9.2 Types of well logs and their interpretation
5 min read•august 14, 2024
Well logs are essential tools in borehole geophysics, providing crucial data about subsurface formations. They measure various properties like electrical resistivity, radioactivity, and acoustic velocity, helping geologists identify rock types, estimate , and determine fluid content.
Interpreting well logs requires analyzing multiple datasets to build a comprehensive picture of the subsurface. By combining different log types and integrating them with other geological data, geoscientists can create detailed subsurface models, characterize reservoirs, and make informed decisions about resource exploration and extraction.
Well Log Types
Electrical Logs
Top images from around the web for Electrical Logs
Processing and Interpretation — Electromagnetic Geophysics View original
Is this image relevant?
HESS - Exploring the regolith with electrical resistivity tomography in large-scale surveys ... View original
Is this image relevant?
Processing and Interpretation — Electromagnetic Geophysics View original
Is this image relevant?
HESS - Exploring the regolith with electrical resistivity tomography in large-scale surveys ... View original
Is this image relevant?
1 of 2
Top images from around the web for Electrical Logs
Processing and Interpretation — Electromagnetic Geophysics View original
Is this image relevant?
HESS - Exploring the regolith with electrical resistivity tomography in large-scale surveys ... View original
Is this image relevant?
Processing and Interpretation — Electromagnetic Geophysics View original
Is this image relevant?
HESS - Exploring the regolith with electrical resistivity tomography in large-scale surveys ... View original
Is this image relevant?
1 of 2
- Measure the electrical properties of subsurface formations, including resistivity and spontaneous potential (SP)
- Used to identify permeable zones and fluid content
- Resistivity logs help distinguish between water-bearing zones (lower resistivity) and hydrocarbon-bearing zones (higher resistivity)
- SP logs measure the natural electrical potential difference between the borehole and the formation, indicating permeable beds and lithology changes
Nuclear Logs
- Measure the natural radioactivity and induced radioactivity of subsurface formations
- Include gamma ray, neutron, and density logs
- Gamma ray logs measure the natural radioactivity of formations, helping to identify lithology (shales have higher gamma ray values)
- Neutron logs measure the hydrogen content of formations, providing information on porosity and fluid content
- Density logs measure the bulk density of formations, which is influenced by lithology, porosity, and fluid content
Acoustic Logs
- Measure the velocity and attenuation of sound waves in subsurface formations
- Used to determine porosity, lithology, and mechanical properties
- Acoustic velocity is influenced by rock matrix, porosity, and fluid content
- Attenuation of sound waves can indicate fractures or other inhomogeneities in the formation
Other Log Types
- Caliper logs measure borehole diameter, identifying washouts, cavings, and fractures
- Temperature logs measure borehole temperature, detecting geothermal gradients and fluid flow zones
- Dipmeter logs measure the orientation of bedding planes, providing information on structural dip and stratigraphic features
- Image logs (resistivity or acoustic) provide high-resolution images of the borehole wall, revealing sedimentary structures, fractures, and faults
Interpreting Well Log Data
Lithology Determination
- Analyze responses of gamma ray, density, and neutron logs to identify rock types
- Different rock types have characteristic log signatures based on mineral composition and texture
- Shales typically have high gamma ray values, low density, and high neutron porosity
- Sandstones and carbonates have lower gamma ray values, higher density, and lower neutron porosity
- Combinations of log responses help to distinguish between different lithologies (limestone vs. dolomite, quartz vs. arkosic sandstone)
Porosity Estimation
- Use density, neutron, and acoustic logs to estimate porosity
- Density logs respond to the presence of pore spaces filled with fluids or gases (lower density indicates higher porosity)
- Neutron logs measure the hydrogen content, which is related to the amount of pore space (higher neutron porosity indicates higher total porosity)
- Acoustic logs measure the velocity of sound waves, which is influenced by the rock matrix and the presence of pore spaces (lower velocity indicates higher porosity)
- Porosity can be calculated using appropriate equations and assumptions based on lithology and fluid content
Fluid Content Interpretation
- Infer fluid content from resistivity logs and the combination of other log responses
- Formation water is typically more conductive than hydrocarbons, resulting in lower resistivity values for water-bearing zones
- Hydrocarbons (oil and gas) have higher resistivity values compared to water-bearing zones
- The separation between neutron and density logs can help distinguish between gas, oil, and water-bearing zones (gas zones have larger separation)
- Resistivity logs, in combination with porosity logs, can be used to estimate water saturation and hydrocarbon saturation in reservoir rocks
Identifying Subsurface Features
Stratigraphic Features
- Identify bedding planes, unconformities, and lateral facies changes by examining variations in log responses across different depth intervals
- Gamma ray logs can be used to identify intervals, which often serve as marker beds for correlating stratigraphic units between wells
- Abrupt changes in log responses may indicate unconformities or sequence boundaries
- Gradual changes in log responses may represent lateral facies changes or gradational contacts between stratigraphic units
Structural Features
- Recognize faults and folds by abrupt changes in log responses, repeated sections, or missing sections in the well log data
- Faults may be indicated by sudden offsets in log responses, changes in dip angles, or the presence of fault gouge or breccia
- Folds may be identified by repeated sections or gradual changes in dip angles across the well log
- Dipmeter logs can provide direct measurements of the orientation of bedding planes, helping to identify and characterize structural features
Reservoir Characterization
- Identify permeable zones, such as sandstones or carbonates, which may act as reservoir rocks for hydrocarbons or groundwater
- Resistivity logs can help identify zones with high and potential fluid flow
- Porosity logs (density, neutron, acoustic) can be used to estimate the storage capacity of reservoir rocks
- Combination of log responses and other data (core, seismic) can be used to assess reservoir quality and continuity
Integrating Well Log Datasets
Subsurface Modeling
- Correlate well logs from multiple wells across an area to develop 2D and 3D subsurface models
