5.5 Enhanced geothermal systems (EGS)
9 min read•august 21, 2024
(EGS) expand geothermal energy production beyond traditional resources. By creating engineered reservoirs in hot, dry rock formations, EGS technology enables from low- or low-fluid content areas, significantly increasing potential geothermal energy sites.
EGS involves fracturing low-permeability rock to create fluid pathways for heat extraction. This process requires careful reservoir engineering, well design, and site selection to optimize heat extraction efficiency while addressing environmental and economic considerations.
Fundamentals of EGS
- Enhanced Geothermal Systems (EGS) expand geothermal energy production capabilities beyond traditional hydrothermal resources
- EGS technology enables heat extraction from low-permeability or low-fluid content geothermal reservoirs, significantly increasing potential geothermal energy sites
Definition and concept
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- Engineered reservoir systems created in hot, dry rock formations to extract geothermal energy
- Involves fracturing low-permeability rock to create fluid pathways for heat extraction
- Utilizes injected water to absorb heat from fractured rock and produce steam for power generation
- Requires minimum temperatures of 150-200°C at depths of 3-5 km for economic viability
Comparison vs conventional geothermal
- Conventional geothermal limited to naturally occurring hydrothermal reservoirs
- EGS applicable to wider range of geological settings, expanding geothermal potential
- Higher initial capital costs for EGS due to reservoir engineering requirements
- EGS offers greater control over reservoir management and longevity
Historical development
- Concept originated in 1970s at Los Alamos National Laboratory ( project)
- Fenton Hill project (New Mexico, USA) first EGS demonstration site, operated from 1974 to 1995
- Soultz-sous-Forêts project (France) advanced EGS technology from 1987 to 2008
- Recent developments include successful commercial EGS projects (Cooper Basin, Australia)
Reservoir engineering for EGS
- Reservoir engineering crucial for creating and maintaining artificial geothermal reservoirs in EGS
- Involves complex interplay of geological, hydraulic, and thermal processes to optimize heat extraction
Fracture network creation
- Utilizes hydraulic fracturing to create interconnected fracture networks in low-permeability rock
- Aims to increase rock permeability and surface area for heat exchange
- Fracture orientation controlled by in-situ stress field and pre-existing geological structures
- Requires careful design to maximize reservoir volume while minimizing fluid losses
Hydraulic stimulation techniques
- High-pressure fluid injection to induce shear failure along pre-existing fractures
- Cyclic stimulation involves alternating periods of injection and shut-in to enhance fracture propagation
- Chemical stimulation uses acids or other reactive fluids to dissolve minerals and increase permeability
- Proppants (sand or ceramic particles) used to keep fractures open after pressure release
Thermal-hydraulic-mechanical coupling
- Describes complex interactions between thermal, hydraulic, and mechanical processes in EGS reservoirs
- Thermal contraction of rock due to cooling can induce additional fracturing and permeability changes
- Fluid pressure changes affect stress state and can lead to further fracture propagation or closure
- Modeling these coupled processes essential for predicting long-term reservoir behavior and performance
EGS site selection
- Site selection critical for EGS project success and economic viability
- Requires comprehensive geological, geophysical, and thermal characterization of potential sites
Geological criteria
- Target formations typically crystalline basement rocks or deep sedimentary basins
- Favorable lithologies include granites, metamorphic rocks with high quartz content
- Absence of major faults or excessive natural fracturing to minimize fluid losses
- Sufficient depth to achieve required temperatures while minimizing drilling costs
Geothermal gradient assessment
- Measures rate of temperature increase with depth, typically expressed in °C/km
- Higher geothermal gradients (>30°C/km) more favorable for EGS development
- Utilizes temperature logs from existing wells or heat flow measurements
- Considers local variations due to radiogenic heat production or groundwater circulation
Stress field analysis
- Determines orientation and magnitude of principal stresses in target formation
- Critical for predicting fracture orientation and optimizing well placement
- Methods include analysis of borehole breakouts, drilling-induced fractures, and focal mechanism studies
- Stress regime (normal, strike-slip, or thrust faulting) influences design
Well design and drilling
- Well design and drilling techniques crucial for accessing and exploiting EGS reservoirs
