September/October 2007

Rock On

Continued from page 1

By Gregory Mone

Successes like Cooper Basin are attributable to new technology that takes much of the guesswork out of siting and building EGS plants. When engineers pump down water to expand the cracks in the rock, they can map the spreading fractures with tools widely used in the oil industry. Seismome­ters record vibrations, and simulation software generates a map of the new cracks. The map helps engineers pick the best spot for the second hole--the spot amid those veins of frac­tures that will yield the best flow from one hole's pipe to the next. Such tools are "a very crucial thing for us to make this whole process feasible and economically viable," says MIT geophysics professor M. Nafi Toksöz, a report coauthor.

Drilling is one of the biggest costs associated with building a geothermal plant, but that's a problem ­Augustine and Tester are addressing. Conventional drill bits wear down and must be replaced frequently. That can sideline drill rigs--which are rented on a daily basis--for the better part of a day, Augustine says. While deep-pocketed oil companies can afford expensive delays, the prospect of absorbing extra costs makes would-be EGS investors wary. One alternative is a different kind of drilling, called thermal spallation, which replaces the drill bit with a jet flame. The rock is heated to about 500 ºC, at which point it begins to spall, or chip off in tiny chunks. "If you put the right amount of heat on the right type of rock," Augustine says, "it'll just fly away."

In Tester's basement lab, Augustine is experimenting with this flamethrower technique in an apparatus that he built to simulate the extremes of temperature (upwards of 2,000 ºC) and pressure at the bottom of a borehole. Working under such conditions isn't for the faint of heart; he jokes that he encased the rig in two layers of bulletproof material to appease his lab mates. But Augustine's research could make EGS installation much cheaper. With thermal spallation, there are no bits to wear out, and thus no delays while they're replaced. It's also more efficient; other researchers' work suggests that the technique may be more than twice as fast as normal drilling.

The process of exploring and drilling a site poses a few potential environmental risks, but the report deems them manageable. For example, coauthor ­Ronald DiPippo, a mechanical engineer and author of Geothermal Power Plants, says expanding the fractures could induce tremors registering two or three on the Richter scale--something people living nearby should be aware of but would have no cause to fear. MIT chemical engineer Elisabeth Drake '58, ScD '66, led the drafting of the report's section on the environmental impact of EGS. "Once the facility is operational," she says, "the impacts are small compared with other energy technologies."

Although geothermal alone won't meet our energy needs in the coming decades, a relatively small investment in EGS research and development over 15 years would easily pay off. Tester says investing $800 million to $1 billion­--less than the cost of one new clean coal-burning power plant­--could make EGS an economically viable energy source. And as part of a portfolio of energy alternatives, it could substantially decrease our dependence on fossil fuels. "If we think about what made America strong over its history, it has largely been a diversity of the ideas and the people and the resources we have," he says. We "haven't capitalized on that" in the realm of energy, but developing geothermal would be one way to start.