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Hot dry rock goes supercritical

By proposing a method for using carbon dioxide under high pressure to extract energy from geothermal reservoirs, a Laboratory scientist has put a new twist on a historic Laboratory project. The proposed invention has the potential to take global geothermal energy science in new and exciting directions.

Based on expertise gained during the development of the Laboratory's Hot Dry Rock Project, Los Alamos geoscientist Donald W. Brown of Geophysics (EES-11) has proposed a method for producing geothermal energy using supercritical fluids such as carbon dioxide for the stimulation of the underground reservoir, production of the geothermal energy and heat transport. The Hot Dry Rock Project was a geothermal energy experiment that Los Alamos conducted between 1970 and 1996.

Brown's process stimulates underground reservoirs by pumping a supercritical fluid into a formation to fracture the rock. Generally, this is done at depths ranging from about 5,000 feet to about 20,000 feet below surface depending upon underground thermal conditions. At such depths, underground temperatures are in the range of roughly 200 to 600 degrees Fahrenheit in the western United States. A patent been granted on the process.

Once the reservoir is accessed, the supercritical fluid is allowed to heat up and expand. It is then pumped out of the reservoir to transfer the heat to a surface power generating plant or other application requiring heat. The recovered fluid is sent back down into the reservoir and the heat-extraction process is repeated. Any loss of the carbon dioxide is slowly diffused in the Earth into the surrounding rock mass.

A supercritical fluid is a liquid that has been raised beyond a temperature and pressure at which the liquid and gas densities are equal -- its critical temperature and pressure. For carbon dioxide, the supercritical conditions are a pressure of 1,074 pounds per square inch and a temperature of 88 degrees Fahrenheit.

Carbon dioxide is useful as the supercritical fluid because it is readily available, economical and easy to store and handle when not in contact with water. It also is environmentally benign because it is relatively inert, nontoxic and nonflammable. The process can readily use waste carbon dioxide from industrial processes, locking up excess carbon dioxide, a known greenhouse gas. When carbon dioxide is used as the supercritical fluid, any mineral constituents such as silicates or chlorides within the reservoir are left behind as mineral precipitates since most minerals are not soluble in supercritical carbon dioxide.

-- Todd Hanson

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