Casing wave relative amplitude correction method based on cement sheath thickness

Abstract

The invention discloses a method for correcting the relative amplitude of casing waves based on the thickness of a cement sheath, and belongs to the field of oil-gas exploration and development. The method comprises the following steps: firstly, selecting a depth interval of a to-be-analyzed well, and simulating and calculating a full-wave waveform of a sound field in the well by synthesizing obtained data such as formation speed, density and well diameter in the depth interval according to actually measured logging information of CBL and VDL of the depth interval; the well depth-amplitude under the actual condition is compared with the normalized amplitude of different cementing conditions obtained through simulation calculation, and cementing types of different well sections are determined; according to different cementing conditions, the casing wave amplitude of the cement thickness is simulated, calculated and compared with the casing wave amplitude simulated through actual parameters, and a correction value needed by correction is obtained; and according to the cementation types and the correction values of different well sections, a corrected sound amplitude curve can be obtained. The method solves the problem that the influence on the wave amplitude of the casing due to the fact that the specification of the casing is not changed and the cement thickness is changed is not considered when the well cementation quality is evaluated in practice.

Description

Technical Field

[0001] The invention belongs to the field of oil and gas exploration and development, and in particular is a method for correcting the relative amplitude of casing waves based on cement sheath thickness. Background Art

[0002] In order to ensure the safe production of oil and gas wells and extend the service life of casing, the annulus between casing and formation is generally filled with cement for wellbore integrity and stratum isolation. Casing, cement and formation should be tightly bonded. Among them, cementing cement is a key material to ensure the functional integrity of oil and gas wells. It plays an important role in supporting casing, sealing underground complex formations, sealing underground oil, gas and water layers, preventing stratum collusion and protecting production layers. The quality of cement consolidation is directly related to whether oil and gas resources can be mined safely, reliably, economically and efficiently. Generally, when conducting actual cementing evaluation, the interpretation chart includes the influence of casing specifications on casing wave amplitude, that is, the influence of casing thickness and outer diameter on the relative amplitude of casing wave, while ignoring the influence of cement ring thickness change on casing wave relative amplitude. However, the change of cement thickness will also cause the casing wave amplitude to change. In actual situations, factors that cause cement thickness changes are also common, such as diameter expansion caused by well wall collapse, and comparison of wellbores with different cement thicknesses.

[0003] In the field, the acoustic logging method is mainly used to detect and evaluate the cementing quality. The non-directional cement bond logging, acoustic amplitude logging (CBL) and acoustic variable density logging (VDL) are often used. Acoustic amplitude logging mainly relies on the casing wave amplitude measured by acoustic logging for evaluation. The casing wave amplitude is affected by many factors: casing specifications, cement specifications and thickness, formation lithology, etc. Cement bond logging uses the amplitude of these casing waves to evaluate the quality of cement and casing bonding, but the amplitude or attenuation change of a type of casing wave may be caused by many factors. In fact, when evaluating the quality of cementing, the casing specifications, that is, the influence of casing thickness and outer diameter on the relative amplitude of casing waves, are often concerned, and the influence of changes in cement thickness on the casing wave amplitude is not considered. However, it was found in the study that the size of the casing wave amplitude is not only affected by the type of bonding, but also by the thickness of cement. When the bonding type is different, the change of the casing wave amplitude is not linear with the increase of cement thickness, such as lightweight cement (1.3g / cm 3 ) In a well-bonded section in a hard formation under the condition of 30 mm, the casing wave amplitude increases with the increase of cement thickness when it is less than 30 mm, but decreases when it exceeds 30 mm; while in free casing, the boundary thickness of this cement is 20 mm. In soft formations, the situation is different from that in hard formations. For example, when the bonding is good, the casing wave amplitude will decrease with the increase of cement thickness. It can be seen that the influence of cement thickness needs to be judged in combination with actual conditions. In the field, the industry standard is generally 20 mm. In order to eliminate the influence of cement thickness, the measured casing wave amplitude needs to be corrected. Summary of the invention

[0004] The purpose of the present invention is to overcome the above-mentioned shortcomings of the prior art and provide a method for correcting the relative amplitude of casing waves based on cement sheath thickness.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for correcting the relative amplitude of casing waves based on cement sheath thickness comprises the following steps:

[0007] 1) According to the measured acoustic waveforms at different depths, the first wave amplitude of the casing wave is extracted to obtain the well depth-acoustic amplitude curve L0; the well depth-acoustic amplitude curve L0 is normalized with the acoustic amplitude of the free casing section as 100% to obtain the normalized well depth-acoustic amplitude curve L1;

