Ultra-soft formation well cementation quality evaluation method

Abstract

The invention discloses an ultra-soft formation well cementation quality evaluation method, and relates to the technical field of geophysical logging. According to the method, array acoustic logging is carried out on a target well section based on well condition data of the cased well, a cased well acoustic logging actually-measured waveform curve is obtained, a cased well acoustic logging theoretical full-wave waveform curve of the target well section under different cased well cementing conditions is obtained in cooperation with an ultra-soft formation cased well acoustic logging simulation model, and the cased well acoustic logging theoretical full-wave waveform curve of the target well section under different cased well cementing conditions is obtained. According to the depth points of the target well section, waveform feature matching is carried out on the cased well acoustic logging actual measurement waveform curve and the cased well acoustic logging theoretical full-wave waveform curve, full-wave waveform response features of the ultra-soft formation under different cased well cementing conditions are extracted, the ultra-soft formation well cementing quality of the target well section is accurately evaluated, and the well cementing quality of the target well section is accurately evaluated. And generating an ultra-soft formation well cementation quality evaluation result map. According to the method, the problem that the well cementation quality of the second cementation surface of the cement sheath in the ultra-soft formation cased well is difficult to evaluate is effectively solved, and accurate evaluation of the well cementation quality of the ultra-soft formation is achieved.

Description

Technical Field

[0001] This invention relates to the field of geophysical logging technology, specifically to a method for evaluating cementing quality in ultra-soft formations. Background Technology

[0002] With the further development of offshore oil and gas exploration, deepwater oil and gas has received increasing attention. Due to the shallow burial depth and loose cementation of rock particles in deepwater shallow formations, the P-wave and S-wave velocities are lower than those in conventional soft formations. That is, the P-wave velocity is greater than the velocity of the drilling mud in the well, and the S-wave velocity is less than the velocity of the drilling mud in the well. Especially in gas-bearing strata, the P-wave velocity of the formation may be lower than the velocity of the drilling mud in the well. Such formations are defined as ultrasoft formations.

[0003] Sonic logging, as one of the main technologies for cementing quality evaluation, primarily assesses the cementing quality between the casing and cement (i.e., the first cementing surface) and between the cement and the formation (i.e., the second cementing surface). The sonic logging response characteristics in casing wells in ultra-soft formations differ from those in conventional formations. Existing sonic variable-density logging techniques cannot detect formation waves, increasing the difficulty of evaluating the cementing quality at the second cementing surface.

[0004] Therefore, it is urgent to clarify the acoustic logging response characteristics of casing wells in ultra-soft formations and to propose a cementing quality evaluation method applicable to ultra-soft formations. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes a method for evaluating cementing quality in ultra-soft formations. This method involves obtaining measured waveform curves from acoustic logging of the casing well using array acoustic logging. Utilizing the full-wave response characteristics of these waveform curves, and combining them with theoretical full-wave waveform curves from acoustic logging of various cementing conditions within the casing well, the cementing status of the first and second cementing surfaces of the cement sheath is evaluated. This effectively solves the current problem of difficulty in evaluating the cementing quality of the second interface of the cement sheath using variable-density logging in ultra-soft formations.

[0006] The present invention adopts the following technical solution: A method for evaluating cementing quality in ultra-soft formations includes the following steps: Step 1: Select the target well section in the casing well, and perform array acoustic logging in the target well section using monopole acoustic sources and dipole acoustic sources respectively to obtain the measured waveform curves of the casing well acoustic logging, including the measured full waveform of monopole acoustic sources and the measured full waveform of dipole acoustic sources. Step 2: Establish a casing well model based on the well condition data of the target well section to simulate the cementation of casing wells in ultra-soft formations. Use the ultra-soft formation casing well acoustic logging simulation model to perform theoretical calculations for various casing well cementation conditions to obtain the theoretical full-wave waveform curves of casing well acoustic logging in the target well section, including the theoretical full-wave waveform curves of monopole acoustic sources and the theoretical full-wave waveform curves of dipole acoustic sources. Step 3: For each depth point of the target well section, perform waveform feature matching on the measured waveform curve of the casing well acoustic logging and the theoretical full-wave waveform curve of the casing well acoustic logging, extract the full-wave waveform response characteristics under different cementation conditions of the ultra-soft formation, and evaluate the cementing quality of the ultra-soft formation based on the extracted full-wave waveform response characteristics. Step 4: Based on the cementing quality evaluation results of the ultra-soft formation in the target well section, generate an ultra-soft formation cementing quality evaluation result map.

