4.4 Geothermal energy exploration and exploitation

Geothermal energy taps into Earth's heat for power and heating. It's a key part of understanding our planet's thermal structure. From hot springs to power plants, geothermal systems offer a glimpse into Earth's inner workings.

Exploring and using geothermal energy involves geology, chemistry, and physics. We'll look at how scientists find these hot spots and turn them into clean energy sources. It's a fascinating blend of Earth science and sustainable technology.

Geothermal Systems and Characteristics

Types of Geothermal Systems

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• Geothermal systems are classified based on their geological setting, temperature, and fluid characteristics into three main types: hydrothermal systems, enhanced geothermal systems (EGS), and sedimentary basin geothermal systems
• Hydrothermal systems are the most common and commercially viable type characterized by the presence of naturally occurring hot water or steam in permeable rocks (Larderello field in Italy, The Geysers in California)
  • Vapor-dominated systems produce dry steam and are rare but highly productive with temperatures typically exceeding 240°C
  • Liquid-dominated systems produce hot water with temperatures ranging from 90°C to 240°C and are exploited for both electricity generation and direct heat applications (Wairakei field in New Zealand, Salton Sea field in California)
• Enhanced Geothermal Systems (EGS) involve the creation of artificial geothermal reservoirs by fracturing hot, dry rock at depths of 3-10 km and circulating fluid through the fractures to extract heat, potentially expanding the geographic range of geothermal energy utilization
• Sedimentary basin geothermal systems are found in deep sedimentary basins with high heat flow where hot water is trapped in permeable sedimentary layers, typically having lower temperatures (30-150°C) compared to hydrothermal systems but can be used for direct heat applications and, in some cases, electricity generation using binary cycle power plants

Characteristics of Geothermal Systems

• Geothermal systems require a heat source, fluid, and permeable pathways to facilitate the circulation of geothermal fluids
• Heat sources can be magmatic intrusions, radioactive decay of minerals, or high geothermal gradients in tectonically active areas
• Geothermal fluids are typically meteoric water that has percolated into the Earth's crust and been heated by the heat source, often containing dissolved minerals and gases (CO2, H2S)
• Permeable pathways, such as faults, fractures, and porous rock layers, allow the heated fluids to circulate and rise towards the surface
• The temperature, pressure, and fluid composition of geothermal systems determine their potential for energy production and the appropriate technology for exploitation

Geothermal Exploration Methods

Geological and Geochemical Methods

• Geological mapping and structural analysis identify favorable geologic settings, such as recent volcanic activity, faults, and fractures that can act as fluid pathways, while remote sensing techniques (satellite imagery, aerial photography) aid in regional-scale mapping and the identification of surface manifestations (hot springs, fumaroles)
• Geochemical surveys analyze the composition and temperature of geothermal fluids (hot springs, fumaroles) and gases (CO2, H2S) to estimate reservoir temperatures and fluid origin, with isotope geochemistry (oxygen and hydrogen isotopes) helping to determine the source and age of the geothermal fluids

Geophysical Surveys

• Geophysical surveys are crucial for characterizing subsurface properties and include a range of methods:
  • Gravity surveys identify density variations associated with geothermal reservoirs and can delineate the extent of the reservoir and the presence of faults
  • Magnetic surveys detect variations in the Earth's magnetic field caused by the presence of magnetic minerals, which can be altered by geothermal activity
  • Electrical and electromagnetic methods, such as magnetotellurics (MT) and controlled-source electromagnetics (CSEM), map the electrical conductivity of the subsurface, which is influenced by the presence of geothermal fluids and clay minerals
  • Seismic surveys, including reflection and refraction methods, provide information on subsurface structure, stratigraphy, and the presence of faults or fractures that can act as fluid pathways
• Well logging involves the measurement of physical properties (temperature, pressure, electrical conductivity, gamma radiation) in geothermal wells using specialized tools to characterize reservoir properties, such as temperature gradient, permeability, and fluid composition, and to guide further exploration and development decisions

Geothermal Energy Production

Electricity Generation

• Electricity generation from high-temperature (>150°C) geothermal resources typically involves the use of steam turbines in conventional power plants, with vapor-dominated systems using dry steam directly to drive the turbines and liquid-dominated systems using a mixture of hot water and steam, separating the steam to drive the turbines
• Binary cycle power plants are used for lower-temperature (90-150°C) geothermal resources, where the heat from the geothermal fluid is transferred to a secondary working fluid (isobutane or pentane) with a lower boiling point, which vaporizes and drives a turbine before being condensed and reused in a closed loop

Direct Use and Heat Pumps

• Direct use of geothermal energy involves the utilization of low-to-moderate temperature (30-150°C) geothermal resources for heating and cooling applications, such as space heating, greenhouse heating, aquaculture, and industrial processes, using heat exchangers to transfer the heat from the geothermal fluid to the target application
• Geothermal heat pumps (GHPs) use the relatively constant temperature of the shallow subsurface (< 100 m depth) to provide heating and cooling for buildings by circulating a fluid through a closed loop of pipes buried in the ground, extracting heat in the winter and rejecting heat in the summer

Enhanced Geothermal Systems (EGS)

• Enhanced Geothermal Systems (EGS) involve the creation of artificial geothermal reservoirs by fracturing hot, dry rock and circulating a fluid (typically water) through the fractures to extract heat
• EGS technologies are still in the development stage but have the potential to greatly expand the geographic range of geothermal energy utilization by enabling the exploitation of geothermal resources in areas without naturally occurring hydrothermal systems

Environmental and Economic Aspects of Geothermal Energy

Environmental Considerations

• Geothermal energy is considered a renewable and sustainable energy source, as the heat extracted from the Earth is continuously replenished by natural processes, but the rate of replenishment is slow compared to the rate of extraction, so geothermal resources must be carefully managed to ensure long-term sustainability
• Geothermal energy has a low carbon footprint compared to fossil fuels, as it does not involve the combustion of hydrocarbons, but geothermal fluids can contain dissolved gases (CO2, H2S) that can be released into the atmosphere during energy production, which can be mitigated by advanced technologies, such as CO2 capture and storage
• Geothermal energy production can have local environmental impacts, such as land subsidence due to fluid withdrawal, induced seismicity from fluid injection or reservoir stimulation, and the release of geothermal fluids containing heavy metals or other contaminants, but careful site selection, monitoring, and management practices can help minimize these impacts

Economic Aspects

• The economic viability of geothermal energy projects depends on factors such as the resource temperature, depth, and permeability, as well as the proximity to energy markets and infrastructure
• High upfront costs for exploration and drilling can be a barrier to development, but geothermal energy can provide a stable, baseload power source with low operational costs over the long term
• Geothermal energy can contribute to energy security and diversification, particularly in countries with significant geothermal resources (Iceland, New Zealand), and provide local economic benefits, such as job creation and increased tax revenues
• Policy support, such as feed-in tariffs, tax incentives, and research and development funding, can help promote the growth of the geothermal energy industry and make it more competitive with other energy sources