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A major increase in seismic activity has risen alongside the growth of oil and gas production in areas like Oklahoma, Kansas and Colorado.

Scientist have found that wastewater injection increases pressure deep in the earth to cause an increase in tremors. A new study on earthquakes in Oklahoma shows that not only have earthquakes become more frequent in the region, but they have also been going deeper. Researchers from Virginia Polytechnic Institute suggest that in that specific area, stretching from Oklahoma City to Tulsa and north to Wichita, Kansas, the increased pressure that can trigger earthquakes will continue for another 10 years after oil and gas companies stop wastewater injection.

Oddly, in comparison, the same will not be true for the Raton Basin. Here, though tremors have increased along with oil and gas production in the same manner, the quakes are not getting deeper due to the fact that the wastewater is not denser than the fluids in the deep rock formations. So, the wastewater doesn’t sink and earthquakes in the Raton Basin are not getting deeper, the team reports.

But in Oklahoma, quakes become systematically deeper soon after fossil fuel companies inject this high-density wastewater. The finding implies a surprising long-term consequence of wastewater disposal in the area.

“Our study implies that simply turning off the injection wells will not immediately stop the earthquakes,” said Virginia Polytechnic Institute hydrogeologist Ryan Pollyea, who led the new research. “Our models show that fluid pressure caused by high-density wastewater disposal can persist in the environment over time scales exceeding 10 to 15 years,” he said.

Pollyea’s idea for the study came while being in the cold at a workshop in Oklahoma. “It occurred to me that any wastewater stored above ground was probably getting cold,” he said. When it’s cold, a fluid’s density increases and as density rises, fluid pressure builds up. This led Pollyea to the idea that water density might play a role in the seismic activity associated with wastewater injections. If the wastewater is denser than the fluid already sitting in deep geologic formations, then injecting high-density wastewater into underground wells could create enough pressure to set off quakes. And, as it turns out, the oilfield wastewater in Oklahoma and Kansas is denser than the fluid in the subterranean geologic formations.

Pollyea was curious to know how this difference in density would affect the underground pressure. To find out, he and his team built a model that reproduces the physics of the system.

The model revealed that high-density wastewater sinks and displaces the lower density fluids in the deep rock formations there in Oklahoma, while in the Raton Basin the fluids in the deep rock formations were more dense than the injected wastewater.

Pollyea and his team were published in the journal of Nature Communications.