[0007] Preferably, in step 2, a casing well model is established in a cylindrical coordinate system based on the well condition data of the target well section. The casing well model is used to simulate the cementation of the formation and the casing, including the casing and the formation. The formation is an infinitely large solid medium, and the casing of the casing well model is filled with wellbore fluid. There are four types of cement sheath bonding conditions: free casing, cement sheath with no bonding at the first bonding surface, cement sheath with no bonding at the second bonding surface, and cement sheath with complete bonding. When no cement sheath is present between the casing and the formation, a fluid layer is placed between the outer wall of the casing and the formation to simulate a free casing. When a cement sheath is present between the casing and the formation, the bonding interface between the cement sheath and the outer wall of the casing is the first bonding surface, and the bonding interface between the cement sheath and the formation is the second bonding surface. A fluid layer is placed between the cement sheath and the outer wall of the casing to simulate the first bonding surface not being bonded, a fluid layer is placed between the cement sheath and the formation to simulate the second bonding surface not being bonded, and the inner wall of the cement sheath is in close contact with the outer wall of the casing, and the outer wall of the cement sheath is in close contact with the formation to simulate a complete cement sheath bonding.

[0008] Preferably, the acoustic logging simulation model for casing wells in ultra-soft formations acquires the response characteristics of the full-wave waveform of acoustic logging in casing wells, including the full-wave waveform curves of monopole source theory and dipole source theory, expressed as: ; ; In the formula, This represents the full-wave waveform of a monopole sound source. The radius of the medium; This is the axial distance; For time; Radial wave number; The radius of the sound source; For the circumferential ordinal number, when When corresponding to a monopole sound source, when The time corresponds to the dipole sound source; The density of the fluid in the wellbore; For frequency; It is the amplitude coefficient of the sound wave propagating from the outside to the inside in the fluid medium inside the casing; Category 1 Bessel function of order 1; The source coefficient; Category II Bessel function of order 1; is the frequency spectrum function of the sound source; It is a natural constant; The imaginary unit; Here is the axial wave number; The waveform curve is the full-wave waveform of the dipole sound source theory.

[0009] Preferably, in step 3, for each depth point of the target well section, the casing wave velocity measured by the monopole sound source is extracted using the measured full-wave waveform, and the casing bending wave velocity and formation bending wave dispersion curve measured by the dipole sound source are extracted using the measured full-wave waveform, so as to obtain the full-wave waveform response characteristics of the casing well acoustic logging. Based on the casing wave velocity measured by the monopole sound source, the casing bending wave velocity measured by the dipole sound source, and the dispersion curve of the formation bending wave, combined with the formation shear wave velocity obtained from the well condition data of the target well section, the cementing quality of the ultra-soft formation is evaluated, the cementing status of the casing well at each depth point is determined, and the cementing quality evaluation results of the ultra-soft formation are obtained.

[0010] Preferably, the criteria for determining the cementation status of the casing well are as follows: When the casing wave velocity measured by the monopole source is 5386~5586m / s and the casing bending wave velocity measured by the dipole source is 1506~1606m / s, and the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is greater than the formation shear wave velocity at a frequency greater than 5.85kHz and there is dispersion in the 0~30kHz frequency domain, and the measured waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the first cementing surface of the cement sheath is not cemented, then the cementing condition of the casing well is determined to be that the first cementing surface of the cement sheath is not cemented. When the casing wave velocity measured by the monopole sound source is 4671~4871m / s and the casing bending wave velocity measured by the dipole sound source is 1404~1504m / s, and the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is less than the formation shear wave velocity in the 0~30kHz frequency domain and there is a dispersion phenomenon in the 0~10kHz frequency domain, and the measured waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the second cementing surface of the cement sheath is not cemented, then the cementing condition of the casing well is determined to be that the second cementing surface of the cement sheath is not cemented. When the casing wave velocity measured by the monopole sound source is 4671~4871m / s and the casing bending wave velocity measured by the dipole sound source is 1404~1504m / s, and there is no formation bending wave in the extracted formation bending wave dispersion curve, and the actual waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the cement sheath is completely cemented, then the casing well cementation is determined to be complete cement sheath cementation. When the casing wave velocity measured by the monopole sound source is 5386~5586m / s and the casing bending wave velocity measured by the dipole sound source is 1404~1504m / s, and the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is less than the formation shear wave velocity when the frequency is greater than 10kHz and there is no dispersion phenomenon when the frequency is greater than 1kHz, and the actual waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the casing is free, then the casing well cementation is determined to be free casing.

