A numerical simulation method for the integrity of cement sheath system in oil and gas wells under alternating temperature and pressure.

Abstract

A numerical simulation method for the integrity of cement sheath systems in oil and gas wells under alternating temperature and pressure is characterized by the use of extended finite element method (XFEM), concrete damage plasticity model theory, Cohesive element method, and bilinear traction separation criterion, based on elastoplastic theory and plastic damage theory. This method can simulate not only the failure process of the cement sheath-casing interface under alternating temperature and pressure and characterize the mechanical behavior of interface failure, but also the failure process of the cement sheath itself and characterize its mechanical behavior. By comprehensively analyzing the failure behavior of the cement sheath system integrity in oil and gas wells, this method enables a realistic evaluation, prediction, and diagnosis of the integrity of the casing-cement sheath-casing assembly system under alternating temperature and pressure, which helps optimize cement slurry systems, improve cementing quality, and prevent cement sheath integrity failure. This invention is applicable to the field of oil and gas drilling and production engineering technology.

Description

Technical Field

[0001] This invention relates to the field of oil and gas drilling and production engineering technology, specifically a numerical simulation method for the integrity of the cement sheath system in oil and gas wells under alternating temperature and pressure. Background Technology

[0002] Cementing is one of the most important aspects of oil and gas well drilling. Its main purpose is to isolate oil, gas, and water layers within the wellbore, protect the well casing, increase well lifespan, and improve oil and gas production. With the development of my country's oil and gas industry, the concept of wellbore integrity has evolved, and the requirements for wellbore integrity have become more stringent, including the integrity of the cement sheath.

[0003] To stabilize and increase production, oilfields have successively carried out downhole engineering operations such as water injection, fracturing, acidizing, and perforation repair. Different operations inevitably change wellbore conditions. Changes in casing pressure, formation rock pressure, and temperature stress caused by changes in wellbore temperature alter the stress state of the cement sheath, which may lead to cracks in the cement sheath itself, micro-annular gaps at the cement sheath-casing interface or cement sheath-formation interface, and failure of the cement sheath's sealing function.

[0004] When the cement sheath fails due to integrity issues, its sealing performance is significantly reduced, leading to many complex operating conditions such as annular pressure buildup, reduced casing life, and oil-gas-water exchange. To mitigate the problems caused by cement sheath integrity failure, researchers have analyzed its failure process using theoretical models and finite element analysis. However, a systematic simulation method for cement sheath system integrity failure using finite element analysis is still lacking. Therefore, this invention proposes a numerical simulation method for the integrity of oil and gas well cement sheath systems under alternating temperature and pressure conditions. Summary of the Invention

[0005] The purpose of this invention is to provide a numerical simulation method for the integrity of the cement sheath system in oil and gas wells under alternating temperature and pressure. This method can not only simulate the failure process of the cement sheath-casing interface under alternating temperature and pressure and characterize the mechanical behavior of interface failure, but also simulate the failure process of the cement sheath itself and characterize the mechanical behavior of body failure. It comprehensively reflects the failure behavior of the cement sheath system integrity in oil and gas wells, and realizes the realistic evaluation, prediction, and diagnosis of the integrity of the casing-cement sheath-formation assembly system under alternating temperature and pressure. This helps to optimize the cement slurry system, improve cementing quality, and prevent cement sheath integrity failure.

[0006] To achieve the above objectives, the present invention provides a numerical simulation method for the integrity of an oil and gas well cement sheath system under alternating temperature and pressure. The method is characterized by employing the extended finite element method (XFEM), a concrete damage plasticity model, the Cohesive element method, and a bilinear traction-separation criterion, based on elastoplastic theory and plastic damage theory. Based on elastoplastic theory and plastic damage theory, a three-dimensional mechanical model of the "casing-cement sheath-casing" assembly was established. The extended finite element method and a concrete damage plasticity model were used to simulate and analyze the cracking, propagation, and complete failure processes of the cement sheath. Maintaining the same number of load alternations, the minimum peak load corresponding to crack initiation, propagation, and complete failure of the cement sheath was obtained. Maintaining the same peak load, the minimum number of load alternations corresponding to crack initiation, propagation, and complete failure of the cement sheath was obtained. The Cohesive element method and bilinear traction-separation criterion were used to simulate the failure process of damage initiation and micro-annular gap formation at the cement sheath-casing interface. Maintaining the same number of load alternations, the minimum peak load corresponding to damage initiation and micro-annular gap formation at the cement sheath-casing interface was obtained. Maintaining the same peak load, the minimum number of load alternations corresponding to damage initiation and micro-annular gap formation at the cement sheath-casing interface was obtained. The cracking, propagation, and complete failure mechanisms of the cement sheath and the cement sheath-interface peeling and complete failure mechanisms under alternating temperature and pressure were studied. The integrity of the cement sheath system in oil and gas wells under alternating temperature and pressure was evaluated. The specific technical solutions adopted are as follows:

