🌋Geothermal Systems Engineering Unit 8 – Geothermal Resource Assessment & Exploration

Key Concepts and Definitions

• Geothermal energy originates from the Earth's interior heat, which is generated by radioactive decay and residual heat from planetary formation
• Geothermal systems require a heat source, a reservoir, and a fluid to transfer the heat to the surface
  • Heat sources can include magmatic intrusions, deep circulation of groundwater, or radioactive decay in the crust
  • Reservoirs are permeable and porous rock formations that store the geothermal fluid
  • Fluids can be water, steam, or a mixture of both, depending on the temperature and pressure conditions
• Geothermal gradients represent the increase in temperature with depth in the Earth's crust, typically ranging from 20 to 30°C/km
• Permeability is a measure of a rock's ability to allow fluid flow through its interconnected pores or fractures, expressed in units of darcy (D)
• Porosity refers to the fraction of void space within a rock that can store fluids, expressed as a percentage
• Geothermal power plants convert the heat energy from geothermal fluids into electricity using various technologies (flash steam, dry steam, binary cycle)
• Geothermal resource assessment involves estimating the potential energy that can be extracted from a geothermal system over its lifetime

Geothermal System Types

• Hydrothermal systems are the most common type of geothermal system, characterized by the presence of hot water or steam in a reservoir
  • High-temperature hydrothermal systems (>150°C) are suitable for electricity generation
  • Low-temperature hydrothermal systems (<150°C) are used for direct heating applications (space heating, greenhouses, aquaculture)
• Enhanced Geothermal Systems (EGS) involve creating an artificial reservoir by fracturing hot, dry rock and circulating a fluid to extract heat
• Sedimentary basin geothermal systems rely on deep circulation of groundwater in permeable sedimentary formations, often with lower temperatures than hydrothermal systems
• Magmatic geothermal systems are associated with recent volcanic activity and can have extremely high temperatures (>300°C)
• Geo-pressured systems contain high-pressure, high-temperature fluids trapped in deep sedimentary formations, often with dissolved methane
• Hot dry rock (HDR) systems have high temperatures but lack a natural reservoir and fluid, requiring hydraulic fracturing to create an artificial reservoir
• Shallow geothermal systems, such as ground source heat pumps, utilize the relatively constant temperature of the shallow subsurface for heating and cooling applications

Geological and Geophysical Exploration Methods

• Geological mapping involves identifying surface features (hot springs, altered rocks, fault zones) that indicate the presence of a geothermal system
• Geochemical surveys analyze the composition of geothermal fluids and gases to estimate reservoir temperature and fluid origin
  • Geothermometers use the concentration of certain elements (silica, sodium, potassium, calcium) to estimate reservoir temperature
  • Isotope analysis can help determine the source and age of the geothermal fluids
• Geophysical methods provide subsurface information about the geothermal system's structure, temperature, and fluid content
  • Gravity surveys measure variations in the Earth's gravitational field to identify density contrasts associated with geothermal reservoirs
  • Magnetic surveys detect variations in the Earth's magnetic field caused by the presence of magnetic minerals or high-temperature zones
  • Electrical resistivity surveys map the subsurface electrical conductivity, which is influenced by the presence of geothermal fluids and clay minerals
• Seismic methods use the propagation of elastic waves to image subsurface structures and identify potential geothermal reservoirs
  • Passive seismic monitoring detects natural microearthquakes associated with fluid movement and fracturing in the geothermal system
  • Active seismic surveys involve generating seismic waves and recording their reflections and refractions to create a detailed image of the subsurface
• Remote sensing techniques, such as satellite imagery and aerial photography, can identify surface manifestations of geothermal activity and map regional geological structures

Drilling and Well Testing Techniques

• Exploration drilling involves drilling slim holes or core holes to confirm the presence of a geothermal reservoir and gather subsurface data
  • Temperature logs measure the temperature profile along the well to identify hot zones and estimate geothermal gradients
  • Pressure logs record the fluid pressure in the well to determine reservoir permeability and identify flow zones
  • Fluid sampling and analysis provide information about the geothermal fluid's composition, origin, and thermodynamic properties
• Production drilling creates wells to extract geothermal fluids from the reservoir for energy production
  • Directional drilling techniques allow for the creation of deviated or horizontal wells to intersect multiple fractures or target specific zones
  • Well completion methods, such as casing and cementing, ensure well integrity and prevent fluid loss or contamination
• Injection wells are drilled to reinject cooled geothermal fluids back into the reservoir, maintaining pressure and extending the system's productive life
• Well testing techniques evaluate the reservoir's properties and performance
  • Flow tests measure the well's production rate, fluid enthalpy, and pressure response to assess reservoir permeability and productivity
  • Interference tests monitor pressure changes in observation wells during production or injection to estimate reservoir connectivity and boundary conditions
  • Tracer tests involve injecting chemical or radioactive tracers into the reservoir and monitoring their arrival in production wells to characterize fluid flow paths and velocities
• Well stimulation methods, such as hydraulic fracturing or acid stimulation, can enhance the permeability of the reservoir and improve well productivity