[0011] The present invention has the following beneficial effects: (1) This invention proposes a method for evaluating the cementing quality of ultra-soft formations. Because the full-wave characteristics measured by monopole and dipole acoustic sources in casing wells of ultra-soft formations differ significantly under different cementing conditions of the cement sheath, when measured by monopole acoustic sources, the full-wave characteristics of the acoustic logging in the casing well contain strong casing waves, consistent with conventional formations, regardless of whether the cement sheath interface is uncemented or at the first or second cementing surface. However, the full-wave characteristics of the acoustic logging in the casing well differ when the cement sheath is fully cemented. In conventional casing wells, when the cement sheath is fully cemented, most of the energy of the casing wave is radiated into the formation through the cement sheath, generating a strong formation signal. However, in ultra-soft formations, due to the very low formation velocity, the formation signal generated by the radiated energy is very weak and arrives late. Therefore, it is difficult to qualitatively determine the cementation status of the second cementing surface of the cement sheath based on the strength of the formation wave. However, when the second cement sheath is not cemented, the velocity of the casing wave will be significantly reduced. Therefore, this characteristic can be used to distinguish the cementation status of the first and second cementing surfaces of the cement sheath. When excited by a dipole sound source, regardless of the cement sheath's cementation status, the first wave to arrive in the waveform is the casing bending wave, mainly contributed by the casing bending wave, followed by the formation longitudinal wave. The velocity of the casing bending wave varies depending on the cement sheath bonding condition in the casing well, resulting in different final waveforms. In both free casing and unbonded cement sheath first bonding surfaces, the final arriving wave is a formation bending wave; however, the casing bending wave exhibits strong dispersion when the first bonding surface is unbonded. In the same case, the final arriving wave in the full waveform is a higher-order casing bending wave, with even more pronounced dispersion, as the weaker formation bending wave excitation intensity is masked by the higher-order casing bending wave. When the cement sheath is fully bonded, the final arriving wave in the full waveform is a formation shear wave, at which point the formation bending wave disappears. Therefore, this invention utilizes the waveform characteristics of the full waveform curves in sonic logging of casing wells under different cement sheath bonding conditions, combined with theoretically calculated full waveform curves, to achieve accurate evaluation of the bonding conditions at the first and second bonding surfaces of the cement sheath.

[0012] (2) This invention proposes a method for evaluating cementing quality in ultra-soft formations. When measuring the cement sheath in casing wells under different cementing conditions in ultra-soft formations using array acoustic logging, the response characteristics of the full waveform of the measured casing well acoustic logging excited by monopole and dipole acoustic sources are different. The cementing condition of the first cementing surface and the second cementing surface of the cement sheath in casing wells in ultra-soft formations can be accurately evaluated using the full waveform in the array acoustic logging data. This solves the problem of difficulty in evaluating cementing quality in ultra-soft formations and provides a technical basis for evaluating cementing quality in ultra-soft formations. Attached Figure Description

[0013] Figure 1 This is a flowchart of a cementing quality evaluation method for ultra-soft formations according to the present invention.

[0014] Figure 2 The diagram shows the structure of the cemented well. In the diagram, (a) is a free casing model, (b) is a model of the first cemented surface of the cement sheath that is not cemented, (c) is a model of the second cemented surface of the cement sheath that is not cemented, and (d) is a model of the cement sheath that is fully cemented.

[0015] Figure 3 Figure 1 shows the full-wave waveform curve of the casing well acoustic logging theory for the free casing model; in the figure, (a) is the full-wave waveform curve of the monopole acoustic source theory for the free casing model, and (b) is the full-wave waveform curve of the dipole acoustic source theory for the free casing model.

[0016] Figure 4 The dispersion curve of the mode wave in the free sleeve model when excited by a dipole sound source.

[0017] Figure 5 Figure 1 shows the full-wave waveform curve of the casing well acoustic logging theory for the fully cemented cement sheath model; in the figure, (a) is the full-wave waveform curve of the monopole acoustic source theory for the fully cemented cement sheath model, and (b) is the full-wave waveform curve of the dipole acoustic source theory for the fully cemented cement sheath model.

[0018] Figure 6 The dispersion curve of the mode wave in the model of a fully cemented cement ring when excited by a dipole sound source.

[0019] Figure 7 Figure 1 shows the full-wave waveform curve of the casing well acoustic logging theory for the unbonded model of the first cemented surface of the cement sheath; in the figure, (a) is the full-wave waveform curve of the monopole acoustic source theory for the unbonded model of the first cemented surface of the cement sheath, and (b) is the full-wave waveform curve of the dipole acoustic source theory for the unbonded model of the first cemented surface of the cement sheath.

[0020] Figure 8 The dispersion curve of the mode wave in the unbonded model of the first cemented surface of the cement ring when excited by a dipole sound source.

[0021] Figure 9 Figure 1 shows the full-wave waveform curves of the acoustic logging theory for the unbonded model of the second cemented surface of the cement sheath in the casing well. In the figure, (a) is the full-wave waveform curve of the monopole acoustic source theory for the unbonded model of the second cemented surface of the cement sheath, and (b) is the full-wave waveform curve of the dipole acoustic source theory for the unbonded model of the second cemented surface of the cement sheath.

[0022] Figure 10 The dispersion curve of the mode wave in the unbonded model of the second cemented surface of the cement ring when excited by a dipole sound source.

[0023] Figure 11 This is a diagram showing the cementing quality evaluation results for the target well section in ultra-soft formations. Detailed Implementation

[0024] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings: This invention proposes a method for evaluating cementing quality in ultra-soft formations, such as... Figure 1 As shown, the specific steps include: Step 1: Select the target well section in the casing well, and perform array acoustic logging in the target well section using monopole acoustic sources and dipole acoustic sources respectively to obtain the measured waveform curves of the casing well acoustic logging, including the measured full waveform of monopole acoustic sources and the measured full waveform of dipole acoustic sources.

