Method for evaluating packer capacity of cased well cement sheath

Abstract

The invention relates to a method for evaluating the packer capacity of a cement sheath of a cased well. The method comprises the following steps: collecting multi-source logging data; the cement sheath packing length is determined; inverting the apparent compressive strength of the cement; calculating the coupling rigidity of the casing-cement sheath interface; calculating formation pressure; determining the extrusion stress between the cement sheath and the sleeve; finite element modeling is carried out, the actual cementing condition of a target well section is simulated, and the packing pressure difference is calculated in a simulation mode. Through the method, a standardized quantitative evaluation system can be established, the development safety and efficiency are improved, and multi-scene application requirements are met.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field exploration and development technology, specifically to a method for evaluating the sealing capacity of cement sheaths in casing wells. This method is used to accurately determine the sealing effectiveness of cement sheaths between test layers and adjacent water layers in oil and gas wells, providing technical support for the safety and efficiency of oil and gas well development. Background Technology

[0002] In oil and gas field exploration and development, the cement sheath in casing wells serves as a critical barrier, and its sealing capacity directly determines oil and gas extraction efficiency, formation pressure control effectiveness, and wellbore integrity. Currently, the industry's evaluation of cement sheath sealing capacity largely relies on oilfield crossflow data, forming an evaluation system primarily based on qualitative descriptions. The core of this system is to indirectly determine sealing failure through crossflow phenomena, without establishing a quantitative analysis method based on the actual cementation parameters of the target well.

[0003] In existing technologies, some evaluation methods refer to basic logging data such as cement-bonded logging (CBL), but they only use simple parameters such as relative acoustic amplitude, without systematically integrating information from multiple types of logging, and without combining key factors such as formation pressure and interface coupling characteristics for comprehensive analysis.

[0004] The existing technology has the following main drawbacks:

[0005] 1. Limitations of the evaluation method: It is mainly based on qualitative evaluation and lacks quantitative data support. It cannot accurately give the sealing pressure difference that the cement ring can withstand, resulting in vague evaluation results and difficulty in guiding the design of on-site construction parameters.

[0006] 2. Incomplete parameter consideration: The synergistic effect of multiple factors such as formation pressure, apparent compressive strength of cement, and coupling stiffness of casing-cement sheath interface is ignored, resulting in a large deviation between the evaluation results and the actual working conditions, which can easily lead to misjudgment.

[0007] 3. Lack of standardization in the process: There is no unified process for processing and simulating multi-source logging data, and the evaluation results of different well sections and different logging methods lack comparability and have limited applicability. Summary of the Invention

[0008] The purpose of this invention is to establish a standardized processing flow based on multi-source logging data (CBL, SBT, back-casing imaging logging, etc.) to accurately extract key parameters of the cement sheath (apparent compressive strength, interface coupling stiffness, etc.); to construct a multi-parameter coupled evaluation model by combining formation pressure calculation and finite element simulation technology, thereby achieving quantitative calculation of cement sheath isolation pressure differential and improving the accuracy and reliability of the evaluation results; to provide a clear basis for judging the isolation capacity between the test layer and adjacent water layer in oil and gas wells, guiding on-site production pressure differential control, avoiding problems such as water channeling and formation pressure runaway, and ensuring the safety and efficiency of oil and gas well development. This addresses the problems of existing technologies in evaluating the isolation capacity of casing well cement sheaths, which are primarily qualitative, use only a single parameter, and have non-standardized processes.

[0009] To achieve the above objectives, the present invention provides a method for evaluating the sealing capacity of a cement sheath in a casing well, comprising the following steps:

[0010] Step 1: Well logging data acquisition;

[0011] Step 2: Determine the sealing length of the cement ring between the test layer and the adjacent water layer;

[0012] Step 3: Calculate the apparent compressive strength of the cement after application;

[0013] Step 4: Calculate the coupling stiffness of the sleeve-cement ring interface;

[0014] Step 5: Calculate formation pressure;

[0015] Step 6: Determine the extrusion stress between the cement ring and the sleeve;

[0016] Step 7: Use finite element simulation to model and replicate the actual cementation morphology in the imaging logging diagram, and use finite element simulation to calculate the sealing pressure difference.

