15.2 Vertical seismic profiling and well logging
2 min read•august 9, 2024
and well logging are crucial tools in exploration geophysics. These methods provide detailed subsurface information, helping geologists understand rock properties, fluid content, and geological structures with high precision.
By combining surface and downhole measurements, these techniques bridge the gap between seismic surveys and well data. They enable better interpretation of seismic data, accurate depth conversion, and improved understanding of reservoir characteristics.
Vertical Seismic Profiling (VSP)
Fundamentals and Setup of VSP
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- Vertical seismic profiling measures seismic properties along a vertical path in a wellbore
- Involves placing at various depths within the borehole
- generates waves detected by geophones
- Provides high-resolution data on subsurface geology and rock properties
- Offers advantages over surface seismic surveys includes improved vertical resolution and reduced noise
Types and Applications of VSP
- serves as a basic form of
- Determines accurate for seismic interpretation
- uses source near wellhead to image area around borehole
- employs multiple source positions at increasing offsets from well
- Allows imaging of subsurface structures away from the wellbore
- use VSP to detect nearby salt domes or diapirs
Data Processing and Interpretation
- Time-depth relationship derived from VSP data calibrates surface seismic data
- Enables accurate depth conversion of seismic reflections
- generated from VSP data
- Compares modeled seismic response to actual recorded data
- Aids in identifying and correlating with geologic formations
- VSP data processing includes separation of upgoing and downgoing wavefields
- Corridor stacks created from VSP data enhance subsurface imaging
Well Logging Techniques
Sonic Logging and Applications
- Well logging involves recording various physical properties of rock formations
- measures of acoustic waves through formations
- Determines and
- capture entire acoustic wave train
- Provides information on rock mechanical properties (, )
- Sonic logs used in seismic-well tie processes and synthetic seismogram generation
Density and Neutron Logging Methods
- measures bulk density of formations using gamma-ray source
- assesses in formations indicates porosity
- Combination of density and neutron logs helps identify lithology and fluid content
- Density logs contribute to synthetic seismogram creation and calculations
- used for advanced lithology identification (limestone, dolomite, sandstone)
Gamma Ray and Resistivity Logging
- measures of formations
- Distinguishes between shales (high gamma) and clean formations (low gamma)
- Used for correlation between wells and identifying formation boundaries
- measure formation's resistance to electric current flow
- Indicates presence of (high resistivity) or water-bearing zones (low resistivity)
- Combination of gamma ray and resistivity logs aids in interpretation
Key Terms to Review (29)
Acoustic Impedance: Acoustic impedance is a measure of how much resistance a medium offers to the passage of sound waves through it. It is defined as the product of the medium's density and the speed of sound in that medium. This concept is crucial for understanding how seismic waves interact with different geological layers, affecting the reflection and refraction at interfaces.
Checkshot survey: A checkshot survey is a method used in geophysical exploration, particularly in vertical seismic profiling, to determine the interval velocity of subsurface layers by measuring the travel time of seismic waves. This technique involves recording the arrival times of seismic waves generated at the surface and detected at various depths in a borehole, allowing for an accurate assessment of the geological structure and properties beneath the surface.
Density log: A density log is a well logging tool that measures the density of geological formations by detecting the amount of gamma radiation emitted by naturally occurring isotopes in the rock. This data helps geologists and engineers interpret subsurface conditions, determine lithology, and assess hydrocarbon reserves by providing a profile of density variations with depth.
Downhole geophones: Downhole geophones are specialized sensors placed within boreholes to detect and record seismic waves traveling through the subsurface. They play a crucial role in vertical seismic profiling and well logging by providing high-quality data on the physical properties of geological formations, helping in the assessment of potential resources such as oil and gas.
Formation Porosity: Formation porosity refers to the volume fraction of void spaces within a rock or sediment that can hold fluids, such as water, oil, or gas. This characteristic is crucial for understanding the storage capacity of geological formations and has significant implications for resource exploration and extraction.