[0025] Step 2: Establish a casing well model based on the well condition data of the target well section to simulate the cementation of casing wells in ultra-soft formations. Use the ultra-soft formation casing well acoustic logging simulation model to perform theoretical calculations for various casing well cementation conditions to obtain the theoretical full-wave waveform curves of casing well acoustic logging in the target well section, including the theoretical full-wave waveform curves of monopole acoustic sources and the theoretical full-wave waveform curves of dipole acoustic sources. Specifically, the well condition data for the target well section includes the radius, density, P-wave velocity, and S-wave velocity of the mud, casing, cement sheath, and formation within the casing well. In this embodiment, a casing well model is established in a cylindrical coordinate system based on the well condition data of the target well section. The casing well model includes the casing and the formation, wherein the formation is an infinitely large solid medium, and wellbore fluid is installed inside the casing of the casing well model to simulate the cementation between the formation and the casing.

[0026] There are four types of cementation in the casing well, such as... Figure 2 The figures show the conditions of a free casing, an unbonded first cement sheath, an unbonded second cement sheath, and a fully bonded cement sheath. When no cement sheath is present between the casing and the formation, a fluid layer is placed between the outer wall of the casing and the formation to simulate a free casing. When a cement sheath is present between the casing and the formation, the bonding interface between the cement sheath and the outer wall of the casing is the first bonding interface, and the bonding interface between the cement sheath and the formation is the second bonding interface. A fluid layer is placed between the cement sheath and the outer wall of the casing to simulate the unbonded first bonding interface, and a fluid layer is placed between the cement sheath and the formation to simulate the unbonded second bonding interface. A fully bonded cement sheath is simulated by the inner wall of the cement sheath being in close contact with the outer wall of the casing and the outer wall of the cement sheath being in close contact with the formation.

[0027] In this embodiment, the displacement potential function of the acoustic wave in the casing well model is set as follows: ; in, ; In the formula, , , These are the longitudinal wave displacement potentials in solid media, transverse wave displacement potential and Transverse wave displacement potential, where, A transverse wave is a transverse wave polarized along the horizontal direction. A transverse wave is a transverse wave that is polarized along the vertical direction; It is a natural constant; The imaginary unit; Here is the axial wave number; This is the axial distance; For the circumferential ordinal number, when When corresponding to a monopole sound source, when The time corresponds to the dipole sound source; Radial wave number; The radius of the medium; The radius of the sound source; , , Both are amplitude coefficients of sound waves propagating from the outside to the inside in a solid medium; , , Both are amplitude coefficients of sound waves propagating from the inside out in a solid medium; Category 1 Bessel function of order 1; Category II Bessel function of order 1; The radial wave number of the longitudinal wave; For the longitudinal wave number; The radial wave number of the transverse wave; The transverse wave number; For reference angle; This refers to the azimuth angle; It is a sine function; It is a cosine function.

[0028] In the cased well model, longitudinal and transverse waves propagate in the solid medium. Based on the displacement potential function of the acoustic wave, the displacement and stress vectors of the acoustic wave in the solid medium are determined as follows: ; In the formula, The displacement and stress vectors of the sound wave in the solid medium; , , These are the radial, circumferential, and vertical displacement components, respectively. , , These are the radial, circumferential, and vertical stress components, respectively. It is the transpose matrix; This is the amplitude coefficient matrix of sound waves in a solid medium. ; The solid medium coefficient matrix has dimensions of . .

[0029] In the cased well model, only longitudinal waves propagate in the fluid medium. Based on the displacement potential function of the acoustic wave, the displacement and stress vectors of the acoustic wave in the fluid medium are determined as follows: ; In the formula, The displacement and stress vectors of the sound wave in the fluid medium; The fluid medium coefficient matrix has dimensions of . ; Let be the amplitude coefficient matrix of sound waves in a fluid medium. .

[0030] The boundary conditions of the casing well model are set, including the boundary conditions at the solid-solid interface formed by solid media and the boundary conditions at the solid-liquid interface formed by solid media and fluid media. The displacement and stress components of the sound wave at the solid-solid interface are continuous, and the radial displacement and radial stress of the sound wave at the solid-liquid interface are continuous, while the tangential displacement is zero.

[0031] Based on the boundary conditions of the casing well model, the equation for the medium amplitude coefficient of the casing well model is determined using the global matrix method as follows: ; In the formula, Regarding frequency and axial wavenumber The function; is the amplitude coefficient of the medium.

[0032] When the equation for the amplitude coefficient of the medium has a non-zero solution, the dispersion equation is obtained as follows: ,in, Given the determinant of the matrix, the axial wavenumber is determined by solving the dispersion equation. The phase velocity of the mode wave is obtained. , .

[0033] When a sound source exists within the wellbore of a casing well model, the fluid in the wellbore contains both a direct wave field generated by the sound source radiation and a reflected wave field propagating inward due to reflection from the inner wall of the wellbore. The boundary conditions for the inner wall of the wellbore are set as follows: ; In the formula, The displacement and stress of the fluid inside the well at the wellbore wall due to the reflected wave field; The displacement and stress of the fluid inside the well at the wellbore wall are directly related to the wave field. This represents the displacement and stress of the solid at the well wall.

