Understanding deep geothermal energy
Understanding deep geothermal energy

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Understanding deep geothermal energy

2 High- to medium-enthalpy steam fields

When the geothermal gradient heats water above the temperature at which it boils at atmospheric pressure, at a depth accessible to drilling, conditions can favour using natural geothermal steam to generate electricity. Typically, the pressure can be several tens to hundreds of times that of the atmosphere. Even at 200 °C, high pressure can ensure that much of the fluid in a geothermally heated aquifer remains in the liquid state. Figure 4 shows why that seemingly odd situation occurs; the physical states of water (solid, liquid and vapour) transform to one another at different temperatures and pressures. The critical point of water is the temperature above which liquid and vapour are no longer valid concepts, irrespective of pressure. Instead, water exists at higher temperatures as a supercritical fluid.

Figure 4
Figure 4 How the different states of water depend on temperature and pressure (the vertical scale is logarithmic). Above a temperature of 374 °C (the 'critical point' of water) there is no distinction between the liquid and vapour states of water, and it exists as a 'supercritical fluid'. ( Note: A pressure of 1000 times that of the atmosphere at sea-level is equivalent to a depth of about 3.57 km in the crust. Pressures less than one atmosphere do not apply to the Earth's crust, but to water in the atmosphere.)
  • In an area with a geothermal gradient of 60 K km−1 will water trapped in an aquifer exist as a liquid or steam at a depth of 2 km?

  • A temperature of 120 ° C that occurs between 0.357 to 3.57 km deep plots in the liquid field of Figure 4.

  • What would happen to that liquid water if a well connected the aquifer to the surface?

  • Pressure will drop, and as it rises toward the surface the water will become steam.

Effectively, deep groundwater that is above the boiling temperature at atmospheric pressure is superheated. Once pressure is released it 'flashes' to steam, and jets to the surface. Many potential geothermal areas naturally connect to the surface along faults, so that steam is generated at depth and drives hot water to the surface, where it appears as hot springs and sometimes geysers that emit high-pressure steam and boiling water. To exploit this power-generating potential requires that such a water source be effectively trapped so that it cannot leak away naturally.

In Figure 5, traps below the Lardarello and Wairakei geothermal fields are at shallow enough depths for high-pressure steam to be present naturally in the aquifer (see Figure 4). Deeper systems contain superheated water that 'flashes' to steam when penetrated by wells. The fluids that issue from geothermal wells range from hot water and wet steam (close to 100 °C and easily condensed) to dry steam far above boiling temperature. Different kinds of fluid demand different electricity-generating technologies, so that each geothermal field is unique and is exploited on its own merits.

Figure 5
Figure 5 Cross sections of high-enthalpy geothermal fields in volcanic areas: (a) The Lardarello field in Tuscany, Italy. Note the favourable dome-like structure over the active igneous intrusions, where a limestone aquifer forms a trap beneath shale and clay cap rocks. (b) The Wairakei field in New Zealand has several aquifers sealed by impermeable lavas, volcanic ash and intrusive igneous rocks. In both cases some hot water and steam escapes naturally to the surface along faults.

High- to medium-enthalpy geothermal resources are conventionally divided into water- or vapour-dominated fields. Vapour-dominated systems where the steam is at high temperature and dry tend to be the most economically viable geothermal resources, because the PV term in the enthalpy equation is high. Vapour-dominated fields also suffer less from problems of corrosion from the mineral-laden waters that are typical of deep aquifers. When such hot water passes through pipelines, not only is it highly corrosive, but solids dissolved in it precipitate in the pipes as temperature decreases, thereby clogging them in the manner of a 'furred' kettle.

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