A new study from the Massachusetts Institute of Technology has found some potential issues with injecting carbon dioxide deep into the earth.

Carbon sequestration—paired with carbon capture—is seen by some as a technology that will allow coal-fired power plants to keep operating in the United States, despite stricter environmental regulations that limit greenhouse gas emissions. Carbon capture isolates the gas from power plant emissions, and then the CO2 is sequestered thousands of feet underground. Different ways to capture the carbon are being tested at some power plants—including one in Kentucky—but so far, the technology has proved too expensive to do on a large scale.

The MIT study published this week found that once it’s injected underground and reacts chemically with the geology, less of the CO2 becomes solid rock than previously thought. Instead, much of it stays in a gaseous or liquid form.

“So when it’s solid it’s very stable because it’s a rock,” said MIT postdoc Yossi Cohen, one of the study’s authors. “But when it’s a gaseous phase or a liquid phase, it can escape at some point back to the atmosphere.”

And that defeats the purpose of injecting the carbon dioxide underground to begin with.

The Kentucky Geological Survey has drilled two wells to test carbon dioxide sequestration in Kentucky. CO2 was actually injected into the first well, drilled in Hancock County in 2009. The second well was drilled to test the geology in Carter County in 2013.

Rick Bowersox of the Kentucky Geological Survey said the MIT study confirms the findings of a similar study by KGS looking at CO2 sequestration in Western Kentucky. He said the test wells drilled in Kentucky suggest the geology here (using thick caprock over the reservoir where the CO2 is stored) does a good job of keeping the carbon underground.

“We cored and had lab analyses performed on the caprock shales in both our recent Carter County test well in 2013 and the Hancock County test well in 2009,” Bowersox said via email. “ Results of the analyses of the caprock shale from the Carter County well are still trickling in, but they show, like what we found in the Hancock County well, that we have shales in Kentucky that is far exceed the requirements of a good caprock.”

That being said, Bowersox added that it’s unlikely carbon dioxide will ever be sequestered in Kentucky. Several of the state’s coal-fired power plants are retiring, and the geology in nearby states—like Illinois—is better suited to holding the gas.

MIT’s Yossi Cohen said his study doesn’t show that carbon sequestration doesn’t work—just that it’s complicated, and that there are certain types of geology which are better suited to holding the CO2 long-term. But he said further research is needed to figure out what geologic formations prompt which chemical reactions to keep carbon sequestered.