Offsetting the Greenhouse Effect: CEE Achievements in Carbon Storage Research
This article is Part 1 of a three-part series that explores CEE research on carbon capture, utilization, and storage (CCUS). Part 2 features CEE researchers who are studying how CCUS could impact the underground environment, while Part 3 focuses on CEE efforts to develop a risk assessment framework for use in CCUS operations.
Carbon dioxide is in the air, and no one is smiling about it. 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.
CCUS: What it is and why it matters
In carbon capture, utilization, and storage, carbon dioxide is captured from emissions from industrial sites such as power plants, physically compressed, and injected into brine-filled aquifers and reservoirs far beneath the earth’s surface. This prevents the captured carbon dioxide from entering the atmosphere and contributing to the greenhouse effect, in which heat bouncing off the earth’s surface encounters greenhouse gases such as CO2 and is reflected back toward the earth, warming the atmosphere.
An advantage of CCUS is the sheer scale of its operations; the Intergovernmental Panel on Climate Change estimated in a 2005 report that CCUS could account for up to 55% of world efforts to mitigate the greenhouse effect. However, because the process is complex, researchers need a good understanding of the associated risks—in particular, how and where CO2 leakage might occur—before it is widely implemented.
CEE Department Head and Walter J. Blenko, Sr. University Professor Dave Dzombak is one of the CEE faculty members researching CCUS as part of NETL-RUA’s wellbore integrity program. In 2005, he was contacted by NETL researcher Brian Strazisar, who was leading the NETL efforts on wellbore integrity, to collaborate on CCUS because of Dzombak’s reputation in this area of research. The team evolved to include CEE Professor Greg Lowry and NETL Research Scientist Barbara Kutchko (CEE ’08), then a CEE doctoral candidate. Dzombak noted that the scale of the project has allowed those involved to form a clear picture of the technique’s challenges and opportunities. “What is unique about the CEE approach is that we have a critical mass of people under one roof working on key components of the issue of CO2 leakage and risk assessment,” he said. “We have the whole picture here, from the high-level risk model down to the individual processes involved in CO2 storage.”
The unlikely tie between CCUS and cement
Dzombak’s CCUS-related research deals with the integrity of wellbore cement, a critical factor in the technique’s success. Oil and gas fields contain hundreds of abandoned wells that are typically filled with cement—known as wellbore cement—once exploitation of the underlying reservoirs is completed. These depleted oil and gas reservoirs are desirable CO2 storage sites for several reasons: They have a high storage capacity, and injecting CO2 into a reservoir makes it easier to extract the remaining oil and gas in a process known as enhanced oil and gas recovery. Because the cement-filled wellbores penetrate the reservoirs, they are considered to be a likely point of CO2 leakage, and Dzombak and his collaborators at CMU and NETL are studying the cement’s ability to withstand the effects of stored CO2.
“When CO2 is compressed and injected into the ground, it may become supercritical – a high-pressure substance between a gas and liquid state,” he explained. “We are looking at the potential of supercritical CO2 mixed with brine contained in the reservoirs to degrade the cement used to fill these wells.”
From 2005 to 2008, the team recreated the temperature and pressure conditions found in these reservoirs and then exposed cement to those conditions in a laboratory setting. Though they expected to find evidence of rapid erosion of the cement, their one-year experiment yielded more positive results: The alteration occurring in the cement as a result of the CO2 was extremely slow, and did not feature the rapid degradation they were expecting.
“By understanding how CO2 interacts with the wellbores, and what the impact is on overlying aquifers, we can improve the science base for CCUS risk assessment,” Dzombak said. “This knowledge will be critical to the public dialogue on reducing atmospheric CO2.”
How a cement additive might affect carbon storage
Liwei Zhang, who completed his PhD in CEE in March 2013, built on the research conducted by his advisors Dzombak and Kutchko and studied an additive commonly found in wellbore cement. Pozzolan is a type of coal ash that may be added to cement to reduce its permeability and lower its production cost. Depending on the intended use of the cement, pozzolan can account for more than half of a cement mixture.
