Cracking Rock to Get More from Geothermal Fields

An enhanced geothermal project fractures hot rocks at unproductive wells, an approach with low financial risk. 

Growth in conventional geothermal power last year was a tepid five percent. A recently-demonstrated enhanced geothermal method could squeeze more usable energy from existing wells. 

Geothermal company Ormat last week said it connected the first enhanced geothermal well to the electricity grid, adding an additional 1.7 megawatts to the Desert Peak project in the Nevada desert. It’s one of a number of enhanced geothermal projects partially funded by the Department of Energy. The approach used at this project, which increased production by 38 percent, could be replicated to expand production at many other existing wells, says Paul Thompson, director of policy and business development at Ormat.

Conventional geothermal wells tap underground reservoirs of hot water and convert the heat into steam, which drives electricity-producing turbine. With enhanced geothermal, fluids are pumped underground to fracture underground rock and stimulate the flow of liquids across hot rocks. The energy source–underground heat–is renewable and clean, but unlike wind and solar, geothermal wells can operate around the clock and provide baseload power.

A 2006 MIT study, which has become famous and geothermal circles, found that the geothermal potential miles underground in the United States is vast. The U.S. Geological Survey estimates that the country’s potential for EGS is between 100 and 500 gigawatts. But until the small Desert Peak project, enhanced geothermal systems have remained in the realm of exploration and research.

The Desert Peak project is more of an incremental approach to tapping enhanced geothermal resources. But it’s significant given that it takes on average seven and a half years to determine if the well has the attributes—the heat, the water, and the permeability within the rock layer– to make it economic. Product developers can’t begin to raise money and actually construct projects until after feasbility testing is done. Using enhanced geothermal “in field” offers a route to essentially getting more production from a field where there has already been an initial investment.

“If we can go to all the hundreds or thousands of wells that are unproductive and tinker with them to make them productive, this is a game changer,” Thompson says. Drilling a brand-new well costs between $4 million and $7 million, whereas expanding existing resources is significantly less, he says.

In this project, Ormat pumped water from its existing hot-water source at a pressure between 800 and 1,000 pounds per square inch to create fractures in the rock. That allowed water to flow through a web of cracks at a much higher rate, going from 4 gallons per minute to 1600 gallons per minute, a change that made it commercially viable.

Part of the project was to implement a protocol for monitoring and communicating the effects of fracturing rock with high-pressure liquids. Concern over tremors caused by enhanced geothermal has derailed other projects. In this case, the “micro seismic events” were minor and didn’t affect people because they occurred in the middle of the desert, Thompson says. More detailed data on the project is expected to be published next month. The technical data from this project, such as how to stimulate fractures at the well bore, can be used to advance all types of EGS projects, Thompson says.

http://www.technologyreview.com/view/513816/cracking-rock-to-get-more-from-geothermal-fields