Directional Sonic Logging With Orthogonal Dipole Stacking
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Solution Overview
Problem
Conventional LWD dipole measurements suffer from low signal-to-noise ratio (SNR) due to the averaging out of azimuthal information, making them azimuthally insensitive and ineffective for detecting transverse anisotropy in formations.
Innovation Solution
A method involving a directional sonic logging tool with a multipole transmitter and receiver arrays, which rotates in the wellbore to make dipole measurements, identifies orthogonal pairs, screens out noisy waveforms based on monopole components, and stacks the remaining waveforms in both time and frequency domains to improve SNR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional stacking methods are used for LWD dipole measurements, then signal-to-noise ratio is improved, but azimuthal information is lost making measurements azimuthally insensitive
Solution Approach 1:
The patent segments the dipole measurements into orthogonal pairs (XX and YY components) based on their azimuthal orientations. By identifying and processing orthogonal pairs separately, the method preserves the azimuthal information contained in each pair while enabling stacking within each pair to improve signal-to-noise ratio. This segmentation allows simultaneous achievement of both goals: noise reduction through stacking and azimuthal sensitivity preservation through selective processing of orthogonal components.
2Loss of information
If un-stacked dipole waveforms are used, then azimuthal sensitivity is maintained, but signal-to-noise ratio becomes very low
Solution Approach 1:
The patent divides the un-stacked dipole waveforms into orthogonal pairs and performs stacking within each pair separately. This segmentation enables the maintenance of azimuthal sensitivity by preserving the distinct characteristics of each orthogonal pair while improving signal-to-noise ratio through the stacking process applied to each pair. The method thus resolves the contradiction by applying stacking selectively rather than uniformly to all measurements.
Solution Approach 2:
The patent changes the processing parameter from uniform stacking of all dipole measurements to selective stacking of orthogonal pairs. By identifying measurements that form orthogonal pairs (with azimuthal separation of approximately 90 degrees) and stacking only those pairs, the method transforms the processing approach to simultaneously achieve noise reduction and azimuthal sensitivity preservation.
Data Source
AI summary
A method for acoustic logging a wellbore includes making a plurality of directional sonic logging measurements while rotating an acoustic logging tool in a wellbore. A plurality of orthogonal pairs of measurements are identified among the measurements. Each of the orthogonal pairs includes a first measurement having a measured angle of a transmitter firing direction that is perpendicular with a measured angle of a transmitter firing direction of a second measurement within a predetermined tolerance. A set of 4C component waveforms is compiled for each of the identified orthogonal pairs and mathematically rotated to align with predefined axes. Selected ones of the rotated waveforms are transformed to a frequency domain and stacked in the frequency domain to compute a median or average slowness at each of a plurality of frequencies.