2.4 Fracture systems
8 min read•august 21, 2024
Fracture systems are crucial for geothermal energy extraction, providing pathways for fluid flow and heat transfer. Understanding different types of fractures, their mechanics, and characterization methods helps engineers optimize reservoir performance and production strategies.
Fracture modeling, imaging, and stimulation techniques are essential for predicting and enhancing reservoir behavior. Continuous monitoring of fracture systems informs reservoir management decisions, while environmental and safety considerations ensure sustainable geothermal energy production.
Types of fracture systems
- Fracture systems play a crucial role in geothermal energy extraction by providing pathways for fluid flow and heat transfer
- Understanding different types of fractures helps engineers optimize reservoir characterization and production strategies
- Fracture systems significantly impact the overall and productivity of geothermal reservoirs
Natural vs induced fractures
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- Natural fractures form due to tectonic forces, thermal stresses, or rock deformation over geological time scales
- Induced fractures result from human interventions (hydraulic fracturing, thermal stimulation)
- Natural fractures often exhibit complex geometries and orientations, while induced fractures can be more controlled
- Interaction between natural and induced fractures affects overall reservoir performance
Open vs closed fractures
- Open fractures allow fluid flow and heat transfer, enhancing reservoir productivity
- Closed fractures may act as barriers to fluid flow or reopen under certain conditions
- Fracture aperture determines the degree of openness and fluid conductivity
- Stress state and mineral precipitation influence the open or closed state of fractures
Fracture networks vs single fractures
- Fracture networks consist of interconnected fractures forming complex fluid flow pathways
- Single fractures may dominate fluid flow in certain reservoir zones or well intersections
- Fracture network connectivity impacts overall reservoir permeability and heat extraction efficiency
- Characterizing fracture network properties (density, orientation, connectivity) is crucial for reservoir modeling
Fracture mechanics
- Fracture mechanics principles govern the behavior and propagation of fractures in geothermal reservoirs
- Understanding fracture mechanics helps predict reservoir response to stimulation and production activities
- Fracture mechanics concepts are essential for designing effective fracture stimulation treatments
Stress and strain concepts
- Principal stresses (, , ) determine fracture orientation and propagation direction
- Stress tensor describes the complete state of stress at a point in the rock mass
- Strain represents the deformation of rock in response to applied stresses
- Stress-strain relationships vary depending on rock type and properties (elastic, plastic, brittle)
Fracture propagation mechanisms
- Tensile fracturing occurs when the tensile stress exceeds the rock's tensile strength
- Shear fracturing results from differential stresses exceeding the rock's shear strength
- Mixed-mode fracturing combines tensile and shear components
- Subcritical crack growth can occur at stress intensities below the critical value
Rock failure criteria
- Mohr-Coulomb failure criterion relates shear stress to normal stress and friction angle
- Griffith criterion considers the energy balance for crack propagation
- Hoek-Brown criterion accounts for rock mass properties and discontinuities
- Failure envelopes help predict rock failure under various stress conditions
Fracture characterization
- Accurate fracture characterization is essential for understanding geothermal reservoir behavior
- Fracture properties directly influence fluid flow, heat transfer, and reservoir productivity
- Characterization techniques combine various data sources to build a comprehensive fracture model
Fracture orientation and dip
- Strike and dip measurements describe the three-dimensional orientation of fractures
- Rose diagrams and stereonets visualize fracture orientation distributions
- Fracture sets with similar orientations may indicate specific stress regimes or geological events
- Fracture orientation affects fluid flow anisotropy and well placement strategies
Fracture aperture and roughness
- Fracture aperture determines the flow capacity of individual fractures
- Hydraulic aperture accounts for fracture roughness and tortuosity
- Joint Roughness Coefficient (JRC) quantifies fracture surface roughness
- Fracture roughness influences fluid flow behavior and heat transfer efficiency
Fracture spacing and density
- Fracture spacing describes the distance between adjacent fractures in a set
- Fracture density represents the number of fractures per unit volume of rock
- P32 (fracture area per unit volume) and P33 (fracture ) are common density measures
- Fracture spacing and density impact overall reservoir permeability and heat extraction potential
Fluid flow in fractures
- Fluid flow in fractures is a fundamental process in geothermal energy extraction
- Understanding fracture flow behavior is crucial for predicting reservoir performance and optimizing production
- Fracture flow characteristics differ significantly from flow in porous media
Single-phase vs multi-phase flow
- Single-phase flow involves only one fluid (water or steam) in the fracture system
- occurs when multiple fluids (water, steam, non-condensable gases) coexist
- Relative permeability concepts apply to multi-phase flow in fractures
