Method for inverting casing behind pipe cementing medium velocity based on ultrasonic lamb wave

Abstract

This invention discloses a method for inverting the velocity of the cemented medium behind the casing based on ultrasonic Lamb waves, relating to the field of geophysical logging. This invention utilizes a back-casing scanning imaging logging instrument to perform ultrasonic Lamb wave logging and ultrasonic pulse reflection logging at various depth points, acquiring waveform records at each depth point. The casing thickness and acoustic impedance of the cemented medium behind the casing at specified azimuths are inverted and calculated. Based on the ultrasonic Lamb wave logging waveform records, Fourier transforms are performed on the near-curved and far-curved Lamb wave full-wave waveforms at each specified azimuth of each depth point, calculating the phase difference between near and far-curved Lamb waves and performing group delay processing. Combining this with the operating frequency band of the back-casing scanning imaging logging instrument, the frequency values ​​corresponding to the minimum values ​​of the group delay at each azimuth of each depth point are obtained. After obtaining the frequency set, the wave velocity and medium type of the cemented medium behind the casing are determined based on the frequency values, the number of minimum frequencies, and the acoustic impedance values ​​at each azimuth of the frequency set.

Description

Technical Field

[0001] This invention relates to the field of geophysical logging technology, specifically to a method for inverting the velocity of cemented media behind the casing based on ultrasonic Lamb waves. Background Technology

[0002] Post-casing scanning imaging logging is a 360° all-round, high-resolution logging imaging technology that uses sonic or electromagnetic logging instruments to detect the cement sheath and formation outside the casing after the cement sheath in the downhole casing has solidified.

[0003] When using back-casing scanning imaging logging combined with resonant wave acoustic impedance and ultrasonic curved Lamb wave attenuation to identify the phase state of the back-casing cemented medium, the back-casing scanning imaging logging instruments used include a single-transmitter, single-receiver resonant acoustic system and a single-transmitter, dual-receiver Lamb wave acoustic system. When measuring from bottom to top in the well, the acoustic system of the back-casing scanning imaging logging instrument rotates, and 36 ultrasonic Lamb wave curves and 72 resonant wave curves are obtained along the well circumference. When the curved Lamb wave excited by the back-casing scanning imaging logging instrument propagates along the casing, its phase velocity gradually increases with the increase of frequency. When its phase velocity is equal to the transverse or longitudinal wave velocity of the back-casing cemented medium, it will cause an abnormality in the propagation phase of the curved Lamb wave at that frequency. This phenomenon provides assistance in inverting the velocity of the back-casing cemented medium.

[0004] Therefore, there is an urgent need to propose a method for inverting the velocity of the cementing medium after the jacket based on ultrasonic Lamb waves. By inverting the velocity of the cementing medium after the jacket, it is helpful to further determine whether the cementing medium after the jacket is lightweight cement or mud, and provide a basis for reliably identifying the type of cementing medium after the jacket. Summary of the Invention

[0005] To accurately obtain the velocity of the cemented medium behind the casing and identify its type, this invention provides a method for inverting the velocity of the cemented medium behind the casing based on ultrasonic Lamb waves. This method combines logging results from ultrasonic Lamb wave resonance mode and curved Lamb wave mode, achieving precise acquisition of the velocity of the cemented medium behind the casing. This facilitates the identification of the type of cemented medium behind the casing and provides technical support for indicating whether cementing is contaminated during the solidification process and its potential impact on interlayer sealing and mechanical properties.

[0006] The present invention adopts the following technical solution:

[0007] A method for retrieving the velocity of cemented media behind casing based on ultrasonic Lamb wave inversion, wherein the behind-casing scanning imaging logging instrument includes a resonant acoustic system and a Lamb wave acoustic system, comprising the following steps:

[0008] Step 1: Select the preset depth range of the well to be logged, and use the back-casing scanning imaging logging instrument to perform ultrasonic Lamb wave logging and ultrasonic pulse reflection logging at each depth point to obtain the waveform records measured by the back-casing scanning imaging logging instrument at each depth point, including the ultrasonic Lamb wave logging waveform records and ultrasonic pulse reflection logging waveform records at each specified azimuth.

