The most important question, according to Petro, is how supercritical carbon dioxide will interact with rock and minerals. Supercritical CO2 also has a particularly complex relationship with water. On its own, supercritical CO2 is not expected to dissolve minerals from rocks - a major problem encountered in the water-based approach. But, says Petro, adding a fraction of water to supercritical CO2 could form a super-dissolving “acidic soda water.”
At least one developer, meanwhile, is seeking financing for a field demonstration of carbon dioxide-based geothermal. In September, Salt Lake City-based geothermal developer GreenFire Energy announced a joint venture with small oil developer, Enhanced Oil Resources, to build a two-megawatt CO2-based demonstration plant near the Arizona-New Mexico border. The companies propose to commence drilling wells in 2010 to access hot rock underlying a natural underground carbon dioxide reservoir. They project that the location could yield enough heat to generate up to 800 megawatts of power and, in the process, could absorb much of the carbon dioxide generated by the six large coal-fired power plants in the region.
Instead of adding CO2 to geothermal energy plans, the University of Minnesota’s geofluids research group, one of the DOE’s awardees, proposes to add geothermal energy extraction to existing plans for carbon capture and storage. Martin Saar, the University of Minnesota geophysicist who leads the geofluids group, says this scheme 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. That carbon dioxide will pick up heat from the surrounding rocks, says Saar, so why not circulate some of it to generate power? This eliminates the need to fracture rocks. And it takes advantage of existing equipment and drilled wells, thus reducing the cost of the geothermal plant.
Saar is researching how supercritical CO2 interacts with rock, minerals, and water. Understanding the latter is critical to the Minnesota scheme, since carbon dioxide injected into a saline aquifer will mix with water. However, Saar says that may be less of a problem than it appears, because large volumes of CO2 injected into a saline aquifer should separate to form a distinct layer: “Supercritical CO2 is actually less dense than the brine, so in an aquifer it will rise and pool underneath the cap rock.”
If the lab work confirms that and other predictions, Saar says, they could be testing CO2 geothermal in the field in as few as three years.