9.1 Principles of well logging and borehole geophysics
6 min read•august 14, 2024
and are crucial tools in subsurface exploration. They provide detailed data on rock properties, fluid content, and geological features by measuring physical and chemical characteristics of formations penetrated by a borehole.
These techniques use various tools to record electrical, acoustic, nuclear, and magnetic properties. The data helps identify hydrocarbon-bearing zones, estimate reserves, and optimize well placement. It's essential for building accurate subsurface models and making informed decisions in resource exploration and production.
Principles and Techniques of Well Logging
Basic Principles of Well Logging
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- Well logging records various physical, chemical, and geological properties of subsurface formations penetrated by a borehole
- Principles of well logging are based on the interaction of with the formation (response to electrical, acoustic, or nuclear stimulation)
- Logging tools interact with the formation through electrical, acoustic, or nuclear stimulation to measure properties
- Formation properties measured include , , , fluid content, and rock mechanics
Techniques Used in Well Logging
- lowers logging tools into the borehole on a wireline cable to record formation properties
- (LWD) records formation properties during the drilling process using tools integrated into the drill string
- (MWD) transmits real-time data from the drilling assembly to the surface for monitoring and decision-making
- Borehole geophysics involves measuring physical properties of the subsurface using sensors lowered into the borehole
- Well logging and borehole geophysics provide valuable information about subsurface geology (lithology, porosity, permeability, fluid content, rock mechanics)
Types of Well Logging Tools
Electrical and Acoustic Logging Tools
- Electrical logging tools measure electrical properties of the formation (resistivity, conductivity) to determine the presence of hydrocarbons and water
- Resistivity logs measure the resistance of the formation to electrical current flow, indicating the presence of hydrocarbons or water
- measure the ability of the formation to conduct electrical current, influenced by the presence of water and clay content
- tools use sound waves to measure the velocity and attenuation of the formation, providing information about porosity, fractures, and mechanical properties
- measure the travel time of sound waves through the formation, related to porosity and lithology
- combine density and sonic velocity data to characterize the acoustic properties of the formation
Nuclear and Magnetic Resonance Logging Tools
- tools utilize radioactive sources and detectors to measure the density, porosity, and lithology of the formation
- measure the natural radioactivity of the formation, indicating the presence of shale or clay
- measure the of the formation using gamma-ray attenuation, related to porosity and lithology
- measure the hydrogen index of the formation, indicating the presence of water or hydrocarbons
- tools use the principles of nuclear magnetic resonance to determine the porosity, permeability, and fluid content of the formation
- measure the response of hydrogen nuclei in the formation to magnetic fields, providing information about pore size distribution and fluid properties
Formation Testing and Borehole Imaging Tools
- , such as (RFT) and (MDT), collect fluid samples and measure formation pressure and permeability
- RFT and MDT tools isolate a portion of the borehole and extract fluid samples for analysis, providing information about fluid composition and reservoir properties
- , such as the (FMI) and (BHTV), provide high-resolution images of the borehole wall
- FMI uses microelectrodes to create a detailed electrical image of the borehole wall, revealing fractures, bedding planes, and other geological features
- BHTV uses acoustic waves to generate a 360-degree image of the borehole wall, providing information about borehole geometry and rock fabric
Physical Properties Measured by Well Logging
Electrical and Acoustic Properties
- Electrical logging methods measure the electrical resistivity and conductivity of the formation, influenced by the presence of hydrocarbons, water, and clay content
- Resistivity is a measure of the formation's resistance to electrical current flow, with high resistivity indicating the presence of hydrocarbons and low resistivity indicating the presence of water or conductive minerals
- Conductivity is the reciprocal of resistivity and measures the ability of the formation to conduct electrical current, with high conductivity associated with water-bearing or clay-rich formations
- Acoustic logging methods measure the velocity and attenuation of sound waves in the formation, related to the porosity, lithology, and mechanical properties of the rocks
- Sonic velocity is the speed at which sound waves travel through the formation, with higher velocities associated with denser and less porous rocks
- Acoustic attenuation is the reduction in amplitude of sound waves as they propagate through the formation, influenced by the presence of fractures, vugs, or other heterogeneities
Nuclear and Magnetic Resonance Properties
- Nuclear logging methods measure the density, porosity, and lithology of the formation by detecting the interaction of gamma rays and neutrons with the formation
