	Introduction

Carbon capture and sequestration (or storage)—known as CCS—is a physical process that involves capturing manmade carbon dioxide (CO 2 ) at its source and storing it before its release to the atmosphere. CCS could reduce the amount of CO 2 emitted to the atmosphere despite the continued use of fossil fuels. An integrated CCS system would include three main steps: (1) capturing CO 2 and separating it from other gases; (2) purifying, compressing, and transporting the captured CO 2 to the sequestration site; and (3) injecting the CO 2 in subsurface geological reservoirs or storing it in the oceans. As a measure for mitigating global climate change, CCS has attracted congressional interest and support because several projects in the United States and abroad—typically associated with oil and gas production—are successfully capturing, injecting, and storing CO 2 underground, albeit at relatively small scales. The oil and gas industry in the United States injects nearly 50 million metric tons of CO 2 underground each year to help recover oil and gas resources (a process known as enhanced oil recovery, or EOR). Electricity-generating plants are among the most likely initial candidates for capture, separation, and storage or reuse of CO 2 because they are predominantly large, stationary, single-point sources of emissions. The capture phase of the CCS process, however, may be 80% or more of the total costs for CCS. U.S. commercial electricity-generating plants currently do not capture large volumes of CO 2 because they are not required to and there are no economic incentives to do so. The U.S. Department of Energy's (DOE's) flagship CCS demonstration project, FutureGen, plans to retrofit an existing power unit with an oxy-fuel combustion unit. CO2 Sequestration

Three main types of geological formations are being considered for carbon sequestration: (1) depleted oil and gas reservoirs, (2) deep saline reservoirs, and (3) unmineable coal seams. Advantages and Disadvantages

Depleted or abandoned oil and gas fields, especially in the United States, are considered prime candidates for CO 2 storage for several reasons:
oil and gas originally trapped did not escape for millions of years, demonstrating the structural integrity of the reservoir; extensive studies for oil and gas typically have characterized the geology of the reservoir; computer models have often been developed to understand how hydrocarbons move in the reservoir, and the models could be applied to predicting how CO 2 could move; and infrastructure and wells from oil and gas extraction may be in place and might be used for handling CO 2 storage. The sheer volume of CO 2 envisioned for CCS as a climate mitigation option is overwhelming compared to the amount of CO 2 used for EOR. From estimates of the potential storage capacity in saline reservoirs, it is likely that the vast majority of CO 2 injected underground would be stored in these formations, assuming that CCS were deployed on a commercial scale across the United States. One disadvantage is therefore the possibility that displaced brine could leak into underground sources of drinking water. The total lower estimate (sum of the three reservoir types) from the 2012 Carbon Sequestration Atlas shown in Table 2 indicates the potential to store the equivalent of over 1,100 years of CO 2 emissions from electricity generation in the United States at current emission rates (2.1 billion tons per year). Although DOE has identified substantial potential storage capacity for CO 2 , particularly in deep saline formations, large-scale injection experiments are only beginning in the United States to test how different types of reservoirs perform during CO 2 injection. Data from the experiments will undoubtedly be crucial to future permitting and site approval regulations. Acceptance by the general public of large-scale deployment of CCS may be a significant challenge if the majority of CCS projects involve private land. Some of the large-scale injection tests could garner information about public acceptance, as local communities become familiar with the concept, process, and results of CO 2 injection tests.
Carbon capture and sequestration (or storage)—known as CCS—has attracted congressional interest as a measure for mitigating global climate change because large amounts of carbon dioxide (CO2) emitted from fossil fuel use in the United States are potentially available to be captured and stored underground and prevented from reaching the atmosphere. Large, industrial sources of CO2, such as electricity-generating plants, are likely initial candidates for CCS because they are predominantly stationary, single-point sources. Electricity generation contributes over 40% of U.S. CO2 emissions from fossil fuels. Currently, U.S. power plants do not capture large volumes of CO2 for CCS.
Several projects in the United States and abroad—typically associated with oil and gas production—are successfully capturing, injecting, and storing CO2 underground, albeit at relatively small scales. The oil and gas industry in the United States injects nearly 50 million tons of CO2 underground each year for the purpose of enhanced oil recovery (EOR). The volume of CO2 envisioned for CCS as a climate mitigation option is overwhelming compared to the amount of CO2 used for EOR. According to the U.S. Department of Energy (DOE), the United States has the potential to store billions of tons of CO2 underground and keep the gas trapped there indefinitely. Capturing and storing the equivalent of decades or even centuries of CO2 emissions from power plants (at current levels of emissions) suggests that CCS has the potential to reduce U.S. greenhouse gas emissions substantially while allowing the continued use of fossil fuels.
An integrated CCS system would include three main steps: (1) capturing and separating CO2 from other gases; (2) purifying, compressing, and transporting the captured CO2 to the sequestration site; and (3) injecting the CO2 in subsurface geological reservoirs or storing it in the oceans. Deploying CCS technology on a commercial scale would be a vast undertaking. The CCS process, although simple in concept, would require significant investments of capital and of time. Capital investment would be required for the technology to capture CO2 and for the pipeline network to transport the captured CO2 to the disposal site. Time would be required to assess the potential CO2 storage reservoir, inject the captured CO2, and monitor the injected plume to ensure against leaks to the atmosphere or to underground sources of drinking water, potentially for years or decades until injection activities cease and the injected plume stabilizes.
Three main types of geological formations in the United States are being considered for storing large amounts of CO2: oil and gas reservoirs, deep saline reservoirs, and unmineable coal seams. The deep ocean also has a huge potential to store carbon; however, direct injection of CO2 into the deep ocean is controversial, and environmental concerns have forestalled planned experiments in the open ocean. Mineral carbonation—reacting minerals with a stream of concentrated CO2 to form a solid carbonate—is well understood, but it is still an experimental process for storing large quantities of CO2.
Large-scale CCS injection experiments are underway in the United States to test how different types of reservoirs perform during CO2 injection of 1 million tons of CO2 per year or more. Results from the experiments will undoubtedly be crucial to future permitting and site approval regulations. Acceptance by the general public of large-scale deployment of CCS may be a significant challenge. Some of the large-scale injection tests could garner information about public acceptance, as citizens become familiar with the concept, process, and results of CO2 injection tests in their local communities.