Ultrasonic Reverse Time Migration for Cased-Hole Cement Imaging
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Solution Overview
Problem
Conventional ultrasonic Lamb data processing methods fail to provide complete information of the second and third acoustic interfaces of the borehole structure, and are unable to accurately evaluate the cement bond quality between the casing and formation due to issues like tool eccentricity and complex downhole environments.
Innovation Solution
A reverse time migration imaging method is proposed, which interpolates ultrasonic Lamb wave data from two conventional receivers into array waveforms using phase-shift interpolation. This method employs a zero-lag cross-correlation imaging condition, calculated using Hilbert transforms, to suppress image artifacts caused by dispersion and enhance the imaging of the casing-cement and cement-formation interfaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional ultrasonic Lamb data processing methods are used, then the processing is simple, but complete information of the second and third acoustic interfaces cannot be provided and cement bond quality cannot be accurately evaluated
Solution Approach 1:
The patent segments the ultrasonic wavefield into forward-propagating and backward-propagating components using one-way wave equations. This segmentation allows separate processing of different wave directions, enabling accurate imaging of multiple acoustic interfaces (casing-cement and cement-formation) while maintaining manageable computational complexity through directional decomposition of the wavefield
Solution Approach 2:
The patent transforms the processing from conventional time-domain analysis to a space-time domain approach by solving one-way wave equations. This dimensional transformation enables the method to handle complex downhole environments and tool eccentricity by propagating wavefields through a velocity model, providing complete interface information that conventional methods cannot obtain
2Adaptability or versatility
If oblique incidence ultrasonic Lamb wave is used, then large thickness range of casing can be covered, but time-position conversion cannot be performed and exact location of acoustic interfaces cannot be determined
Solution Approach 1:
The patent replaces conventional time-domain signal processing with a physics-based wave propagation model (one-way wave equations). Instead of attempting time-position conversion of measured signals, the method numerically propagates wavefields through a velocity model to predict wave arrival times and positions, accurately determining acoustic interface locations while maintaining adaptability to various casing thicknesses
Solution Approach 2:
The patent changes the fundamental parameter from time-domain signal analysis to space-domain wavefield propagation. By solving one-way wave equations with velocity models, the method transforms the problem from converting measurement time to position into directly computing wavefield distribution in space, simultaneously achieving broad casing thickness coverage and precise interface location determination
3Ease of operation
If tool eccentricity is present, then measurement can still be performed, but amplitude of A0 reflection wave is affected and cement bond conditions cannot be directly observed
Solution Approach 1:
The patent creates a virtual copy of the measurement process through numerical wavefield propagation. Instead of directly interpreting measured wave amplitudes (which are affected by tool eccentricity), the method numerically propagates wavefields through a velocity model that incorporates casing and formation properties. This virtual copying allows accurate imaging of acoustic interfaces and cement bond conditions by comparing simulated wavefields with actual measurements, effectively compensating for tool eccentricity effects
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
The method effectively images the casing, casing-cement, and cement-formation interfaces, providing accurate evaluation of cement bond quality and overcoming limitations such as tool eccentricity and complex downhole environments.
Implementation Method 1
employs one single oblique incidence transmitter with two receivers, where a certain incident angle is selected to excite low-order anti-symmetric A0-mode Lamb wave in the range of 200-500 kHz in the casing
Implementation Method 2
During the propagation in the casing, the A0-mode wave will leak into the medium between the casing and formation
Implementation Method 3
the A0-mode wave leaking from the casing into the annulus is reflected into the well by the cement-formation interface to form a third interface echo (TIE)
Implementation Method 4
employs a zero-lag cross-correlation imaging condition, calculated using Hilbert transforms, to suppress image artifacts caused by dispersion
Implementation Method 5
interpolates ultrasonic Lamb wave data from two conventional receivers into array waveforms using phase-shift interpolation
Implementation Method 6
A reverse time migration imaging method is proposed, which interpolates ultrasonic Lamb wave data from two conventional receivers into array waveforms using phase-shift interpolation
Data Source
AI summary
A reverse time migration imaging method for cased-hole based on ultrasonic pitch-catch measurement, including: calculating a theoretical dispersion curve; expanding original Lamb data of two receivers into array waveform data based on phase-shift interpolation; establishing a two-dimensional migration velocity model including density, P-wave velocity and S-wave velocity of a target area; generating and storing a forward propagating ultrasonic wavefield for each time step; reversing a time axis; generating and storing a reversely propagating ultrasonic Lamb wavefield for the two receivers after phase-shift interpolation; calculating envelopes of the forward propagating ultrasonic Lamb wavefield and the reversely propagating ultrasonic Lamb wavefield; applying a zero-lag cross-correlation imaging condition to obtain reverse time migration imaging results; and applying Laplace filtering to suppress low-frequency imaging noises in the imaging results.