In recent years, advances in horizontal drilling and hydraulic fracturing have accelerated the extraction of natural gas from shale formations, ushering in a new era of energy productivity. But, once areas have been depleted of their hydrocarbons, is there a good use for the fractured shale formations left behind? NETL researchers are using complex experiments to determine if the formations can accommodate a new role as a reservoir for carbon dioxide (CO₂) captured from fossil fuel burning power plants and other industries.

The idea seems attractive because depleted shale gas reservoirs already have much of the infrastructure needed to perform CO₂ injection like pipelines, wells, and developed well sites that were used to remove natural gas. However, there isn’t a clear picture of how the geochemistry of CO₂ can affect the shale. That’s where NETL research enters the picture. The Laboratory is attempting to fill this knowledge gap through shale characterization efforts.

Initial NETL research has shown that injected CO₂ may change the rock’s porosity (amount of empty spaces) and permeability (how easily fluid can move through the rock). Understanding these and other effects is key to developing successful carbon storage techniques and achieving more accurate predictions of the formation’s storage potential.

A recent NETL study looked at samples of the Utica shale using a variety of tools and techniques such as in-situ infrared spectroscopy and feature relocation scanning electron microscopy that can characterize samples at subsurface storage temperature and pressure conditions.

“Because 47–91 percent of fracturing fluids remain in the formation after production, we are looking at wet (CO₂ and fracturing fluids) as well as dry CO₂ interaction” Dr. Angela Goodman said. “The results of the study demonstrated, contrary to current literature, that even dry CO₂ injection can cause etching in the rock. That’s significant because etching has the potential to alter the physical characteristics of the rock over time, affecting the storage potential of the formation.”

Many shale types contain approximately 1–20 percent organic material in the form of waxy material called kerogen as well as abundant amounts of clay. Injected CO₂ also interacts with these shale components through chemical alterations of the rock, swelling or shrinkage of the matrix, and other geomechanical effects.

“Our study found that shales with a higher content of kerogen and certain clays would be expected to have the highest CO₂ storage capacity provided these constituents were accessible for interaction,” Dr. Goodman said.

Researchers are optimistic about the research and its implications not only for carbon storage, but also for determining if CO₂ can be safely and effectively used as a fracturing fluid in future hydraulic fracturing operations.

The work also supports an Energy Department goal to increase the ability to predict CO₂ storage capacity in geologic formations to within ± 30 percent. Understanding CO₂ interactions on shale is key to that goal. Expanding knowledge about the chemical reactions that occur underground in shales where hydraulic fracturing has been performed is just one of the ways that NETL works to discover and disseminate technology approaches that support the nation’s energy security.