The U.S. Department of Energy supports scientists at several Energy Frontier Research Centers (EFRCs) who are investigating creative solutions to solve the problems of carbon sequestration, carbon recycling, and the use of carbon in hydraulic fracturing. This fundamental science research, and the resulting new technologies, could provide answers for industrialized nations seeking to limit carbon emissions, and for industries looking for innovative carbon-capturing technologies.
We store it, like in a refrigerator
Injection of captured carbon emissions into porous rock underground, a form of carbon sequestration, is one approach to lowering carbon dioxide levels in the atmosphere. Young-Shin Jun, who worked on seal integrity at the Center for Nanoscale Controls on Geologic CO2 (NCGC), said the ideal location for carbon sequestration has layers of highly porous rock beneath impermeable layers of rock with much smaller pores.
“When we inject pressurized carbon dioxide, we store it, like in a refrigerator,” explained Jun, who is also a professor at Washington University in St. Louis.
Like food in a refrigerator, the stored carbon dioxide changes over time. When it dissolves, carbon dioxide forms carbonic acid, which then dissolves pre-existing minerals in rocks. This process creates new pores with more room to store carbon dioxide, but it can also cause new cracks in the rock that could leak carbon dioxide into the air and water. EFRC scientists investigated how carbon dioxide changes rocks and what those changes mean for our ability to safely store carbon dioxide in rock.
Geologists at the Center for Frontiers of Subsurface Energy Security (CFSES) investigated how the interactions among carbon dioxide, water, and rock affected storage of carbon dioxide underground. They analyzed the mineral components of rocks near naturally occurring carbon dioxide vents and found significant changes compared to nearby rocks that were only exposed to carbon dioxide in the air.
“It's a matter of what the rocks are composed of initially, and what geochemical reactions are favored given those ingredients and the conditions under which they interact over time,” said Jonathan Major, a geologist who worked on the project.
The scientists found that depending on the mineral components of the rock, temperature, and pressure, the carbon dioxide either creates cracks or forms a cement that seals the gas more securely within the rock.
The carbonic acid that forms when carbon dioxide reacts with water can dissolve some of the minerals in rocks and introduce metal ions to water. The metal ions react with carbonic acid to form new carbonate minerals, which can securely store carbon dioxide for ages. Several cave formations have developed this way, as mineral-rich water dripped through caves and slowly deposited new rock. Scientists at NCGC, studied the very first step in new mineral formation when dissolved minerals come together to form new crystals, a process called nucleation.
Jun said, “Understanding nucleation tells us where the mineral will form and gives us great power to control the system.”
In some places, the formation of a new mineral can create a bottleneck and prevent the further flow of carbon dioxide. While in other areas, new mineral formation can create a seal and prevent leaking.
Recycle it to make new fuel
Another way to lower carbon dioxide emissions is to recycle the carbon dioxide into a useful product. Plants are expert carbon dioxide recyclers and convert the gas into sugars and starches using photosynthesis. Several scientists have attempted similar carbon dioxide recycling and are able to generate useful products, but the process still requires too much energy. Chemists at the Alliance for Molecular PhotoElectrode Design for Solar Fuels (AMPED) recently coupled carbon dioxide recycling to another chemical reaction that is useful for manufacturing.
“You can imagine ways to exploit technology by attaching carbon dioxide reduction to a power plant and recycle it to make new fuel,” said lead researcher Tom Meyer.
Coupling carbon dioxide recycling to a chemical reaction that is already used in manufacturing could make recycling carbon emissions a more economically viable option. The recent results from AMPED show it is possible to use a lower total amount of energy if a manufacturing process is coupled to carbon dioxide recycling.
A closed loop system
Carbon dioxide emissions may also be captured and used for hydraulic fracturing, simultaneously lowering carbon levels and mitigating the environmental impact of extracting natural gas from shale. It may even be possible to store carbon emissions in the shale after the natural gas is extracted.
Currently, hydraulic fracturing injects a mixture of water and other chemicals into shale to force natural gas out of the rock. Unfortunately, large volumes of water are required, and some of the water that is injected cannot be recovered. Using carbon dioxide instead of water would conserve water and effectively reduce atmospheric carbon levels at the same time.
Scientists at two new EFRCs, the Center for Mechanistic Control of Water-Hydrocarbon-Rock Interactions in Unconventional and Tight Oil Formations (CMC-UF) and Multi-Scale Fluid-Solid Interactions in Architected and Natural Materials (MUSE), are studying the fundamental science involved with using carbon dioxide for hydraulic fracturing.
“We could have a closed loop system,” said CMC-UF Director Tony Kovscek.
Although hydraulic fracturing extracts fossil fuels, a closed loop system means that additional carbon emissions would not be introduced to the atmosphere because the amount of carbon dioxide going underground would equal the amount of carbon-based fuel coming up.
“We’re very much focused on fundamental science,” said Kovscek, “so we can make models of more complicated systems to make better decisions.”
Greeshma Gadikota, a scientist at MUSE, stresses the importance of understanding fundamental science before implementing new technology to use carbon dioxide for subsurface energy recovery operations and evaluate potential carbon storage sites.
“We need to understand how carbon dioxide will interact with other fluids and solid surfaces,” Gadikota said.
In a potential operation using carbon dioxide for hydraulic fracturing, the interactions among carbon dioxide, water, and rock would be further complicated by the presence of natural gas or heavier hydrocarbons. MUSE scientists are working to improve our understanding of those interactions.
A global need
As EFRC scientists continue to invent and examine creative approaches for carbon capture, development of the new technologies made possible by the EFRC research could benefit the economy. Around the world, nations committed to lowering carbon dioxide emissions are looking for new, efficient carbon-capture technologies. The EFRCs’ multifaceted approach to solve the problems associated with carbon capture provides the information necessary to develop new carbon-capturing technologies.