- Requires specialized approaches to overcome challenges of deep, high-temperature environments
Multi-well configurations
- Typically involves at least one injection well and one or more production wells
- Doublet system consists of single injection-production well pair
- Triplet configurations use one injection well and two production wells for improved heat extraction
- Well spacing optimized to balance heat extraction efficiency and thermal breakthrough time
Directional drilling techniques
- Enables precise well placement to intersect target fracture zones
- Utilizes steerable downhole motors and measurement-while-drilling (MWD) technology
- Allows creation of multilateral wells to access multiple reservoir zones from single surface location
- Horizontal sections can increase wellbore contact with fractured reservoir, enhancing productivity
Wellbore stability challenges
- High temperatures and pressures in EGS environments pose risks to wellbore integrity
- Thermal cycling during stimulation and production can induce casing failures
- Corrosive geothermal fluids necessitate use of specialized casing materials and cements
- Mitigating measures include use of thermal wellhead expansion spools and flexible casing connections
Reservoir characterization
- Comprehensive understanding of reservoir properties essential for EGS design and optimization
- Employs various geophysical and geochemical techniques to map subsurface structure and properties
Geophysical imaging methods
- Seismic reflection surveys provide detailed structural information of subsurface geology
- Magnetotelluric (MT) surveys map electrical resistivity variations related to temperature and fluid content
- Gravity and magnetic surveys help identify major structural features and intrusive bodies
- Integration of multiple geophysical methods improves overall reservoir characterization accuracy
Tracer testing
- Involves injection of chemical or radioactive tracers to study fluid flow paths in reservoir
- Measures tracer breakthrough curves at production wells to estimate flow velocities and reservoir volume
- Thermally degrading tracers provide information on temperature distribution within reservoir
- Helps identify short-circuiting pathways and optimize injection-production well configurations
Microseismic monitoring
- Records and analyzes small-magnitude seismic events induced during hydraulic stimulation
- Provides real-time mapping of fracture network growth and orientation
- Helps optimize stimulation parameters and assess potential risks
- Long-term monitoring used to track reservoir evolution and identify need for re-stimulation
Fluid circulation systems
- Designed to efficiently circulate working fluid through EGS reservoir for heat extraction
- Critical component in determining overall system performance and power output
Injection and production wells
- Injection wells pump cooler fluid into reservoir to absorb heat from fractured rock
- Production wells extract heated fluid from reservoir for power generation
- Well completion techniques (e.g., slotted liners, sand control) optimize fluid flow and prevent formation damage
- Downhole pumps often required in production wells to overcome reservoir pressure drawdown
Working fluid selection
- Water most common working fluid due to availability and thermal properties
- Supercritical CO2 proposed as alternative fluid for enhanced heat extraction and potential carbon sequestration
- Consideration of fluid chemistry to minimize scaling and corrosion issues
- Additives used to improve fluid properties (e.g., viscosity modifiers, scale inhibitors)
Circulation pump requirements
- High-pressure pumps needed to overcome reservoir friction losses and maintain circulation
- Pump selection based on required flow rates, pressure differentials, and fluid properties
- Variable frequency drives used to adjust pump output for optimal reservoir management
- Redundancy and backup systems critical to ensure continuous circulation and prevent thermal shock to reservoir
Heat extraction efficiency
- Maximizing heat extraction efficiency key to economic viability of EGS projects
- Requires optimization of reservoir engineering, well design, and surface plant operations
Heat transfer mechanisms
- Convective heat transfer dominates in fractured reservoir, enhanced by turbulent flow in fractures
- Conductive heat transfer from surrounding rock matrix replenishes heat in fractures
- Heat transfer rate influenced by fluid velocity, fracture aperture, and rock thermal properties
- Mathematical models (e.g., parallel plate model) used to estimate heat transfer coefficients
Reservoir thermal depletion
- Gradual cooling of reservoir rock as heat extracted over time
- Thermal front propagation from injection to production wells (thermal breakthrough)
- Cooling rate depends on heat extraction rate, reservoir volume, and rock thermal properties
- Strategies to mitigate depletion include periodic shut-ins, alternating injection patterns, and reservoir stimulation