[0034] In the casing well model, acoustic waves in different media are connected through boundary conditions at the interfaces. When the boundary of the casing well model is a solid-solid interface, the displacement and stress components of the acoustic waves at the interface are continuous. When the boundary is a solid-liquid interface, the radial displacement and radial stress of the acoustic waves at the interface are continuous, and the tangential stress is zero. By combining the boundary conditions in the casing well model using the global matrix method, the amplitude coefficient equations of each layer of the casing well model are obtained as follows: ; In the equations for the amplitude coefficients of the medium in each layer of the casing well model, The first two terms are the radial displacement and radial stress generated by the direct wave field at the inner wall of the casing, while the other terms are all zero.

[0035] By solving the amplitude coefficient equations of each medium layer in the casing well model, the acoustic amplitude coefficients of each medium layer in the casing well are obtained. Substituting the acoustic amplitude coefficients of the reflected waves from the fluid in the well into the acoustic logging simulation model of the casing well in ultra-soft formation shown below, the theoretical full-wave waveform curve of the casing well acoustic logging in the time domain is obtained as follows: ; ; In the formula, This represents the full-wave waveform of a monopole sound source. The radius of the medium; This is the axial distance; For time; Radial wave number; The radius of the sound source; For the circumferential ordinal number, when When corresponding to a monopole sound source, when The time corresponds to the dipole sound source; The density of the fluid in the wellbore; For frequency; It is the amplitude coefficient of the sound wave propagating from the outside to the inside in the fluid medium inside the casing; Category 1 Bessel function of order 1; The source coefficient; Category II Bessel function of order 1; Let be the frequency spectrum function of the sound source. In this embodiment, a Gaussian sound source is used. ,in, The center frequency of the sound source; It is a natural constant; The imaginary unit; Here is the axial wave number; The waveform curve is the full-wave waveform of the dipole sound source theory.

[0036] The full-wave response characteristics of the acoustic source excited under different cementation conditions in casing wells in ultra-soft formations were analyzed using an acoustic logging simulation model.

[0037] When the casing well model has no cement sheath between the casing and the formation, and a fluid layer is present between the outer wall of the casing and the formation, the casing well model is specifically a free casing model. Based on the center frequencies of the monopole and dipole sound sources used in array acoustic logging, the sound source function of the ultra-soft formation casing well acoustic logging simulation model is set. In this embodiment, the monopole sound source is set to excite at a main frequency of 8kHz. Theoretical calculations are performed using the ultra-soft formation casing well acoustic logging simulation model to obtain the theoretical full-wave waveform curve of the monopole sound source for the free casing model, as shown below. Figure 3 As shown in (a), the theoretical full-wave waveform curve of the monopole acoustic source in the free casing model contains casing waves C1-C2, Nestorley waves In_St-F1-1, and leakage longitudinal waves Lp. The casing wave with the highest amplitude arrives first, with a velocity of approximately 5487 m / s. A dipole acoustic source is set up to excite at a dominant frequency of 3 kHz. Theoretical calculations are performed using a casing well acoustic logging simulation model in ultra-soft formations to obtain the theoretical full-wave waveform curve of the dipole acoustic source in the free casing model, as shown in [example image]. Figure 3 As shown in (b), the full-wave waveform curve of the dipole sound source theory of the free casing model contains casing bending wave CF1, formation longitudinal wave P, and formation bending wave FF, with corresponding velocities of 1456 m / s, 1102 m / s, and 434 m / s, respectively. The casing bending wave has the largest amplitude and arrives earliest. At this time, the formation bending wave has almost no dispersion except for extremely low frequencies, such as... Figure 4 As shown.

[0038] When a cement sheath is installed between the casing and the formation in the casing well model, the complete cement sheath bonding is simulated by having the inner wall of the cement sheath in close contact with the outer wall of the casing and the outer wall of the cement sheath in close contact with the formation. That is, both the first and second bonding surfaces are completely bonded. In this case, the casing well model is specifically a model with a completely bonded cement sheath. Based on the center frequencies of the monopole and dipole sound sources used in array acoustic logging, the sound source function of the acoustic logging simulation model for casing wells in ultra-soft formations is set. In this embodiment, the monopole sound source is set to excite at a main frequency of 8kHz. Theoretical calculations are performed using the acoustic logging simulation model for casing wells in ultra-soft formations to obtain the theoretical full-wave waveform curve of the monopole sound source for the model with a completely bonded cement sheath, as shown below. Figure 5As shown in (a), the full-wave waveform curve of the monopole acoustic source in the fully cemented cement sheath model contains casing waves C1-C2, Nestorley waves In_St, and formation longitudinal waves P. The casing waves arrive earliest, but their amplitude is significantly reduced, and their velocity decreases to approximately 4771 m / s. A dipole acoustic source is set up to excite at a dominant frequency of 3 kHz. Theoretical calculations are performed using a simulation model of acoustic logging in ultra-soft formation casing wells to obtain the full-wave waveform curve of the dipole acoustic source in the fully cemented cement sheath model, as shown below. Figure 5 As shown in (b), the full-wave waveform curve of the dipole sound source theory of the fully cemented cement sheath model mainly consists of formation longitudinal wave P with a velocity of 1108 m / s. The casing bending wave CF1 and formation transverse wave S have low amplitudes, and at this time the formation bending wave disappears, as shown in (b). Figure 6 As shown.