[0008] 2) Obtaining the formation velocity, density and wellbore parameters in the depth interval, and establishing cased well models corresponding to different cementation types; using the real axis integration method to calculate the acoustic waveform of the cased well model, extracting the first wave amplitude of the casing wave, and normalizing it with the free casing section as 100%, to obtain the cementation degree-acoustic amplitude curve L2;

[0009] 3) According to the corresponding relationship of the sound amplitude, the actually measured well depth-amplitude curve L1 and the cementation degree-sound amplitude curve L2 are compared to obtain the cementation degree at different depths; different well sections are divided according to different cementation degrees; 4) According to the cementation conditions of different well sections, only the cement thickness is changed when other conditions remain unchanged to calculate the sound wave waveform, and the ratio of the calculated casing wave amplitude of the sound wave waveform to the casing wave amplitude method of the cased well model is used as the correction value T m ;

[0010] 5) Multiply the well depth-amplitude normalization curve L1 by the correction value T according to the cementation condition m The well depth-acoustic amplitude curve L3 after correction is obtained.

[0011] Furthermore, in step 1), the measured acoustic amplitude logging data are obtained from non-directional cement bond logging, acoustic amplitude logging CBL and acoustic variable density logging VDL.

[0012] Furthermore, in step 2), the established cased well models include a free casing model, an I interface poorly bonded cased well model represented by the thickness of the fluid annulus, an I interface relatively poorly bonded cased well model represented by the shear coupling stiffness of the slip interface, and a well-bonded model.

[0013] Further, in step 3), the formation velocity, density and well diameter in the depth interval are obtained according to the array acoustic logging, density logging and well diameter logging data in the depth interval, and the acoustic waveforms of different cementation conditions are simulated and calculated according to the formation velocity, density and well diameter data in the depth interval. The calculation formula is:

[0014]

[0015] Where n is the circumferential azimuth number, n=0 represents a monopole sound source; r represents the radial distance; k is the axial wave number; z is the axial distance; ω is the angular frequency; S(ω) is the sound source function; t is time; I n is the first kind of n-order modified Bessel function; A n It is the acoustic amplitude coefficient in the well, and the calculation method of the acoustic amplitude coefficient varies with different cementation types.

[0016] Furthermore, when the bonding model is good, A n satisfy:

[0017]

[0018] Where fl, ca, ce, for represent the well fluid, casing, cement and formation respectively; r0, r1, r2 represent the radii of the well wall, interface I and interface II; T represents the coefficient matrix of the medium; Q represents the acoustic amplitude coefficient of casing, cement and formation; u f and σ f represents the displacement and stress of the direct wave;

[0019] When the I interface is a relatively poorly bonded cased well model represented by the shear coupling stiffness of the slip interface, A n satisfy:

[0020]

[0021] Among them, M is the matrix of coupling stiffness coefficients:

[0022]

[0023] According to the different stresses, the coupling stiffness can be divided into shear coupling stiffness η T and the normal coupling stiffness η N , normal coupling stiffness η N Take infinity;

[0024] When the I interface poorly bonded cased well model is represented by the fluid annulus thickness, A n satisfy:

[0025]

[0026] Where, d represents the thickness of the fluid annulus;

[0027] When it is a free casing model, A n satisfy:

[0028]

[0029] Furthermore, in step 4), the cement thickness is changed to 20 mm.

[0030] Further, in step 4), the correction value T m It is the ratio of the casing head wave amplitude calculated according to the comparison cement thickness under the same bonding condition to the casing head wave amplitude calculated according to the actual cement thickness. The calculation formula is:

[0031]