Long-term sustainability
- Balancing heat extraction rate with natural heat recharge to maintain reservoir productivity
- Typical EGS project lifetimes range from 20-30 years before significant thermal depletion
- Potential for heat farming by allowing reservoir recovery periods between production cycles
- Integration with seasonal energy storage concepts to improve overall system efficiency
Environmental considerations
- EGS development must address potential environmental impacts to ensure sustainable implementation
- Requires comprehensive monitoring and mitigation strategies throughout project lifecycle
Induced seismicity risks
- Potential for triggering small to moderate earthquakes during hydraulic stimulation or long-term operation
- Magnitude typically limited to M<3, but larger events possible in certain geological settings
- Mitigation strategies include careful site selection, staged stimulation approaches, and real-time seismic monitoring
- Traffic light systems implemented to adjust or halt operations based on observed seismicity levels
Water resource management
- Significant water requirements for initial reservoir stimulation and ongoing circulation
- Potential competition with other water users in water-scarce regions
- Strategies include use of non-potable water sources, water recycling, and closed-loop systems
- Monitoring of groundwater quality to detect potential contamination from geothermal fluids
Emissions comparison vs fossil fuels
- EGS power plants produce significantly lower greenhouse gas emissions than fossil fuel alternatives
- Lifecycle emissions primarily from construction and drilling phases, minimal during operation
- Potential for minor emissions of non-condensable gases (e.g., CO2, H2S) from geothermal fluids
- Net emissions reduction potential increased when EGS used for direct heating applications
Economic aspects of EGS
- Economic viability of EGS projects depends on balancing high upfront costs with long-term energy production
- Requires careful financial modeling and risk assessment throughout project development
Capital costs vs conventional geothermal
- EGS projects typically have higher initial capital costs due to reservoir engineering requirements
- Major cost components include well drilling, stimulation operations, and surface plant construction
- Economies of scale can reduce per-MW costs for larger EGS projects
- Technology improvements and learning curve effects expected to reduce costs over time
Operational expenses
- Ongoing costs include well maintenance, pump electricity consumption, and chemical treatments
- Periodic reservoir re-stimulation may be required to maintain productivity
- Skilled workforce needed for specialized operations and maintenance tasks
- Potential for reduced fuel costs compared to conventional power plants offsets higher operational expenses
Levelized cost of electricity
- Metric used to compare EGS economics with other energy sources on per-kWh basis
- Current LCOE estimates for EGS range from $100-200/MWh, higher than conventional geothermal
- Projected to decrease to $50-100/MWh with technology improvements and economies of scale
- Competitive with other baseload low-carbon energy sources in favorable geological settings
EGS project management
- Successful EGS projects require integrated management approach across multiple disciplines
- Long development timelines and high upfront costs necessitate careful planning and risk management
Feasibility studies
- Comprehensive assessment of geological, technical, and economic factors influencing project viability
- Includes detailed site characterization, reservoir modeling, and preliminary well design
- Economic analysis considering various scenarios for reservoir performance and energy prices
- Identification of critical success factors and potential showstoppers for project advancement
Regulatory compliance
- Navigation of complex regulatory landscape covering drilling, water use, and power generation
- required to address potential risks and mitigation measures
- Engagement with local communities and stakeholders to address concerns and build support
- Compliance with evolving regulations related to induced seismicity and groundwater protection
Risk assessment and mitigation
- Identification and quantification of geological, technical, and financial risks throughout project lifecycle
- Use of probabilistic models to assess range of potential outcomes and inform decision-making
- Implementation of staged development approach to manage risks and optimize capital deployment
- Development of contingency plans for various scenarios (e.g., lower-than-expected reservoir performance)
Future prospects and challenges
- EGS technology continues to evolve, with potential for significant expansion of geothermal energy utilization
- Overcoming technical and economic challenges key to widespread commercial deployment
Technological advancements
