It is now claimed that another renewable energy source, geothermal, could supply all of our energy needs. According to Jefferson Tester, professor of chemical engineering at the MIT Laboratory for Energy and the Environment, the heat generated deep within the earth by the decay of naturally occurring isotopes has the potential to supply a tremendous amount of power -- thousands of times more than we now consume each year. The quantity for the whole world is on the order of 100 million quads (a quad is one quadrillion BTUs). This is the part that would be useable. We now use worldwide just over 400 quads per year.
Tester calls it "universal geothermal" energy because the reservoirs could be located wherever they're needed, such as near power-hungry cities worldwide.
So far, we've been able to harvest only a tiny fraction of geothermal energy resources, taking advantage of places where local geology brings hot water and steam near the surface, such as in Iceland or California. New oil-field stimulation technology, developed for extracting oil from sources such as shale, makes it possible to harvest much more of this energy by allowing engineers to create artificial geothermal reservoirs many kilometers underground.
The heat could be harvested by emulating what nature has provided in existing geothermal installations. When we go very deep, (rocks) are crystalline. They're very impermeable. What is needed is to create porosity and permeability. The rock is filled with small fractures, so what you're trying to do is find those weak zones and reopen them. We need to engineer good connectivity between an injection set of wells and a production set of wells, and sweep fluid, in this case, water, over that rock surface so that we extract the thermal energy and bring it up another well.
The technology that is used in drilling and completing oil and gas production systems, (such as) stimulation of wells, hydraulic fracturing, deep-well completion, and multiple horizontal laterals, could in principle be extended to deep heat mining. Hydraulic methods hold the most promise, where you go into the system and you pressurize the rock -- just water pressure. If you go higher than the confinement stress, you will reopen the small fractures. This is a technique that's used almost every single day to stimulate oil and gas reservoirs.
With sufficient financing and a well-characterized field, you can go into existing areas right now and build a plant, getting it operational within a few years. To get universal heat mining to the point that it can be readily applied commercially might take 10 or 15 years of investment.
Resource: Abundant Power from Universal Geothermal Energy, Kevin Bullis, MIT Technology Review, August 1, 2006