[0017] In step 1, well logging is performed between the test layer and the adjacent water layer. One or a combination of four methods can be selected: traditional cement bonded logging (CBL), sector cement bonded logging (SBT) attached to the well wall, behind-casing imaging logging, and ultrasonic pulse reflection logging.

[0018] In step 2: the sealing length of the cement sheath between the test layer and the adjacent water layer is determined based on the cementing acoustic logging results. Specifically, the determination method is to determine the continuous length of the well section if the relative amplitude of the CBL casing wave is less than 60%, or the apparent compressive strength of the cement obtained by the SBT attenuation rate of the cement bonded sector attached to the well wall is greater than 100 psi, or the phase of the medium behind the casing obtained by the IBC inversion shows solid components, or the impedance obtained by the CAST inversion is greater than 2.6 M ayl.

[0019] In step 3: based on the data source characteristics of different logging methods, the corresponding inversion method is used to obtain the apparent compressive strength of the cement behind the casing, specifically including four methods:

[0020] The apparent compressive strength of cement was obtained from the CBL logging interpretation chart:

[0021] First, define the casing outer diameter and casing thickness parameters for the section to be interpreted, and extract the relative acoustic amplitude value (with the free casing acoustic amplitude as 100%) for that section from the CBL logging curve. Then, in the "Casing Outer Diameter - Relative Acoustic Amplitude - Casing Thickness" relationship diagram on the left side of the chart, determine the attenuation rate for that section based on the correspondence between casing outer diameter, thickness, and relative acoustic amplitude value. Finally, map the attenuation rate to the compressive strength on the right side of the chart using the casing wall thickness to obtain the corresponding apparent cement compressive strength.

[0022] The apparent compressive strength of cement can also be calculated directly from the relative acoustic amplitude value:

[0023] (1)

[0024] (2)

[0025] (3)

[0026] (4)

[0027] In the formula, S represents the apparent compressive strength of cement, in MPa; x cs and d o , respectively, are the casing wall thickness and casing outer diameter, mm; p index; a acoustic attenuation rate, dB / m; U relative acoustic amplitude, %; k coefficient related to source distance L.

[0028] The apparent compressive strength of cement in the circumferential direction is obtained based on the attenuation rate of SBT logging:

[0029] First, the casing thickness (in) of the section to be analyzed is determined, and the average casing wave attenuation (dB / ft) for this section is extracted from SBT logging data. Then, in the two-dimensional relationship of "casing thickness – casing wave attenuation" on the chart, the intersection of the casing thickness and casing wave attenuation is located. The compressive strength marked on the curve corresponding to this intersection point is the apparent compressive strength of the cement (psi), from which the corresponding apparent compressive strength value of the cement can be directly read. This process, through the correlation between casing structural parameters and acoustic response, achieves a quantitative characterization of the cement's mechanical properties, providing an intuitive interpretive basis for the application of SBT logging in evaluating the cement bonding quality of casing wells.

[0030] The apparent compressive strength of the cement is obtained from the average impedance of the entire well section obtained by inversion from the behind-casing imaging logging:

[0031] (5)

[0032] In the formula, Z is the average impedance after the set, MRayl; S is the apparent compressive strength of cement, psi.

[0033] The apparent compressive strength of cement was obtained from the average impedance obtained by CAST ultrasonic reflection logging inversion.

[0034] (6)

[0035] In the formula, Z is the average impedance after the set, MRayl; S is the apparent compressive strength of cement, psi.

[0036] In step 4: the coupling stiffness of the casing-cement sheath interface is obtained according to different logging methods.

