Geothermal energy production has historically been limited to relatively few sites, such as The Geysers, where naturally occurring steam or hot water reservoirs exist. But the dream is to find a way to develop geothermal power production over a much wider range of locations.
Scientists and engineers funded by the U.S. Department of Energy (DOE) are working on that. Their strategy, still experimental, is to pump water into hot “basement rock” (deep crustal rock that currently lacks permeability) creating new fractures and opening and widening existing fractures. Once the fractures are open, water is pumped into the wells. It circulates through the hot rock absorbing heat and generates steam in a process similar to what happens naturally at The Geysers.
The first enhanced geothermal system test site in France is already online. If all goes well, the payoff could be huge, with the potential to produce 10% of our nation’s electric energy.
When creating an enhanced geothermal system (an EGS), the goal is to apply small forces to stimulate rock layers to slip. Once that happens, fractures in the rock are opened and water can flow through.
The methods used to create enhanced geothermal systems are typically different from the “fracking” process used in the oil and gas industry to release hydrocarbons from tight rock. Fracking involves pumping fluids into rock under extreme pressures, physically breaking the rock.
Enhanced geothermal systems utilize the temperature difference between the hot rock and cooler water introduced to the formation to increase permeability. The cooled rock undergoes “thermal contraction,” which causes it to shrink and allows cracks to open. In addition to the thermal stresses, increased fluid pressure (usually much less than is used in fracking) assists in opening cracks, allowing opposing rock surfaces to slip.
The DOE is currently supporting an enhanced geothermal system on the flank of Newberry Volcano, Oregon. The site has plenty of shallow heat, as evidenced by at least 25 eruptive vents less than 10,000 years old (very recent in geological terms) on the summit and flanks of the volcano. However, the site lacks the permeable rock necessary to provide a geothermal reservoir. The goal of the experiment is to create permeability in this "tight" rock by injecting water and causing the opening of existing fractures. Once permeability is established, all of the components of a geothermal system are in place: heat, permeability and fluids (water or steam).
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