December 1, 2009

Assessing the Earthquake Risk of Enhanced Geothermal Systems

Sonal Patel

The Marriage of EGS and CO₂ Sequestration

EGS may have garnered much recent attention for its appeal as a renewable energy source, but as a relatively new technology, it continues to pose numerous technical challenges. Governments are injecting billions of dollars into its development, which could help solve some of the puzzles. The DOE in late October, for example, committed $338 million in funding from the American Recovery and Reinvestment Act of 2009 to advance geothermal technologies.

Yet industry experts like Dr. Subir K. Sanyal, president of the California-based geothermal consulting and services firm GeothermEx Inc., assert that it will take decades to reduce levelized costs of EGS power enough to make EGS commercial, even with federal support. In a 2007 paper, Sanyal put the total capital cost — including exploration and drilling, stimulation, power plant, and surface facilities costs — for all sizes and configurations of hypothetical EGS projects at higher than $4,000 per kilowatt of installed capacity. This compares to a typical cost of $3,500 per kilowatt for conventional geothermal projects.

Nevertheless, findings from federally backed projects could be beneficial and could even lead to the integration of EGS and another relatively new concept that uses similar technologies and has similar challenges: geologic carbon sequestration. Here’s how it might work.

The primary working fluid circulated through an EGS in a continuous loop is typically brine. Heat extracted from the deep subsurface system by the brine is brought to the surface and then transferred to a secondary fluid — typically isopentane — which is then expanded through a turbine-generator to generate power. But as research continues on this basic system, efforts are also under way around the world to develop an EGS concept that uses supercritical carbon dioxide (CO₂).

Such a system is said to have many theoretical advantages: Not only would it help sequester the greenhouse gas, but it could also minimize water use and offer a greater power output with minimal parasitic losses from pumping and cooling, say scientists at the recently established Queensland Geothermal Energy Center of Excellence at the University of Queensland in Australia.

The center is developing an approach that circulates highly pressurized CO₂ through superheated rocks within the subsurface reservoir to extract geothermal heat. The scientists say that supercriticaCO₂   would behave like a liquid under pressure in the injection well and like a gas on the surface. The hot fluid will then rise as in a flash geothermal plant, and then — without an intermediate heat exchanger — expand to a gas at the surface to turn a turbine and produce electricity.

In theory, the center says, the thermal cycle efficiency of what it calls the "supercritical CO₂ geothermal siphon system," could be 50% higher than the efficiency of a conventional geothermal binary power plant working between similar reservoir and ambient temperatures — an improvement that could make a big impact on the financial viability of a given EGS project.

Asked if using CO₂ in the reservoir as opposed to water or brine could induce large seismic events, University of Queensland Professor Hal Gurgenci, who is also interim director of the Geothermal Center of Excellence, responded that CO₂ in the reservoir would be at the current reservoir pressures, and that in terms of the pressure loading on the surrounding rock, there would be no difference between CO₂ or water. "Therefore, as far as earthquake risk is concerned, there is no difference between water or CO₂ as the geothermal heat exchange fluid," he said.

Do EGS and CO₂ Sequestration Share a Fault?

The question of whether and how induced seismicity will affect the future of EGS is a question also being asked about geologic storage of CO₂ .

The USGS, in a March 2009 paper evaluating CO₂ storage, deems earthquakes a potential hazard from large-scale CO₂  injection, ranking it as important a concern as contamination of shallow groundwater and transport of contaminants out of the storage formation.

Dr. Christian Klose, a geophysical hazards research scientist at Columbia University, says the risk of stored CO₂  causing quakes is very real. "Carbon sequestration can trigger earthquakes of varying magnitudes [less than magnitude 5], as any fluid injection can do," he told POWER. Three processes could trigger seismic activity, large and small: pore fluid pressure changes; fluid mass (volume) changes, which can cause stress on the rock; and migration of theCO₂ through the rock over decades to centuries. "CO₂ is buoyant, since its density is [lower] than saline water deep in the crust," he said. "Thus it will come upward through cracks and fractures and faults — even in so-called cap rocks there are rock discontinuities that cause leakages."

The Earth Sciences division at LBNL disagrees, however, saying that there are too few large-scale injections of CO₂ for sequestration to practically assess the risk. It also points out on its website that CO₂ has been injected safely underground for years in many oil and gas reservoirs with little seismic impact, adding that the lessons from EGS-induced seismicity will particularly apply to CO₂   injections into deep saline water formations and coal gas and shale formations.

Moreover, according to Traci Rodosta, a geological sequestration project manager for the National Energy Technology Laboratory, quake risk is well-assessed during research and development of any given project. "Potential sequestration reservoirs are thoroughly characterized prior to injection," she told POWER. "In order to eliminate and reduce the potential for fault activation and slippage along preexisting fractures that could be caused when injecting fluids at high pressures, regulatory agencies limit injection rates and pressure to avoid unintentional hydrofracturing. CO₂  storage projects would operate under similar guidelines, and the risk [would be] managed through site characterization, injection design, and monitoring."

For Majer, the likelihood of seismic activity as it relates to carbon sequestration — even at larger volumes — would be minimal. "Most CO₂ sequestration sites shouldn’t be near faults. In geothermal you don’t really have a choice, whereas in sequestration, you do," he explains. Perhaps what will be of more importance is to address with urgency — as for EGS — public perceptions of potential hazards as they apply to induced earthquakes. He points out that, unlike a geothermal plant, which provides a source of revenue and development for any given community, a carbon sequestration site may not. "With CO₂  sequestration, if annoying seismicity starts, it may be more difficult to deal with than geothermal, because [the community is] not getting too much out of a particular project even though it’s suffering the annoyance," he said. "There won’t be too many jobs associated with it, they’re not getting any tax revenue from it — it’s really just helping us save the planet."

—Sonal Patel is POWER’s senior writer.