[0009] Step 2: Based on the ultrasonic pulse reflection logging waveform records at each depth point, invert and calculate the casing thickness and acoustic impedance of the cemented medium behind the casing at a specified orientation at each depth point to obtain a three-dimensional imaging map of the casing thickness and an acoustic impedance imaging map of the cemented medium behind the casing.

[0010] Step 3: Based on the ultrasonic Lamb wave logging waveform records at each depth point, perform Fourier transforms on the full waveforms of the near-curved Lamb wave and the far-curved Lamb wave at each specified azimuth of each depth point to calculate the phase spectrum of the near-curved Lamb wave and the phase spectrum of the far-curved Lamb wave. Subtract the phase spectrum of the far-curved Lamb wave from the phase spectrum of the near-curved Lamb wave to obtain the phase difference between the near and far curved Lamb waves. Then calculate the derivative of the phase difference with respect to the actual measurement frequency to obtain the group delay. Combined with the working frequency band of the back-mounted scanning imaging logging instrument, obtain the frequency values ​​corresponding to the minimum values ​​of the group delay at each azimuth of each depth point to obtain the frequency set.

[0011] Step 4: Based on the global matrix method and combined with the amplitude coefficients of the media in each layer from the inside to the outside of the casing well, obtain the phase velocity dispersion curve V of the Lamb wave in the casing well under the preset measurement mode. phase Group velocity dispersion curve V group ;

[0012] Step 5: For each depth point within the measurement depth range, determine the wave velocity of the bonded medium after the sleeve based on the frequency of the minimum value at each location in the frequency set.

[0013] Step 6: For each depth point within the measurement depth range, determine the medium type of the bonding medium after the sleeve based on the number of minimum frequencies at each location in the frequency set and the acoustic impedance value at each location.

[0014] Preferably, when the post-casing scanning imaging logging instrument performs ultrasonic Lamb wave logging, it measures the full-wave waveform of Lamb wave in 36 azimuths; when the post-casing scanning imaging logging instrument performs ultrasonic pulse reflection logging, it measures the full-wave waveform of resonance in 72 azimuths.

[0015] The ultrasonic Lamb wave logging waveform record is a full waveform of 36 curved Lamb waves, and the ultrasonic pulse reflection logging waveform record is a full waveform of 72 resonant waves.

[0016] The full waveform of the curved Lamb wave includes the near curved Lamb wave full waveform measured by the near detector in the Lamb wave acoustic system and the far curved Lamb wave full waveform measured by the far detector.

[0017] Preferably, the formula for calculating the sleeve thickness is:

[0018]

[0019] In the formula, H is the sleeve thickness; v is the propagation speed of the resonant wave in the sleeve; and f is the resonant frequency.

[0020] The formula for calculating the acoustic impedance of the adhesive medium after the sleeve is:

[0021]

[0022] in,

[0023]

[0024] In the formula, Zc is the acoustic impedance value of the cemented medium after the sleeve; a and d are both fitting constants; c is the instrument constant; Zm is the mud impedance; Bw is the resonance efficiency of pure water; b is the resonance efficiency in the measurement depth range. This is the start time of the resonance window; is the end time of the resonance window; Amp is the amplitude of the resonance wave; Max is the maximum value function.

[0025] Preferably, in step 3, the phase spectrum calculation formulas for the near-curved Lamb wave and the far-curved Lamb wave are as follows:

[0026]

[0027] in,

[0028]

[0029] In the formula, n is the sequence number of the input waveform data; N is the total number of input waveform data; k is the actual measured frequency; φ(·) is the phase spectrum; X(·) is the Fourier transform function; arctan(·) is the arctangent function; Im(·) is the function for taking the imaginary part; Re(·) is the function for taking the real part; j is the imaginary unit;

[0030] The phase difference of the near and far curved Lamb waves at each specified azimuth of each depth point is:

[0031] dφ(k)=φ(k) f -φ(k) n ;

[0032] In the formula, dφ(·) represents the phase difference between the near and far curved Lamb waves; φ(·)f The phase spectrum of the far-curved Lamb wave; φ(·) n The phase spectrum is a near-curved Lamb wave.