[0007] Step 1: Preparation of test samples for mechanical property testing of the "casing-cement ring-casing": Using an on-site cement grout system, the "casing-cement ring-casing" assembly was prepared under normal temperature and pressure curing conditions, and mechanical property testing experiments were conducted on the "casing-cement ring-casing" assembly to obtain the cement ring-casing interface bonding strength t. n .

[0008] Step 2: Preparation of test samples for testing the mechanical properties of cement stone: Standard cement stone samples (25 mm in diameter and 50 mm in height) are prepared using an on-site cement slurry system under normal temperature and pressure curing conditions.

[0009] Step 3: Test the mechanical properties of cement stone: (1) Determine the uniaxial compression / tension and triaxial compression stress-strain curves of cement stone according to the "Standard for Test Methods of Engineering Rock Mass" GB / T50266-2013; (2) Calculate the uniaxial compressive / tensile strength, triaxial compressive strength, elastic modulus E0, and Poisson's ratio μ0 of cement stone; (3) Based on the stress-strain curves, calculate the compressive inelastic strain using the formula for calculating the inelastic strain under compression and the formula for the cracking strain under tension of cement stone. Tensile crack strain

[0010] The formula for calculating the inelastic strain of cement stone under compression is shown in Equation 1.

[0011]

[0012] In the formula, For compressive inelastic strain; σ c ε c Compressive stress and compressive strain are defined in GB50010—2012. This represents the elastic compressive strain corresponding to the initial elastic modulus. This represents the plastic strain under compression.

[0013] The formula for calculating the tensile cracking strain of cement stone is shown in Equation 2.

[0014]

[0015] In the formula, For tensile cracking strain; σ t ε t The tensile stress and tensile strain are specified in GB50010—2012. This represents the elastic tensile strain corresponding to the initial elastic modulus. This represents the plastic strain under tension.

[0016] Step 4: Based on the compressive inelastic strain obtained in Step 3 and tensile crack strain The compressive damage factor d was calculated using both the compressive damage factor formula and the tensile damage factor formula. c Tensile damage factor d t .

[0017] The formula for calculating the compressive damage factor of cement stone is shown in Equation 3.

[0018]

[0019] In the formula, σ c ε c These are the compressive stress and compressive strain as defined in GB50010—2012. It is an inelastic strain under compression; This represents the plastic strain under compression.

[0020] The formula for calculating the tensile damage factor of cement stone is shown in Equation 4.

[0021]

[0022] In the formula, σ t ε t These are the tensile stress and tensile strain specified in GB50010—2012, respectively. For tensile cracking strain; This represents the plastic strain under tension.

[0023] Step 5: Based on the elastic-plastic theory and plastic damage theory, two three-dimensional mechanical models of the "sleeve-cement ring-sleeve" assembly are established using ABAQUS finite element software. Model 1 and Model 2 are used to carry out simulation analysis of cement ring body integrity failure and cement ring-sleeve interface integrity failure, respectively.

[0024] Step 6: Use C3D8R elements (eight-node linear hexahedral elements with reduced integrals) to sweep and mesh the model, and refine the cement ring part by local densification seeding.

[0025] Step 7: Define boundary conditions and loads. Apply a fully fixed constraint to the outer boundary of the technical casing and apply a uniform alternating load to the inner wall of the production casing.

[0026] Step 8: Input the stress-strain constitutive relationship of the casing material and define the mechanical properties of the casing material.

