THE PROMISE OF GEOTHERMAL

Geothermal power search holds promise, threat
David R. Baker and David Perlman, August 16, 2009 (SF Chronicle)

"On a high ridge in the Mayacamas Mountains, a drill slowly bores into the earth…[AltaRock Energy] has chosen this ridge to try a new form of geothermal power, using the heat of the Earth to produce energy. The surrounding hills…north of San Francisco…hold more than a dozen older geothermal plants that tap underground pockets of steam to turn turbines and generate electricity.

"But AltaRock will drill below the steam pockets, burrowing into deep rocks hotter than 500 degrees Fahrenheit…fracture those rocks with high-pressure water, creating a network of cracks...[and] then pump more water into the cracks, using the rocks to heat the water and create steam. Geologists monitoring the $17 million project say it will create earthquakes, but most will be too small to notice…[If successful] the technique could work most anywhere. Geothermal energy, which produces almost no greenhouse gas emissions, would no longer be limited to rare places…that have steam or super-heated water underground…"

Schematic of dry hot rock geothermal. (click to enlarge)

"…[Residents near the drilling] worry about the repercussions…Their homes have been rattled by frequent earthquakes for years. The shaking increased after Lake County and the city of Santa Rosa started pumping treated waste water into nearby wells to recharge the underground steam fields…

"Fracturing deep rocks for geothermal energy touched off a 3.4-magnitude earthquake in Basel, Switzerland, in 2006…[S]imilarities between AltaRock's project and the one in Basel prompted the federal Bureau of Land Management and the U.S. Department of Energy to take another look…[It allowed the] drilling [but] the government won't let AltaRock pump water into the well and fracture rock until federal officials are satisfied that the project is safe…"

The deeper you drill, the hotter it gets. (click to enlarge)

"…AltaRock [says it] isn't drilling as close to big fault lines as the Basel drillers did, and the water pressure the company will use to fracture underground rocks will be lower than in Basel…[The area has had] more than 300 quakes greater than a magnitude 2 hitting each year for the last five years…The number of quakes has been growing in the last decade, a rise many residents blame on the injection of waste water into the steam fields. But some of the area's seismicity is natural…A magma bubble lies just 4 miles below…[making it] an ideal place to test…hot dry rock [geothermal]...

"AltaRock will use high-pressure water to lubricate the rock, allowing natural pressures within the rocks to create a network of cracks that can be used to heat water…Each fresh fracture inevitably creates new "micro-earthquakes" …[but] evidence shows there are no faults in [the area]… Most places don't have 500-degree rocks so close to the surface. But "hot dry rock" geothermal power could still work in other parts of the country, provided the wells are drilled much deeper…The benefits for a world grappling with climate change could be huge….[G]eothermal plants can run nonstop, producing a steady stream of power…"