Full waveform sonic logs: Full waveform sonic logs are detailed measurements that capture the complete acoustic waveforms generated by sonic tools deployed in boreholes. These logs provide essential information on the subsurface geological formations by recording the time it takes for sound waves to travel through various materials, offering insights into the rock properties and fluid content. This technology is vital for understanding subsurface structures and aids in the interpretation of vertical seismic profiling data.
Gamma ray log: A gamma ray log is a well logging tool that measures the natural radioactivity of rock formations, specifically detecting gamma radiation emitted by radioactive isotopes present in the rocks. This log helps geologists and engineers identify different rock types, assess the presence of hydrocarbons, and evaluate the properties of subsurface materials. By analyzing the gamma ray response, one can infer the characteristics of formations and their potential for resource extraction.
Hydrocarbons: Hydrocarbons are organic compounds made up solely of hydrogen and carbon atoms, forming the backbone of many natural resources such as oil and natural gas. These compounds can exist in various forms, including gases, liquids, and solids, and are significant in energy production and geological studies. Their properties and behaviors under various conditions make them essential for understanding subsurface formations.
Hydrogen content: Hydrogen content refers to the amount of hydrogen present in a geological formation, which is crucial for understanding the composition and behavior of fluids within the subsurface. This measure is significant in evaluating reservoirs, as it influences properties like density and porosity, and plays a key role in the interpretation of data collected through various techniques such as vertical seismic profiling and well logging.
Interval Transit Time: Interval transit time refers to the time it takes for seismic waves to travel between two specific points within a geological formation. This measurement is crucial in understanding subsurface structures and can provide valuable insights into rock properties, fluid reservoirs, and other geological features.
Lithology: Lithology is the study of the physical and chemical characteristics of rocks, particularly their mineral composition and texture. Understanding lithology is crucial as it influences the behavior of seismic waves, which is important for constructing velocity models and travel time calculations. The type of rock material impacts how seismic reflection and refraction methods are applied in exploration, and detailed lithological analysis is essential for interpreting results in vertical seismic profiling and well logging.
Natural radioactivity: Natural radioactivity is the process by which unstable atomic nuclei spontaneously decay, releasing energy in the form of radiation. This phenomenon occurs in certain elements, primarily uranium, thorium, and radon, and plays a crucial role in geophysical processes and subsurface characterization methods.
Neutron log: A neutron log is a well logging technique that measures the concentration of hydrogen atoms, primarily in the form of water, within geological formations. This technique is crucial for understanding the porosity of rock formations, as it helps distinguish between oil, gas, and water in reservoirs. By sending neutrons into the surrounding formation and detecting the resulting gamma rays, neutron logs provide valuable information for evaluating hydrocarbon reserves.
Neutron-density crossplots: Neutron-density crossplots are graphical representations used in well logging that compare the measurements of neutron porosity and density porosity from subsurface formations. These plots are essential in identifying rock types and fluid content, as they can indicate whether a formation is gas, oil, or water-bearing based on the relationship between the two measurements. The shape of the crossplot can help in distinguishing between different lithologies and their respective fluid saturations.
Poisson's Ratio: Poisson's ratio is a measure of the proportional relationship between lateral strain and axial strain when a material is deformed elastically. It helps to understand how materials behave under stress, influencing seismic wave velocities, elasticity, and the response of geological materials during stress events like earthquakes.
Resistivity logs: Resistivity logs are measurements taken in boreholes that provide insights into the electrical resistance of subsurface materials. These logs help geoscientists identify the presence of hydrocarbons, water saturation, and lithology by measuring how easily electric current passes through the geological formations, which can vary based on material composition and fluid content.
Salt proximity surveys: Salt proximity surveys are geophysical techniques used to detect and characterize the presence of salt deposits in the subsurface. These surveys are crucial in oil and gas exploration, as salt can create significant challenges in imaging subsurface structures due to its unique acoustic properties, which can affect seismic wave propagation. Understanding the location and extent of salt bodies helps in optimizing drilling strategies and interpreting seismic data more accurately.