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention discloses a correction method for the relative amplitude of casing waves based on cement sheath thickness. First, the depth interval of the well to be analyzed is selected. According to the logging data of acoustic amplitude logging (CBL) and acoustic variable density logging (VDL) actually measured in the depth interval, the formation velocity, density and well diameter and other data in the depth interval are obtained by integrating array acoustic logging, density logging and well diameter logging data, and the full wave waveform of the acoustic field in the well is simulated and calculated; the well depth-amplitude under actual conditions is compared with the normalized amplitude of different cementation conditions calculated by simulation, and the cementation types of different well sections are determined, and divided into free well sections and free well sections. Casing, poor I interface bonding, relatively poor I interface bonding (expressed by the coupling stiffness of the slip interface) and good bonding; according to different bonding conditions, the full wave waveform of the simulated calculation is different from the measured waveform. This is because the simulated waveform is calculated under ideal conditions, but they are the same in studying the propagation law of the acoustic field in the well. Therefore, the casing wave amplitude of the simulated calculation comparison cement thickness is compared with the casing wave amplitude simulated by actual parameters to obtain the correction value required for correction; according to the bonding type and correction value of different well sections, the corrected sound amplitude curve can be obtained. The present invention solves the problem that the influence of the change of cement thickness on the casing wave amplitude when the casing specifications remain unchanged is not considered when evaluating the cementing quality in practice. The full wave waveform of the acoustic wave received in the well is simulated by real axis integration and other methods, and the classical elastic wave theory and the slip interface theory are combined to determine a method with strong versatility to correct the casing wave amplitude of the measured cement thickness to the casing wave amplitude of the comparative cement thickness. According to the simulation results of the acoustic field in the well, it is shown that the correction method has a reliable theoretical basis. The present invention combines actual strata and rock physics models, has convenient and concise calculations, strong versatility and wide applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A flow chart of a method for correcting the relative amplitude of casing waves based on cement sheath thickness provided by the present invention;

[0035] Figure 2 A schematic diagram of the actual wellbore established;

[0036] Figure 3 is the variation law of coupling stiffness and relative amplitude of casing wave

[0037] Figure 4 The acoustic waveforms calculated using wellbore parameters when the cement thickness is 19 mm and 50 mm;

[0038] Figure 5 is the correction result of the relative amplitude of the wellbore. DETAILED DESCRIPTION

[0039] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0040] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0041] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0042] See also Figure 1 , a flowchart of a method for correcting the relative amplitude of casing waves based on cement sheath thickness provided by an embodiment of the present invention comprises the following steps:

[0043] 1) According to the acoustic waveforms at different depths obtained from the acoustic amplitude logging data, the first wave amplitude of the casing wave is extracted to obtain the well depth-acoustic amplitude curve L0; the acoustic amplitude of the free casing section is normalized as 1 (100%) to obtain the well depth-acoustic amplitude curve L1 after normalization;

[0044] 2) Establish a cased well model based on the velocity-density parameters of the well fluid, casing, cement and formation obtained from casing specifications, cement specifications, time difference curves and wellbore data; use the real axis integration method to calculate the acoustic waveform of the cased well model, extract the first wave amplitude of the casing wave, and perform normalization with the free casing section as 1 (100%) to obtain the cementation degree-acoustic amplitude curve L2;

[0045] 3) According to the corresponding relationship of acoustic amplitude, the actual measured well depth-amplitude curve L1 is compared with the cementation degree-acoustic amplitude curve to obtain the cementation degree at different depths; different well sections are divided according to the cementation degree (divided into fluid ring sections and coupling stiffness sections), and numbered from top to bottom (represented by m);

[0046] 4) According to the cementation conditions of different well sections, calculate the acoustic waveform when only the cement thickness is changed while the casing specifications and cement specifications remain unchanged (the industry standard is 20 mm, the same below), and regard the ratio of the casing wave amplitude of the waveform to the casing wave amplitude method of the original cased well model as the correction value T m ;

[0047] 5) The well depth-acoustic amplitude normalized curve L1 is multiplied by the correction value T according to the cementation condition to obtain the well depth-acoustic amplitude curve L3 after correction.

[0048] Preferably, in step 1), the acoustic amplitude logging data is obtained by non-directional cement bond logging acoustic amplitude logging (CBL) and acoustic variable density logging (VDL);

[0049] Preferably, in step 2), the established cased well models include a free casing model, an I interface poorly bonded cased well model (expressed by fluid annulus thickness), an I interface relatively poorly bonded cased well model (expressed by shear coupling stiffness of the slip interface), and a well-bonded model;

[0050] Preferably, in step 3), according to the array acoustic logging, density logging and caliper logging data of the depth interval, the real data such as the formation velocity, density and caliper in the depth interval are obtained, and the acoustic waveforms of different cementation conditions are simulated and calculated according to the above data, and the calculation formula is:

[0051]

[0052] Where n is the circumferential azimuth number, n=0 represents a monopole sound source, r represents the radial distance, k is the axial wave number, z is the axial distance, ω is the angular frequency, S(ω) is the sound source function, and I n is the first kind of n-order modified Bessel function; A n It is the acoustic amplitude coefficient in the well. The calculation method of the acoustic amplitude coefficient of different cementation types is different.