[0033] The formula for calculating the group delay is:

[0034]

[0035] In the formula, Group delay.

[0036] Preferably, in step 4, the media in the casing well from the inside out are, in sequence, wellbore fluid, casing, cement sheath, and formation, and the equations for the amplitude coefficient L of each media layer are:

[0037] D(ω,k z L = 0;

[0038] In the formula, ω is the operating frequency of the back-mounted scanning imaging logging instrument, and k z Let ω be the axial wave number, and D(·) be the wave number with respect to frequency ω and wave number k. z The function L is the amplitude coefficient of the medium;

[0039] Based on matrix D, when the determinant ΔD(ω,k) z When ) = 0, the phase velocity dispersion curve and group velocity dispersion curve of the curved Lamb wave in the casing well under the preset measurement mode are calculated;

[0040] The phase velocity V of the curved Lamb wave phase The calculation formula is:

[0041]

[0042] In the formula, V phase (·) is the phase velocity calculation function; Re(·) is the function for taking the real part;

[0043] The group velocity V of the curved Lamb wave group The calculation formula is:

[0044]

[0045] In the formula, V phase (·) is the group velocity calculation function.

[0046] Preferably, in step 5, based on the frequency set of the measurement depth interval, the number of frequency values ​​at each depth point in each direction is obtained; when two frequency values ​​f exist simultaneously in a certain direction... min1 and f min2 And f min2 >f min1 When, calculate the frequency value f.min1 and frequency value f min2 The corresponding phase velocity V of the curved Lamb wave phase1 and V phase2 At this time, the phase velocity V phase1 and V phase2 These correspond to the transverse and longitudinal wave velocities of the cemented medium after the sleeve, respectively; when only one frequency value f exists in a certain azimuth. min1 When, calculate the frequency value f. min1 The corresponding phase velocity V of the curved Lamb wave phase1 Based on the acoustic impedance value of the cemented medium at the current location, if the acoustic impedance value exceeds 3.8 Mrayls and the phase velocity V phase1 If the velocity is less than 2000 m / s, then the phase velocity V is... phase1 For the transverse wave velocity of the cemented medium after the sleeve, if the acoustic impedance value does not exceed 3.8 Mrayls and the phase velocity V phase1 If the velocity is greater than 2000 m / s, then the phase velocity V at this time is... phase1 The longitudinal wave velocity is the velocity of the cemented medium after the sleeve is applied.

[0047] Preferably, in step 6, based on the frequency set of the measurement depth range, the number of frequency values ​​at each depth point is obtained in each direction; when two frequency values ​​exist in a certain direction, the bonding medium after the sleeve is determined to be slow cement; when only one frequency value exists in a certain direction, combined with the acoustic impedance value of that direction, if the acoustic impedance value exceeds 3.8Mrayls, the bonding medium after the sleeve is determined to be fast cement; otherwise, the bonding medium after the sleeve is determined to be liquid.

[0048] Preferably, the slow-speed cement is lightweight cement or contaminated fast-speed cement; the fast-speed cement is conventional cement or high-density cement.

[0049] The present invention has the following beneficial effects:

[0050] This invention proposes a method for inverting the velocity of cemented media behind casing based on ultrasonic Lamb waves. Utilizing ultrasonic Lamb wave cementing quality evaluation technology, it employs the ultrasonic Lamb wave resonance mode and the curved Lamb wave mode of a behind-casing scanning imaging logging instrument for measurement. By combining the processed results of ultrasonic Lamb wave logging waveform records and ultrasonic pulse reflection logging waveform records, it achieves rapid inversion of the longitudinal wave velocity of cement in the annulus between the casing and the formation. This method is beneficial for indicating whether cement is contaminated during the solidification process and its potential impact on interlayer sealing and mechanical properties.

