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"It is a win-win situation."
Battery Cage
Researchers have successfully turned an abandoned oil and gas well into a geothermal energy storage system, repurposing a once-polluting resource extraction site into what they say amounts to a green energy battery.
As detailed in a new study published in the journal Renewable Energy, the researchers from the University of Illinois Urbana-Champaign were able to make use of the deep subsurface structure, despite the fact that it doesn't actually produce geothermal energy.
That's because they found it was the perfect place to build an artificial geothermal reservoir, which stores energy in the form of heat in the surrounding rocks.
"Many of the same properties that make a subsurface rock formation ideal for oil and gas extraction also make it ideal for geothermal storage," said lead researcher Tugce Baser, an environmental engineering professor at the University of Illinois, in a statement. "And because our test site is a former gas well, it already has most of the needed infrastructure in place."
Win-Win
The long-term vision is to store excess heat from nearby industry underground and release it as electric power when demand is high.
"The underground reservoir essentially acts as a large underground battery while repurposing abandoned oil and gas wells," Baser said. "It is a win-win situation."
The Illinois Basin, a large geological feature that stretches underneath almost the entire state, contains spongelike rock and minerals with excellent thermal conductivity. Insulating layers ensure that all the heat doesn't get dissipated immediately.
Heat Injection
In a test, Baser and his team injected water preheated to 122 degrees Fahrenheit into a layer of porous sandstone 3,000 feet under the surface using the abandoned oil well.
The results were surprising.
"Our field results, combined with further numerical modeling, find that the process can sustain a thermal storage efficiency of 82 percent," Baser said.
According to the new study, it would even be an economically viable and even profitable system, producing electricity at a competitive $0.138 per kilowatt-hour.
"Our findings show that the Illinois Basin can be an effective means to store excess heat energy from industrial sources and eventually more sustainable sources like wind and solar," Baser concluded.
READ MORE: Geothermal 'battery' repurposes abandoned oil and gas well in Illinois, researchers report [University of Illinois]