Carnegie Mellon University
October 07, 2013

Carbon Storage and the Underground Environment

Carbon Storage and the Underground Environment
This article is Part 2 of a three-part series that explores CEE research on carbon capture, utilization, and storage (CCUS), an innovative technique being considered for its potential to slow climate change. Part 3 will focus on CEE efforts to develop a risk assessment framework for use in CCUS operations. Click here to read Part 1.

As levels of atmospheric carbon dioxide (CO2) continue to rise at a sobering rate, CEE researchers are exploring a bold technique that could play a key role in global greenhouse gas reduction. The technique is known as carbon capture, utilization, and storage, or CCUS, and has a straightforward objective: capturing CO2 and storing it underground before it makes its way to the atmosphere. Through the collaborative efforts of the NETL-RUA, a partnership between the National Energy Technology Laboratory and five nationally recognized universities (CMU, Penn State, Pitt, Virginia Tech, and WVU), a team of CEE researchers and NETL scientists are researching the potential benefits and risks associated with CCUS.

The role of bacteria in CCUS

CEE Associate Professor Kelvin Gregory studies the potential effects of carbon storage on microbial populations living in the deep subsurface. “There are bacteria living in geologic formations two miles below the earth’s surface,” he said. “It’s likely that these deep subsurface microbial communities are going to be impacted by CO2 storage—we want to find out how they will be affected.” 

So why do these bacteria matter? Gregory offered some background. Some bacteria in the deep subsurface can convert CO2 gas into a highly dense, stable mineral form, increasing the storage capacity of the reservoirs and making it unlikely that the CO2 will escape to the surface. However, others produce acids that erode surrounding mineral deposits and free heavy metals such as lead and arsenic. This reduces the storage capacity of the reservoirs and pushes the heavy metals out of the reservoirs toward overlying aquifers. 

“Depending on which bacteria survive the CO2 storage process, this could have either a beneficial or a detrimental impact on the security and storage capacity of these reservoirs,” Gregory explained. He noted that his research group’s experiments so far have shown that there is a rapid drop in microbial diversity when deep subsurface bacteria are exposed to CO2. “The more sensitive populations go away quickly, and we’re left with a community of virtually pure cultures. And these bacteria are the likely targets of future research.”

The same bacteria that survive CO2 exposure may also be able to seal cracks that occur naturally in the caprock, preventing CO2 from leaking to the surface. Because the cracks are a critical component of the long-term safety of CO2 storage, knowing whether the bacteria that survive can reduce CO2 leakage through their metabolic activity will give scientists a better idea of how to manage the impacts of leakage—for instance, by stimulating the growth of these “sealing” bacteria. 

“Ultimately, we want to make sure that the next body of microbiology research is focusing on the right organisms,” Gregory said. “Once we know which bacteria can survive these challenging environments, we can explore the roles they are going to play.”

DJUNA GULLIVER

Learning from potential storage sites

PhD candidate Djuna Gulliver is working to gather and analyze microbial data from potential carbon storage sites around the United States. Data on the deep subsurface can give scientists valuable insight into the impact of CO2 exposure on microbial communities. It’s also hard to come by. “This is one of the few studies that is actually using native samples,” she said. “I’m hoping that as more research occurs, we’ll see trends in what types of microorganisms survive and therefore what bioprocesses we can expect to see in these carbon storage environments.”

Gulliver has taken environmental samples from potential carbon storage sites which include two depleted oil reservoirs and a saline aquifer. Her experiments have helped her to identify the microorganisms that can survive in each environment. “To start out knowing very little about what’s down there, and then to end up with a list of species – that’s the most exciting part,” she said. 

“In the subsurface sequestration environment, whether it’s a saline aquifer or a depleted oil reservoir for enhanced oil recovery, microbes are going to play a role in the performance of that system,” said CEE Professor Greg Lowry, who co-advises Gulliver with Gregory. “Djuna’s work is opening our eyes to what is in the deep subsurface.” 

Carbon storage and groundwater

Oil isn’t the only valuable liquid beneath our feet. When CO2 is stored underground – typically in spacious saline formations or depleted oil and gas reservoirs – it shares those spaces with brine that is nearly 35% salt by weight (by comparison, the Atlantic Ocean is about 3% salt). The high pressure and temperature of the reservoirs cause the CO2 to dissolve into the brine, forming a highly acidic mixture which begins to dissolve the surrounding rocks and free major and trace metals within them. If the brine then makes its way to an overlying aquifer, those metals could enter the groundwater supply.  