- Phase transitions (boiling, condensation) can significantly impact fracture flow behavior
Fracture permeability and conductivity
- Fracture permeability depends on aperture, roughness, and connectivity
- Cubic law relates fracture aperture to permeability (, where is aperture)
- Fracture conductivity combines permeability and aperture ()
- Stress-dependent permeability accounts for changes in fracture aperture with effective stress
Fracture-matrix interaction
- Dual-porosity systems consider both fracture and matrix contributions to flow
- Matrix blocks act as fluid storage, while fractures provide flow pathways
- Fracture-matrix transfer functions model fluid exchange between domains
- Thermal drawdown in fractured reservoirs is influenced by heat conduction from matrix to fractures
Fracture modeling techniques
- Fracture modeling is essential for predicting geothermal reservoir behavior and optimizing production strategies
- Various modeling approaches balance computational efficiency with accurate representation of fracture systems
- Integration of multiple data sources improves the reliability of fracture models
Discrete fracture network models
- Explicitly represent individual fractures as planar or curved surfaces
- Stochastic generation of fractures based on statistical distributions of properties
- DFN models capture complex fracture network geometries and connectivity
- Computationally intensive but provide detailed representation of fracture systems
Equivalent continuum models
- Represent fractured rock mass as a continuous medium with effective properties
- Upscaling techniques derive equivalent permeability tensors from fracture data
- Dual-porosity and dual-permeability models account for fracture-matrix interactions
- Computationally efficient for large-scale simulations but may oversimplify fracture behavior
Hybrid modeling approaches
- Combine discrete and continuum methods to balance accuracy and efficiency
- Embedded discrete fracture models (EDFM) represent major fractures explicitly within a continuum
- Multi-scale models use different approaches at different scales (wellbore, near-well, far-field)
- Adaptive mesh refinement techniques focus computational resources on areas of interest
Fracture imaging methods
- Fracture imaging provides crucial data for characterizing geothermal reservoirs
- Multiple imaging techniques are often combined to build a comprehensive fracture model
- Imaging methods vary in resolution, depth of investigation, and applicability to different environments
Borehole imaging techniques
- Acoustic televiewer creates high-resolution images of borehole walls using sound waves
- Optical televiewer provides visual images of borehole walls in clear fluids
- Formation microimager (FMI) uses electrical resistivity to map fractures and bedding
- Borehole video cameras offer direct visual inspection of fractures intersecting the wellbore
Seismic fracture detection
- 3D seismic surveys identify large-scale fracture zones and faults
- Seismic attributes (coherence, curvature) highlight fracture-related features
- Shear-wave splitting analysis indicates fracture orientation and density
- Microseismic monitoring detects small-scale fracturing events during stimulation
Surface outcrop analysis
- Provides analogs for subsurface fracture systems in similar geological settings
- Photogrammetry and LiDAR scanning create detailed 3D models of fracture networks
- Fracture mapping techniques quantify orientation, spacing, and connectivity
- Outcrop studies inform stochastic fracture modeling and upscaling approaches
Fracture stimulation techniques
- Fracture stimulation enhances the productivity of geothermal reservoirs by improving fluid flow pathways
- Various stimulation methods target different aspects of fracture systems and reservoir properties
- Selecting appropriate stimulation techniques depends on reservoir characteristics and project objectives
Hydraulic fracturing principles
- Fluid injection at high pressure creates new fractures or reopens existing ones
- Breakdown pressure initiates fracturing, while propagation pressure extends fractures
- Proppants (sand, ceramic particles) keep fractures open after pressure release
- Fracture geometry (height, length, width) depends on in-situ stresses and rock properties
Chemical stimulation methods
- Acid stimulation dissolves minerals to enhance fracture aperture and connectivity
- Chelating agents target specific minerals without damaging formation
- Chemical stimulation can remove near-wellbore damage and improve injectivity
- Sequential injection of multiple chemicals may optimize stimulation effectiveness
Thermal stimulation approaches
- Cold water injection into hot reservoirs induces thermal contraction and fracturing
- Cyclic thermal stimulation alternates injection and shut-in periods to enhance fracturing
- Thermal shock techniques rapidly change wellbore temperature to induce fracturing
- Thermal stimulation can be combined with hydraulic or chemical methods for synergistic effects
Fracture system monitoring
- Continuous monitoring of fracture systems is crucial for optimizing geothermal reservoir performance
- Monitoring techniques provide insights into fracture behavior during stimulation and production
- Data from monitoring informs reservoir management decisions and model updates
Microseismic monitoring
- Detects small-scale seismic events associated with fracture creation or reactivation
- Passive seismic arrays record and locate microseismic events in real-time
- Moment tensor analysis provides information on fracture orientation and mechanism
