Geologic carbon capture, utilization and storage (CCUS) is the process of capturing carbon dioxide (CO2) from the point of release and permanently storing it in deep, underground rock formations that typically include enhanced oil recovery (EOR) techniques. This method prevents the release of CO2 into the atmosphere and encourages the efficient use of our natural resources. Although scientists generally agree that there is no ‘silver bullet’ solution to the greenhouse gas emissions problem, geologic CCUS is one strategy that could be deployed to reduce emissions from burning fossil fuels while meeting our rising energy demand. Geologic formations that are well-suited for CCUS include deep saline formations, basalt formations, oil and gas reservoirs and unmineable coal beds.
In geologic CCUS, CO2 is separated from other gases, pressurized to a nearly liquid state, transported to an appropriate storage location, such as an oil or gas field, and injected deep underground for permanent isolation from the atmosphere. In this way, carbon can be used to bring new life to natural resources while reducing the impact of carbon emissions on our world.
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, which functions as a seal. Deep saline formations are estimated to have the largest storage capacity of CO2.
Flood basalts are formed when very large volcanic eruptions create extensive lava flows that cool and solidify into thick basaltic rock formations. The giant eruptions create stacks of individual flows tens to hundreds of layers thick. The fast-cooling tops of these flows are full of cracks and holes, while the slower-cooling interiors form dense, impermeable barriers. The porous tops of the flows are well suited to store CO2, while the dense interiors function to trap the CO2. Laboratory experiments have shown that basalt rocks are unique in that they can rapidly convert injected CO2 to solid carbonate minerals, thereby permanently trapping and securing the CO2.
Carbon dioxide has been injected into underground oil reservoirs for more than 30 years to increase oil production, a process known as enhanced oil recovery (EOR). As CO2 is injected into the oil field, it mixes with the oil. This mix causes the oil to swell and decrease its viscosity, allowing it to flow towards the production well. EOR is an attractive option for underground CCUS because the cost of CCUS the carbon is offset by the recovery of additional oil that may otherwise have remained in the ground.
Coal seams that are too deep or too thin to be economically viable are referred to as unmineable coal seams. CCUS in unmineable coal seams is usually done in conjunction with coal bed methane recovery. As CO2 is injected into the coal, it adheres to the coal surface, displacing methane-rich gas that was previously adsorbed to the coal 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 costs associated with the CCUS process are offset by the capital gained from the methane.