[0053] When the bonding model is good, A n satisfy:

[0054]

[0055] Where fl, ca, ce, for represent the well fluid, casing, cement and formation respectively; r0, r1, r2 represent the radii of the well wall, interface I and interface II; T represents the coefficient matrix of the medium; Q represents the acoustic amplitude coefficient of casing, cement and formation; u f and σ f represents the displacement and stress of the direct wave;

[0056] When the I interface is a relatively poorly bonded cased well model represented by the shear coupling stiffness of the slip interface, A n satisfy:

[0057]

[0058] Among them, M is the matrix of coupling stiffness coefficients:

[0059]

[0060] According to the different stresses, the coupling stiffness can be divided into shear coupling stiffness η T and the normal coupling stiffness η N , normal coupling stiffness η N Take infinity;

[0061] When the I interface poorly bonded cased well model is represented by the fluid annulus thickness, A n satisfy:

[0062]

[0063] Where, d represents the thickness of the fluid annulus;

[0064] When it is a free casing model, A n satisfy:

[0065]

[0066] Preferably, in step 3), the cementation condition of the well section is determined based on the well depth-amplitude curve L1 extracted from the acoustic amplitude logging waveform and the normalized amplitudes of different cementation types calculated by simulation, and the cementation conditions are sorted from top to bottom. The cementation conditions determined in this way are more accurate and more in line with reality.

[0067] Preferably, in step 4), the cased well model for comparing cement thickness under other conditions unchanged is the model compared in the present invention. mIt is the ratio of the casing head wave amplitude calculated according to the comparison cement thickness under the same bonding condition to the casing head wave amplitude calculated according to the actual cement thickness. The calculation formula is:

[0068]

[0069] Preferably, in step 5), the depth-amplitude curve L1 is multiplied by the correction value T of the well section in each section. m , the well depth-acoustic amplitude curve L3 of cement thickness can be obtained by correcting the cement thickness from the actual casing wave contrast amplitude to the contrast cement thickness.

[0070] Below, the specific embodiments of the embodiment are further described in detail to support the technical problem to be solved by the present invention. The implementation steps of the embodiment are as follows:

[0071] 1. Figure 2 This is a schematic diagram of the calibration wellbore. The reason for selecting this well is that the wellbore contains two cement rings of different thicknesses and has two formations, sandstone and mudstone. The ability to clear the wellbore indicates the correctness of the calibration method.

[0072] 2. Figure 2 The calibration wells are sandstone 0 (fluid ring thickness 19mm, abbreviated below), mudstone 1 (19mm), mudstone 2 (50mm), sandstone 1 (50mm) and sandstone 2 (19mm) from top to bottom. Except for sandstone 0, which is completely unbonded (free casing), the other formations are completely bonded (well bonded). Since the wellbore is a calibration well, the formation velocity, density, well diameter and other parameters as well as the bonding condition of the wellbore are known. Therefore, the present invention uses the well section with a cement thickness of 50mm as the calibration layer section, and the well section with a cement thickness of 19mm as the comparison well section;

[0073] 3. The ratio of the casing wave amplitude of sandstone 0 to that of sandstone 2 in the wellbore is about 1.8%, which can be described as good bonding in practice. In order to accurately determine the bonding conditions, the formation parameters, casing parameters, cement parameters, and casing parameters such as well diameter of sandstone 0 and sandstone 2 are substituted into formulas (1) to (6), and the relative amplitudes of free casing and coupling stiffness increase can be calculated. The results are: Figure 3 According to this figure, it can be judged that the relative amplitude is 1.8% when the shear coupling stiffness is 65Gpa / m.

[0074] 4. According to Figure 5 From the first amplitude curve in the well, we can judge that the sandstone 1, sandstone 2, mudstone 1 and mudstone 2 in the well are in good bonding state. According to step 3, for accurate calculation, the bonding condition can be regarded as shear coupling stiffness of 65Gpa / m.

[0075] 5. Substitute the casing well parameters and shear coupling stiffness of sandstone 1, sandstone 2, mudstone 1 and mudstone 2 into formulas (3) to (4) to calculate the theoretical waveforms under different models. The results are as follows: Figure 4 As shown. Comparing the wave amplitude of the casing with cement thickness of 19mm and 50mm, the correction values ​​of sandstone formation and mudstone formation are 1.35 and 0.62 (the casing wave amplitude calculated by simulation is based on Figure 2 The wellbore from top to bottom are 1.095, 0.0094, 0.0152, 0.0136 and 0.0184;

[0076] 6. According to the correction value Figure 5 The relative amplitude curve of the first track can be corrected to obtain Figure 5 The fourth curve in .

