6.3 Geothermal energy exploration and exploitation
4 min read•Last Updated on August 14, 2024
Geothermal energy taps into Earth's heat for power and heating. This renewable resource is found in volcanic areas, plate boundaries, and hot spots. Exploring and exploiting geothermal energy involves identifying high heat flow zones and using advanced technologies.
Geophysical methods like seismic and electromagnetic surveys help locate geothermal reservoirs. Power plants then extract heat to generate electricity or provide direct heating. Environmental impacts are generally low, but initial costs can be high.
Favorable Geological Settings for Geothermal Energy
High Heat Flow Areas
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Geothermal energy resources are typically associated with areas of high heat flow, such as volcanic regions (Yellowstone National Park), tectonic plate boundaries (Ring of Fire), and hot spots (Hawaii)
The presence of cap rocks, such as impermeable clay layers or dense volcanic rocks, can help to trap and concentrate geothermal fluids in reservoirs, increasing the potential for geothermal energy development
Hydrothermal Systems and Enhanced Geothermal Systems (EGS)
Hydrothermal systems, which involve the circulation of hot water or steam through permeable rocks, are the most common type of geothermal resource exploited for energy production
High-temperature hydrothermal systems (>150°C) are often found in volcanic regions and are suitable for electricity generation
Low-temperature hydrothermal systems (<150°C) are more widespread and are used for direct heating applications, such as space heating (district heating systems) and agricultural processes (greenhouses)
Enhanced Geothermal Systems (EGS) involve the creation of artificial geothermal reservoirs by fracturing hot, dry rock and circulating fluid through the fractures to extract heat
Sedimentary basins with high geothermal gradients and thick, permeable aquifers (Williston Basin) can also be potential targets for geothermal energy development
Geophysical Methods in Geothermal Exploration
Seismic, Gravity, and Magnetic Surveys
Geophysical methods are used to identify and characterize geothermal resources by measuring various physical properties of the subsurface
Seismic surveys, including reflection and refraction methods, can provide information on subsurface geology, fault structures, and the presence of geothermal reservoirs
Gravity surveys measure variations in the Earth's gravitational field, which can indicate the presence of low-density, high-temperature rocks and fluids associated with geothermal systems
Magnetic surveys detect variations in the Earth's magnetic field, which can be influenced by the presence of magnetic minerals altered by geothermal fluids
Electrical, Electromagnetic, and Thermal Methods
Electrical and electromagnetic methods, such as magnetotellurics (MT) and controlled-source electromagnetic (CSEM) surveys, can map the electrical conductivity of the subsurface, which is often enhanced by the presence of geothermal fluids
Heat flow measurements and temperature gradient surveys can directly assess the thermal regime of an area and help identify regions with elevated geothermal potential
Geochemical analysis of surface manifestations (hot springs, fumaroles) can provide insights into the temperature, composition, and origin of geothermal fluids
Principles and Technologies of Geothermal Exploitation
Geothermal Power Generation
Geothermal energy exploitation involves the extraction of heat from geothermal reservoirs and its conversion into usable forms of energy, such as electricity or direct heat
Conventional geothermal power plants use steam or hot water from hydrothermal reservoirs to drive turbines and generate electricity
Flash steam plants are used for high-temperature resources, where the geothermal fluid is rapidly depressurized to produce steam that drives the turbines
Binary cycle plants are used for lower-temperature resources, where the geothermal fluid heats a secondary working fluid with a lower boiling point (isobutane), which then drives the turbines
Direct Use and Advanced Technologies
Direct use applications involve the direct utilization of geothermal heat for space heating, greenhouse heating, aquaculture (fish farming), and industrial processes (food dehydration)
Ground-source heat pumps (GSHPs) use the relatively constant temperature of the shallow subsurface to provide heating and cooling for buildings
Enhanced Geothermal Systems (EGS) involve the creation of artificial geothermal reservoirs by fracturing hot, dry rock and circulating fluid through the fractures to extract heat
Hydraulic stimulation techniques, such as hydroshearing and hydraulic fracturing, are used to enhance the permeability of the rock and create a heat exchange system
Directional drilling technologies are employed to create injection and production wells in the artificially created reservoir
Environmental and Economic Aspects of Geothermal Development
Environmental Considerations
Geothermal energy is considered a renewable and low-carbon energy source, as the heat extracted from the Earth is continuously replenished by natural processes
Geothermal power plants have a relatively small land footprint compared to other energy sources, and they can provide baseload power with high capacity factors
Geothermal fluids can contain dissolved gases, such as carbon dioxide and hydrogen sulfide, which may need to be properly managed to minimize air pollution
The extraction of geothermal fluids can lead to subsidence if not properly managed, as the removal of fluids can cause the ground to sink
Induced seismicity is a potential concern in geothermal development, particularly in EGS projects, where hydraulic stimulation techniques are used to enhance reservoir permeability
Economic Factors
The initial capital costs of geothermal projects can be high due to the need for exploration, drilling, and plant construction
However, geothermal plants have low operational costs and long lifetimes (30-50 years), which can make them economically competitive in the long run
Government incentives (tax credits, grants) and supportive policies can help to reduce the financial risks and encourage geothermal energy development
The economic viability of geothermal projects depends on factors such as the resource quality, location (proximity to transmission lines), market conditions (electricity prices), and the availability of transmission infrastructure
Geothermal energy can provide a stable and predictable source of revenue for local communities through job creation, tax revenues, and royalty payments