As states seek out much-needed supplies of clean, reliable energy, some are looking to an unconventional source: abandoned oil and gas wells harnessed for geothermal heat.

Millions of inactive wells are littered across the United States, the relics of earlier eras of fossil fuel production. A large number of the sites have no official owner, and many are still polluting groundwater and leaking heat-trapping methane. The country has barely scratched the surface in dealing with this problem.

Policymakers in both Republican- and Democratic-led states are exploring whether these sites could instead be converted into new wells for producing geothermal energy. The holes are already drilled in the ground, after all. And regions with widespread oil and gas development have rich subsurface data that geothermal firms need in order to determine where and how to build their carbon-free systems.

The concept is relatively new and largely untested, though scientists and startups are working to change that. States are also laying the groundwork for action by lifting regulatory hurdles and launching in-depth studies.

In Oklahoma, the state Senate is considering a bill that would create a process for companies to buy abandoned oil and gas wells and repurpose them for geothermal energy or underground energy storage. Oklahoma has identified over 20,000 such wells, and state regulators estimate that it would take 235 years and hundreds of millions of dollars to plug all of them. Fixing a single old well can cost anywhere from $75,000 to $150,000 or more, by some calculations, depending on where it’s located and how complicated it is to clean up.

The Well Repurposing Act, which passed Oklahoma’s House in March, is modeled after a similar law that New Mexico adopted last year to address its 2,000-plus orphan wells.

The Oklahoma bill ​“recognizes that these wells are a liability, and that there may be a way to turn them into some sort of revenue generation and give them value,” said Dave Tragethon, communications director for the nonprofit Well Done Foundation, which works to find and cap abandoned oil and gas wells nationwide. ​“And if there’s value, that means there’s more of a willingness to address them and more of an opportunity to raise funding.”

In Alabama, legislators passed a law last month that allows the state to approve and regulate the conversion of oil and gas wells to tap alternative energy resources like geothermal. North Dakota adopted a bill last year requiring a legislative council to study the feasibility of using nonproductive wells to generate geothermal power. And in Colorado, state agencies just launched a technical study to evaluate the potential of repurposing old wells for geothermal development and carbon capture and sequestration.

These efforts reflect the growing bipartisan support for geothermal energy, which has largely remained unscathed by the Trump administration’s efforts to block renewable energy projects. The energy resource has the potential to help meet the nation’s soaring energy demand while also slashing planet-warming emissions from electricity and heating.

Converting Wells Is Enticing but Complicated

Geothermal systems work by circulating fluids underground to capture naturally occurring heat, which can then be used to drive turbines for generating electricity or to directly warm the air and water in buildings. The industry is gaining momentum thanks to recent advances in drilling methods and technologies that are making it technically possible or financially viable to access geothermal energy in more places.

Many of those breakthroughs have come from the oil and gas industry, whose skilled workforce of drilling engineers and geoscientists, and deep corporate pockets, have helped launch startups and deploy cutting-edge systems. However, most of that expertise and funding is being poured into building new projects—not figuring out how to retool leaky wells left behind by earlier generations.