- Microseismic data helps map stimulated reservoir volume and optimize well placement
Tracer testing in fractures
- Inert or reactive tracers injected into the reservoir track fluid movement
- Tracer breakthrough curves provide information on flow paths and
- Multiple tracer types (chemical, thermal, radioactive) offer different insights
- Tracer data helps calibrate fracture models and estimate heat transfer surface area
Pressure transient analysis
- Pressure buildup and falloff tests characterize fracture system properties
- Derivative analysis identifies flow regimes (fracture linear, bilinear, radial)
- Pressure transient data provides estimates of fracture conductivity and skin factor
- Interference testing between wells maps large-scale fracture connectivity
Fracture systems in geothermal reservoirs
- Fracture systems are critical components of geothermal reservoirs, controlling fluid flow and heat transfer
- Understanding fracture behavior in geothermal environments is essential for efficient resource exploitation
- Fracture characteristics evolve over time due to thermal, chemical, and mechanical processes
Fracture contribution to permeability
- Fractures often dominate fluid flow in low-permeability geothermal reservoirs
- Fracture network connectivity determines overall reservoir permeability
- Stress-dependent fracture apertures lead to non-linear permeability behavior
- Fracture-dominated permeability exhibits strong anisotropy and scale dependence
Heat transfer in fractured media
- Convective heat transfer along fractures is the primary mechanism for energy extraction
- Conductive heat transfer from matrix to fractures replenishes thermal energy
- Fracture surface area and spacing influence overall heat transfer efficiency
- Thermal breakthrough time depends on fracture network properties and flow rates
Fracture evolution with time
- Mineral precipitation and dissolution alter fracture aperture and roughness
- Thermal contraction and expansion affect fracture aperture and stress state
- Chemical reactions between injected fluids and rock can create or seal fractures
- Long-term reservoir cooling may induce thermal fracturing and permeability changes
Environmental and safety considerations
- Fracture-related activities in geothermal development require careful environmental and safety management
- Understanding and mitigating potential risks is crucial for sustainable geothermal energy production
- Regulatory compliance and public acceptance depend on addressing environmental and safety concerns
Induced seismicity risks
- Fluid injection and withdrawal can trigger small-scale seismic events
- Traffic light systems monitor and control injection based on seismicity thresholds
- Proper site characterization and injection management minimize induced seismicity risks
- Public outreach and communication are essential for addressing community concerns
Groundwater contamination potential
- Fracture systems can potentially create pathways for fluid migration to aquifers
- Proper well construction and cementing prevent fluid leakage along wellbores
- Monitoring programs track potential changes in groundwater quality
- Risk assessment models evaluate potential contamination scenarios and mitigation strategies
Fracture containment strategies
- Stress barriers and lithological boundaries can limit fracture growth
- Microseismic monitoring helps verify fracture containment within target zones
- Staged stimulation treatments allow better control of fracture growth
- Pressure management techniques balance reservoir productivity with containment goals
Key Terms to Review (18)
Continuum modeling: Continuum modeling is a mathematical and physical representation of materials that treats them as continuous distributions, rather than discrete particles or structures. This approach allows for the analysis of complex behaviors in materials, such as fluid flow and stress distribution, particularly in geological formations like those found in geothermal systems. By using continuum modeling, engineers can better understand how heat and fluids interact with fracture systems in the subsurface, leading to more effective resource management and extraction strategies.
Darcy's Law: Darcy's Law is a fundamental equation that describes the flow of fluid through porous media, establishing a linear relationship between the flow rate and the pressure gradient. This principle is crucial for understanding how fluids move through geological formations, as it connects the concepts of permeability, porosity, and fluid dynamics. Darcy's Law plays an essential role in geothermal systems by helping to predict fluid behavior and flow rates within various subsurface conditions, informing the design and operation of geothermal energy extraction processes.
Discrete Fracture Network Modeling: Discrete fracture network modeling is a technique used to represent and analyze the complex geometries and interactions of fractures within a rock mass. This approach allows for a detailed understanding of fluid flow, heat transfer, and mechanical behavior in fractured geological formations, which is essential for applications like geothermal energy extraction and groundwater management.
Fault fractures: Fault fractures are cracks or breaks in geological formations that occur along fault lines, where there has been a displacement of rock masses due to tectonic forces. These fractures can significantly affect the movement of fluids and heat within the Earth's crust, which is crucial for understanding geothermal systems and reservoir behavior.