This process is well understood by CEE Assistant Research Professor Athanasios Karamalidis, who studies the potential impact of CCUS on groundwater. Recently, Karamalidis collaborated with researchers at NETL and two national labs on a project designed to shed light on the movement of heavy metals in carbon storage environments. Using samples that they gathered from fifteen CCUS demonstration sites around the world, the researchers conducted experiments and identified the average concentrations of the metals that are commonly found in this CO2-saturated brine. 

“Previously there were no data on the contents of this brine, and that was limiting the accuracy of models,” he explained. “We were able to propose average concentrations of these trace metals for different carbon storage scenarios – for instance, sandstone sites versus shale or basalt – so that modelers can use these values in the absence of site-specific data.” The team found that lead, chromium, and arsenic, along with other metals, were mobilized in some of those cases.  In other words, were the brine to leak into an overlying aquifer, these metals could potentially come with it.

Karamalidis hopes the group’s findings can be used not only to improve modeling accuracy, but also to paint a clearer picture of the ways minerals behave under CO2 storage conditions. “We need more information, and that is the main reason for conducting these experiments,” he said. “Not to populate the models, but to understand the fundamental science underlying CO2 storage.” 

Aniela Burant

Organic compounds on the move

PhD candidate Aniela Burant, who is advised by Karamalidis and Lowry, studies organic compounds found in brine-filled, depleted oil reservoirs being used primarily for enhanced oil recovery and eventually for carbon storage. The brine in these reservoirs contains a small amount of dissolved oil, and organic compounds from that oil can partition, or move, from the brine into the injected CO2 once the CO2 has become supercritical (between a liquid and a gas). Burant is working to pinpoint the environmental conditions that cause those compounds to enter the supercritical CO2. 

“We’ve known for decades that supercritical CO2 is a good solvent for organic compounds, so it has the potential to mobilize, or get, the hydrocarbons in those reservoirs, some of which are hazardous,” she explained.  “If that CO2 leaks and migrates to a shallow aquifer, we’ll need to know exactly how much of those compounds it contains so we can figure out what needs to be done to keep those aquifers safe.” After conducting a literature review of data gathered through laboratory experiments, she was able to identify trends in the behavior of a variety of organic compounds in carbon storage environments. This information will be a valuable resource to scientists working to model CO2 leakage.

In addition to conducting research, Burant organizes a weekly discussion group for CMU graduate students studying CCUS and Marcellus shale. In the meetings, students present their latest research to their peers and receive feedback and support. “Many times, we use the meeting as an opportunity to say, ‘Here’s what I’m doing in my research, and I need help,’ or, ‘I’d like your feedback on my thesis defense presentation,’” Burant explained. “It’s also a good way to learn about what other students are doing and how that might affect your own work.” 

Filling the knowledge gaps

CEE Professor Greg Lowry, who advises Gulliver and Burant, is collaborating with Gregory and Karamalidis to characterize the biological and chemical processes taking place in carbon storage reservoirs. Lowry noted that their research addresses significant knowledge gaps in the CCUS field. “We’ve found that there are very little partitioning data available collected at the conditions of interest to us, which are high-pressure, high-salinity environments,” he said. “We’re collecting the fundamental solubility and partitioning data that are needed to predict behaviors of selected chemicals in the deep subsurface. It’s exciting.”

As CEE scientists research the components of CCUS, they’re finding that their work could also be useful in other contexts. Lowry noted that many of the CCUS processes being studied in CEE are relevant to shale gas research. “In many cases, you’re talking about similar concerns,” he said. “If you’re fracking the subsurface, you need to understand the leakage pathways and various fate scenarios. What are the potential problems that you hadn’t anticipated?”

CEE research on the impact of CCUS on the underground environment is laying the groundwork for more informed modeling and site selection. “The deep subsurface microbiology is really a relatively unexplored part of the planet,” Lowry explained. “This is important work and as far as we know, it’s the first of its kind.“

Top graphic: Larry Scott, Colorado Geological Survey

In carbon capture, utilization, and storage (CCUS), carbon dioxide emitted from industrial sites such as power plants is captured, compressed, and injected into underground reservoirs. Potential storage sites range from depleted oil and gas reservoirs to saline aquifers and unmineable coal deposits.