- Improved reservoir stimulation techniques to enhance fracture network creation and control
- Advanced drilling technologies to reduce costs and access deeper, hotter resources
- Development of novel working fluids and circulation systems for improved heat extraction efficiency
- Enhanced reservoir modeling and real-time monitoring capabilities for optimized reservoir management
Scalability potential
- EGS technology could dramatically increase accessible geothermal resources worldwide
- Potential for development in regions previously considered unsuitable for geothermal energy
- Modular plant designs and standardized development approaches to reduce costs and deployment times
- Integration with existing oil and gas infrastructure for repurposing of depleted hydrocarbon fields
Integration with renewable energy systems
- EGS as baseload complement to variable renewable sources (wind, solar)
- Potential for hybrid geothermal systems combining EGS with other renewable technologies
- Use of excess renewable energy for reservoir thermal energy storage or enhanced stimulation
- Integration with district heating systems and industrial processes for improved overall energy efficiency
Key Terms to Review (18)
Brine: Brine is a concentrated solution of salt in water, often found in geothermal systems where it plays a crucial role in heat and mineral transport. In geothermal contexts, brine usually contains dissolved minerals and is used as a working fluid in energy extraction, influencing both the thermal and chemical properties of the fluid. Understanding brine's characteristics is essential for analyzing geothermal fluid behavior, its geochemical interactions, and the performance of enhanced geothermal systems.
Capital Investment: Capital investment refers to the funds used by companies or governments to acquire, upgrade, and maintain physical assets such as buildings, machinery, and technology. In the context of enhanced geothermal systems (EGS), capital investment is crucial as it influences the feasibility and success of developing geothermal energy projects by covering costs associated with exploration, drilling, and infrastructure development.
Enhanced Geothermal Systems: Enhanced Geothermal Systems (EGS) are engineered geothermal reservoirs created to extract heat from the Earth by enhancing or creating permeability in hot, dry rock formations. This technology allows for the utilization of geothermal energy in areas where conventional geothermal resources are not readily available, linking it to concepts like geothermal gradient, heat flow, and energy conversion principles.
Environmental Impact Assessments: Environmental impact assessments (EIAs) are systematic processes used to evaluate the potential environmental effects of proposed projects or actions before they are carried out. They aim to identify, predict, and assess the likely environmental impacts, ensuring that decision-makers consider environmental factors in their planning and development processes. This is particularly important when dealing with advanced technologies, policies, and large-scale energy systems, where potential effects on ecosystems, communities, and resources must be carefully analyzed and mitigated.
G. p. w.: g. p. w. stands for 'geothermal power generation capacity per unit of water' and is a critical metric in evaluating the efficiency and sustainability of Enhanced Geothermal Systems (EGS). This term connects the volume of water used in the geothermal process with the electricity generated, helping to assess how effectively resources are utilized and the potential for energy production.
Geothermal lease agreements: Geothermal lease agreements are legal contracts between landowners and energy companies that allow the latter to explore, develop, and extract geothermal resources from a specified land area. These agreements outline terms related to compensation, rights to subsurface resources, and responsibilities of both parties, making them essential for the sustainable management of geothermal energy projects, especially in enhanced geothermal systems.
Geothermal reservoir: A geothermal reservoir is a subsurface volume of rock and fluid that can store and transmit heat, primarily from the Earth's interior, which can be harnessed for energy production or heating. These reservoirs are formed by geological processes that create pockets of hot water or steam, often associated with volcanic or tectonic activity, and are essential for the extraction of geothermal energy.
Heat exchanger design: Heat exchanger design refers to the engineering process of creating devices that transfer heat between two or more fluids without mixing them. This process is essential in various applications, including enhanced geothermal systems and greenhouse heating, as it maximizes energy efficiency by efficiently transferring heat from one fluid to another while maintaining desired temperatures for operational effectiveness.
