Scientists from the National Laboratory of the Rockies have decided to find out whether U.S. geological formations could be converted into large-scale energy storage facilities and what volumes could be expected. To that end, they collected data on the country’s depleted oil fields and aquifers to build a mathematical model that estimates how much heat, as well as electricity, could be stored in each formation. This is a practical matter for the U.S., since the country’s power demand is growing rapidly due to data centers and industry, while gas turbines, which are typically used to cover peak loads, have been sold out until 2030.
The technology, known as geological thermal energy storage (GeoTES), is not new. In simplified form, it has been used in Europe for heating and cooling buildings via aquifers. Until now, solutions like this have been localized. The U.S. researchers have posed a more far-reaching question: could these geological structures be used to store energy in volumes comparable to power grid needs? The operating principle remains the same: excess heat generated by solar facilities or industrial processes is injected underground into water-saturated porous rock, where it is accumulated and then extracted when needed. In terms of their potential energy efficiency, these formations significantly outperform lithium-ion batteries, which are designed for a few hours of energy output and cannot be used for long-term storage on a daily or seasonal basis.
To calculate the potential of underground storage, the U.S. scientists analyzed the formations (their thickness, porosity, heat capacity and temperature regimes) and accounted for data uncertainty using Monte Carlo methods to simulate various scenarios repeatedly.
They selected four sites for detailed calculations: two depleted oil fields in California and two aquifers in Texas.
The resulting estimates showed that even individual formations can accumulate tens to hundreds of terawatt-hours of thermal energy. In the best-case scenario for the Carrizo-Wilcox Aquifer in the southern United States, the upper estimate reached 7,160 terawatt-hours (TWh) of heat. Taking losses into account, this corresponds to tens or hundreds of TWh when converted into electricity. For comparison, the entire United States consumed roughly 4,000 TWh of electricity in 2022. This means that one such geological storage could theoretically provide the entire country with energy for nearly a week.
These systems are also cost-efficient. Since the storage device already exists as a geological formation, the main costs are associated with drilling and equipping the wells. As a result, the cost of energy storage becomes significantly lower than battery-based solutions, especially for long-term power backup.
The biggest limitation has to do with well preparation and infrastructure deployment. A storage facility with a capacity of about 1 TWh may require from several dozen to some 200 injection and production wells. However, this is hardly significant given the scale of the U.S. oil-and-gas sector, where up to 12,000 wells are drilled nationwide every year.