[0039] When a cement sheath is present between the casing and the formation in the casing well model, a fluid layer is set between the cement sheath and the outer wall of the casing to simulate the unbonded first cemented surface of the cement sheath. In this case, the casing well model is specifically a model of the unbonded first cemented surface of the cement sheath. Based on the center frequencies of the monopole and dipole sound sources used in array acoustic logging, the sound source function of the acoustic logging simulation model for ultra-soft formation casing wells is set. In this embodiment, the monopole sound source is set to excite at a main frequency of 8kHz. Theoretical calculations are performed using the acoustic logging simulation model for ultra-soft formation casing wells to obtain the theoretical full-wave waveform curve of the monopole sound source for the unbonded first cemented surface model of the cement sheath, as shown below. Figure 7 As shown in (a), the full-wave waveform curve of the monopole sound source in the unbonded model of the first cemented surface of the cement sheath contains casing waves C1-C2-C3-C4 and Nesttonley waves In_St-F1-1. The casing waves arrive earliest but their amplitude is lower than that of the free casing model, and the velocity of the casing waves is about 5486 m / s. A dipole sound source is set to excite at a main frequency of 3 kHz, and theoretical calculations are performed using a casing well acoustic logging simulation model for ultra-soft formations to obtain the full-wave waveform curve of the dipole sound source in the unbonded model of the first cemented surface of the cement sheath, as shown in the figure. Figure 7 As shown in (b), the full-wave waveform curve of the dipole sound source theory of the unbonded model of the first cemented surface of the cement sheath contains a casing bending wave CF1, a formation longitudinal wave P, and a formation bending wave FF. The velocities of the casing bending wave and the formation longitudinal wave are 1556 m / s and 1107 m / s, respectively. Compared with the casing bending wave amplitude of the free casing model, the amplitude is significantly reduced. At this time, the formation bending wave exhibits strong dispersion characteristics, such as... Figure 8 As shown.

[0040] When a cement sheath is present between the casing and the formation in the casing well model, a fluid layer is set between the cement sheath and the formation to simulate the unbonded second cemented surface of the cement sheath. In this case, the casing well model is specifically a model of the unbonded second cemented surface of the cement sheath. Based on the center frequencies of the monopole and dipole sound sources used in array acoustic logging, the sound source function of the ultrasonic logging simulation model for ultra-soft formation casing wells is set. In this embodiment, the monopole sound source is set to excite at a main frequency of 8kHz. Theoretical calculations are performed using the ultrasonic logging simulation model for ultra-soft formation casing wells to obtain the theoretical full-wave waveform curve of the monopole sound source for the unbonded second cemented surface model of the cement sheath, as shown below. Figure 9 As shown in (a), the full-wave waveform of the monopole sound source in the unbonded model of the second cemented surface of the cement sheath contains casing waves C1-C2, Nestorley waves In_St, and leakage longitudinal waves Lp. Compared with the free casing and the unbonded model of the first interface, the casing waves have a time delay and a reduced amplitude, while their velocity is reduced to about 4771 m / s. A dipole sound source is set to excite at a main frequency of 3 kHz. Theoretical calculations are performed using the acoustic logging simulation model of the casing well in ultra-soft formation to obtain the full-wave waveform of the dipole sound source in the unbonded model of the second cemented surface of the cement sheath, as shown in the figure. Figure 9 As shown in (b), the full-wave waveform curve of the dipole sound source theory of the unbonded model of the second cemented surface of the cement sheath contains a low-order casing bending wave CF1, a formation longitudinal wave P, and a high-order casing bending wave CF2. The velocities of the low-order casing bending wave CF1 and the formation longitudinal wave P are 1454 m / s and 1108 m / s, respectively. At this time, the high-order casing bending wave CF2 has strong dispersion in the low-frequency band, and the excitation intensity of the formation bending wave FF is weak and is masked by the high-order casing bending wave CF2 in the waveform. Figure 10 As shown.

[0041] Step 3: For each depth point of the target well section, perform waveform feature matching on the measured waveform curve of the casing well sonic logging and the theoretical full-wave waveform curve of the casing well sonic logging, extract the full-wave waveform response characteristics under different cementation conditions of the ultra-soft formation, and evaluate the cementing quality of the ultra-soft formation based on the extracted full-wave waveform response characteristics.

