Geologic CO2 Sequestration Research at the USGS
Helpful Definitions
Concepts of Geologic Sequestration of CO2:
Enhanced coalbed methane production (ECBM): When CO2 is injected into coal beds, it displaces methane molecules that are attached to the surface of the coal. This methane that is dislodged from the coal is then free to move about in the coal, and it can be pumped out of the bed. This methane extraction process is referred to as “Enhanced Coal Bed Methane” or “ECBM”.
Deep coal seam: Deep coal seams are too deep to be mined economically and that depth varies with locality. When injected, CO2 is adsorbed by the coal, typically displacing methane as a result. This coal bed methane can then be recovered (ECBM, enhanced coal bed methane).
Brine formation: A brine (saline) formation is an underground body of rock which contains water with more than 10,000 ppm total dissolved solids (unsuitable for drinking water and often too deep to be economically pumped). CO2 pumped into a saline formation displaces the existing fluid and is trapped.
Depleted hydrocarbon reservoir: The production of oil and/or natural gas has become uneconomic in some hydrocarbon reservoirs. This type of hydrocarbon reservoir is then termed “depleted”. CO2 can be pumped into these hydrocarbon reservoirs to enhance the production of the oil or natural gas. This type of operation is termed either Enhanced Oil Recovery (EOR) or Enhanced Gas Recovery (EGR). As the hydrocarbons are produced, some of the CO2 remains in the formation, or in other words, some of the CO2 remains sequestered.
Reservoir trap/seal: Hydrocarbon accumulations (oil and natural gas) are typically found trapped within reservoirs which can be either formed structurally (i.e. an anticline) or stratigraphically (i.e. formation “pinch out”). The accumulation is kept within the reservoir by a low permeability bounding unit, which is referred to as a “seal”. Deep reservoirs are, for the purposes of CO2 sequestration, defined to be deeper than 1 kilometer.
Reservoirs: Hydrocarbon accumulations (oil and natural gas) are typically found within reservoirs which can be either formed structurally (i.e. an anticline) or stratigraphically (i.e. formation “pinch out”). The accumulation is kept within the reservoir by a low permeability bounding unit, which is referred to as a “seal”.
Brine formation: A brine (saline) formation is an underground body of rock which contains water with more than 10,000 ppm total dissolved solids (unsuitable for drinking water and often too deep to be economically pumped). CO2 pumped into a saline formation displaces the existing fluid and is trapped.
Injection of CO2 into geologic reservoirs: CO2 will be pumped into geologic formations from the surface, most likely as a dense, liquid-like fluid (also known as a “supercritical fluid”) into either a coal bed, or a saline or hydrocarbon reservoir. If the CO2 is injected into a depleted hydrocarbon reservoir, then additional petroleum or natural gas could be extracted (Enhanced Oil Recovery or Enhanced Gas Recovery). If the CO2 is injected into a coal bed, then methane could be liberated and extracted (Enhanced Coal Bed Methane).
Pipeline transporting CO2: Separation of CO2 in flue gas takes place at power plant. The gas must be compressed to a liquid and can be transported by pipeline to injection site.
Offshore natural gas production: Natural gas produced in the offshore Norway Sleipner field is high in CO2. This CO2 is removed from the natural gas stream and is then pumped into the Utsira Formation, which is a highly permeable sandstone. The operator of the Sleipner field, Statoil, geologically sequesters the CO2 because the sequestration cost is less than the Norwegian carbon emission tax.
Enhanced Oil Recovery (EOR): CO2 can be injected into depleted oil reservoirs to enhance oil recovery from the reservoir. CO2 will dissolve into the residual oil in place, which lowers the viscosity of the oil. The lower viscosity enables the oil to flow more easily, which makes it possible to extract more oil from reservoirs.
Enhanced Gas Recovery (EGR): As gas is removed from natural gas reservoirs, the pressure of the reservoir decreases. As the pressure within the reservoir decreases, it becomes more difficult to recover more gas. By injecting CO2 into the natural gas reservoir, the pressure of the reservoir is increased, and more gas can be recovered.
Top of Page
U.S. Department of the Interior |
U.S. Geological Survey
