Feb 12, 2008
Storing carbon: the options
Currently, large amounts of carbon dioxide and methane are being released to the atmosphere through power plants, cement plants, car emissions, and changes in vegetation cover – from deforestation and human-guided agricultural practices. As might be expected, researchers are observing increased concentrations of carbon dioxide and methane all over the world. These increases are being observed in both air collection stations and also in air trapped in the upper ice fields or packed snow, known as "firn". This observed increase is greater than anything previously captured in ice cores dating from between 450,000 years ago and 1850.
This observation is important because both carbon dioxide and methane are so-called greenhouse gases, meaning they can trap or capture long-wave radiation (in the infra-red wavelength) that bounces off the Earth’s surface. This trapped radiation, or heat, remains in the atmosphere and increases the atmospheric temperature. We are currently measuring a 1°Celsius increase in the annual average temperature worldwide since 1850. In fact, there is a heightened trend in temperature over the last 12 years, where 11 of the last 12 years (1995 to 2006) were among the 12 warmest years in the instrumental record of global surface temperature (IPCC Report 2007).
Rising sea levels are also consistent with warming. Since 1993, the global average rate of rise has been 1.8 mm a year, which has been attributed to melting of glaciers, ice caps, and ice sheets, as well as to thermal expansion of the oceans (IPCC Report 2007). It’s reasonable to infer that increased atmospheric concentrations of carbon dioxide and methane may be causing global warming, which is now considered to be the cause for the rise in global sea levels. The consequences are very important for our island communities, our cities by the sea, our fisheries (as migration patterns are starting to change), and for our energy infrastructures that rely on coastal resources. As a result, the world is now looking at ways to stop global warming or at least to stop the emissions of carbon gases into the atmosphere.
One option, which is being researched by local, regional and national governments as well as academic, research, and industrial centers, is carbon sequestration – the long-term or permanent storage of carbon that could otherwise give rise to greenhouse gases. During the last five years, much work has been done to understand carbon storage in terrestrial (soil, land, plants), oceanic (deep sea), and geologic (deep underground regions) systems.
Terrestrial carbon sequestration is an option that can be launched immediately, and much work has been performed over the last 40 years in understanding soil and plant uptake of carbon. Very little infrastructure is required to use this sequestration method and it’s relatively easy to monitor carbon at the ground surface. The amount of time that carbon can be stored in the terrestrial environment ranges between two and 200 years depending upon the storage reservoir targeted (soils or woody plants).
Oceanic carbon sequestration requires a bit more infrastructure to pipe the greenhouse gases into deep ocean basins and sediments. Here, there is contention regarding the impact of these gases to the local ecosystem – biology living in the sediments – as well as the political implications in storing gas in international waters. Carbon dioxide can be stored in these systems from 50 to 1,000 years depending upon the proximity of the storage reservoir to upwelling currents, ocean temperature anomalies (ocean warming scenarios or even local temperature anomalies in the sediments from magmatic release or "hot spots"), and also to bioturbation (local biologic activity that could release pockets of gas).
Geologic sequestration offers the longest storage option: gas can be stored for hundreds to millions of years in the deep underground. This technique requires infrastructure such as wells and pipelines to get the gas under the ground, and the systems are very remote for research as they can be as deep as 5,000 to 20,000 feet underground. That said, new monitoring technologies are being created that can look at gas movement and rock chemical changes due to injected gas in the deep underground.
Large research efforts are now underway to look at methods for transferring emission gases from power plants to storage reservoirs. These efforts focus on how best to inject these greenhouse gases deep underground (5,000 to 20,000 feet), how these gases will move within the subterranean systems, and how the gases will react with the surrounding rock and, perhaps, become part of the rocks themselves. Specifically, work is being performed in university, industry, and government research laboratories on: improving infrastructure for long-term carbon dioxide storage such as creating new material compounds for pipelines and well bores to sustain long-term exposure to carbon dioxide; placement of pipelines and wells in varying reservoirs; in situ sensing of gas movement in the deep underground for example, using pressure, temperature, and chemical probes that can track gas within the storage reservoir; and finally, coring techniques and seismic sensing to monitor the carbon dioxide interaction with the rock matrix over time.
The United States, Norway, China, Africa, Germany, Canada, Japan, and Korea are all conducting carbon sequestration pilot programs, where small amounts of carbon dioxide are being injected into various types of geologic systems, from carbonate, silicate, basalt and saline aquifer reservoirs. The storage reservoir types vary in size and have different capacities to store carbon dioxide. For example, unmineable coal seams are considered to be able to store as much as 40 to 150 GtCO2 (1 Gt = 1 billion tons) or 2% of global carbon dioxide emissions. It’s thought that gas and oil fields can store as much as 920 GtCO2 or 45% of the world’s emissions of carbon dioxide. And deep saline reservoirs can store as much as 10,000 Gt of carbon dioxide or 500% of the world emissions (IPCC report).
The carbon dioxide is monitored with various tools both before and after injection – at periods of three to 10 years – to better understand how it interacts and moves in deep subsurface geologic reservoirs. Carbon dioxide or any injected liquid/gas will both interact and become incorporated into the rock matrix as well as move in the pore spaces of the rock it is injected into. Depending upon the permeability – or the joining of – the pores within the rock, the injected gas/liquid will either stay close to the injection well or will move away to other regions in the rock. So it is important to understand the rock matrix and the chemical nature of the rock as well as the pore structure and pore connectivity before any liquid/gas is injected.
These pilot studies bring together researchers from academic, industrial and government laboratories with regulators, land owners, industrial operators (usually oil/gas companies where existing infrastructure and expertise exist), politicians, and government officials. These large and diverse teams are working together to create a system where carbon dioxide and potentially methane can be safely stored for 10,000 to 100,000,000 years.
In the future, research efforts will be directed toward pulling these gases directly out of the air or out of emission stacks of power plants and then piping them to geologic storage sites for permanent storage. The people involved with carbon sequestration are exploring ways to stop global warming. It’s a humanitarian goal to determine the viability of long-term storage in geologic systems in the hope that mankind will be able to launch large storage programs within the next 20 years. Our future depends on it.
About the author
Julianna Fessenden received her PhD in Earth Sciences from Scripps Institution of Oceanography at the University of California, San Diego in 1999. She was a postdoctoral student at the University of Utah and at California Institute of Technology from 1999 to 2002, then moved to Los Alamos National Laboratory where she is now a technical staff member in the Geochemistry, Geology, and Hydrology Group in the Earth Environmental Sciences Division.