[0027] Step 9: Based on Model 1 established in Step 5, define the plastic damage model parameters, elastic modulus, and Poisson's ratio of the cement ring. Use the extended finite element method and the concrete damage plastic model to study the crack initiation and crack propagation process of the cement ring body.

[0028] Step 10: The Maxpe damage mode based on energy and linear softening is used to control the damage evolution of the cement sheath, and XFEM-type cracks are used to characterize the cracks in the cement sheath. The displacement u(x) of any crack node is approximated using an approximation formula. Among them, u i It is the nodal displacement; a j b kl It is a node-strengthening variable; N i N j N k I is the nodal shape function; I is the set of all nodes in the discrete domain; J is the set of nodes through which the crack completely penetrates the element.

[0029] Step 11: Based on Model 2 established in Step 5, add a Cohesive unit layer of type COH3D8 to the cement ring-sleeve interface, and define the mechanical properties of the Cohesive layer, including the interfacial bonding strength.

[0030] Step 12: Select the maximum nominal stress criterion Controlling initial damage to cohesive elements. Among other things, It is the critical stress in the normal direction of the Cohesive Element, i.e., the interfacial bonding strength (MPa); These are the first and second critical tangential stresses (MPa) of the element; <> represents Macaulay brackets, indicating that no damage occurs at the interface under pure compressive stress. The BK fracture criterion is selected. Controlling the damage evolution process of Cohesive units. Among them, It is the normal critical strain energy release rate at cracking (J / m) 2 ); The first and second tangential critical strain energy release rates (J / m) at crack initiation 2 );G n G s G t It is the strain energy release rate (J / m) in the crack normal and two tangential directions. 2 ); β is a material parameter.

[0031] Step 13: The maximum nominal stress criterion, BK fracture criterion, and Maxs damage mode based on energy and linear softening are used to control the interface damage evolution and micro-annulus formation mechanism.

[0032] Step Fourteen: Use DAMAGEC (compressive damage value) and DAMAGET (tensile damage value) to characterize the degree of compressive and tensile damage to the cement ring body. DAMAGEC and DAMAGET are between 0 and 1. If DAMAGEC or DAMAGET = 0, the cement ring body has not experienced compressive or tensile damage. If DAMAGEC or DAMAGET > 0 and is close to 1, the cement ring body has reached the compressive or tensile damage limit. Use STATUSXFEM state value (state of XFEM element) to characterize the crack initiation and propagation process of the cement ring. STATUSXFEM state value is between 0 and 1. If STATUSXFEM = 0, the cement ring is intact and has no cracks. If 0 < STATUSXFEM < 1, the cement ring is partially initiating and propagating cracks. If STATUSXFEM = 1, the cement ring is completely cracked.

[0033] Step 15: Use the SDEG state value (stiffness reduction rate) to characterize the damage initiation and micro-annular gap generation process at the cement sheath-casing interface. If SDEG = 0, the interface is intact. If 0 < SDEG < 1, the interface has not generated micro-annular gaps but damage has initiated. If SDEG = 1 and element deletion occurs, the interface generates micro-annular gaps.

[0034] Step 16: Apply loads to the two models to obtain the minimum load peak value for cement sheath crack initiation and the minimum load peak value for cement sheath-casing interface peeling. Determine the type of failure that occurs first in the "casing-cement sheath-casing" mechanical model based on the magnitude of the minimum load peak value. If the cement sheath body fails first, proceed to Step 17. If the cement sheath-casing interface fails first, proceed to Step 18.

[0035] Step 17: Based on Model 1 in Step 8, use the amplitude function (in tabular form) to apply loads with different peak values ​​and different number of cycles, keeping the number of load cycles the same, to obtain the minimum peak load corresponding to the initiation of the cement sheath crack. Keeping the peak load value the same, obtain the minimum number of load cycles corresponding to the initiation of the cement sheath crack, and record the propagation process of the cement sheath crack as the peak load value increases and the number of cycles increases, then stop the calculation.