- Density is a measure of the bulk density of the formation, determined by the attenuation of gamma rays emitted from a radioactive source, with higher densities associated with lower porosity and denser minerals
- Porosity is the fraction of the rock volume occupied by pores or voids, estimated using density and neutron logs, with higher porosity indicating greater storage capacity for fluids
- Lithology is the physical characteristics and composition of the rock formation, inferred from the combined responses of gamma-ray, density, and neutron logs
- Magnetic resonance logging methods measure the response of hydrogen nuclei in the formation to magnetic fields, providing information about porosity, permeability, and fluid content
- NMR porosity is the fraction of the rock volume occupied by hydrogen-bearing fluids, determined by measuring the amplitude of the NMR signal
- NMR permeability is an estimate of the formation's ability to transmit fluids, derived from the decay characteristics of the NMR signal and related to pore size distribution
- NMR fluid typing distinguishes between water, oil, and gas in the formation based on their different relaxation times and diffusion coefficients
Formation Testing and Borehole Imaging Properties
- Formation testing methods measure the pressure, permeability, and fluid properties of the formation by collecting fluid samples and conducting in-situ measurements
- Formation pressure is the pressure exerted by the fluids in the pore spaces of the rock, measured using formation testing tools (RFT, MDT) to determine reservoir connectivity and fluid gradients
- Formation permeability is a measure of the ease with which fluids can flow through the rock, estimated from pressure buildup or drawdown tests conducted using formation testing tools
- Fluid properties, such as composition, viscosity, and phase behavior, are determined by analyzing fluid samples collected using formation testing tools
- Borehole imaging methods provide high-resolution images of the borehole wall, revealing fractures, bedding planes, and other geological features
- Fracture characterization involves identifying the orientation, aperture, and density of fractures from borehole image logs (FMI, BHTV), which are essential for understanding fluid flow and well stability
- Structural analysis uses borehole image logs to determine the orientation of bedding planes, folds, and faults, providing insights into the tectonic history and stress regime of the formation
Importance of Borehole Geophysics in Subsurface Exploration
Reservoir Characterization and Well Placement
- Borehole geophysics provides detailed information about subsurface geology, essential for exploration, development, and production of oil, gas, and other natural resources
- Well logging data helps identify hydrocarbon-bearing zones, estimate reserves, and optimize well placement and completion strategies
- Porosity, permeability, and fluid saturation data from borehole geophysics are crucial for reservoir modeling and simulation, guiding field development decisions
- Well logging and borehole geophysics data are used to correlate between wells, construct subsurface maps and cross-sections, and understand the lateral and vertical variability of the reservoir
- Borehole geophysics plays a vital role in identifying and characterizing fractures, faults, and other geological features that impact fluid flow and well performance (natural fractures, induced fractures, fault sealing)
Integration with Other Subsurface Data
- The integration of borehole geophysics with other subsurface data, such as seismic and core data, provides a comprehensive understanding of the subsurface geology and reduces exploration and development risks
- Borehole geophysics data can be tied to seismic data to improve the interpretation of seismic attributes and generate more accurate subsurface models (seismic-well tie, velocity models)
- Core data provides ground truth for calibrating and validating borehole geophysics interpretations, ensuring the accuracy and reliability of subsurface characterization (core-log integration)
- The integration of borehole geophysics, seismic, and core data enables the construction of 3D geological models that capture the heterogeneity and complexity of the subsurface (static reservoir models)
- Integrated subsurface characterization using borehole geophysics and other data types optimizes field development strategies, reduces uncertainty, and maximizes hydrocarbon recovery (reservoir management, enhanced oil recovery)
Key Terms to Review (38)
Acoustic Impedance Logs: Acoustic impedance logs are measurements that reflect the resistance of geological formations to the passage of acoustic waves, which are used to assess subsurface characteristics. They combine the velocity of sound in the formation with its density, providing critical data about rock properties and fluid content. These logs are essential for identifying lithology, porosity, and potential hydrocarbon zones, thereby aiding in efficient resource exploration and extraction.
Acoustic Logging: Acoustic logging is a technique used in well logging to measure the acoustic properties of geological formations by sending sound waves through the borehole and analyzing their responses. This method provides valuable information about the type and condition of rock, fluid content, and porosity, which are essential for understanding subsurface formations. The data collected helps in the evaluation of potential hydrocarbon reservoirs and in making informed decisions during drilling operations.