Resource Assessment Models

• Volumetric methods estimate the geothermal resource potential based on the reservoir's volume, temperature, and fluid properties
  • The heat-in-place method calculates the total thermal energy stored in the reservoir using the equation: $Q = ρ × c × V × (T_r - T_0)$, where $Q$ is the heat content, $ρ$ is the rock density, $c$ is the specific heat capacity, $V$ is the reservoir volume, $T_r$ is the reservoir temperature, and $T_0$ is the reference temperature
  • The recoverable resource is estimated by applying a recovery factor to the heat-in-place, accounting for technical and economic constraints
• Numerical reservoir models simulate the geothermal system's behavior over time, considering fluid flow, heat transfer, and geomechanical processes
  • Finite difference or finite element methods discretize the reservoir into grid blocks or elements and solve the governing equations for mass and energy conservation
  • Coupled models incorporate the interactions between fluid flow, heat transfer, and rock deformation to predict reservoir performance and sustainability
• Decline curve analysis extrapolates future production rates based on historical production data, assuming a certain decline trend (exponential, hyperbolic, or harmonic)
• Monte Carlo simulation is a probabilistic method that accounts for uncertainties in reservoir parameters by generating multiple realizations and calculating a range of possible outcomes
• Analogue methods compare the geothermal system under investigation with similar, well-characterized systems to infer its properties and potential

Environmental and Economic Considerations

• Geothermal energy is considered a clean and renewable resource, as it does not produce significant greenhouse gas emissions during operation
• Environmental impacts of geothermal development may include:
  • Land subsidence due to fluid withdrawal and pressure decline in the reservoir
  • Induced seismicity caused by fluid injection or reservoir stimulation
  • Water and air pollution from the release of geothermal fluids containing dissolved gases (hydrogen sulfide, carbon dioxide) or toxic elements (mercury, arsenic)
  • Noise and visual impact of drilling and power plant operations
• Mitigation measures, such as reinjection, seismic monitoring, and air and water treatment systems, can minimize the environmental impacts of geothermal projects
• Geothermal energy's economic viability depends on factors such as resource quality, project scale, technology, and market conditions
  • High upfront costs for exploration, drilling, and power plant construction can be a barrier to geothermal development
  • Economies of scale favor larger projects, as the cost per unit of energy decreases with increasing plant size
  • Geothermal power plants have high capacity factors (typically >90%) and provide baseload electricity, which can command higher prices than intermittent renewable sources
• Policy incentives, such as feed-in tariffs, tax credits, and risk mitigation schemes, can support the development of geothermal projects and attract private investment
• Life-cycle assessment (LCA) is a tool to evaluate the environmental and economic performance of geothermal systems over their entire lifespan, from exploration to decommissioning

Case Studies and Real-World Applications

• The Geysers, California, USA, is the world's largest geothermal field, with an installed capacity of over 1,500 MW across 22 power plants
  • The field produces dry steam from a vapor-dominated reservoir in fractured metamorphic rocks
  • Reservoir management strategies, such as reinjection and steam field optimization, have been crucial to sustaining production and extending the field's life
• Larderello, Italy, is the oldest geothermal field in the world, in operation since 1913
  • The field produces superheated steam from a carbonate-evaporite reservoir, with temperatures up to 260°C
  • The geothermal system is associated with a granitic intrusion and a regional extensional tectonic setting
• Krafla, Iceland, is a high-temperature geothermal field located in a basaltic volcanic system
  • The field produces a mixture of steam and hot water from a shallow, two-phase reservoir with temperatures up to 350°C
  • The geothermal power plant has an installed capacity of 60 MW and also supplies heat for district heating in the nearby town of Reykjahlíð
• Soultz-sous-Forêts, France, is a pioneering Enhanced Geothermal System (EGS) project
  • The project involved drilling deep wells (up to 5 km) into a granitic basement and stimulating the reservoir through hydraulic fracturing
  • The EGS plant has an installed capacity of 1.5 MW and demonstrates the feasibility of creating artificial geothermal reservoirs in hot, dry rock formations
• Geothermal heat pumps (GHPs) are a widespread application of shallow geothermal energy for heating and cooling buildings
  • GHPs use the relatively constant temperature of the shallow subsurface (10-20 m depth) to provide efficient heating in winter and cooling in summer
  • The technology can be applied in a wide range of climates and geologic settings, and has been successfully implemented in residential, commercial, and institutional buildings worldwide

Future Trends and Challenges

• Advanced drilling technologies, such as laser drilling or plasma drilling, could significantly reduce the cost and time required for geothermal well construction
• Improved reservoir stimulation methods, such as multi-stage hydraulic fracturing or thermal fracturing, could enhance the productivity and longevity of geothermal systems
• Supercritical geothermal systems, with reservoir temperatures and pressures above the critical point of water (374°C, 22 MPa), offer the potential for higher power output and efficiency
  • Accessing and harnessing supercritical geothermal resources requires advanced drilling and well completion technologies to withstand extreme conditions
  • Iceland's Iceland Deep Drilling Project (IDDP) has successfully tapped into supercritical fluids at a depth of 4.6 km, with temperatures up to 427°C
• Hybrid geothermal systems, which combine geothermal energy with other renewable sources (solar, biomass) or fossil fuels, can provide flexible and dispatchable power generation
• Mineral extraction from geothermal brines, such as lithium, zinc, or silica, can provide additional revenue streams and improve the economics of geothermal projects
• Geothermal energy storage, using the subsurface as a heat storage medium, can help balance supply and demand and increase the penetration of renewable energy in the grid
• Integrated geothermal energy systems, which cascade the use of geothermal heat for multiple applications (electricity, heating, cooling, agriculture, industrial processes), can maximize resource utilization and overall system efficiency
• Expansion of geothermal development in underexplored or unconventional settings, such as offshore geothermal resources or geothermal systems in sedimentary basins, can unlock vast untapped potential