Zhang wanted to know whether adding pozzolan to a cement mixture would make it more vulnerable to supercritical CO2 and the extreme conditions found in depleted reservoirs. He mixed a series of cement samples that varied in their ratio of pozzolan to straight cement and exposed those samples to CO2 in a high-pressure, high-temperature environment that mimicked the conditions of the deep subsurface. He then examined the cement samples for damage.
Zhang’s hunch was correct. He found that the ratio of pozzolan to cement did affect how quickly the cement degraded. Samples that contained two-thirds pozzolan showed more pronounced signs of degradation than samples that were only one-third pozzolan. In other words, if a wellbore in a CCUS site is filled with pozzolan-heavy cement, that cement may not be strong enough to prevent eventual leakage of CO2 from its underlying reservoirs.
“This is important information that decision makers should consider when choosing safe sites for CO2 storage,” said Zhang. Now a post-doctoral researcher at NETL, he plans to test the integrity of additional pozzolan-amended cement samples, and may also expand his focus to include other potential CO2 leakage points.
Modeling the behavior of foamed cement
Continuing the partnership between CMU and NETL, CEE Associate Professor Craig Maloney recently began working with Kutchko and CEE PhD candidate Eilis Rosenbaum on foamed cement, a material used to seal the gap between the pipe and the rock formation in an oil or gas well. The cement is said to be “foamed” because it has been injected with nitrogen to lower its density and prevent the rock formation from being damaged.
“We’re mainly interested in the short-term stability of the foam,” Maloney explained. “The loss of stability in the short term is thought to be one of the main contributing factors to the Macondo well blowout in the Gulf of Mexico. In addition to affecting short-term stability, the structure of the foam, after the cement sets, will also impact the likelihood of CO2 escape in carbon storage.”
Maloney and his collaborators are using computer simulations to try to characterize the structure and rheology (how the foam flows in response to applied loads) of cement foams. He noted that modeling the behavior of foam cement is complicated by several factors. First, the air bubbles in the foam are compressible, meaning they will shrink when pressure on the cement is increased. Second, the cement itself is a complex fluid that is difficult to model, even without any air bubbles present. “These are two challenging issues that we’re dealing with on the modeling side that haven’t really been studied before,” he explained.
While there has been a fair amount of research on the behavior of incompressible droplets in a simple fluid (for instance, oil-in-water emulsions), Maloney’s team is the first to tackle foamed cement modeling on such a detailed level. Though the research is in its initial stages, Maloney hopes that their work will lead to a quantitative tool that petroleum engineers can use when designing protocols for cementing oil and gas wells.
The big role of tiny fractures
Craig Griffith, who completed his PhD in CEE in May 2012, studied another potential point of CO2 leakage: Tiny fractures in the caprock, or seal, which is the layer of rock overlying these reservoir formations. Griffith looked at this slow CO2 leakage process, known as seal release, by analyzing data from more than twenty proposed CCUS sites in the U.S. and Canada. His research question was deceptively simple: How many microfractures do you need before you start worrying?
Griffith developed a model to evaluate sites for microfracture existence and density, and found that every site had some degree of micro-fracturing. “The lithological and mineralogical properties of a seal can change significantly from one end of a basin to the other,” he explained. “I identified the dominant mineral species that would characterize a seal – quartz or calcite, for example – and then developed a model to simulate the geochemical behaviors and fractures that might occur in each site’s seal.”
Griffith’s findings, which include data on the permeability and lithological behaviors of mineral species often found in the seal, contribute to the growing body of CCUS knowledge. “I hope for my research to serve as a resource to help guide basin modeling and experiments in the laboratory,” he said. Griffith is currently a Senior Petroleum Engineer with the U.S. Bureau of Ocean Energy Management in New Orleans.
Climate change is an issue of great global importance, and the number of minds in CEE at work on the topic of CCUS reflects that reality. For Dzombak, the potential of CCUS to mitigate climate change is reason enough to persevere. “If we as a society are going to get serious about CO2 control, CCUS must be an important element,” he said. “It will be done at a very large scale, and must be done in an environmentally protective way.”
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.