[0037] Based on the coupling stiffness of the casing-cement sheath interface obtained from CBL logging inversion:

[0038] Based on known physical parameters of the casing, well mud, cement, and formation (such as dimensions, density, and sonic velocity), the casing wave amplitude under free casing conditions is first simulated using a classical model. Then, using a casing well model characterized by a slip interface, the minimum shear coupling stiffness at the maximum casing wave amplitude and the maximum shear coupling stiffness when the full wave characteristics conform to a well-cemented model are determined. Subsequently, 7-15 values ​​are uniformly selected within this stiffness range to simulate the corresponding casing wave amplitude and calculate its relative value (Ra) with the free casing wave amplitude. The functional relationship between shear coupling stiffness and Ra is then fitted. Finally, combined with the relative casing wave amplitude measured by CBL logging, the shear coupling stiffness of the casing-cement sheath interface is inverted through this functional relationship, thereby achieving a quantitative evaluation of cementing quality. Alternatively, the measured casing wave amplitude can be theoretically calibrated to construct an objective function and adjust the shear coupling stiffness until the objective function reaches its minimum value, thus determining the interface coupling stiffness at the corresponding depth.

[0039] The coupling stiffness at the bushing-cement sheath interface was retrieved based on the attenuation rate measured by SBT:

[0040] Based on the relationship between attenuation and interface coupling stiffness measured by SBT, the apparent compressive strength of the cement is obtained by intersecting the attenuation rate of the casing body with that measured by SBT. The attenuation value at a casing thickness of 10.36 mm is determined along the curve of this strength versus casing thickness. Based on this attenuation value, the interface coupling stiffness value is read from the relationship diagram, with the shear coupling stiffness and normal coupling stiffness taking the same value. If the attenuation value is less than the minimum attenuation value in the relationship diagram, the minimum value is used; if it is greater than the maximum value, the maximum value is used.

[0041] The coupling stiffness of the casing-cement sheath interface is determined based on the impedance inverted from back-casing imaging logging or ultrasonic reflection logging.

[0042] Based on the impedance values ​​obtained from ultrasonic reflection logging or back-casing imaging logging, the coupling stiffness of the interface can be directly read. The shear coupling stiffness and normal coupling stiffness are taken to be the same. When the back-casing impedance is greater than or equal to 6.2 MPayl, the interface coupling stiffness is taken to be 10000 GPa / m. If it is less than the minimum impedance value in the relationship diagram, the minimum critical impedance value is taken.

[0043] In step 5: the formation pressure is calculated using the formation pressure coefficient based on the vertical depth of the target well section from the surface.

[0044] (7)

[0045] In the formula, P: formation pressure (Pa)

[0046] Density of fresh water (1000 kg / m³)

[0047] g: acceleration due to gravity (9.81 m / s²)

[0048] h: Vertical depth of well section (m, note: measured from the ground, not the depth measured)

[0049] k: Formation pressure coefficient (dimensionless, i.e., the ratio of formation pressure to hydrostatic pressure at the same depth).

[0050] In step 6: the extrusion stress between the cement sheath and the casing is determined based on the formation pressure;

[0051] During the solidification process of cement grout, the formation pressure will generate extrusion stress on the outer wall of the casing through the cement grout. The magnitude of the extrusion stress has a good linear relationship with the cement compressive strength and the formation pressure. Based on the cement compressive strength and the formation pressure, the formula for calculating the extrusion stress is as follows:

[0052] (8)

[0053] In the formula, : Extrusion stress (Pa).

[0054] In step 7: the apparent compressive strength of cement, bonding strength, interface coupling stiffness, and extrusion stress are input into finite element simulation software such as COMSOL or Abaqus. A casing well model simulating the sealing performance of the cement sheath is established using cohesive elements. The bonding morphology of the cement sheath outside the casing is referenced from the cement bonding imaging image of the well section between the test layer and the adjacent water layer obtained by back-casing imaging logging, ultrasonic reflection logging, or SBT logging. If there is no ultrasonic imaging logging and only conventional cement bonding logging (CBL) is available, a circumferentially uniform casing well model can be established for the bonding between the casing and cement interface. A zero-thickness adhesion layer is pre-placed between the cement sheath and the casing to simulate the interface bonding. The degree of bonding is reflected by parameters such as the apparent compressive strength of cement and coupling stiffness. A point at the bottom of the adhesion layer is taken as the formation fluid crossflow point. During the simulation, the fluid pressure at the crossflow point is continuously increased until the pressure transmission breaks through the entire sealing length. The breakthrough pressure at this point is recorded as the sealing pressure difference at that cement sheath length.

