HIGHLIGHTS
• Storing renewable energy generated from solar and wind in the form of compressed air is potentially achievable in abandoned oil and gas wells. 
• By using geothermal heat present in the abandoned wells, GA-CAES will enhance efficiency and reduce dependency on external energy sources.
• Conceptual, mathematical and numerical models were generated to determine the benefits of potentially using GA-CAES. 

The transition to renewable energy generation has the potential to significantly reduce greenhouse gas emission but still presents challenges. Solar and wind energy are the two primary renewable energy sources, but both are not always present as the sun does not always shine, and the wind does not always blow.

This dilemma is leading researchers to develop strategies for storing energy generated from renewable sources during periods of low demand for use at times when there is need for it. One potential energy storage uses hydropower. In a previous TLT article,¹ researchers developed an approach to improve the efficiency of generating electricity from pumping water. This method starts by pumping water from a low land elevation point to a higher land elevation point on earth during a period of low electricity demand. When more electricity is needed, the process reverses as the potential energy stored in the water at the higher reservoir moves to the lower reservoir under gravity. Kinetic energy forms which is used to then turn a turbine producing electricity.

To reduce cost, the researchers demonstrated that prefabricated structural steel modules and a closed loop system can be used. 

Arash Dahi Taleghani, professor of petroleum and natural gas engineering at Penn State University in State College, Pa., says, “Another approach for storing energy involves the use of compressed air. The technique, known as compressed air energy storage (CAES), starts by using electricity to compress air into a specific storage container using energy and then releasing it to generate electricity during periods of high demand. CAES has an advantage in not relying on a water source and is able to integrate wind, solar and geothermal energy sources into the electric grid.”

But there are limitations with CAES as indicated by Dahi Taleghani. He says, “CAES systems are diabatic which means they lose heat during compression which cannot be recovered. Fossil fuels are needed to provide energy when the air goes into the electricity generating turbines leading to added greenhouse gas emissions and reducing system efficiency.”

Adiabatic compressed air energy storage (A-CAES) has emerged as a viable option that stores heat generated during compression and reuses it prior to the air expansion cycle. This has led to an increase in efficiency but still leads to the issue of how to cost effectively identify suitable locations for storing large volumes of compressed air in high pressure containers.

Dahi Taleghani and his colleagues conducted a study to determine if using abandoned oil and gas wells is a viable option.



GA-CAES
The researchers believe that oil and gas wells are suitable for A-CAES because they offer advantages such as stability and natural insulation, allowing them to retain compressed air. Another benefit is that the number of abandoned oil and gas wells in the U.S. is estimated to be in the 2 to 3 million range with approximately 130,000 wells documented as orphaned, meaning they have been abandoned. 

Dahi Taleghani says, “Abandoned oil and gas wells represent environmental hazards because they lack appropriate maintenance, monitoring, an owner and they can represent a high risk for emission such as methane which is a potent greenhouse gas and ground water contamination. Our proposal to use them in a productive manner is an example of an upcycling process.”

The key to this approach is to utilize the geothermal heat present underground. This heat source will be able to increase the pressure of the stored air leading directly to a higher release of stored energy, assist with improving heat conductivity in the bottom of the well and improve heat insulation in the top half of the wellbore. 

As a result, the researchers’ objective became to better understand the viability of geothermal-assisted compressed air energy storage (GA-CAES). Dahi Taleghani says, “An abandoned well with a depth of 2,000 meters is needed for GA-CAES. The temperature at the bottom of this well is probably about 70°C, which will increase the energy that can be released when needed. The cost of developing GA-CAES should be lower since the infrastructure is also in place and the economic efficiency will increase due to the availability of geothermal heat.”

He adds, “We believe GA-CAES has four main potential advantages. Abandoned oil and gas wells can be repurposed for energy storage. In the U.S. state of Pennsylvania, we estimate there are greater than 400,000 idle wells. Using geothermal energy will enhance efficiency and reduce reliance on external energy sources. Finally, substantial cost savings by using existing infrastructure can be achieved.” 

To determine the feasibility of using abandoned oil and gas wells, the researchers prepared a conceptual model, then produced a mathematical model and finally generated a numerical model. System efficiency was found to be a function of wellbore depths. With a wall length of only 500 meters, the researchers calculated an efficiency value of 53%. This can increase to 65% if the wellbore depth is extended to 2,500 meters. 

The researchers found that system efficiency increased by approximately 9.5% when further pressurizing the air in the well with geothermal heat increased pressure by approximately 0.5 megapascals. An additional benefit in using GA-CAES is that abandoned oil and gas wells are present in clusters which will further improve the efficiency of pressurization and of utilizing the stored air as an energy source when needed. 

Dahi Taleghani indicates that he and his colleagues are working with interested parties in an effort to find a partner to commercialize GA-CAES. Additional information can be found in a recent article² or by contacting Dahi Taleghani at aud440@psu.edu. 

Dahi Taleghani is a member of Penn State’s Repurposing Center for Energy Transition (ReCET). The objective of ReCET is to identify approaches for repurposing abandoned oil and gas wells and abandoned surface and subsurface mines so they can facilitate use of renewable energy sources. 
REFERENCES
1. Canter, N. (2024), “Energy storage using pumped storage hydropower,” TLT, 80 (21), pp. 12-13. Available at www.stle.org/files/TLTArchives/2024/02_February/Tech_Beat_I.aspx.
2. Zhang, Q., Dahi Taleghani, A. and Elsworth, D. (2025), “Underground energy storage using abandoned oil & gas wells assisted by geothermal,” Journal of Energy Storage 110, 115317.