[0077] contrast Figure 4 From the first and fourth curves in the figure, it can be clearly observed that the corrected curve is smoother than the first curve, and the amplitude of the corrected cement thickness of 50 mm is similar to that of the cement thickness of 19 mm, which illustrates the correctness and strong adaptability of the invented correction method.

[0078] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for correcting the relative amplitude of casing waves based on cement sheath thickness, characterized in that: The following steps are involved: 1) According to the measured acoustic waveforms at different depths, the first wave amplitude of the casing wave is extracted to obtain the well depth-acoustic amplitude curve L0; the well depth-acoustic amplitude curve L0 is normalized with the acoustic amplitude of the free casing section as 100% to obtain the normalized well depth-acoustic amplitude curve L1; 2) Obtaining the formation velocity, density and wellbore parameters in the depth interval, and establishing cased well models corresponding to different cementation types; using the real axis integration method to calculate the acoustic waveform of the cased well model, extracting the first wave amplitude of the casing wave, and normalizing it with the free casing section as 100%, to obtain the cementation degree-acoustic amplitude curve L2; 3) According to the corresponding relationship of the sound amplitude, the actually measured well depth-amplitude curve L1 and the cementation degree-sound amplitude curve L2 are compared to obtain the cementation degree at different depths; different well sections are divided according to different cementation degrees; 4) According to the cementation conditions of different well sections, only the cement thickness is changed when other conditions remain unchanged to calculate the sound wave waveform, and the ratio of the calculated casing wave amplitude of the sound wave waveform to the casing wave amplitude method of the cased well model is used as the correction value T m ; 5) Multiply the well depth-amplitude normalization curve L1 by the correction value T according to the cementation condition m The corrected well depth-acoustic amplitude curve L3 is obtained.

2. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 1, characterized in that: In step 1), the measured acoustic amplitude logging data are obtained from non-directional cement bond logging, acoustic amplitude logging CBL and acoustic variable density logging VDL.

3. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 1, characterized in that: In step 2), the cased well models established include a free casing model, an I interface poorly bonded cased well model represented by the fluid annulus thickness, an I interface relatively poorly bonded cased well model represented by the shear coupling stiffness of the slip interface, and a well-bonded model.

4. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 1, characterized in that: In step 3), the formation velocity, density and well diameter in the depth interval are obtained according to the array acoustic logging, density logging and well diameter logging data in the depth interval. The acoustic waveforms of different cementation conditions are simulated and calculated according to the formation velocity, density and well diameter data in the depth interval. The calculation formula is: Where n is the circumferential azimuth number, n=0 represents a monopole sound source; r represents the radial distance; k is the axial wave number; z is the axial distance; ω is the angular frequency; S(ω) is the sound source function; t is time; I n is the first kind of n-order modified Bessel function; A n It is the acoustic amplitude coefficient in the well, and the calculation method of the acoustic amplitude coefficient varies with different cementation types.

5. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 4, characterized in that: When the bonding model is good, A n satisfy: Where fl, ca, ce, for represent the well fluid, casing, cement and formation respectively; r0, r1, r2 represent the radii of the well wall, interface I and interface II; T represents the coefficient matrix of the medium; Q represents the acoustic amplitude coefficient of casing, cement and formation; u f and σ f represents the displacement and stress of the direct wave.

6. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 5, characterized in that: When the I interface is a relatively poorly bonded cased well model represented by the shear coupling stiffness of the slip interface, A n satisfy: Among them, M is the matrix of coupling stiffness coefficients: According to the different stresses, the coupling stiffness can be divided into shear coupling stiffness η T and the normal coupling stiffness η N , normal coupling stiffness η N Take infinity.

7. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 5, characterized in that: When the I interface poorly bonded cased well model is represented by the fluid annulus thickness, A n satisfy: Where d represents the thickness of the fluid annulus.

8. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 5, characterized in that: When it is a free casing model, A n satisfy:

9. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 1, characterized in that: In step 4), the cement thickness was changed to 20 mm.

10. The method for correcting the relative amplitude of casing waves based on cement sheath thickness according to claim 1, characterized in that: In step 4), the correction value T m It is the ratio of the casing head wave amplitude calculated according to the comparison cement thickness under the same bonding condition to the casing head wave amplitude calculated according to the actual cement thickness. The calculation formula is:

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