[0051] This invention proposes a method for inverting the velocity of cemented media behind the casing based on ultrasonic Lamb waves. This method can not only accurately obtain the velocity of cemented media behind the casing, but also quickly and accurately invert the state of cemented media behind the casing and identify the type of cemented media behind the casing. This lays the foundation for ultrasonic Lamb wave cement seal integrity evaluation and provides a new technical means for cementing quality evaluation. Attached Figure Description

[0052] Figure 1 This is a flowchart of a method for inverting the velocity of a bonded medium after lamination based on ultrasonic Lamb wave inversion according to the present invention.

[0053] Figure 2 This is a three-dimensional imaging image of the casing thickness.

[0054] Figure 3 The image shows the inversion results of the cemented medium after the sleeve is applied. In the image, the first channel represents the depth, the second channel represents the full waveform of the far-curved Lamb wave in the first orientation, the third channel represents the full waveform of the far-curved Lamb wave in the 19th orientation, the fourth channel represents the velocity of the cemented medium after the sleeve is applied, and the fifth channel represents the acoustic impedance imaging of the cemented medium after the sleeve is applied.

[0055] Figure 4 This is a three-dimensional imaging diagram of the velocity of the cementing medium after the sleeve is applied.

[0056] Figure 5 This is a schematic diagram of the group delay curve obtained during the treatment of fast cement bonding after the sleeve.

[0057] Figure 6 This is a schematic diagram of the group delay curve obtained when the slow cement is bonded after the sleeve is applied. Detailed Implementation

[0058] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0059] This invention proposes a method for inverting the velocity of the cemented medium after lamination using ultrasonic Lamb waves, such as... Figure 1 As shown, the post-casing scanning imaging logging instrument includes a resonant acoustic system and a Lamb wave acoustic system. The resonant acoustic system adopts a single-transmitter, single-receiver structure, that is, the resonant acoustic system is equipped with a transmitter and a receiver. The Lamb wave acoustic system adopts a single-transmitter, dual-receiver structure, that is, the Lamb wave acoustic system is equipped with a transmitter, a near detector, and a far detector. The velocity of the post-casing cemented medium is determined by the waveform records measured by the post-casing scanning imaging logging instrument, and the type of post-casing cemented medium is identified.

[0060] Applying the method of this invention to a certain casing well specifically includes the following steps:

[0061] Step 1: Select the preset depth range of the well to be logged, and use the back-casing scanning imaging logging instrument to perform ultrasonic Lamb wave logging and ultrasonic pulse reflection logging at each depth point to obtain the waveform records measured by the back-casing scanning imaging logging instrument at each depth point, including ultrasonic Lamb wave logging waveform records and ultrasonic pulse reflection logging waveform records at each specified azimuth.

[0062] In this embodiment, when the back-casing scanning imaging logging instrument performs ultrasonic Lamb wave logging at various depth points, it measures at 36 equidistant locations along the circumference of the wellbore, obtaining the full-wave waveforms of the Lamb wave at these 36 locations. Specifically, the ultrasonic Lamb wave logging waveforms are recorded as 36 full-wave waveforms of curved Lamb waves. These curved Lamb wave full-wave waveforms specifically refer to the near-curved Lamb wave full-wave waveform measured by the near detector and the far-curved Lamb wave full-wave waveform measured by the far detector in the Lamb wave acoustic system. When the back-casing scanning imaging logging instrument performs ultrasonic pulse reflection logging at various depth points, it measures at 72 equidistant locations along the circumference of the wellbore, obtaining the resonant full-wave waveforms at these 72 locations. Therefore, the ultrasonic pulse reflection logging waveforms are recorded as 72 resonant full-wave waveforms.

[0063] Step 2: Based on the ultrasonic pulse reflection logging waveform records at each depth point, invert and calculate the casing thickness and acoustic impedance of the cemented medium behind the casing at a specified orientation at each depth point to obtain a three-dimensional imaging map of the casing thickness and an acoustic impedance imaging map of the cemented medium behind the casing.

