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Injecting CO2 Instead of Water in Geothermal Wells Could Triple-Serve the Environment

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U.S. Patent 6668554: Geothermal energy production with supercritical fluids

When we hear about carbon dioxide, it’s like hearing of a serial killer. LBNL (Lawrence Berkeley National Laboratory), and a CA-based company Symyx Technologies, instead, aided by several U.S. universities, have changed the image of this greenhouse gas a bit by putting it into a green job: geothermal energy harvesting.

The above-mentioned research groups have been funded $16 million to make a system that would cycle carbon dioxide through hot regions in geothermal wells (having a depth of several km), make it gather the heat, and bring it to the surface, where the heat would be extracted, and then the CO2 reused. The even more interesting fact is that part of the carbon dioxide they put inside the well stays in the well, thus accomplishing two jobs at once: CO2 sequestration and geothermal energy harvesting.

The project had been thought at since 2000, when physicist Donald Brown, from the Los Alamos National Laboratory proposed replacing the water they would have normally had injected into the ground to fracture hot rocks, by supercritical carbon dioxide, which is part gas, part liquid (in pressurized form). The advantage is that supercritical CO2 is less viscous than water and flows more easily through the rocks. He also observed that a siphoning effect should help cycle the CO2, because of the density difference between the supercritical CO2 pumped down and the hotter gas coming up, slashing power losses from pumping fluid.

The second green advantage is that the project would sequester the equivalent of 70 years work of CO2 from a 500 MW coal-powered plant, which is not anything to discard.

The third advantage would be saving important quantities of water, otherwise used in pumping the heat out of the geothermal well.

Investors like GreenFire Energy and Enhanced Oil Resources are planning to build a 2 MW CO2-based prototype plant near the Arizona-New Mexico border, in 2010. The location they chose has a natural underground carbon dioxide reservoir, under which hot rocks are located, and if used properly, they could get 800 MW of power out of it, while still absorbing much of the CO2 generated by the six large coal-fired power plants in the region.

A geofluids research group from the University of Minnesota had another way of looking at this solution: they proposed adding geothermal energy extraction capabilities to already existing plans for carbon capture and storage. They say this plan will yield additional value out of operations that already pump supercritical CO2 into deep saline aquifers for storage, or into oil and gas formations to accelerate production.

This method would eliminate the need of fractured rocks and would also take advantage of existing equipment and drilled wells, reducing the cost of a geothermal plant. Martin Saar, the lead researcher of this latter project, says real life testing will be available in as little as three years.

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