GEOTHERMAL AND EARTHQUAKES – A SCIENTIFIC TAKE

Geothermal quake risks must be faced
Domenico Giardini, 17 December 2009 (Nature)

"Deep geothermal energy is increasingly being explored…An enhanced geothermal system (EGS), originally called a 'hot dry rock' system, involves drilling a hole at least 3 kilometres deep into a layer of non-porous rock where temperatures are higher than 100 °C. Fluids are pumped under high pressure into the rock…which induces it to fracture, generating micro-earthquakes, thereby increasing its permeability and creating a reservoir for the fluid. The ruptures generate elastic waves that are detectable by sensitive seismic networks. Once a reservoir of permeable rock larger than a cubic kilometre has been formed, additional holes are drilled to extract heat from the rock mass by circulating fluids through the fracture network.

"The brute-force approach of EGS is attractively simple. And it has, theoretically, the capacity to generate large amounts of alternative energy by tapping a virtually unlimited source — the heat stored deep inside Earth…The drawback is that such enhanced geothermal systems can induce earthquakes…The probability of this happening is not large, but needs to be considered. In addition, geothermal energy is more profitable if it generates electricity and heating at the same time. That means that customers have to be close to the energy source…"

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"Thousands of deep geothermal sites will have to be developed for geothermal energy to supply a sizeable component of the global energy need. If a significant fraction of these induce seismic action under dense urban areas that is felt or is damaging, this will exceed the natural rate of activity in stable continental areas…In a recent case in California, a planned EGS site at the Geysers, a geothermal power field about 100 kilometres north of San Francisco, met with public resistance…[T]hat project was suspended because of technical difficulties…Open and comprehensive information and education needs to be provided to the public and to authorities before, during and after the project…

"One of the first purely commercially oriented EGS projects — the Deep Heat Mining project — was initiated in Basel, Switzerland, in 1996 by the Geopower Basel (GPB) consortium…It is a textbook example of how the failure to come to terms fully with the possibility of producing earthquakes in an urban area (by everyone involved — including the public) became in itself the largest risk to the whole concept…Basel…has a history of earthquakes…GPB had a valid permit, and had met all that permit's conditions…As expected, thousands of micro-earthquakes were recorded…[I]njection was stopped on 7 December. A few hours later, a magnitude-3.4 event rattled the local population…"

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"Since the water injection stopped, seismicity in the area has slowly decayed. Three years later, sporadic seismicity inside the stimulated rock volume is still being detected…EGS projects need to be thought of both as pilot projects with scientific unknowns and as commercial ventures with technological and financial risks…[T]he scientific and engineering community is hard pressed to provide a consensus opinion on how seismic hazards can be assessed with confidence and minimized…The Basel programme is likely to have a strong effect on the insurance cost of future projects…The damage claims in Basel amounted to more than $9 million…for a local magnitude-3.4 event…How would we treat a magnitude-5.5 earthquake hitting Basel in, say, 30 years?

"…It is now becoming clear to the public, local authorities, the geothermal industry and regulatory agencies that deep geothermal systems carry a small risk — as do most technologies in the energy sector. Dams can break, nuclear power plants may fail, carbon dioxide released from the oil and gas contributes to global warming, and EGS projects can create damage through induced earthquakes. The open question is whether or not society is able to find ways to balance and accept these risks. A well-informed discussion is needed to find out."