[0064] In this embodiment, for each depth point within the depth range, based on the ultrasonic pulse reflection logging waveform records, the casing thickness H at a total of 72 azimuth locations at equal intervals along the well perimeter is calculated. (azim,dep) ,azim=1,2…72 and the acoustic impedance value Zc of the cemented medium after the sleeve. (azim,dep) ,azim=1,2…72, where azim is the orientation and dep is the depth.

[0065] Specifically, the formula for calculating the sleeve thickness is as follows:

[0066]

[0067] In the formula, H is the sleeve thickness; v is the propagation speed of the resonant wave in the sleeve; and f is the resonant frequency.

[0068] The formula for calculating the acoustic impedance of the adhesive medium after the sleeve is:

[0069]

[0070] in,

[0071]

[0072] In the formula, Zc is the acoustic impedance value of the cemented medium after casing; a and d are both fitting constants; c is the instrument constant, which is set according to the measurement mode of the scanning imaging logging instrument after casing; Zm is the mud impedance; Bw is the resonance efficiency of pure water; b is the resonance efficiency in the measurement depth range; Nt1 is the start time of the resonance window; Nt2 is the end time of the resonance window; Amp is the resonance wave amplitude; Max is the maximum value function.

[0073] Then, based on the casing thickness values ​​at 72 azimuth positions within the measured depth range, a three-dimensional imaging map of the casing thickness is drawn, such as... Figure 2 As shown, acoustic impedance imaging diagrams of the cemented medium behind the sheath were drawn based on the acoustic impedance values ​​of the cemented medium at 72 azimuth positions at each depth point within the measured depth range. Figure 3 As shown.

[0074] Step 3: Based on the ultrasonic Lamb wave logging waveform records at each depth point, Fourier transforms are performed on the full waveforms of the near-curved Lamb wave and the far-curved Lamb wave at 36 azimuths along the wellbore at each depth point to calculate the phase spectrum of the near-curved Lamb wave and the phase spectrum of the far-curved Lamb wave.

[0075] Specifically, the phase spectrum calculation formulas for the near-curved Lamb wave and the far-curved Lamb wave are as follows:

[0076]

[0077] in,

[0078]

[0079] In the formula, n is the sequence number of the input waveform data; N is the total number of input waveform data; k is the actual measurement frequency, and further, in this embodiment, k is the actual physical frequency corresponding to the frequency index; φ(·) is the phase spectrum; X(·) is the Fourier transform function; arctan(·) is the arctangent function; Im(·) is the function for taking the imaginary part; Re(·) is the function for taking the real part; and j is the imaginary unit.

[0080] Subtracting the phase spectrum of the far-curved Lamb wave from the phase spectrum of the near-curved Lamb wave yields the phase difference between the far and near curved Lamb waves, thus determining the variation law of the phase difference between the far and near curved Lamb waves with the actual measurement frequency.

[0081] The formula for calculating the phase difference of the near and far curved Lamb waves at each depth point in 36 azimuth directions along the wellbore is as follows:

[0082] dφ(k)=φ(k) f -φ(k) n ;

[0083] In the formula, dφ(·) represents the phase difference between the near and far curved Lamb waves; φ(·) f The phase spectrum of the far-curved Lamb wave; φ(·) n It is a near-curved Lamb wave phase spectrum.

[0084] The group delay is obtained by calculating the derivative of the phase difference with respect to the actual measured frequency. The formula for calculating the group delay is as follows:

[0085]

[0086] In the formula, Group delay.

[0087] By combining the operating frequency band of the back-casing scanning imaging logging instrument, the frequency values ​​corresponding to the minimum values ​​of the group delay at 36 azimuth positions along the wellbore circumference at each depth point are obtained, thus obtaining the frequency set f. min(azim,dep) ,azim=1,2…36, that is, the frequency set described in this embodiment includes the minimum values ​​of the group delay at all depth points along the 36 azimuth directions of the wellbore.

