Wednesday, July 15, 2009

Water, Underground & Above

One thing that struck me about this article from the Sacramento Bee about getting geothermal energy from underground by breaking up rock—and thereby causing earthquakes, though said to be small—is the conjunction with another source of energy developed from above ground water storage, hydropower from dams.

A major objection about building Auburn Dam is the speculation, here and here, that it would cause earthquakes, with evidence for and against, as noted by this government report that: “Based on analysis of 55 reported cases of reservoir-induced seismicity worldwide and geologic and seismologic data for 16 selected dams and reservoirs in the Sierran foothills, WCC (1977) reached two sets of conclusions about reservoir-induced seismicity associated with Auburn Dam as proposed at that time. First, if the 1975 Oroville earthquake is assumed not to be reservoir induced, then the likelihood of an induced earthquake of the magnitude of the Oroville event (5.7) or larger is 2% to 5 % during the life of the dam. Second, if the Oroville earthquake is assumed to be reservoir induced, then the likelihood of an induced earthquake of the magnitude of the Oroville earthquake is 30 % during the lifetime of the dam. We have not been able to reconstruct the basis for these probabilities and do not endorse them. The specific probabilities of an induced earthquake of M 5.7 or larger associated with an Auburn reservoir are open for revaluation.”

So, by building a dam that would produce hydroelectric power there is at most and after a big if is determined, a 30% chance of an earthquake, which current dam construction technology could largely counteract, as opposed to drilling through rock underground that with “little doubt” will cause earthquakes, it would seem the safer bet is the dam.

An excerpt.

“ANDERSON SPRINGS – Residents in this tiny Lake County community have complained for years about the earthquakes touched off by the geothermal energy projects that tap the vast reservoir of steam in the mountains behind their homes.

“Now, with the federal government, Google and some of Silicon Valley's top venture capital firms committing millions to test a new way to mine clean energy from the earth here, the locals are finally getting some attention.

“On a ridgetop above Anderson Springs, Bay Area startup AltaRock Energy Inc. is drilling a hole more than 2 miles deep. As soon as August, the company plans to inject high-pressure water to crack the solid, 500-degree Fahrenheit bedrock, creating an artificial reservoir of superheated water. The steam will then be used to drive electrical turbines.

“If the test works, it could pave the way for essentially limitless exploitation of the heat energy in Earth's crust….

“In Anderson Springs, though, the project has homeowners worried that the regular quake activity they already contend with would get even worse. Over the past two decades, the region has experienced between 13 and 32 earthquakes each year greater than magnitude 3.0 – including six in the past two weeks – according to U.S. Geological Service data, as well as thousands of smaller quakes.

“While these are relatively small quakes, they originate near the surface and can feel stronger than the numbers suggest.

“There's little debate that the quakes are caused by the existing geothermal projects, as water is injected and withdrawn, causing rock to shift.”