**Depleted oil and gas wells could be repurposed as compressed-air energy storage (CAES) sites for stockpiling excess energy from renewables for use when needed.**
CAES plants compress air and store it underground when energy demand is low and then extract the air to create electricity when demand is high. But start-up costs currently limit the commercial development of these projects.
“The problem is that sometimes when we need energy, there is no sunshine or there is no wind. That’s why it’s very important to have some storage capacity to support the grid,” said Arash Dahi Taleghani, a researcher at Penn State University.
His team proposes a new geothermal-assisted CAES system that makes use of depleted oil and gas wells. Estimates suggest there are a vast number of abandoned oil and gas wells worldwide, with [a Stanford University study](https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/Aydin1.pdf) from last year finding at least 3.2 million in the US alone and over 200,000 in China.
The researchers estimate that a geothermally-assisted CAES system, which makes use of the deep cavities already extracted from pre-existing wells, could improve efficiency by 9.5% over existing technologies. This means a larger percentage of the energy stored in the system can be recovered and turned into electricity, potentially boosting profits for operators.
“This improvement in efficiency can be a game changer to justify the economics of CAES projects,” said Taleghani added. “And on top of that, we could significantly avoid the upfront cost by using existing oil and gas wells that are no longer in production. This could be a win-win situation.”
Depleted oil and gas wells would give operators easy access to geothermal heat in hot rock formations underground, eliminating the upfront costs of drilling new wells and potentially making the technology more appealing to industry, the scientists have said.
Gases such as compressed air increase in pressure as temperatures increase, meaning the heated wells could potentially store more energy. When electricity is needed, the heated compressed air is released, driving a turbine to produce power.
The team used numerical modelling simulations to find that placing CAES systems in abandoned oil and gas wells significantly increased the air temperature in the systems.
Energy storage options such as CAES are particularly important in the transition to clean energy, according to the researchers, because they help address the intermittent nature of renewable sources. By storing excess renewable energy and releasing it when needed, energy storage contributes to grid stability and reliability.
Repurposing depleted oil and gas wells may also help mitigate potential environmental impacts of abandoned wells as many continue to emit methane long past their operating lifespan. Methane is a potent greenhouse gas, with 80 times the climate warming potential of carbon dioxide.