Borehole geophysics: Borehole geophysics is the study of the physical properties of the subsurface materials surrounding a borehole, using various logging techniques to gather data. This method provides critical information about geological formations, including their composition, porosity, permeability, and fluid content. By utilizing tools and sensors within boreholes, it enhances our understanding of subsurface conditions for resource extraction, groundwater management, and environmental monitoring.
Borehole imaging tools: Borehole imaging tools are specialized instruments used to visualize the walls of a borehole, capturing detailed images and data about the geological formations surrounding it. These tools are crucial for analyzing rock properties, identifying fractures, and assessing reservoir characteristics, as well as monitoring groundwater conditions. By providing high-resolution imagery, borehole imaging tools help geophysicists make informed decisions about resource extraction and environmental management.
Borehole televiewer: A borehole televiewer is a geophysical tool used to obtain high-resolution images of the borehole wall, providing detailed information about its geological and structural features. It employs acoustic or optical methods to capture data, which is essential for understanding subsurface conditions, guiding drilling operations, and evaluating resource potential.
Bulk density: Bulk density refers to the mass of a material per unit volume, including the spaces between particles and any pore spaces within a sample. This property is crucial in geophysics, particularly in the context of well logging and borehole analysis, as it provides insights into the physical composition of geological formations. Understanding bulk density helps in estimating the porosity and permeability of rocks, which are essential for resource exploration and evaluation.
Conductivity logs: Conductivity logs are a type of well logging tool that measures the electrical conductivity of geological formations surrounding a borehole. This measurement helps in identifying the presence of fluids within the formations, particularly water and hydrocarbons, which is essential for resource exploration and reservoir evaluation.
Crossplotting: Crossplotting is a graphical technique used in geophysics to compare two or more variables from well log data on a single plot, allowing for the analysis of relationships between different parameters. This method helps in identifying trends, correlations, and anomalies in the subsurface geology by visualizing the data in a clear and interpretable format. It is especially valuable in evaluating reservoir properties and making informed decisions during the interpretation of well logs.
Data inversion: Data inversion is a mathematical process used to extract meaningful information from geophysical data by transforming observed measurements into a model of the Earth's subsurface properties. This technique allows scientists to interpret complex datasets, such as resistivity or electromagnetic responses, and derive insights about geological structures and material distributions. It is essential for accurately modeling the subsurface in various geophysical applications, enabling improved understanding and decision-making in fields like resource exploration and environmental assessment.
Density log: A density log is a type of well log used in geophysics to measure the bulk density of subsurface materials, which helps identify different rock types and fluid content. This measurement is crucial for understanding the geological formations encountered during drilling, as it assists in evaluating reservoir potential and characterizing stratigraphy. Density logs provide insights into porosity and are often used in conjunction with other logging techniques to create a more comprehensive picture of the subsurface.
Density logs: Density logs are measurements taken during well logging that indicate the density of the geological formations surrounding a borehole. These logs provide crucial information on the composition and characteristics of subsurface materials, which is essential for various applications, including understanding reservoir properties and groundwater studies. By utilizing density logs, geophysicists can interpret the lithology, porosity, and even the fluid content within rocks, aiding in efficient resource management and environmental assessments.
Formation Micro-Imager: A formation micro-imager is a sophisticated logging tool used in borehole geophysics to capture high-resolution images of the borehole wall, revealing detailed geological features and structures. This technology employs electrical or acoustic sensors to generate images that help in understanding the composition and characteristics of subsurface formations, leading to better decision-making for exploration and production.
Formation Resistivity: Formation resistivity is a measure of how strongly a geological formation resists the flow of electric current, which can indicate the presence and distribution of hydrocarbons, water, and other fluids within the rock. This property is crucial in understanding the electrical characteristics of subsurface materials and plays a significant role in well logging techniques used to evaluate and interpret subsurface formations.
Formation testing tools: Formation testing tools are specialized instruments used in well logging to evaluate the properties of subsurface geological formations by collecting fluid and rock samples. These tools are critical for understanding the formation's permeability, porosity, and fluid composition, which inform decisions related to reservoir evaluation and hydrocarbon extraction.