[0055] The core technical advantage of this invention lies in providing a systematic method for evaluating the cement sealing capacity of casing wells. By integrating multiple logging data and combining them with finite element simulation technology, the actual cementation morphology in the imaging logging diagram can be replicated during finite element simulation modeling, enabling accurate assessment of the cement sheath sealing capacity between actual well sections. Specific benefits are as follows:

[0056] (1) A standardized multi-parameter coupled data processing flow was established:

[0057] This invention not only focuses on the single cement strength parameter but also introduces the key indicator of the coupling stiffness of the cement interface after sheathing. Through finite element simulation technology, it comprehensively analyzes the interaction between cement strength, formation pressure, and coupling stiffness to ultimately derive the sealing pressure difference of the cement sheath. This method can more realistically reflect actual working conditions and avoids evaluation errors caused by neglecting the coupling effects of multiple factors in traditional methods.

[0058] (2) Improve the efficiency and safety of oil and gas well development:

[0059] By quantitatively assessing the sealing pressure differential of the cement sheath, this invention can effectively guide the development and construction design of oil and gas wells, avoiding problems such as water channeling and formation pressure runaway caused by cement seal failure. This not only improves the efficiency of oil and gas resource extraction but also significantly reduces the risk of wellbore failure, ensuring the safety and economy of oil and gas field development. Attached Figure Description

[0060] A more complete understanding of the invention and its accompanying advantages and features will be more readily apparent from the accompanying drawings and the following detailed description, wherein:

[0061] Figure 1 This invention relates to a CBL logging interpretation chart of cement compressive strength.

[0062] Figure 2 This invention relates to the SBT logging interpretation chart of cement compressive strength.

[0063] Figure 3 This is the SBT inversion coupling stiffness relationship curve of the present invention.

[0064] Figure 4 This is the ultrasonic reflection logging impedance inversion coupling stiffness relationship curve of the present invention.

[0065] Figure 5 This invention relates to a casing well model for simulating the sealing performance of cement sheaths.

[0066] Figure 6 This is a diagram showing the bonding failure when the interfacial bonding strength is different according to the present invention.

[0067] Figure 7 This is the cementing quality logging diagram of the present invention.

[0068] Figure 8 This invention describes the complete process from well logging data acquisition to determining the isolation pressure differential. Detailed Implementation

[0069] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0070] Target well: Development well in an oil and gas field; Test interval: 3970.7-3989.0m (interpreted as gas layer); Adjacent water layers: Upper aquifer (10m from test layer), Lower aquifer (2.3m from test layer); Formation pressure coefficient k=1.2.

[0071] Specific implementation steps

[0072] 1. Well logging data acquisition: SBT logging was used to obtain data such as casing thickness (0.5in) and SBT average attenuation rate.

[0073] 2. Determine the isolation length: The apparent compressive strength of cement in both the upper and lower sections of SBT inversion is ≥150psi, and the effective isolation lengths are determined to be 10m and 2.3m respectively.

[0074] 3. Inversion of cement compressive strength: through... Figure 2(SBT logging interpretation chart), casing thickness 0.5in, average attenuation rate 6.5dB / ft → cement compressive strength of 250psi on the upper aquifer side; average attenuation rate 5.6dB / ft → cement compressive strength of 150psi on the lower aquifer side.

[0075] 4. Calculate the interface coupling stiffness: After correction according to the SBT data, from Figure 3 The lateral coupling stiffness of the upper aquifer was read as 350 GPa / m; the lateral coupling stiffness of the lower aquifer was read as 200 GPa / m.

