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In search of cracks

MIT is developing computer programs to aid in the evaluation of geothermal sites

Most energy analysts agree that geothermal energy — tapping the heat of bedrock deep underground to generate electricity — have enormous potential because it is available all the time, almost anywhere on Earth, and there is enough of it available, in theory, to supply all of the world’s energy needs for many centuries. But there are still some unanswered questions about it that require further research.

But what is geothermal energy? Everywhere on Earth, a few miles below the surface, the bedrock is hot and the deeper you go the hotter it gets. In some places, water heated by this hot rock comes naturally to the surface or close to it, where it can be easily tapped to drive a turbine and generate electricity.

But where naturally heated water is not available at or near the surface, this process can be recreated by drilling one very deep well to inject water into the ground, and another well nearby to pump that water back to the surface after it has been heated by passing through cracks in the hot rock. Such systems are known as Engineered Geothermal Systems, or EGS.

But the trick is to find these cracks. One man leading the search is Herbert Einstein, MIT professor of civil and environmental engineering. Einstein studies fracturing in rocks, which are crucial in creating a new EGS site: After drilling the necessary wells, water must be pumped into one of them under very high pressure to create a network of fractures in the deep rock to allow the water to move from the injection well to the extraction well. But exactly how that process works at different depths and in different types of rock is not yet well understood.

Einstein is developing computer programs that can aid in the evaluation of geothermal sites, assessing both the potential power output and any potential risks, such as the triggering of seismic activity. Such triggering has already resulted in the premature closing two years ago of one test installation, in Basel, Switzerland, after some minor earthquakes (the largest being magnitude 3.4) were felt in the area.

The planned software is based on programs Einstein has developed to assess proposed tunnel sites and landslide risks.

“What these decision tools do is allow you to consider the uncertainties, of which there are a lot,” he says.

Einstein suggests that the risks from seismic triggering are largely sociological, because the events seen so far, at least, are too small to produce any serious damage. “I think that’s a red herring,” he said. “We know that every time we drill into the Earth, we alter the state of the stress in the rock.” As a result, small earthquakes do occur regularly near oil and gas wells, deep mine shafts for coal or minerals, and even from the pressure of water when a reservoir fills up behind a new dam. “Wherever there are existing faults, they will induce mostly minor quakes.”