Neutrons shine a light on geological nanostructures
Big issues of the day often turn out to be closely linked to small, seemingly mundane facts of life. Consider carbon sequestration, the practice of locking carbon dioxide (CO2) away in geological formations in hopes of limiting climate change, and hydraulic fracturing, a method of releasing natural gas by cracking layers of rock with highpressure fluids. Both processes are politically contentious, have global implications, and depend heavily on understanding how liquids and gases behave in the nanopores of rocks located hundreds or thousands of feet underground.
ORNL physicist Yuri Melnichenko and postdoctoral associate Lilin He investigate how liquids and gases behave in nanopores using the General Purpose Small-Angle Neutron Scattering (GP-SANS) Diffractometer at ORNL's High Flux Isotope Reactor. Photo: Jason Richards
Melnichenko's recent forays into the realm of the nanoscale involve investigating how fluids, liquids and gases change when confined in nanopores. These changes can be illustrated by imagining a bucket of water, he explains. If you have a big bucket, most of the water molecules aren't affected by the bucket walls. But when you have a nanobucket, there are fewer molecules, and virtually all of them come in contact with the walls. As a result, they don't behave the way as they do in a big bucket. "This is exactly the point of our research," Melnichenko says. "We are trying to understand why various properties of liquids and gases are different, depending on the size of the bucket, the material of the walls, and how molecules interact with the walls."
The properties of nanopores play a major role in carbon sequestration. "Most scientists agree that carbon dioxide emissions constitute some level of threat to the environment by building up in the atmosphere and raising the surface temperature of the earth," Melnichenko says. One approach to limiting the amount of CO2 in the environment is to collect it as it is produced at places like power plants, condense it to a liquid, and pump it into deep, unmineable coal seams where it would permeate the pores of the coal. The hope among proponents of carbon sequestration is that while in these pores, condensed CO2 would eventually transform into a more stable compound.
"This is all theory of course," Melnichenko says, "so a couple years ago we started a project to study how CO2 interacts with coal." The research compared the structure of "dry" coal with CO2-saturated coal to determine CO2 adsorption rates. The research team also made this comparison for different types of coal and for coals mined from different depths. Although they found variability in adsorption rates, Melnichenko says, they still couldn't determine what makes one type of coal more adsorbent than another or how different nanopore sizes contribute to the adsorption process.
Searching for answers, Melnichenko and his colleagues devised an experiment using neutrons generated by ORNL's High Flux Isotope Reactor and applied the General Purpose Small-Angle Neutron Scattering (GP SANS) Diffractometer to monitor the behavior of CO2 in small pores. "We built a high-pressure cell to reproduce the high-pressure, high-temperature conditions many hundreds of feet underground," he says. "Then we used the instrument's unique capabilities to analyze samples of various coals under different conditions."
Based on this research, the team developed a method of calculating not only how much CO2 was adsorbed by the various samples, but also how nanopore size contributed to the adsorption process. This baseline information about the effects of pore size on adsorption is of particular interest to geologists because it provides unprecedented insight into which types of coal are most amenable to carbon sequestration, as well as information about various other processes related to the porosity of coal and other kinds of rock.
One form of sequestration, called enhanced coalbed methane recovery, involves pumping CO2 into unmineable methane-saturated coal seams, displacing the methane from the nanopores and forcing it out through a second well. Although this process hasn't been demonstrated on a commercial scale, researchers anticipate that some sequestration costs could be recovered by selling the recovered methane. "Before that happens on a large scale," Melnichenko says, "we will need to know how to make the process effective, what the optimal conditions for CO2 to replace methane are, and how CO2 and methane interact with each other and the walls of the nanopores." Research conducted on the GP SANS helps illuminate these questions as well.
In addition to being trapped in coal seams, methane is found in large quantities in geologic formations called "tight gas shales." Methane in this sort of rock is sometimes accessed by a process called hydraulic fracturing or "fracking." It is estimated that the Marcellus Shale formation in the eastern United States, for example, contains a trillion tons of natural gas trapped in small, discrete pores. "It's an enormous amount of clean energy," Melnichenko says. Using their SANS techniques, his team can uncover information about both the structure of the shale itself and the behavior of the methane contained in its pores. This information can then be used by geologists to answer questions about the best way to extract the methane; how the process can be made more efficient; and why there is more methane in one place than another.
Research conducted on the SANS instrument also enables Melnichenko's team to calculate how much methane is contained in non-interconnected pores in any porous material. "So, for instance," Melnichenko suggests, "if 50 percent of the pores of a particular formation are not accessible, perhaps a geologist would rather find another area to work with. There is a lot more research to be done before we can determine how the most efficient release of methane can be achieved."
Judgments based on science
Despite the enthusiasm of commercial geologists for his team's research, Melnichenko makes it clear that the team's focus is on the science, not on solving industry problems. "That's not our job," he says. "But we can provide information to individuals who need to know how fluids behave in nanopores. We are giving these researchers tools that will enable them to understand processes at the fundamental level and let them know what to expect when they drill into coal or shale. Then they can use this information to optimize their processes."
Melnichenko recalls that three or four years ago, when he asked geologists discussing carbon sequestration how they would choose a seam for CO2 injection, they didn't have much data to guide their judgments. "Our research has given them a better understanding of what will happen when they pump CO2 into a coal seam," he says. "Now they can make judgments based on science, rather than just educated guessing."— Jim Pearce