[0076] 5. Calculate the formation pressure: Upper formation pressure: P = 1000 × 9.81 × 3970.7 × 1.2 = 46.74 MPa; Lower formation pressure: P = 1000 × 9.81 × 3989 × 1.2 = 46.96 MPa.

[0077] 6. Determine the extrusion stress: Substitute into formula (8), upper aquifer side =8.52MPa, lower aquifer side =5.14MPa.

[0078] 7. Finite element simulation: The model was constructed using COMSOL, and the cement ring morphology was set with reference to the SBT cementation imaging diagram. The above parameters were input for simulation → the pressure difference between the upper aquifer side and the lower aquifer side was 9.2 MPa and 5.0 MPa (as shown in Figure 5).

[0079] During on-site production, the water output remained within a reasonable range when the production pressure differential was controlled at 0.685 and 3.956 MPa. When the pressure differential rose to 6.786 MPa (exceeding 5.0 MPa), the water output began to increase significantly, reaching 39.9 m³. When the pressure differential rose to 11.698 MPa (exceeding 9.2 MPa), the water output surged to 196.3 m³, verifying the accuracy of the simulation results. After adopting this invention to guide production, the well maintained stable gas production for 12 months without crossflow, resulting in a cumulative increase of 2,300 tons of oil.

[0080] The evaluation results of applying this invention are as follows:

[0081] Upper aquifer (distance 10m): SBT attenuation average 6.5dB / ft → cement compressive strength 250psi, coupling stiffness 350GPa / m → extrusion stress 8.52MPa → simulated sealing pressure difference 9.2MPa;

[0082] Lower aquifer (distance 2.3m): SBT attenuation value 5.6dB / ft → cement compressive strength 150psi, coupling stiffness 200GPa / m → extrusion stress 5.14MPa → simulated sealing pressure difference 5.0MPa; Field verification: When the production pressure difference was 0.685, 3.956, 6.786, and 11.698MPa, the water output was small, 8.7, 9.3, 39.9, and 196.3 m³, respectively. Only when the pressure difference increased to 6.786 MPa (exceeding 5.0MPa) did the water output begin to increase significantly, reaching 39.9 m³; when the pressure difference increased to 11.698MPa (exceeding 9.2MPa), the water output increased sharply to 196.3 m³. Figure 6 The simulated pressure difference of the cement sheath under the model shown is 5.0 MPa. At this pressure, the debonding interface between the casing and the cement sheath expands upward over time. The simulated pressure difference can guide the construction design in testing or production operations, i.e., how to adjust the well pressure of the test or production layer. When the difference between the well pressure of the test or production layer and the formation pressure of the adjacent reservoir is less than 5.0 MPa, crossflow will not occur, ensuring safe extraction. This result is in perfect agreement with the simulation results, proving that the evaluation accuracy of this invention is extremely high.

[0083] This invention can be implemented according to the above embodiments, but its application scope is not limited thereto. The above embodiments are only to explain the implementation process of this invention in evaluating the sealing capacity of cement sheaths in casing wells, but should not be construed as limiting the application scope of this invention.

Claims

1. A method for evaluating the sealing capacity of a cement sheath in a casing well, comprising the following steps: Step 1: Well logging data acquisition; Step 2: Determine the sealing length of the cement ring between the test layer and the adjacent water layer; Step 3: Calculate the apparent compressive strength of the cement after application; Step 4: Calculate the coupling stiffness of the sleeve-cement ring interface; Step 5: Calculate formation pressure; Step 6: Determine the extrusion stress between the cement ring and the sleeve; Step 7: Use finite element simulation to model and replicate the actual cementation morphology in the imaging logging diagram, and use finite element simulation to calculate the sealing pressure difference.

2. The method according to claim 1, characterized in that: In step 1: logging is performed between the test layer and the adjacent water layer. One or a combination of four methods can be selected: traditional cement bonded logging (CBL), sector cement bonded logging (SBT) attached to the well wall, behind-casing imaging logging, and ultrasonic pulse reflection logging.

