interesting and I think important consequences for how we understand the hazard posed by oilfield wastewater disposal."

Oil wastewater disposal from oil and gas production may cause earthquakes for decades even after operations or are reduced, says new research published earlier this week.

Fracking, along with oil and gas production in general, in the United States is a booming industry. With this comes an increase in the rate of oilfield wastewater production. Fracking, the process of shooting water mixed with sand and chemicals into the earth to release the oil and natural gas from rock formations.

Fracking produces huge volumes of wastewater that contains cancer-causing chemicals, salts and naturally occurring radioactive material that can cause earthquakes and contaminate aquifers.

According to the United States Geological Survey, wastewater disposal from oil and gas production is the number one cause of human-induced earthquakes. In Oklahoma alone, where the industry is booming, there has been a 4,000% increase in earthquake activity in the past eleven years.

A team of experts from Virginia Polytechnic and State University studied the wastewater flows in two fracking-heavy states, Kansas and Oklahoma. They concluded that wastewater could be responsible for years to come, possibly decades.

“That has some very interesting and I think important consequences for how we understand the hazard posed by oilfield wastewater disposal,” said Ryan Pollyea, lead author of the study.

Wastewater affects subterranean fluid pressure so much that even after operations have ceased or are scaled back the threat of an earthquake remains.

According to the study:

When SWD operations cease after 10 years of injection, our timeseries results show that maximum fluid pressure accumulation (~80 kPa) occurs at 6 km depth and 3 years after injection stops (Fig. 3). These results also reveal that high TDS wastewater continues downward migration through the seismogenic zone for an additional decade causing increasing fluid pressure at sequentially greater depths (Fig. 4a–c). Because there is no additional wastewater injection to increase the dynamic load, this post-injection pressure accumulation is due solely to the advective transport of the high-density wastewater.

However, even if the number of earthquakes declines, larger earthquakes may occur more often:

Because fluid pressure continues increasing at these depths for over a decade after significant SWD rate reductions (Fig. 4c), our study implies that even though earthquake frequency may decline after reduction of SWD injection rates, the sinking wastewater may induce larger earthquakes. Put differently, mandated SWD rate reductions have effectively decreased the number of injection-induced earthquakes in Oklahoma and Kansas, but the occurrence rate of high-magnitude earthquakes is decreasing more slowly than the overall earthquake rate (Fig. 5c) because density-driven pressure transients remain in the environment for much longer time periods than those governed by pressure diffusion.

Earthquakes of a magnitude 3 or greater have increased in the past 11 years. In 2008 there were around 20 a year but that number has increased to more than 400 annually.

“Our study can be used to improve hazard models for injection-induced earthquakes by accounting for fluid pressure variations that occur after injection operations are reduced or stopped,” Pollyea said.

The researchers hope that the study motivates further research into the relationship between wastewater and seismic activity.