- Create cross-sections by correlating well logs along a profile, showing lateral and vertical variations in lithology, porosity, and fluid content
- Construct isopach maps to display the thickness of specific stratigraphic units or reservoir intervals
- Generate structural contour maps to represent the depth or elevation of key stratigraphic or structural surfaces
Multi-disciplinary Integration
- Integrate different types of well logs (electrical, nuclear, acoustic) to provide a more comprehensive understanding of subsurface properties
- Combine well log data with other geological and geophysical data (seismic, core, outcrop) to refine subsurface models and improve the understanding of regional geology
- Use well log data to calibrate seismic data and improve the interpretation of seismic reflectors and facies
- Incorporate well log data into reservoir models to characterize the spatial distribution of porosity, permeability, and fluid content
Applications
- Use subsurface models developed from well log data for various applications
- Reservoir characterization: Assess the quality, heterogeneity, and continuity of reservoir rocks for hydrocarbon or groundwater exploration and production
- Resource estimation: Calculate the volume and distribution of hydrocarbons or groundwater resources based on well log-derived properties
- Well placement: Optimize the location and trajectory of new wells based on the subsurface models and target zones
- Geohazard assessment: Identify potential drilling hazards, such as overpressured zones, lost circulation zones, or unstable formations, based on well log responses and subsurface models
Key Terms to Review (16)
Acoustic log: An acoustic log is a type of well log that measures the travel time of sound waves through the geological formations surrounding a borehole. By analyzing the speed at which these sound waves move, geophysicists can infer important characteristics about the subsurface, including lithology, porosity, and fluid content. This information is essential for making informed decisions in resource exploration and production.
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.
Fluid type determination: Fluid type determination refers to the process of identifying the composition and characteristics of fluids present in subsurface formations, typically utilizing various well logging techniques. This understanding is crucial for determining the presence of hydrocarbons, water, or other fluids, influencing decisions regarding exploration, production, and reservoir management.
Gamma-ray log: A gamma-ray log is a type of well log that measures the natural gamma radiation emitted by rocks in a borehole. This log is essential for identifying lithology and determining the presence of shale and other formations, which helps in the interpretation of subsurface geology and evaluation of potential hydrocarbon reservoirs.
Geothermal studies: Geothermal studies involve the exploration and analysis of heat from the Earth’s interior, which can be harnessed for various applications, including energy production, heating, and mineral extraction. These studies are crucial for understanding geothermal resources, assessing their viability, and developing sustainable energy solutions. By utilizing different techniques, such as well logging and geophysical surveys, researchers can interpret subsurface conditions and identify potential geothermal reservoirs.
Hydrocarbon exploration: Hydrocarbon exploration refers to the search for oil and natural gas deposits beneath the Earth's surface. This process involves a variety of geological and geophysical techniques to locate potential hydrocarbon reservoirs, assess their size, and determine their economic viability. Successful exploration not only fuels energy production but also significantly contributes to understanding geological formations and resource management.
Lithology Identification: Lithology identification is the process of determining the physical and mineralogical characteristics of rock layers in geological formations. This involves analyzing properties such as texture, color, composition, and grain size to classify rocks and understand their distribution and behavior, particularly in the context of subsurface exploration and resource extraction.
Log correlation: Log correlation is a technique used to compare and correlate different well logs from multiple boreholes to understand subsurface geological formations and properties. This process helps in identifying lateral continuity of strata, evaluating reservoir characteristics, and enhancing the interpretation of geological features based on the recorded data from well logs.
Measurement while drilling (mwd): Measurement while drilling (MWD) refers to a set of technologies used to collect data about the geological formations being drilled in real-time as drilling occurs. This technique allows for immediate analysis of formation properties, such as density, porosity, and resistivity, helping drillers make informed decisions during the drilling process. MWD enhances the efficiency of drilling operations by providing crucial information on formation characteristics and wellbore conditions as they happen.
Neutron density log: A neutron density log is a type of well log that measures the hydrogen content of geological formations by detecting neutrons emitted from a source. This log helps in determining the porosity of formations, especially in hydrocarbons and water reservoirs, by comparing the count of neutrons that are scattered and absorbed by different materials. It's often used in conjunction with other logs to provide a more comprehensive understanding of subsurface conditions.
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.
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.
Resistivity Log: A resistivity log is a geophysical measurement that records the electrical resistivity of subsurface materials as a function of depth. This log helps in identifying the presence of fluids in the pore spaces of rocks, determining lithology, and estimating porosity. Understanding resistivity is essential because it correlates with various petrophysical properties, which allows for better interpretation of geological formations.
Sandstone: Sandstone is a sedimentary rock composed mainly of sand-sized mineral particles or rock fragments, often cemented together by silica, calcium carbonate, or iron oxide. Its formation typically occurs through the compaction and cementation of sand deposits over time, making it an essential rock type for understanding subsurface geological structures and fluid flow in reservoirs.
Shale: Shale is a fine-grained sedimentary rock that is formed from compacted mud, clay, and silt. It is characterized by its thin layers and ability to break into smaller pieces, making it an important geological formation often associated with oil and gas deposits. Understanding shale is crucial for interpreting well logs, as its properties can influence the assessment of subsurface formations and their potential for hydrocarbon production.
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.