3. The method according to claim 1, characterized in that: In step 2: Determine the sealing length of the cement sheath between the test layer and the adjacent water layer based on the cementing acoustic logging results. The specific determination method is to determine the continuous length of the well section if the relative amplitude of the CBL casing wave is less than 60%, or the apparent compressive strength of the cement obtained by the SBT attenuation rate of the cement bonded sector of the well wall is greater than 100 psi, or the phase of the medium behind the casing obtained by the IBC inversion shows solid components, or the impedance obtained by the CAST inversion is greater than 2.6 M ayl.

4. The method according to claim 1, characterized in that: In step 3: Based on the data source characteristics of different logging methods, the corresponding inversion method is used to obtain the apparent compressive strength of the cement behind the casing, specifically including four methods: The apparent compressive strength of cement was obtained from the CBL logging interpretation chart. The apparent compressive strength of cement in the circumferential direction is obtained based on the attenuation rate of SBT logging. The apparent compressive strength of cement is obtained from the average impedance of the entire well section inverted by the imaging logging after casing. The apparent compressive strength of cement is obtained from the impedance inversion of CAST ultrasonic reflection logging.

5. The method according to claim 4, characterized in that: According to the CBL logging interpretation chart, the apparent compressive strength of cement is as follows: First, clarify the casing outer diameter and casing thickness parameters of the section to be interpreted, and extract the relative acoustic amplitude of this section from the CBL logging curve, with the free casing acoustic amplitude as 100% as a percentage. Then, in the "Casing Outer Diameter - Relative Acoustic Amplitude - Casing Thickness" relationship diagram on the left side of the chart, determine the attenuation rate of this section based on the correspondence between casing outer diameter, thickness, and relative acoustic amplitude. Finally, map the attenuation rate to the compressive strength on the right side of the chart through the casing wall thickness to read the corresponding apparent cement compressive strength. Alternatively, the apparent compressive strength of cement can be directly calculated based on the relative acoustic amplitude. (1) (2) (3) (4) In the formula, S is the apparent compressive strength of cement, MPa; x cs and d o These are the casing wall thickness and casing outer diameter, respectively, in mm; p is an exponent. a is the sound wave attenuation rate, dB / m; U is the relative sound amplitude, %; k is a coefficient related to the source distance L; The apparent compressive strength of cement in the circumferential direction is obtained from the attenuation rate of SBT logging as follows: First, determine the casing thickness (in) of the well section to be analyzed, and extract the average casing wave attenuation (dB / ft) of that section from the SBT logging data. Then, locate the intersection of the casing thickness and casing wave attenuation in the two-dimensional relationship of "casing thickness - casing wave attenuation" on the chart. The compressive strength marked on the curve to which this intersection point belongs is the apparent compressive strength of cement (psi). Based on this, the corresponding apparent compressive strength value of cement can be directly read. The apparent compressive strength of cement is obtained from the average impedance of the entire well section obtained by the inversion of the casing imaging logging. (5) In the formula, Z is the average impedance after the set, MRayl; S is the apparent compressive strength of cement, psi.

6. The apparent compressive strength of cement is obtained from the impedance inversion of CAST ultrasonic reflection logging as follows: (6) In the formula, Z is the average impedance after the set, MRayl; S is the apparent compressive strength of cement, psi.

