Geologic Storage

What?
Geological sequestration of carbon dioxide (CO₂) could play a major role in the mitigation of greenhouse gas emissions. Also known as carbon capture and storage (CCS), geological sequestration is the process of capturing CO₂ from point-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.

Why?
Current global CO₂ emissions are expected to continue to increase substantially unless action is taken to control carbon emissions. Scientists generally agree that there is no ‘silver bullet’ solution to the emissions problem and that a suite of strategies must be deployed if CO₂ emissions are to be stabilized. Geologic sequestration is one strategy that could be employed to reduce emissions from burning fossil fuels while meeting forecasted increases in energy demand.

How?
One of the primary goals of the DOE carbon sequestration program is to understand the behavior of CO₂ when stored in geologic formations to ensure that geologic storage is safe, secure and environmentally acceptable. CO₂ injected into suitable sites would be trapped by various geologic and geochemical mechanisms. An essential physical trapping mechanism for carbon storage is the presence of a caprock, an overlaying impermeable rock layer that acts as a seal to prevent the CO₂ from returning to the atmosphere. Geologic formations that are well-suited for carbon storage include deep saline formations, basalt formations, oil and gas reservoirs and unmineable coal beds. These formations have stored crude oil, natural gas, brine and CO₂ over millions of years.

Types of Geologic Storage

• Deep saline formations are layers of porous rocks, such as sandstones, that are filled with very salty water called brine. Overlaying the porous rocks is an impermeable rock layer, such as shale or clay, that functions as a seal. Deep saline formations are estimated to have the largest storage capacity of CO₂, but they have not yet been studied as thoroughly as other potential formations.
• Flood basalt formations are layers of volcanic rocks that formed as lava flows cooled and solidified. The cooling process created volcanic provinces that have stacks of many individual flows tens to hundreds of layers thick. Variations during the solidification of the lava flow caused the fast-cooling tops of flows to be full of cracks and holes, while the slower-cooled interior of flows formed dense, impermeable barriers. The porous tops of the flows are well suited to store CO₂, while the dense interiors function to trap the CO₂. Additionally, laboratory experiments have shown that basalt rocks can rapidly convert injected CO₂ to solid carbonate minerals, thereby permanently trapping and securing the CO₂.
• Oil and gas reservoirs have many properties that make them ideal sites to store CO₂. CO₂ has been injected into oil fields for more than 30 years to increase oil recovery, a process known as enhanced oil recovery (EOR). As CO₂ is injected into the oil field, it mixes with the oil, causing the oil to swell and increase its viscosity, allowing it to flow towards the production well. EOR is an attractive option for underground carbon storage because the cost of sequestering the carbon is offset by the additional oil recovered that may otherwise have remained in the ground.
• Unmineable coal seams are coal seams that are too deep or too thin to be mined economically. Carbon storage in unmineable coal seams relies on the adsorption of CO₂ on the coal and the permeability of the coal bed. The more micro holes there are in the coal, the more surface area it has for the CO₂ to accumulate onto. Carbon sequestration in unmineable coal seams is usually done in conjunction with coal bed methane recovery. As CO₂ is injected into the coal, the process releases methane that was previously adsorbed to the coal surface and can be recovered. This process is known as enhanced coal bed methane recovery, or ECBM. ECBM, like EOR, can be a beneficial method of sequestration because the cost of sequestering the carbon can be offset by the capital gained from the methane.