Shear Wave Velocity Correction for Anisotropic Sonic Logging
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Solution Overview
Problem
Existing sonic logging methods face challenges in accurately determining shear wave velocities in anisotropic formations, particularly in inclined wells, due to the discrepancy between measured shear wave velocities and those along the symmetry axis, leading to errors in calculated reflection coefficients.
Innovation Solution
The method calculates the shear wave velocity along the symmetry axis using the quasi-shear velocity, rather than the measured shear velocity, by identifying the quasi-shear mode based on the orientation of the sonic tool and the relative dip direction, incorporating anisotropy parameters and velocities measured by sonic logging tools.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If measured shear wave velocity from sonic logging is used in inclined wells through anisotropic formations, then the measurement process is simple and direct, but the velocity values deviate from the true symmetry axis velocity by up to 10% or more
Solution Approach 1:
The patent introduces an intermediary correction process that uses measured shear wave velocities combined with anisotropy parameters (epsilon and delta) to calculate the true symmetry axis velocity. This intermediary calculation acts as a bridge between the easily measured but inaccurate field data and the accurate symmetry axis velocity needed for reflection coefficient calculations, resolving the contradiction between measurement simplicity and accuracy.
Solution Approach 2:
The patent applies preliminary correction by calculating the symmetry axis velocity before it is needed for reflection coefficient computations. By performing the velocity correction in advance using the measured data and anisotropy parameters, the system eliminates subsequent errors in amplitude calculations, thereby improving measurement precision without adding operational complexity during the main logging process.
2Reliability
If standard sonic logging methods are used without anisotropy correction, then the logging process is straightforward, but errors in reflection coefficient calculations increase significantly
Solution Approach 1:
The patent introduces an intermediary correction process that uses measured shear wave velocities combined with anisotropy parameters (epsilon and delta) to calculate the true symmetry axis velocity. This intermediary calculation acts as a bridge between the easily measured but inaccurate field data and the accurate symmetry axis velocity needed for reflection coefficient calculations, resolving the contradiction between measurement simplicity and accuracy.
Solution Approach 2:
The patent transforms the raw measured shear wave velocity parameter into the corrected symmetry axis velocity parameter by applying anisotropy corrections using epsilon and delta parameters. This parameter transformation changes the velocity value from an inaccurate measured quantity to an accurate geological property, thereby improving reliability of subsequent calculations while managing system complexity through mathematical correction.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides more accurate shear wave velocity estimates along the symmetry axis, reducing errors in reflection coefficient calculations and improving the accuracy of acoustic logging in anisotropic formations.
Implementation Method 1
The sonic tool 10 includes at least one transmitter or source 12 that establishes mechanical disturbances, for example using piezoelectric or magnetostrictive materials
Implementation Method 2
The sonic tool 10 includes at least one transmitter or source 12 that establishes mechanical disturbances, for example using piezoelectric or magnetostrictive materials
Implementation Method 3
The receivers may, for example, be made of piezoelectric ceramics that generate an electic current corresponding to pressure variations around the tool 10
Data Source
Figure 1~2
Figure 3A~3B
Figure 4
AI summary
A method is described for determining a shear wave velocity of a transversely isotropic (TI) region (38) of an earth formation traversed by a wellbore (34) with a non-zero relative dip angle between a longitudinal axis (34a) of the wellbore and an axis of symmetry (40) of the TI region. A sonic tool (10, 20) is used to measure shear velocities in the TI region with polarisations orthogonal to the longitudinal axis of the wellbore. One of the shear velocities is identified as a quasi-shear velocity. A shear wave velocity is calculated, as a function of the quasi-shear velocity, for propagation in a direction along the axis of symmetry (40) of the TI region. Where the orientation of the sonic tool is not known and the measured shear velocities comprises a fast shear velocity and a slow shear velocity, the method comprises identifying either the fast or the slow shear velocity as a quasi-shear mode.