Fracture Toughness: Fracture toughness is a material property that measures a material's ability to resist fracture when subjected to stress. This property is particularly important in the context of fracture systems, as it indicates how well a material can withstand crack propagation and the potential for failure under various loading conditions. Understanding fracture toughness helps in predicting the behavior of materials in geological formations and can influence decisions regarding the design and maintenance of geothermal systems.
Geophysical surveying: Geophysical surveying is a set of techniques used to measure physical properties of the Earth to gather data about its subsurface structures. This method is essential in identifying geological features, such as fracture systems, which can significantly influence fluid flow and geothermal energy production. By utilizing various tools and technologies, geophysical surveying helps in understanding the complex relationships between rock types, fractures, and fluid reservoirs.
Hydraulic testing: Hydraulic testing is a method used to evaluate the integrity and permeability of geological formations, especially in the context of water or fluid flow through subsurface fractures and voids. This process is crucial for understanding how fluids move within geothermal systems, which directly influences resource management and extraction strategies. It helps in assessing fracture systems by determining their connectivity and the hydraulic properties that affect the flow of geothermal fluids.
Hydrothermal Alteration: Hydrothermal alteration refers to the chemical and mineralogical changes that occur in rocks due to the interaction with hot, mineral-rich fluids, typically at elevated temperatures and pressures. This process can lead to the formation of new minerals and can significantly influence the properties of fracture systems, the geochemistry of geothermal fluids, land use, and geological surveys. Understanding hydrothermal alteration is crucial for assessing geothermal energy potential and environmental impacts.
Joint Fractures: Joint fractures refer to breaks that occur in the bones around a joint, impacting the surrounding ligaments, cartilage, and tendons. These fractures can significantly affect joint stability and function, often leading to complications such as arthritis if not treated properly. Understanding joint fractures is crucial for diagnosing injuries and planning effective rehabilitation strategies.
Multi-phase flow: Multi-phase flow refers to the simultaneous movement of more than one phase of matter, such as liquid, gas, and solid, within a single system. This concept is crucial in understanding how geothermal systems operate, as the flow of water, steam, and sometimes solid particles can significantly affect energy extraction and reservoir behavior. It plays an important role in determining the efficiency of heat transfer and influences fluid dynamics in fracture systems.
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.
Porosity: Porosity is the measure of the void spaces in a material, often expressed as a percentage of the total volume. In geothermal systems, porosity is crucial because it determines how much fluid can be stored within reservoir rocks, impacting their ability to retain and transmit heat. A higher porosity typically allows for greater fluid storage, which can enhance the overall efficiency and productivity of geothermal energy extraction.
Production Decline: Production decline refers to the reduction in the output of a geothermal resource over time, often seen as a decrease in pressure and temperature in the reservoir. This decline is influenced by various factors, including reservoir characteristics, extraction rates, and the dynamics of fracture systems within the geothermal field. Understanding production decline is crucial for optimizing resource management and ensuring sustainable energy production.
Reservoir Connectivity: Reservoir connectivity refers to the degree to which different parts of a geothermal reservoir are interconnected and can exchange fluids and heat. This connectivity is crucial because it affects the overall performance and efficiency of geothermal systems, influencing how heat is distributed and how resources can be extracted over time.
Reservoir heterogeneity: Reservoir heterogeneity refers to the variations in the properties of a geothermal reservoir, such as porosity, permeability, and fluid saturation. These differences can arise from geological factors like rock type, fracture systems, and sedimentary structures. Understanding reservoir heterogeneity is crucial as it affects the flow of fluids, heat transfer, and overall geothermal resource potential.
Stress Intensity Factor: The stress intensity factor (SIF) quantifies the stress concentration at the tip of a crack in a material, which is crucial for understanding how cracks propagate under applied loads. It is a key parameter in fracture mechanics that helps predict when a material will fail due to crack growth. By analyzing the SIF, engineers can assess the integrity of materials and structures, particularly in applications involving fatigue and dynamic loading conditions.
Tectonic Activity: Tectonic activity refers to the movement and interaction of the Earth's lithospheric plates, which are responsible for shaping the planet's surface through processes such as earthquakes, volcanic eruptions, and mountain building. This movement is driven by forces such as mantle convection, gravity, and slab pull, leading to significant geological features and events. Understanding tectonic activity is crucial for grasping the formation and evolution of fracture systems within the Earth's crust.
Thermal Conductivity: Thermal conductivity is a material property that indicates how well a substance can conduct heat. It plays a vital role in understanding heat transfer processes, which are crucial for evaluating geothermal systems and their efficiency in energy extraction and utilization.