Geoscientists in Iceland have produced the first realistic simulation of how an energy rich “magmatic intrusion” is created. The intrusions occur when a rising mass of viscous magma becomes stuck in the bedrock, and water above the intrusion reaches a “supercritical” temperature, so hot that it creates a reservoir which can produce massive amounts of geothermal electricity.
The new model predicts these natural phenomena may be widespread in highly volcanic areas like Japan and New Zealand. Somewhere out there is a lot of super-efficient energy just waiting to be tapped, and the Icelandic simulation makes finding it a whole lot easier.
Icelanders love their thin crust
Iceland lies on the Mid-Atlantic Ridge where the Eurasian and American continents are drifting apart. The earth’s crust below Iceland is less than four miles thick in places, and magma can easily reach the surface, giving the country its unique volcanic landscape and abundance of geothermal energy.
In 2008, Icelandic researchers from the Iceland Deep Drilling Project (IDDP) found the first magmatic intrusion while drilling a borehole on the Krafla lava field. But their drilling heads kept getting stuck on something, and directly above the magma chamber was a geothermal reservoir of water reaching a “supercritical” temperature of 450 degrees. (Above 374 degrees, water’s gas and liquid phases become indistinguishable and the resulting fluid can be as dense as a liquid but still flow as easily as gas).
While standard geothermal boreholes only have a capacity of 3 to 5 megawatts, a single borehole in a magmatic intrusion contains enough thermal energy to produce 35 megawatts.
Freak of nature or untapped bounty?
The team wondered if the discovery was just a freak of nature, or if it might mean such reservoirs were widespread. To answer these questions, three ETH Zurich geoscientists simulated this unusual geothermal system using a new computer model. This helped them understand how it occurs and under what conditions. It should also help the search for other similar systems. Their results and the associated modeling have been published in Nature Communications.
“The simulations offer a realistic representation of this reservoir’s behavior, even though we kept the model as simple as possible and built in only a few parameters,” said Thomas Driesner, senior lecturer at ETH Zurich’s Institute of Geochemistry and Petrology.
Driesner said the most satisfying thing about the simulations was that they reproduced what the Icelandic IDDP researchers observed at the original borehole. His doctoral student Samuel Scott used the simulations to show that the formation of such a geothermal reservoir depends on the permeability of the surrounding rock. At 1,000 degrees, the magmatic intrusion is absolutely impermeable to water.
If surrounding rock is highly permeable, water can easily rise through it, taking heat away from the intrusion and cooling the magma more quickly. But if the surrounding rock is only slightly permeable, the water remains trapped above the intrusion and heats up to beyond its critical point. In such a scenario, the magma chamber also stays hot for longer.
Basalt, the perfect incubator
Where temperatures and pressures are high, the host rock can change from brittle fracturing to plastic flow, closing up the cracks and fissures through which water might flow. Different types of rock become “plastic” at different temperatures. Basalt, a typical volcanic rock, becomes plastic and impermeable at around 500 to 800 degrees. But this transformation occurs in more silicon-rich granite at just 350 degrees, so magma intrusions in basaltic rock are more likely to create a geothermal reservoir.
“That’s why we expect this type of heat reservoir to be more common than previously thought in volcanic areas such as Iceland, New Zealand, or Japan,” concluded the researchers.
“The model gives us some idea of the criteria by which these zones develop, and how to recognize them,” Driesner explained.
The Icelandic researchers will soon drill a second test borehole on the Reykjanes Peninsula and the search for more “jackpot” reservoirs continues.
(Image credit: Thinkstock)
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