[0088] Step 4: Since the cement annulus behind the casing is infinitely large, only sound waves propagate from the inside out, and no sound waves propagate from infinity inward. Therefore, based on the global matrix method and combined with the amplitude coefficients of the media in each layer from the inside to the outside of the casing well, the phase velocity dispersion curve V of the Lamb wave in the casing well under the preset measurement mode is obtained. phase Group velocity dispersion curve V group .

[0089] In this embodiment, the media in the casing well, from the inside out, are, in sequence, wellbore fluid, casing, cement sheath, and formation. The equations for the amplitude coefficient L of each media layer are as follows:

[0090] D(ω,k z L = 0;

[0091] In the formula, D(·) represents the operating frequency ω and axial wavenumber k of the back-mounted scanning imaging logging instrument. z The function; ω is the operating frequency of the back-mounted scanning imaging logging instrument, k z Where is the axial wave number, and L is the amplitude coefficient of the medium.

[0092] Based on matrix D, when the determinant ΔD(ω,k) z When ) = 0, the phase velocity dispersion curve and group velocity dispersion curve of the curved Lamb wave in the casing well under the preset measurement mode are calculated, wherein the phase velocity V of the curved Lamb wave is... phase The calculation formula is:

[0093]

[0094] In the formula, V phase (·) is the phase velocity calculation function; Re(·) is the function for taking the real part.

[0095] The group velocity V of the curved Lamb wave group The calculation formula is:

[0096]

[0097] In the formula, V phase (·) is the group velocity calculation function.

[0098] Step 5: For each depth point within the measurement depth range, determine the wave velocity of the cemented medium after the sleeve based on the frequencies of the minimum values ​​at various locations in the frequency set, and draw a three-dimensional velocity image of the cemented medium after the sleeve, such as... Figure 4 As shown.

[0099] In this embodiment, based on the frequency set of the measurement depth interval, the number of minimum frequency values ​​at 36 azimuths along the wellbore circumference is obtained for each depth point, that is, the number of frequency values ​​corresponding to the minimum group delay at each azimuth.

[0100] Furthermore, when two frequency values ​​f exist simultaneously in a certain direction... min1 and f min2 And f min2 >f min1 When, calculate the frequency value f. min1 and frequency value f min2 The corresponding phase velocity V of the curved Lamb wave phase1 and V phase2 At this time, the phase velocity V phase1 and V phase2 These correspond to the transverse and longitudinal wave velocities of the cemented medium after the sleeve, respectively; when only one frequency value f exists in a certain azimuth. min1 When, calculate the frequency value f. min1 The corresponding phase velocity V of the curved Lamb wave phase1 Based on the acoustic impedance value of the cemented medium at the current location, if the acoustic impedance value exceeds 3.8 Mrayls and the phase velocity V phase1 If the velocity is less than 2000 m / s, then the phase velocity V is... phase1 For the transverse wave velocity of the cemented medium after the sleeve, if the acoustic impedance value does not exceed 3.8 Mrayls and the phase velocity V phase1 If the velocity is greater than 2000 m / s, then the phase velocity V at this time is... phase1 The longitudinal wave velocity is the velocity of the cemented medium after the sleeve is applied.

[0101] Step 6: For each depth point within the measurement depth range, determine the medium type of the bonding medium after the sleeve based on the number of frequency values ​​at each location in the frequency set and the acoustic impedance value at each location.

[0102] In this embodiment, based on the frequency set of the measurement depth range, the number of frequency values ​​at 36 azimuths along the wellbore circumference is obtained for each depth point. When two frequency values ​​exist simultaneously at a certain azimuth, that is, the curved Lamb wave propagating along the casing leaks both transverse and longitudinal waves towards the cementing medium behind the casing, considering that the longitudinal wave velocity of cement is less than the phase velocity of the curved wave, the cementing medium behind the casing is determined to be slow cement, i.e., lightweight cement or contaminated fast cement; when only one frequency value exists at a certain azimuth, combined with the acoustic impedance value at that azimuth, if the acoustic impedance value exceeds 3.8Mrayls, the cementing medium behind the casing is determined to be fast cement, i.e., conventional cement or high-density cement; otherwise, the cementing medium behind the casing is determined to be liquid.

