Fracking for Geothermal Heat Instead of Gas
AltaRock has figured out how to use fracking to get more heat out of a geothermal well, but work remains before the energy source can dent carbon emissions.
The use of hydraulic fracturing has unlocked vast new reserves of natural gas. Now Alta Rock, a startup based in Seattle, is developing technology that might do the same for geothermal resources, turning a marginal power source into a major source of carbon-free electricity and heat in the United States.
Earlier this year near the Newberry Volcano in Oregon, Alta Rock demonstrated a key part of that technology, a process akin to fracking. Just as fracking involves pumping high-pressure liquid into underground shale formations to unlock natural gas and oil that’s been trapped there, the new technology could unlock heat trapped deep underground. Unlike solar and wind power, that heat would be available around the clock and in all sorts of weather.
Geothermal power plants now provide a tiny fraction of the world’s energy needs—in the U.S., one of the world’s biggest producers of geothermal energy, the total geothermal capacity is about 1 percent of the country’s coal power capacity.
The main problem is that conventional geothermal plants rely on a rare combination of geological features. Hot rock has to be accompanied by large amounts of hot water or steam that can easily be pumped to the surface, where it would drive steam turbines to generate electricity. The rock formation needs to be porous enough that the water can be continuously recirculated and reheated to keep a power plant running. (Geothermal pumps are sometimes used to heat and cool homes, but these are inadequate for generating electricity because they work at much lower temperatures.)
Although such formations are rare, the amount of heat underground is actually huge (see “Abundant Power from Universal Geothermal Energy”). There’s enough heat trapped under the United States within drilling distance (as deep as 10 kilometers) to supply its energy needs for thousands of years. AltaRock is one of several companies trying to figure out how to access more of that heat (see “Cracking Rock to Get More from Geothermal Fields” and “Using CO2 to Extract Geothermal Energy”).
The basic idea is to modify the rock to allow water to flow through it (researchers call the resulting reservoirs enhanced geothermal systems, or EGS). This involves pumping cold water into rock in just the right way to trigger existing fractures in the rock to expand and allow water to flow through. It’s been tried many times in the past—with efforts stretching back for decades. But it’s been hard to get enough hot water flowing to justify the expense of drilling a well and building a power plant.
AltaRock’s solution borrows a play from the natural gas industry. One of the key advances that allowed companies to produce economic amounts of natural gas from shale rock is the ability to fracture rock at several points along a single well, which reduces the number of wells that need to be drilled. They do this by temporarily plugging up part of a well so that they can apply hydraulic pressure to one section, and then move on to another part.
It’s long been known that doing the same thing could increase hot water production from a geothermal well. But it’s not possible to use the same techniques used in fracking to plug the well. Geothermal wells are typically hotter, and they need to be engineered for higher amounts of water flow.
AltaRock has essentially invented a new plug. At a well near the Newberry Volcano, it has demonstrated that it’s possible to temporarily plug a geothermal well with a special polymer. The material degrades after it’s been down in the hot rock for a certain amount of time, allowing the company to move on to another part of the well. The company fractured three separate areas of one well using the technique. In a future commercial project, it might do seven or more per well, which “could dramatically lower the cost,” says Susan Petty, the president and chief technology officer at AltaRock. She says the technology could be key to making EGS competitive with coal.
But while the AltaRock technology is a key advance, it’s still early days for geothermal power. “AltaRock’s technology is important, but it’s only one part of the puzzle,” says Jefferson Tester, professor of Sustainable Energy Systems at Cornell University. He says there are several remaining engineering challenges, and solving them will require sustained funding, not just for the project AltaRock is working on, but for several others as well. He says what’s needed is a critical mass of demonstrations to prove to businesses that geothermal power plants are a sound investment. He estimates that it will take decades for geothermal to account for even 10 percent of the total power in the United States.
Petty says that the Newberry site could be producing power by as early as 2016, but much work remains. The next step for AltaRock is to drill another well nearby that will intersect with the porous rock it created with its fracturing technique. Engineers will pump water down the first well, which will circulate through the rock and heat up. Then it will be pumped out of the second well and used to produce steam at a power plant.
In past EGS projects, several problems have arisen at this stage. Sometimes the water flows too quickly from one well to the other, and so it doesn’t get hot enough. At other times water disappears down unknown crevices in the rock, never to be seen again. To address these issues, AltaRock is developing new technologies for monitoring where water is flowing.
AltaRock is also working with GE on an improved process for using hot water to generate electricity. It involves improving heat transfer from the hot water to a working fluid that drives a turbine. The approach could increase power output from a geothermal site still more.