7. The method according to claim 1, characterized in that: In step 4 of the claim: the coupling stiffness of the casing-cement sheath interface is obtained according to different logging methods. Based on the coupling stiffness of the casing-cement sheath interface obtained from CBL logging inversion: Based on known physical parameters of the casing, drilling mud, cement, and formation, the casing wave amplitude under free casing conditions is first simulated using a classical model. Then, using a casing well model characterized by slip interfaces, the minimum shear coupling stiffness at the maximum casing wave amplitude and the maximum shear coupling stiffness when the full wave characteristics conform to a well-cemented model are determined. Subsequently, 7-15 values ​​are uniformly selected within this stiffness range to simulate the corresponding casing wave amplitude and calculate its relative value (Ra) with the free casing wave amplitude. The functional relationship between shear coupling stiffness and Ra is then fitted. Finally, combined with the relative casing wave amplitude measured by CBL logging, the shear coupling stiffness of the casing-cement sheath interface is inverted through this functional relationship, thereby achieving a quantitative evaluation of cementing quality. Alternatively, the measured casing wave amplitude can be calibrated theoretically to construct an objective function and adjust the shear coupling stiffness until the objective function reaches its minimum value, thus determining the interface coupling stiffness at the corresponding depth. Based on the attenuation inversion of the coupling stiffness at the interface between the bushing and the cement sheath measured by SBT: Based on the relationship between attenuation and interface coupling stiffness measured by SBT, the apparent compressive strength of cement is obtained by intersecting the casing thickness with the average attenuation rate measured by SBT. The attenuation value at a casing thickness of 10.36 mm is determined along the curve of this strength versus casing thickness. Based on this attenuation value, the interface coupling stiffness value is read from the relationship diagram. The shear coupling stiffness and normal coupling stiffness are taken to be the same value. If the attenuation value is less than the minimum attenuation value in the relationship diagram, the minimum value is taken; if it is greater than the maximum value, the maximum value is taken. The coupling stiffness of the casing-cement sheath interface is determined based on the impedance inverted from back-casing imaging logging or ultrasonic reflection logging. Based on the impedance values ​​obtained from ultrasonic reflection logging or back-casing imaging logging, the coupling stiffness of the interface can be directly read. The shear coupling stiffness and normal coupling stiffness are taken to be the same. When the back-casing impedance is greater than or equal to 6.2 MPayl, the interface coupling stiffness is taken to be 10000 GPa / m. If it is less than the minimum impedance value in the relationship diagram, the minimum critical impedance value is taken.

8. The method according to claim 1, characterized in that: In step 5: Based on the vertical depth of the target well section from the surface, the formation pressure is calculated using the formation pressure coefficient. (7) In the formula, P: formation pressure (Pa) Density of fresh water (1000 kg / m³) g: acceleration due to gravity (9.81 m / s²) h: Vertical depth of well section (m, note: measured from the ground, not the depth measured) k: Formation pressure coefficient (dimensionless, i.e., the ratio of formation pressure to hydrostatic pressure at the same depth).

9. The method according to claim 1, characterized in that: In step 6: Determine the extrusion stress between the cement sheath and the casing based on the formation pressure; during the solidification process of the cement grout, the formation pressure will generate extrusion stress on the outer wall of the casing through the cement grout. The magnitude of the extrusion stress has a good linear relationship with the cement compressive strength and the formation pressure. Based on the cement compressive strength and the formation pressure, the formula for calculating the extrusion stress is: (8) In the formula, : Extrusion stress (Pa).

10. The method according to claim 1, characterized in that: In step 7: The apparent compressive strength of cement, bonding strength, interface coupling stiffness, and extrusion stress are input into finite element simulation software such as COMSOL or Abaqus. A casing well model simulating the sealing performance of the cement sheath is established through cohesive elements. The bonding morphology of the cement sheath outside the casing is referenced from the cement bonding imaging image of the well section between the test layer and the adjacent water layer obtained by back-casing imaging logging, ultrasonic reflection logging, or SBT logging. If there is no ultrasonic imaging logging and only conventional cement bonding logging (CBL) is available, a casing well model with circumferential uniform bonding at the casing and cement interface can be established. A zero-thickness adhesion layer is pre-placed between the cement sheath and the casing to simulate the interface bonding. The bonding degree is reflected by parameters such as the apparent compressive strength of cement and coupling stiffness. A point at the bottom of the adhesion layer is taken as the formation fluid crossflow point. During the simulation, the fluid pressure at the crossflow point is continuously increased until the pressure transmission breaks through the entire sealing length. The breakthrough pressure at this time is recorded as the sealing pressure difference at the length of the cement sheath.

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