A geothermal startup is hoping to use the world's most abundant greenhouse gas to extract heat buried deep below the Arizona high desert, while preventing millions of tons of the gas from ever reaching the atmosphere.
Utah-based GreenFire Energy plans to tap naturally occurring carbon dioxide (C02) from the St. Johns Dome formation about a half-mile underneath the Arizona-New Mexico border near the town of Springerville, Ariz.
The CO2, once compressed to a supercritical state, will then be reinjected into a deeper formation to carry heat to the surface and power a 2-megawatt geothermal plant. Meanwhile, a small portion of the CO2 is expected to become permanently trapped in porous underground rocks.
Power plants and other industrial facilities will also eventually be able to pipe their CO2 emissions to GreenFire's wells, to both boost geothermal power production and reduce overall CO2 emissions, said Randy Balik, the firm's vice president of business development.
"It could be a game-changing electric generation technology," Balik said.
GreenFire plans to drill a series of exploratory wells this year using about 235,000 acres of state and private lands leased by Enhanced Oil Resources Inc., which holds rights to the CO2. The two companies in September announced a joint venture giving GreenFire exclusive access to CO2 within the dome to be used in a geothermal demonstration.
"We're going to be the first ones to attempt proof-of-concept and deployment in the field," Balik said.
If successful, GreenFire hopes to eventually build 50-megawatt modular geothermal plants that would use naturally occurring CO2 from the St. Johns Dome as well as that from industrial suppliers.
Reducing carbon emissions
Even without the additional supply, the dome's naturally occurring CO2 -- created by past volcanic activity -- could power geothermal operations for decades. But Balik said GreenFire is committed to using emissions from area power plants, which could also provide an additional revenue stream for the firm.
Two such plants, the Coronado and Springerville generating stations, are located near the dome and emit a combined 11 million tons of CO2 annually. Those facilities, combined with an additional four coal-fired power plants in the broader region, account for nearly 100 million tons of CO2 each year, all of which could be stored or channeled through the geothermal formation, Balik said.
"The long-term vision is to use anthropogenic CO2 as the geothermal fluid, but there's not a lot of anthropogenic CO2 available right now," Balik said.
Experts say commercial-scale carbon capture at coal plants is still years away, and federal climate change legislation that would cap emissions still awaits passage as Congress focuses on bills to spur jobs and reform the financial and health care systems.
But even if carbon capture technology remains undeveloped, Balik said the St. Johns Dome could theoretically supply enough CO2 to power about 800 megawatts of geothermal energy, or roughly one-fourth of the geothermal capacity in the United States.
"We did an extensive search for these natural CO2 sites globally and we arrived at St. Johns Dome as really the best environment in the world to try this technology," he said. "We needed inexpensive and plentiful CO2, over a heat source and near a power grid."
Just how much heat is available remains to be seen.
Steven Rauzi, administrator of the Arizona Oil and Gas Conservation Commission, said he is unaware of any heat sources near the St. Johns formation that could support a commercial geothermal plant, but that he has not explored the geology at GreenFire's target depth.
"The geothermal resource there is not an area that has really high temperatures," he said, citing a 1994 drilling project near the town of Alpine south of St. Johns that found heat about a mile deep, but not enough to economically produce electricity.
Balik said the firm hopes to both confirm the geothermal resource and demonstrate the core components of what it calls CO2G by the end of 2012. It will take at least three years to prove the technology's commercial viability, he said.
Reducing drilling risks
For decades, the geothermal industry has sought underground reservoirs that contain enough heat, water and permeability to power commercial plants. But those resources are finite and typically limited to remote areas of the West.
In addition, drilling costs can consume up to half of a project's capital budget, and developers always run the risk of drilling a "dry hole."
To reduce that risk, GreenFire and a few other firms are hoping to develop what are known as enhanced geothermal systems (EGS) in which hot, dry rocks are fractured, or "stimulated," to create a porous cavern that can act as a geologic hot pot to heat fluids.
The technology has yet to be demonstrated on a commercial scale, but several federally funded projects are ongoing in Utah, Nevada, Oregon and Alaska.
Using CO2 as a working fluid in EGS could be particularly appealing in water-starved parts of the West, said Susan Petty, president of AltaRock Energy Inc., an EGS startup with projects in Northern California and central Oregon.
AltaRock last month agreed to sublicense its patented CO2G technology to GreenFire for use at the St. Johns Dome and other areas of natural CO2 deposits.
"There are going to be a lot of places in the arid West where EGS can be deployed," she said.
Petty said water consumption was an early concern in developing the nation's first EGS project in the 1970s at the Los Alamos National Laboratory in New Mexico. Project leader Donald Brown, who pioneered the concept of CO2-based geothermal power, said he got the idea while lugging water up to the team's project site at Fenton Hill.
"It became clear to Don that some other fluid being injected into the ground would be great as long as it was readily available and cheap and didn't interfere with people's water supply needs," Petty said.
Lab studies suggest CO2 extracts heat from fractured rock at about 50 percent higher rates than water, Petty said. Supercritical CO2 is also less viscous than water, which could cut down on the amount of energy currently needed to pump water through a system.
Carbon dioxide-based geothermal could offer advantages near the St. Johns site, which is home to some of the finest high-altitude grazing lands in Arizona, but gets only 11 inches of rain a year, according to Rauzi.
While several demonstrations are underway, no EGS projects have reached commercial viability. GreenFire would be the first in the world to deploy EGS using CO2.
The Energy Department in October awarded $133 million in stimulus funding to support EGS technologies and demonstrations across the West. About $16 million of those funds will be shared by nine CO2-related projects led by the Lawrence Berkeley National Laboratory and other federal labs, universities and private firms.
Balik said GreenFire did not seek stimulus funding but is preparing an application bid for a later solicitation at DOE.
The technology needs a chance to be proven, he said, but numerous challenges exist.
Over the next two years, the firm must first confirm the geothermal resource and successfully set up what is known as a "thermosiphon," where the changing density of CO2 is used to guide the fluid through the underground system.
Water in the underground reservoir is also a risk and threatens to turn the CO2 fluid into carbonic acid that could damage the system's geologic integrity and corrode well casings.
"You mix water with carbon dioxide and you're going to create an acidic environment," he said. "But the indication is that the rock formation at the St. Johns Dome is largely dry."