[0042] Specifically, for each depth point in the target well section, the measured full-wave waveform of the monopole acoustic source is processed using the waveform coherent superposition method to extract the casing wave velocity measured by the monopole acoustic source, the measured full-wave waveform of the dipole acoustic source is processed using the waveform coherent superposition method to extract the casing bending wave velocity measured by the dipole acoustic source, and the formation bending wave dispersion curve is extracted using the weighted spectrum coherent method to process the measured full-wave waveform of the dipole acoustic source, thus obtaining the full-wave waveform response characteristics of the casing well acoustic logging.

[0043] Based on the casing wave velocity measured by the monopole sound source, the casing bending wave velocity measured by the dipole sound source, and the dispersion curve of the formation bending wave, combined with the formation shear wave velocity obtained from the well condition data of the target well section, the cementing quality of the ultra-soft formation is evaluated, the cementing status of the casing well at each depth point is determined, and the cementing quality evaluation results of the ultra-soft formation are obtained.

[0044] In this embodiment, the criteria for determining the cementation status of the casing well are as follows: When the casing wave velocity measured by the monopole source is 5386~5586 m / s and the casing bending wave velocity measured by the dipole source is 1506~1606 m / s, the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is greater than the formation shear wave velocity when the frequency is greater than 5.85 kHz, and there is obvious dispersion in the 0~30 kHz frequency domain. In the 0~30 kHz frequency domain, the velocity increases rapidly from zero to 955 m / s. The waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the first cementing surface of the cement sheath is not cemented. Therefore, it is determined that the cementing condition of the casing well is that the first cementing surface of the cement sheath is not cemented.

[0045] When the casing wave velocity measured by the monopole source is 4671~4871 m / s and the casing bending wave velocity measured by the dipole source is 1404~1504 m / s, and the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is less than the formation shear wave velocity in the 0~30 kHz frequency domain and there is obvious dispersion in the 0~10 kHz frequency domain, and the velocity increases rapidly from zero to 363 m / s in the 0~10 kHz frequency domain, and the waveform characteristics of the actual waveform curve of the casing well acoustic logging are consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the second cementing surface of the cement sheath is not cemented, then the cementing condition of the casing well is determined to be that the second cementing surface of the cement sheath is not cemented.

[0046] When the casing wave velocity measured by the monopole sound source is 4671~4871m / s and the casing bending wave velocity measured by the dipole sound source is 1404~1504m / s, and there is no formation bending wave in the extracted formation bending wave dispersion curve, and the waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the cement sheath is completely cemented, then the cementing condition of the casing well is determined to be that the cement sheath is completely cemented.

[0047] When the casing wave velocity measured by the monopole sound source is 5386~5586 m / s and the casing bending wave velocity measured by the dipole sound source is 1404~1504 m / s, and the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is less than the formation shear wave velocity at frequencies greater than 10 kHz and there is no dispersion phenomenon at frequencies greater than 1 kHz, and the measured waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the casing is free, then the cementation condition of the casing well is determined to be free casing.

[0048] Step 4: Based on the cementing quality evaluation results of the target well section in the ultra-soft formation, generate an ultra-soft formation cementing quality evaluation result map, such as... Figure 11 As shown, Figure 11 In the bonding quality of the first and second bonding surfaces of the cement ring, the black part represents good bonding, the diagonal line part represents medium bonding, and the white part represents poor bonding.

[0049] In summary, the method of this invention uses a simulation model of acoustic logging in casing wells in ultra-soft formations to theoretically calculate the full-wave waveform curves of acoustic sources excited in casing wells under different cement sheath cementation conditions. By utilizing the full-wave waveform response characteristics of the acoustic source under different cement sheath cementation conditions, the problem of difficulty in evaluating the cementation quality of the second interface of cement sheath in casing wells in ultra-soft formations through variable density logging is solved, thus realizing the accurate evaluation of cementing quality in casing wells in ultra-soft formations.

[0050] 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 for evaluating cementing quality in ultra-soft formations, characterized in that, Includes the following steps: Step 1: Select the target well section in the casing well, and perform array acoustic logging in the target well section using monopole acoustic sources and dipole acoustic sources respectively to obtain the measured waveform curves of the casing well acoustic logging, including the measured full waveform of monopole acoustic sources and the measured full waveform of dipole acoustic sources. Step 2: Establish a casing well model based on the well condition data of the target well section to simulate the cementation of casing wells in ultra-soft formations. Use the ultra-soft formation casing well acoustic logging simulation model to perform theoretical calculations for various casing well cementation conditions to obtain the theoretical full-wave waveform curves of casing well acoustic logging in the target well section, including the theoretical full-wave waveform curves of monopole acoustic sources and the theoretical full-wave waveform curves of dipole acoustic sources. Step 3: For each depth point of the target well section, perform waveform feature matching on the measured waveform curve of the casing well acoustic logging and the theoretical full-wave waveform curve of the casing well acoustic logging, extract the full-wave waveform response characteristics under different cementation conditions of the ultra-soft formation, and evaluate the cementing quality of the ultra-soft formation based on the extracted full-wave waveform response characteristics. Step 4: Generate a cementing quality evaluation result map of the ultra-soft formation based on the cementing quality evaluation results of the target well section.