Seismic Reflectors: Seismic reflectors are subsurface geological layers that cause seismic waves to bounce back, providing critical information about the Earth's internal structure. These reflectors are often associated with changes in rock properties, such as density or composition, and play a vital role in interpreting seismic data, especially during vertical seismic profiling and well logging processes.
Sequence Stratigraphy: Sequence stratigraphy is a branch of geology that focuses on the interpretation of sedimentary rock layers (strata) in relation to changes in sea level and sediment supply over geological time. It helps geologists understand the depositional environment and the history of sedimentary basins, linking vertical changes in sedimentary layers with horizontal stratigraphic relationships.
Sonic log: A sonic log is a well logging technique that measures the travel time of acoustic waves through geological formations, providing valuable information about the subsurface's physical properties. By analyzing the speed at which these waves travel, geoscientists can infer details about the rock type, porosity, and the presence of fluids. This information is crucial for understanding the geology in oil and gas exploration and other geotechnical applications.
Sonic logging: Sonic logging is a well logging technique that measures the time it takes for sound waves to travel through subsurface materials, providing valuable information about the geological formations surrounding a borehole. This method helps to determine rock properties such as porosity, density, and the presence of fluids, which are crucial for understanding subsurface structures and behaviors.
Surface seismic source: A surface seismic source refers to a point or area on the Earth's surface that generates seismic waves, typically used in geophysical exploration to investigate subsurface structures. These sources can include various methods, such as explosive charges, mechanical impacts, or vibrational devices, which create energy that travels through the Earth, providing valuable information about geological formations. Understanding surface seismic sources is crucial for techniques like vertical seismic profiling and well logging, as they help analyze and characterize subsurface conditions.
Synthetic seismogram: A synthetic seismogram is a computer-generated representation of seismic waves produced by an earthquake or an artificial source, simulating how these waves would travel through different geological materials. This tool is crucial for interpreting seismic data as it helps researchers understand subsurface structures and the properties of geological formations by comparing synthetic data with real recorded seismic data.
Time-depth relationship: The time-depth relationship refers to the correlation between the time it takes for seismic waves to travel through the Earth and the corresponding depth of geological formations. This relationship is crucial in understanding subsurface structures and is established through methods like vertical seismic profiling, which measures the travel time of seismic waves at various depths to create a profile of the geological layers.
Vertical seismic profiling: Vertical seismic profiling (VSP) is a geophysical technique used to obtain high-resolution images of subsurface geological structures by recording seismic waves at various depths within a borehole. This method combines well logging and seismic data acquisition, allowing for better interpretation of geological features and the assessment of subsurface conditions. VSP is particularly valuable for exploring oil and gas reservoirs, as it enhances the accuracy of data obtained from surface seismic surveys.
Vsp: Vertical Seismic Profiling (VSP) is a geophysical technique used to obtain high-resolution seismic data from boreholes. It involves measuring seismic waves generated at the surface and recording how they travel through subsurface formations, providing detailed information about the geological structure and fluid content in the vicinity of the well. VSP complements surface seismic surveys by offering insights into the vertical distribution of rock properties and can help in reservoir characterization and monitoring.
Walkaway vsp: A walkaway vertical seismic profile (VSP) is a geophysical technique used to gather detailed information about subsurface geological formations by measuring seismic waves generated by a source and recorded by geophones placed at different depths within a borehole. This method is particularly valuable in the exploration of oil and gas, as it provides data on the velocity and characteristics of rock layers, helping to identify potential reservoirs.
Young's Modulus: Young's Modulus is a measure of the stiffness of a material, defined as the ratio of stress (force per unit area) to strain (deformation) in the linear elastic region of a material. This property is crucial in understanding how materials respond to stress and strain, influencing seismic wave velocities and the behavior of materials in the earthquake source region.
Zero-offset vsp: A zero-offset vertical seismic profile (VSP) is a technique used in geophysics to gather detailed information about the subsurface by measuring seismic waves generated from a surface source at a single depth level in a borehole. This method helps to obtain high-resolution images of geological structures and is particularly valuable for interpreting complex subsurface features, as it minimizes the effects of wave propagation through varying layers of rock and fluid.