Heat extraction: Heat extraction refers to the process of capturing and utilizing thermal energy from a geothermal reservoir for various applications, such as electricity generation, direct heating, or industrial processes. This process is crucial in geothermal energy systems, as it directly influences the efficiency and sustainability of energy production from the Earth’s heat. Effective heat extraction techniques ensure optimal performance of geothermal systems, whether in traditional geothermal power plants or in enhanced geothermal systems (EGS).
Hot dry rock: Hot dry rock refers to a type of geothermal resource that consists of hot, solid rock formations that are typically found at considerable depths beneath the Earth’s surface. This resource can be exploited for geothermal energy by artificially enhancing permeability and allowing water to circulate through the rock to extract heat, connecting it to the natural heat from the Earth's thermal structure and the geological processes that shape our planet.
Hydraulic stimulation: Hydraulic stimulation is a technique used to enhance the productivity of geothermal reservoirs by injecting high-pressure fluid into the subsurface to create or expand fractures. This process increases permeability in the rock, allowing for better flow of geothermal fluids, which is essential for efficient energy extraction in enhanced geothermal systems. The effectiveness of hydraulic stimulation directly impacts the overall performance and sustainability of these systems.
Induced seismicity: Induced seismicity refers to the earthquakes that are triggered by human activities, particularly those related to resource extraction or subsurface fluid injection. These activities can alter the stress conditions in the Earth's crust, potentially leading to seismic events. Understanding induced seismicity is essential in various fields, including geothermal energy, as it can impact the safety and viability of enhanced geothermal systems, raise concerns about potential hazards, and inform environmental impact assessments.
L. p. b. g. m. f. g. c. k. r. j. b. a.: l. p. b. g. m. f. g. c. k. r. j. b. a. stands for 'low pressure, big geothermal mass flow, gravity controlled, rock-joint big area'. This concept is essential in understanding how geothermal systems operate efficiently, particularly in enhanced geothermal systems (EGS). It emphasizes the significance of managing pressure and mass flow rates to optimize energy extraction while considering geological factors like gravity and rock jointing.
Levelized cost of energy: Levelized cost of energy (LCOE) is a measure used to compare the costs of producing energy from different sources over the lifetime of a project. It considers all costs associated with energy generation, including capital, operational, and maintenance expenses, and divides that by the total energy produced over the project's life. This metric is essential for evaluating the economic viability of various energy systems, including enhanced geothermal systems, resource estimation techniques, and production forecasting.
Permeability: Permeability is the ability of a material, typically rock or soil, to allow fluids to pass through its pores or fractures. This property is crucial for understanding how fluids move within geothermal systems, influencing heat transfer, resource extraction, and reservoir behavior.
Reservoir creation: Reservoir creation refers to the process of forming a subsurface geological formation that can store and facilitate the movement of fluids, particularly in the context of geothermal energy extraction. This process is essential in Enhanced Geothermal Systems (EGS), where reservoirs are artificially stimulated to enhance heat extraction, thus making geothermal energy more accessible and efficient. The creation of these reservoirs involves techniques that modify the permeability and porosity of rock formations, enabling better fluid flow and heat transfer.
Water usage: Water usage refers to the consumption and management of water resources in various processes, particularly in energy production and industrial applications. It plays a vital role in optimizing efficiency and sustainability while minimizing environmental impacts. Understanding water usage is crucial for balancing energy needs with ecological preservation and resource conservation.
Wellbore design: Wellbore design refers to the engineering process of planning and constructing the pathway through which fluids are extracted from or injected into a geothermal reservoir. This design is critical for Enhanced Geothermal Systems (EGS) as it impacts the efficiency, safety, and longevity of geothermal energy extraction. Key considerations in wellbore design include the selection of drilling techniques, materials used for casing and lining, and ensuring adequate spacing for optimal heat exchange between the wellbore and the surrounding rock formation.