[0103] Compared to traditional methods for identifying the type of post-bonding cement, the method of this invention can further subdivide the solid medium type, that is, it can determine whether the type of post-bonding cement is fast-setting cement or slow-setting cement. Specifically, fast-setting cement includes conventional cement and high-density cement. The group delay curve obtained when processing fast-setting cement after bonding exhibits only a minimum value, such as... Figure 5 As shown; the slow-speed cement specifically refers to lightweight cement or contaminated fast-speed cement, and the group delay curve obtained when it is bonded to fast-speed cement after being coated has two local minima, such as... Figure 6 As shown in the figure. The method of this invention can accurately identify the type of bonding medium after the casing is applied, providing technical support for distinguishing between slurry and mud during actual on-site construction.

[0104] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method of inverting the velocity of a post-cemented medium based on ultrasonic Lamb waves, characterized in that, Post-casing scanning imaging logging instruments include resonant acoustic systems and Lamb wave acoustic systems, and include the following steps: Step 1: Select the preset depth range of the well to be logged, and use the back-casing scanning imaging logging instrument to perform ultrasonic Lamb wave logging and ultrasonic pulse reflection logging at each depth point to obtain the waveform records measured by the back-casing scanning imaging logging instrument at each depth point, including the ultrasonic Lamb wave logging waveform records and ultrasonic pulse reflection logging waveform records at each specified azimuth. Step 2: Based on the ultrasonic pulse reflection logging waveform records at each depth point, invert and calculate the casing thickness and acoustic impedance of the cemented medium behind the casing at a specified orientation at each depth point to obtain a three-dimensional imaging map of the casing thickness and an acoustic impedance imaging map of the cemented medium behind the casing. Step 3: Based on the ultrasonic Lamb wave logging waveform records at each depth point, perform Fourier transforms on the full waveforms of the near-curved Lamb wave and the far-curved Lamb wave at each specified azimuth of each depth point to calculate the phase spectrum of the near-curved Lamb wave and the phase spectrum of the far-curved Lamb wave. Subtract the phase spectrum of the far-curved Lamb wave from the phase spectrum of the near-curved Lamb wave to obtain the phase difference between the near and far curved Lamb waves. Then calculate the derivative of the phase difference with respect to the actual measurement frequency to obtain the group delay. Combined with the working frequency band of the back-mounted scanning imaging logging instrument, obtain the frequency values ​​corresponding to the minimum values ​​of the group delay at each azimuth of each depth point to obtain the frequency set. Step 4: Based on the global matrix method and combined with the amplitude coefficients of the media in each layer from the inside to the outside of the casing well, obtain the phase velocity dispersion curve of the Lamb wave in the casing well under the preset measurement mode. Group velocity dispersion curve ; Step 5: For each depth point within the measurement depth range, determine the wave velocity of the bonded medium after the sleeve based on the frequency values ​​at the minimum values ​​in each direction of the frequency set. Step 6: For each depth point within the measurement depth range, determine the medium type of the bonding medium after the sleeve based on the number of minimum frequencies at each location in the frequency set and the acoustic impedance value at each location. In step 5, based on the frequency set of the measurement depth interval, the number of frequency values ​​at each depth point in each direction is obtained; when two frequency values ​​exist simultaneously in a certain direction... and ,and When, calculate the frequency value. and frequency value The phase velocity of the corresponding curved Lamb wave and At this time, phase velocity and These correspond to the transverse and longitudinal wave velocities of the cemented medium after the sleeve, respectively; when only one frequency value exists in a certain azimuth. When, calculate the frequency value. The phase velocity of the corresponding curved Lamb wave Based on the acoustic impedance value of the cemented medium at the current location, if the acoustic impedance value exceeds 3.8 Mrayls and the phase velocity... Then the phase velocity at this time For the transverse wave velocity of the bonded medium after the sleeve, if the acoustic impedance value does not exceed 3.8 Mrayls and the phase velocity... Then the phase velocity at this time The longitudinal wave velocity of the cemented medium after the sleeve is applied; In step 6, based on the frequency set of the measurement depth range, the number of frequency values ​​at each depth point is obtained in each direction. When two frequency values ​​exist in a certain direction, the bonding medium after the sleeve is determined to be slow cement. When only one frequency value exists in a certain direction, the acoustic impedance value of that direction is considered. If the acoustic impedance value exceeds 3.8Mrayls, the bonding medium after the sleeve is determined to be fast cement; otherwise, the bonding medium after the sleeve is determined to be liquid.