Gamma-ray logging: Gamma-ray logging is a geophysical technique used in well logging that measures the natural gamma radiation emitted by rocks in a borehole. This method helps identify different rock types and their mineral content by detecting variations in gamma radiation levels, which can be indicative of the presence of certain elements such as potassium, uranium, and thorium. The data collected through gamma-ray logging is vital for understanding subsurface geology and plays a crucial role in exploration and production of hydrocarbons.
Gamma-ray logs: Gamma-ray logs are a type of well logging measurement that detects the natural gamma radiation emitted by rocks in the subsurface. This logging technique is crucial for identifying and characterizing different rock types, especially in distinguishing between shale and non-shale formations, as shales typically emit higher gamma radiation due to their clay content. Understanding gamma-ray logs is essential for interpreting the geological history and potential resource presence within boreholes.
Hydrocarbon Saturation: Hydrocarbon saturation refers to the proportion of pore space in a rock that is filled with hydrocarbons, typically oil and gas, as opposed to water. Understanding hydrocarbon saturation is crucial for assessing the economic viability of hydrocarbon reservoirs and plays a key role in determining how much recoverable resource is present.
Lithology: Lithology refers to the physical and chemical characteristics of rocks, including their mineral composition, grain size, texture, and color. Understanding lithology is essential for interpreting geological formations and is critical for applications such as well logging and borehole geophysics, where it aids in identifying rock types and their properties. This knowledge also plays a vital role in reservoir characterization and groundwater studies, helping to assess the capacity of geological formations to hold and transmit fluids.
Logging tools: Logging tools are specialized instruments used to measure and record the physical and chemical properties of geological formations encountered while drilling boreholes. These tools help in acquiring detailed information about subsurface conditions, enabling geologists and engineers to make informed decisions regarding resource extraction, environmental assessments, and site characterization.
Logging While Drilling: Logging while drilling (LWD) is a technique used to acquire geological and geophysical data in real-time during the drilling process of boreholes. This method allows for immediate analysis of the subsurface characteristics, such as rock properties and fluid content, which is crucial for making informed decisions while drilling. LWD integrates measurements into the drilling operation, enhancing efficiency and accuracy in resource exploration and extraction.
Magnetic Resonance Logging: Magnetic resonance logging is a well logging technique used to obtain information about the properties of rocks and fluids in boreholes by measuring the magnetic resonance of hydrogen nuclei in water or hydrocarbons. This method provides valuable data on porosity, fluid saturation, and even the chemical composition of the fluids present, making it a powerful tool in hydrocarbon exploration and reservoir characterization.
Measurement while drilling: Measurement while drilling (MWD) refers to the process of gathering geological and engineering data in real-time during the drilling of a borehole. This technique provides critical information such as formation properties, wellbore stability, and drilling parameters, allowing for immediate adjustments and more efficient decision-making. The integration of MWD with well logging enhances the understanding of subsurface conditions and improves the overall drilling performance.
Modular Formation Dynamics Testers: Modular formation dynamics testers are specialized tools used in well logging to gather data about the physical and chemical properties of subsurface formations. They are designed to assess formation pressure and fluid characteristics, allowing for a better understanding of the reservoir's potential. By providing real-time data during drilling operations, these testers play a critical role in optimizing resource extraction and managing reservoir behavior.
Neutron logs: Neutron logs are a type of well logging measurement used to assess the hydrogen content in geological formations by detecting the interaction of neutrons with matter. This technique is especially effective in identifying water and hydrocarbons in reservoir rocks, making it a vital tool in both exploration and production phases. Neutron logging can provide valuable information about the porosity and fluid content of subsurface formations, which are critical factors in understanding reservoir behavior and groundwater resources.
Neutron porosity logging: Neutron porosity logging is a geophysical technique used in well logging to measure the porosity of subsurface formations by analyzing the interaction of neutron radiation with hydrogen nuclei in the formation fluids. This method is particularly effective in determining the amount of fluid-filled pore space within rocks, which helps in assessing the potential for hydrocarbon production. By providing information on both total and effective porosity, neutron porosity logging plays a crucial role in evaluating the reservoir properties and understanding the subsurface geology.
