Stashing the Gas:
Geologic Carbon Sequestration
Michael F. Forlenza, P.G.
Editor HGS Bulletin
Carbon sequestration sounds like something that a judge may do to the jury during a trial of the periodic table. The word sequestration comes from the Latin sequestrare meaning "to hand over to a trustee." Merriam-Webster’s dictionary defines sequestration as "to set apart or segregate." Carbon sequestration is the storage part of the larger strategy referred to as carbon capture and storage (CCS). The objective of CCS is to slow the increase or to stabilize or cause the decrease of the concentration of carbon in the earth’s atmosphere, primarily in the form of carbon dioxide (CO2). Carbon sequestration is a developing technology that can use one or more strategies to isolate captured CO2.
Geologic sequestration involves injecting CO2 into underground reservoirs that have the ability to securely contain it. Ongoing research has focused on several types of geologic formations having this characteristic, including: oil and gas reservoirs, deep saline formations, and unmineable coal seams. Geologists play a key role in the study of sequestration options and will be leaders in future sequestration efforts.
Background
Most energy used to meet human needs is derived from the combustion of fossil fuels such as natural gas, oil, and coal. The combustion process releases waste gases, chiefly water vapor and the greenhouse gas CO2. Greenhouse gases have the characteristic of causing atmospheric warming through a phenomenon known as the "greenhouse effect." Such gases are transparent to longer-wavelength radiation, allowing incoming solar radiation to pass through. But these gases are opaque to scattered and reflected infrared radiation of shorter wavelength, trapping the solar energy near the planet’s surface.
In the 200 years since the industrial revolution, the world’s population has grown from about 800 million to over 6 billion people. During this time, the CO2 concentration in the atmosphere has increased from 280 to 387 parts per million (ppm) by volume, a 30 percent increase. The rate of increase accelerated in the last 50 years, coinciding with the accelerating rate of emissions of CO2.
The earth’s atmospheric temperature has risen in the last century with the most rapid rise occurring in the last few decades. While critics dispute the link between rising concentrations of CO2 and global warming, the majority of the scientific community and governments recognize a causal link and acknowledge the need to take action.
Most large energy companies have accepted a role in addressing rising CO2 emissions. ExxonMobil makes this statement on their website:
There is increasing evidence that the earth's climate has warmed on average about 0.7 C in the last century. Many global ecosystems, especially the polar areas, are showing signs of warming. CO2 emissions have increased during this same time period — and emissions from fossil fuels and land use changes are one source of these emissions.
Climate remains today an extraordinarily complex area of scientific study. The risks to society and ecosystems from increases in CO2 emissions could prove to be significant, so it is prudent to develop and implement strategies that address the risks, keeping in mind the central importance of energy to the economies of the world.
A report from the International Energy Agency urges the world’s governments to invest $20 billion in near-term, full-scale carbon capture and storage demonstrations. The report, entitled "Carbon Dioxide Capture and Storage: A Key Carbon Abatement Option," says that current spending is insufficient to achieve the necessary emissions reductions set by the G8. The United States is one of 189 signatory countries to the United Nations Framework Convention on Climate Change (UNFCC), a treaty which calls for stabilization of atmospheric greenhouse gases at a level that would prevent anthropogenic interference with the world’s climate. However, currently, there is no government policy concerning CO2 emissions in the United States.
Carbon Capture and Storage
Among the plans to reduce the CO2 concentration in the atmosphere are reducing emissions through increases in industrial efficiency and carbon capture. Most carbon capture studies have focused on stripping CO2 from the emissions of fossil fuel burning electrical generating plants, so called large-scale stationary sources. These large-scale sources account for approximately 60 percent of the world’s man-made CO2 emissions. Engineering challenges involve the separation of CO2 from exhaust gases and the compression of the gas into a liquid to allow efficient transport and sequestration.
The mass of CO2 emissions from all sources of combustion is huge. A single 1000-megawatt coal-fired generating plant can emit six million tons of CO2 annually – as much as is emitted by two million cars. In Texas alone, 667 million metric tons of CO2 is emitted annually. The United States emits about seven gigatons (billions of tons) of CO2 each year and the global total is more than 27 gigatons and growing, according to the United Nations Statistics Division.
The International Energy Agency’s World Energy Outlook 2007 projects the growth in energy demand will translate into a 57 percent rise in energy related CO2 emissions by 2030.
Even compressed into a liquid, the volumes of captured CO2 could be vast. Over the 60-year lifetime of a 1,000-megawatt plant, the CO2 emissions will have the equivalent volume of three billion barrels of oil. The storage of large volumes of CO2 will be a growing challenge as more countries adopt carbon credits and cap-and-trade rules.
Storage proposals involve a wide range of technologies employing biological, chemical, and physical processes. Biological processes generally involve encouraging the growth of plants that draw CO2 from the atmosphere and incorporate the carbon into their structure. One proposal involves ocean fertilization to accelerate and enhance the growth of plankton to absorb CO2.
Chemical processes involve removing the CO2 to form stable solid materials. Physical processes involve holding the liquefied gas in a setting that is