2. The method for inverting the velocity of the cemented medium after lamination based on ultrasonic Lamb waves according to claim 1, characterized in that, When the post-casing scanning imaging logging instrument performs ultrasonic Lamb wave logging, it measures the full-wave waveform of Lamb wave in 36 azimuths; when the post-casing scanning imaging logging instrument performs ultrasonic pulse reflection logging, it measures the full-wave waveform of resonance in 72 azimuths. The ultrasonic Lamb wave logging waveform record is a full waveform of 36 curved Lamb waves, and the ultrasonic pulse reflection logging waveform record is a full waveform of 72 resonant waves. The full waveform of the curved Lamb wave includes the near curved Lamb wave full waveform measured by the near detector in the Lamb wave acoustic system and the far curved Lamb wave full waveform measured by the far detector.

3. The method of inverting back the velocity of a cemented medium based on ultrasonic Lamb waves of claim 1, wherein, The formula for calculating the sleeve thickness is: ； wherein is the thickness of the sleeve; is the propagation speed of the resonant wave in the sleeve; is the resonant frequency; The formula for calculating the acoustic impedance of the adhesive medium after the sleeve is: ； in, ； In the formula, The acoustic impedance value of the adhesive medium after the sleeve is applied; , All are fitting constants; This is the instrument constant; For mud resistance; The resonance efficiency of pure water; To measure the resonance efficiency in the depth range; This is the start time of the resonance window; This is the end time of the resonance window; The amplitude of the resonant wave; It is a function for maximizing the value.

4. The method for inverting the velocity of the cemented medium after lamination based on ultrasonic Lamb waves according to claim 1, characterized in that, In step 3, the phase spectrum calculation formulas for the near-curved Lamb wave and the far-curved Lamb wave are as follows: ； in, ； In the formula, The sequence number of the input waveform data; The total number of input waveform data; This refers to the actual measured frequency. Phase spectrum; It is the Fourier transform function; It is the arctangent function; To take the imaginary part of the function; To take the real part of the function; For imaginary units; The phase difference of the near and far curved Lamb waves at each specified azimuth of each depth point is: ； In the formula, The phase difference of the curved Lamb wave at near and far distances; The phase spectrum of the far-curved Lamb wave; The phase spectrum is a near-curved Lamb wave. The formula for calculating the group delay is: ； In the formulae, is the group delay.

5. The method of inverting back the velocity of a cemented medium based on ultrasonic Lamb waves of claim 1, wherein, In step 4, the media in the casing well, from the inside out, are, in sequence, wellbore fluid, casing, cement sheath, and formation. The equations for the amplitude coefficient L of each media layer are: ； In the formula, This refers to the operating frequency of the post-scanning imaging logging instrument. The axial wave number, Regarding frequency and wave number The function; The amplitude coefficient of the medium; Based on matrix When the determinant At that time, the phase velocity dispersion curve and group velocity dispersion curve of the curved Lamb wave in the casing well under the preset measurement mode were calculated; Phase velocity of the bending lamb wave The calculation formula is: ； In the formula, This is a function for calculating phase velocity. To take the real part of the function; Group velocity of the bending lamb wave The calculation formula is: ； In the formula, is the group velocity calculation function.

6. The method of inverting back the velocity of a cemented medium based on ultrasonic Lamb waves of claim 1, wherein, The slow-speed cement is lightweight cement or contaminated fast-speed cement; the fast-speed cement is conventional cement or high-density cement.

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