2. The method for evaluating cementing quality in ultra-soft formations according to claim 1, characterized in that, In step 2, a casing well model is established in a cylindrical coordinate system based on the well condition data of the target well section. The casing well model is used to simulate the cementation of the formation and the casing, including the casing and the formation. The formation is an infinitely large solid medium, and the casing of the casing well model is filled with wellbore fluid. There are four types of cement sheath bonding conditions: free casing, cement sheath with no bonding at the first bonding surface, cement sheath with no bonding at the second bonding surface, and cement sheath with complete bonding. When no cement sheath is present between the casing and the formation, a fluid layer is placed between the outer wall of the casing and the formation to simulate a free casing. When a cement sheath is present between the casing and the formation, the bonding interface between the cement sheath and the outer wall of the casing is the first bonding surface, and the bonding interface between the cement sheath and the formation is the second bonding surface. A fluid layer is placed between the cement sheath and the outer wall of the casing to simulate the first bonding surface not being bonded, a fluid layer is placed between the cement sheath and the formation to simulate the second bonding surface not being bonded, and the inner wall of the cement sheath is in close contact with the outer wall of the casing, and the outer wall of the cement sheath is in close contact with the formation to simulate a complete cement sheath bonding.

3. The method for evaluating cementing quality in ultra-soft formations according to claim 2, characterized in that, The acoustic logging simulation model for casing wells in ultra-soft formations obtains the response characteristics of the full-wave waveform of acoustic logging in casing wells, including the full-wave waveform curves based on the theory of monopole acoustic sources and the full-wave waveform curves based on the theory of dipole acoustic sources. The expression is as follows: ； ； In the formula, This represents the full-wave waveform of a monopole sound source. The radius of the medium; This is the axial distance; For time; Radial wave number; The radius of the sound source; For the circumferential ordinal number, when When corresponding to a monopole sound source, when The time corresponds to the dipole sound source; The density of the fluid in the wellbore; For frequency; It is the amplitude coefficient of the sound wave propagating from the outside to the inside in the fluid medium inside the casing; Category 1 Bessel function of order 1; The source coefficient; Category II Bessel function of order 1; is the frequency spectrum function of the sound source; It is a natural constant; The imaginary unit; Here is the axial wave number; The waveform curve is the full-wave curve of the dipole sound source theory.

4. The method for evaluating cementing quality in ultra-soft formations according to claim 1, characterized in that, In step 3, for each depth point of the target well section, the casing wave velocity measured by the monopole sound source is extracted using the measured full-wave waveform, and the casing bending wave velocity and formation bending wave dispersion curve measured by the dipole sound source are extracted using the measured full-wave waveform, so as to obtain the full-wave waveform response characteristics of the casing well acoustic logging. Based on the casing wave velocity measured by the monopole sound source, the casing bending wave velocity measured by the dipole sound source, and the dispersion curve of the formation bending wave, combined with the formation shear wave velocity obtained from the well condition data of the target well section, the cementing quality of the ultra-soft formation is evaluated, the cementing status of the casing well at each depth point is determined, and the cementing quality evaluation results of the ultra-soft formation are obtained.

5. The method for evaluating cementing quality in ultra-soft formations according to claim 4, characterized in that, The criteria for determining the cementation status of the casing well are as follows: When the casing wave velocity measured by the monopole source is 5386~5586m / s and the casing bending wave velocity measured by the dipole source is 1506~1606m / s, and the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is greater than the formation shear wave velocity at a frequency greater than 5.85kHz and there is dispersion in the 0~30kHz frequency domain, and the measured waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the first cementing surface of the cement sheath is not cemented, then the cementing condition of the casing well is determined to be that the first cementing surface of the cement sheath is not cemented. When the casing wave velocity measured by the monopole sound source is 4671~4871m / s and the casing bending wave velocity measured by the dipole sound source is 1404~1504m / s, and the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is less than the formation shear wave velocity in the 0~30kHz frequency domain and there is a dispersion phenomenon in the 0~10kHz frequency domain, and the measured waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the second cementing surface of the cement sheath is not cemented, then the cementing condition of the casing well is determined to be that the second cementing surface of the cement sheath is not cemented. When the casing wave velocity measured by the monopole sound source is 4671~4871m / s and the casing bending wave velocity measured by the dipole sound source is 1404~1504m / s, and there is no formation bending wave in the extracted formation bending wave dispersion curve, and the actual waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the cement sheath is completely cemented, then the casing well cementation is determined to be complete cement sheath cementation. When the casing wave velocity measured by the monopole sound source is 5386~5586m / s and the casing bending wave velocity measured by the dipole sound source is 1404~1504m / s, and the extracted formation bending wave dispersion curve shows that the formation bending wave velocity is less than the formation shear wave velocity when the frequency is greater than 10kHz and there is no dispersion phenomenon when the frequency is greater than 1kHz, and the actual waveform curve of the casing well acoustic logging is consistent with the waveform characteristics of the theoretical full-wave waveform curve of the casing well acoustic logging when the casing is free, then the casing well cementation is determined to be free casing.

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