Nmr logs: NMR logs, or Nuclear Magnetic Resonance logs, are a type of well logging technique used to evaluate the properties of subsurface formations, particularly for hydrocarbon exploration. This technology measures the magnetic properties of hydrogen nuclei in fluids, providing detailed information about porosity, permeability, and fluid types present in rock formations. By utilizing the principles of nuclear magnetic resonance, NMR logs can help differentiate between movable and bound fluids in reservoirs, significantly impacting the assessment of oil and gas reserves.
Nuclear logging: Nuclear logging is a well logging technique that uses radioactive isotopes to determine the composition and properties of subsurface materials in boreholes. This method provides valuable information about the geological formations, including density, porosity, and mineral content, which are crucial for resource exploration and environmental studies.
Permeability: Permeability is a measure of how easily fluids can flow through a material, particularly in the context of geological formations. It is crucial for understanding the movement of oil, gas, and water in subsurface environments and influences reservoir behavior, groundwater flow, and the design of geotechnical structures.
Petrophysical analysis: Petrophysical analysis is the study of the physical and chemical properties of rocks and their interactions with fluids, which is crucial in understanding subsurface formations. This analysis provides insights into porosity, permeability, and saturation levels, helping to evaluate reservoir potential for oil, gas, and water resources. It integrates data from various sources, including well logs and core samples, to construct a comprehensive picture of the rock's characteristics.
Porosity: Porosity is the measure of void spaces in a material, typically expressed as a percentage, indicating how much space is available for fluids like water or hydrocarbons to occupy. This property is crucial in understanding how reservoirs store and transmit fluids, making it a key factor in various applications such as resource exploration, subsurface characterization, and environmental studies.
Repeat Formation Testers: Repeat formation testers are specialized tools used in well logging to assess the properties of rock formations at different depths by taking measurements multiple times. These devices provide valuable data on fluid saturations, permeability, and pressure, helping to inform decisions related to reservoir management and production optimization. By enabling repeated tests, they allow for a better understanding of the variability and consistency within the geological formations being studied.
Reservoir evaluation: Reservoir evaluation is the process of assessing and analyzing subsurface reservoirs to determine their capacity, quality, and potential for resource extraction. This evaluation involves using various geophysical and geological techniques to gather data about the physical properties of the reservoir, such as porosity, permeability, and fluid saturation, which are critical for effective resource management and exploitation.
Resistivity Logging: Resistivity logging is a geophysical technique used to measure the electrical resistivity of geological formations surrounding a borehole. This method helps in identifying and characterizing subsurface materials, such as hydrocarbons, water, and minerals, based on their resistivity values. By analyzing these measurements, it’s possible to gain insights into the formation's fluid content, porosity, and permeability, which are crucial for resource exploration and reservoir management.
Sonic logging: Sonic logging is a well logging technique used to measure the travel time of acoustic waves through geological formations in boreholes. This method provides essential information about the rock properties, such as porosity, density, and lithology, which are crucial for understanding subsurface conditions and guiding drilling operations. By analyzing the velocity of sound waves in the formation, sonic logging helps in identifying the presence of hydrocarbons and assessing the quality of reservoirs.
Sonic logs: Sonic logs are a type of well log that measures the travel time of sound waves through the rock formations surrounding a borehole. They provide valuable information about the geological properties of the subsurface, particularly in terms of porosity and rock type. By analyzing the velocity of sound waves, sonic logs help in evaluating reservoir characteristics and identifying potential groundwater sources.
Sp curve: The sp curve, or spontaneous potential curve, is a well logging tool used to measure the natural electrical potential differences in the formation surrounding a borehole. This curve provides crucial insights into the geological properties of subsurface formations, including permeability, fluid saturation, and the presence of hydrocarbons. By analyzing the sp curve, geophysicists can make informed decisions regarding the exploration and production of oil and gas resources.
Well logging: Well logging is a geophysical method used to obtain a continuous record of the geological formations encountered in a borehole. It involves measuring the physical properties of the rock and fluid in the borehole, which helps to evaluate subsurface conditions and identify potential resources such as oil, gas, and groundwater. This technique is essential for understanding subsurface geology and plays a critical role in resource exploration and environmental studies.
Wireline logging: Wireline logging is a technique used in the oil and gas industry to gather detailed information about the geological formations encountered during drilling. This process involves lowering instruments on a wire into a borehole to measure various physical properties, such as resistivity, density, and porosity, which helps in evaluating the potential of hydrocarbon reservoirs. The data collected through wireline logging is crucial for making informed decisions about drilling and production operations.