What is CO2 Sequestration

Overview


Carbon Caputure & Storage  (DOE/NETL publication Geologic Storage Formation Classifications)

Carbon Caputure & Storage (DOE/NETL publication Geologic Storage Formation Classifications)


The U.S. is one of 189 countries which are signatories to the United Nations Framework Convention on Climate Change (UNFCC), a treaty which calls for stabilization of atmospheric greenhouse gases (GHG) at a level that would prevent anthropogenic interference with the world’s climate. Conservation, renewable energy, and improvements in the efficiency of power plants, automobiles, and other energy consumption devices are important first steps in any GHG emissions mitigation effort. But those approaches cannot deliver the level of emissions reduction needed to stabilize the concentrations of GHGs in the atmosphere — especially against a growing global demand for energy. Technological approaches are needed that are effective in reducing atmospheric GHG concentrations yet, at the same time, have little or no negative impacts on energy use and economic growth and prosperity.

CO2 sequestration could play a major role in the mitigation of GHG emissions. Also known as carbon capture and sequestration (CCS) and geologic sequestration (GS), CO2 sequestration is the process of capturing CO2 from emission sources, such as a power plant, and storing it permanently in deep underground geological formations. Instead of releasing CO2 to the atmosphere, it is separated from other gases, pressurized to a nearly liquid supercritical state, transported to an appropriate storage location, and injected deep underground for long-term isolation from the atmosphere.

Worldwide CO2 emissions from human activity have increased from an insignificant level two centuries ago to annual emissions of more than 33 billion tons today. The U.S. Energy Information Administration predicts that, if no action is taken, the United States will emit approximately 6,850 million metric tons (7,550 million tons) of CO2 per year by 2030, increasing 2005 emission levels by more than 14 percent.

California ranks second among all states in CO2 emissions. The largest stationary sources in the region are power plants, oil refineries, and cement and lime plants. Power plants are the single largest point source of CO2 emissions, accounting for more than 80 percent of the emissions from the region’s largest stationary sources.

CCS begins with the separation and capture of CO2 from power plant flue gas and other stationary CO2 sources.  At present, this process is costly and energy intensive, accounting for the majority of the cost of sequestration.  However, analysis shows the potential for cost reductions of 30–45 percent for CO2 capture.  Post-combustion, pre-combustion, and oxy-combustion capture systems being developed are expected to be capable of capturing more than 90 percent of flue gas CO2.

After transport and injection, the ultimate step is to sequester (store) the CO2.  The primary means for carbon storage are injecting CO2 into geologic formations.  Geologic formations such as oil and gas reservoirs and underground saline formations are potential options for storing CO2 in Southern California. Many of these formations have naturally stored carbon dioxide and other gases and fluids (i.e., petroleum) for millions of years, and have the potential to store hundreds of years’ worth of human-generated CO2 emissions.

Geologic storage technologies are already in use; natural gas produced for fuel, for example, is routinely re-injected into the ground to be stored until it is needed, and CO2 is often injected into oil reservoirs to increase recovery. With proper site selection based on available subsurface information, a monitoring and verification program, regulatory system, and appropriate mitigation to stop or control CO2 releases should they arise, environmental and safety concerns are minimal. Local health, safety, and environmental risks of geological storage would be less than the risks of current activities such as natural gas storage and enhanced oil recovery due to the fact that CO2 is not toxic, flammable, or explosive.

There is already a strong base of industry experience in enhanced oil recovery (EOR), where water and then CO2 are pumped into depleted oil wells to re-pressurize wells and increase oil production.  Recent sequestration research is building on this experience.  We know that storage sites need to be selected very carefully to ensure that they are located far away from drinking water supplies, that the cap rock is impermeable and leakage will not occur.  Additionally, we are examining the extent to which CO2 moves within the formations, as well as the physical and chemical changes that occur.  Importantly, we are also developing ways to improve monitoring equipment and techniques to ensure that the CO2 is secure.