7.2 Acoustic and seismic logging

Acoustic and seismic logging are powerful tools for peering into Earth's subsurface. These methods use sound waves to reveal rock properties, helping geologists understand what's hidden beneath our feet. From identifying different rock types to detecting fractures, these techniques are essential for exploring and managing underground resources.

By analyzing how waves travel through rocks, we can learn about their density, porosity, and fluid content. This information is crucial for finding oil and gas, assessing groundwater resources, and even understanding Earth's structure. It's like giving geologists X-ray vision to see through solid rock!

Acoustic and Seismic Wave Propagation

Principles of Wave Propagation in the Subsurface

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• Acoustic and seismic waves are mechanical vibrations that propagate through the Earth's subsurface, carrying information about the properties of the rocks they pass through
• Waves propagate differently in various rock types due to their unique elastic properties, such as density, compressibility, and shear strength
• The velocity of P-waves and S-waves depends on the elastic moduli and density of the rock, which can be related to lithology, porosity, and fluid content
• Acoustic and seismic waves undergo reflection, refraction, and attenuation as they encounter boundaries between different rock layers or formations (sedimentary bedding planes, unconformities)

Types of Waves and Their Characteristics

• Compressional (P) waves and shear (S) waves are the two main types of waves used in acoustic and seismic logging
  • P-waves are longitudinal waves that cause compression and rarefaction of the rock particles parallel to the direction of wave propagation
  • S-waves are transverse waves that cause rock particles to oscillate perpendicular to the direction of wave propagation
• The acoustic impedance of a rock, defined as the product of its density and wave velocity, determines the amount of energy reflected or transmitted at an interface
• Stoneley waves are surface waves that propagate along the borehole wall and are sensitive to the permeability of the formation
• Dipole shear waves are generated by dipole sources and can be used to estimate shear wave velocity in slow formations (unconsolidated sediments, heavy oil reservoirs)

Acoustic and Seismic Well Logging Methods

Logging Tools and Their Components

• Acoustic and seismic well logging involves lowering a tool string containing a source and receivers into a borehole to measure the properties of the surrounding rocks
• The acoustic logging tool consists of a transmitter that generates high-frequency sound waves (10-60 kHz) and receivers that detect the arriving waves after they have traveled through the formation
• Seismic logging tools, such as the vertical seismic profile (VSP) tool, use a surface seismic source and downhole receivers to record the seismic waves as they propagate through the subsurface
• Full waveform sonic (FWS) logging tools record the entire waveform, allowing for the analysis of P-waves, S-waves, and Stoneley waves
• Dipole sonic logging tools employ a dipole source to generate flexural waves, which are sensitive to the anisotropic properties of the formation

Data Acquisition and Processing

• The spacing between the transmitter and receivers, as well as the time delay between the transmitted and received signals, provides information about the velocity and attenuation of the waves in the formation
• Acoustic and seismic log data are processed to remove noise, compensate for borehole effects (washouts, breakouts), and enhance the signal-to-noise ratio
• Processing techniques include waveform stacking, filtering, and deconvolution to improve the quality and interpretability of the data
• Velocity analysis is performed to determine the P-wave and S-wave velocities at different depths in the borehole
• Waveform inversion techniques can be used to estimate the elastic properties of the formation from the recorded waveforms (velocity, density, attenuation)

Analyzing Acoustic and Seismic Log Data

Formation Properties and Lithology Determination

• Acoustic and seismic log data provide valuable information about the subsurface formation properties, such as velocity, density, porosity, and lithology
• P-wave velocity (Vp) and S-wave velocity (Vs) can be calculated from the travel times of the respective waves between the transmitter and receivers
• The ratio of Vp to Vs, known as the Poisson's ratio, is indicative of the rock's lithology and can help distinguish between different rock types (sandstones, shales, carbonates)
• Density can be estimated from the P-wave velocity using empirical relationships, such as the Gardner equation, which relates velocity to density for a given lithology
• Porosity can be derived from the sonic log using the Wyllie time-average equation, which relates the transit time of the P-wave to the porosity and mineralogy of the rock

Identifying Heterogeneities and Anomalies

• The attenuation of acoustic and seismic waves can provide information about the presence of fractures, vugs, or other heterogeneities in the formation
• Anomalies in the acoustic and seismic log data, such as abrupt changes in velocity or waveform character, can indicate the presence of fractures, faults, or stratigraphic boundaries (unconformities, pinchouts)
• Stoneley wave analysis can be used to detect and characterize fractures, as these waves are sensitive to the permeability of the formation and the presence of open fractures
• Integrating acoustic and seismic log data with other well logs, such as gamma ray, density, and neutron logs, can improve the accuracy of lithology and fluid content determination

Fracture Identification and Permeability Estimation

Fracture Detection and Characterization

• Acoustic and seismic logging techniques can be used to identify fractures and estimate permeability in the subsurface, which is crucial for reservoir characterization and production optimization
• Fractures appear as discontinuities or anomalies in the acoustic and seismic log data, such as abrupt changes in velocity, attenuation, or waveform character
• The attenuation of Stoneley waves increases in the presence of permeable fractures due to the fluid flow between the borehole and the formation
• Dipole shear wave anisotropy, which is the difference between the fast and slow shear wave velocities, can indicate the presence of aligned fractures or stress-induced anisotropy in the formation
• Formation micro-imager (FMI) logs provide high-resolution images of the borehole wall, allowing for the direct visualization and orientation analysis of fractures

Permeability Estimation Techniques

• The Stoneley wave reflection coefficient, which quantifies the amount of energy reflected at the borehole wall, can be used to estimate the hydraulic conductivity and permeability of the formation
• The attenuation of Stoneley waves can be related to the permeability of the formation using empirical or theoretical models (Biot-Rosenbaum model, Hornby model)
• Dipole shear wave anisotropy can be used to estimate the fracture density and orientation, which can be related to the permeability tensor of the formation
• Integrating acoustic and seismic fracture characterization with other data, such as core analysis and well test data, can provide a more comprehensive understanding of the fracture network and its impact on reservoir performance
• Permeability estimates from acoustic and seismic logs can be used to calibrate reservoir models and optimize well placement and completion strategies (hydraulic fracturing, horizontal drilling)