[0036] Step 18: Based on Model 1 in Step 9, using the amplitude function (tabular form), loads with different peak values ​​and different number of cycles are applied respectively, keeping the number of load cycles the same, to obtain the minimum peak load for the initiation of damage and the generation of micro-annular gaps at the cement sheath-casing interface. Keeping the peak load value the same, the minimum number of cycles for the initiation of damage and the generation of micro-annular gaps at the cement sheath-casing interface is obtained, and the expansion process of micro-annular gaps at the cement sheath-casing interface is recorded as the peak load value increases and the number of cycles increases, and the calculation is stopped.

[0037] Step 19: Determine the environmental loads for cement sheath body failure and cement sheath-casing interface failure through experiments: Apply alternating temperature / pressure loads to the inside of the casing-cement sheath-casing assembly, and record the minimum number of cycles required for cement sheath body integrity failure and interface integrity failure under different loads. Also record the propagation process of cement sheath cracks and the development process of micro-annular gaps as the load and number of cycles increase.

[0038] Step 20: Compare the finite element simulation results with the full-scale experimental results to verify and improve the three-dimensional mechanical model of the "sleeve-cement ring-sleeve" assembly.

[0039] The present invention has the following advantages:

[0040] Based on elastoplastic theory and plastic damage theory, this study utilizes the extended finite element method (XFEM), concrete damage plasticity model, Cohesive element method, and bilinear traction-separation criterion to provide a systematic approach. This approach enables the simulation of the failure process of the cement ring body and the cement ring-sleeve interface in the sleeve-cement ring-sleeve assembly under alternating loads. This provides new methods and basis for indoor integrity evaluation research, cement slurry system evaluation and improvement research, and even for taking targeted measures on site. Attached Figure Description

[0041] Figure 1 This is a technical roadmap for the present invention.

[0042] Figure 2 This is a schematic diagram of the bilinear traction-separation criterion.

[0043] Figure 3To verify the comparison between simulation results and experimental results. Detailed Implementation

[0044] The present invention will now be described in detail with reference to the embodiments.

[0045] This invention proposes a numerical simulation method for the integrity of the cement sheath system in oil and gas wells under alternating temperature and pressure conditions. The method mainly includes the following steps:

[0046] Step 1: Taking a common cement ring with an outer diameter of 152.5 mm and a wall thickness of 12.75 mm, a P110 production casing with an outer diameter of 127.0 mm and a wall thickness of 12.14 mm, and a P110 technical casing with an outer diameter of 177.8 mm and a wall thickness of 12.65 mm as examples, experimental samples for mechanical property testing of the "casing-cement ring-casing" assembly were prepared. Using a cement slurry system from an oilfield in western my country, the "casing-cement ring-casing" assembly was prepared under normal temperature and pressure curing conditions, and mechanical property testing experiments were conducted on the "casing-cement ring-casing" assembly. The bonding strength of the cement ring-casing interface was found to be 1.2 MPa.

[0047] Step 2: Preparation of experimental samples for testing the mechanical properties of cement stone: Based on the GB 19139-2012 standard, standard cement stone samples (25 mm in diameter and 50 mm in height) were prepared using a cement slurry system from an oilfield in western my country under normal temperature and pressure curing conditions.

[0048] Step 3: Test the mechanical properties of cement stone: According to the "Standard for Test Methods of Engineering Rock Mass" GB / T 50266-2013, determine the uniaxial compression / tension and triaxial compression stress-strain curves of cement stone; (2) Calculate the uniaxial compressive strength of cement stone as 20.1 MPa, uniaxial tensile strength as 0.91 MPa and triaxial compressive strength as 49.12 MPa, elastic modulus as 5.61 GPa and Poisson's ratio as 0.13; (3) Based on the stress-strain curve, calculate the compressive inelastic strain of cement stone using the formula for calculating the inelastic strain under compression and the formula for the cracking strain under tension, respectively. Tensile crack strain

[0049] Step 4: Based on the compressive inelastic strain obtained in Step 3 and tensile crack strain The compressive damage factor d is calculated using the formulas for compressive damage factor (Formula 3) and tensile damage factor (Formula 4), respectively. c Tensile damage factor d t .

[0050] Step 5: Based on the elastic-plastic theory and the plastic damage theory, use ABAQUS finite element software to establish two three-dimensional mechanical models of the "sleeve-cement ring-sleeve" assembly, Model 1 and Model 2, respectively, to simulate the failure process of the cement ring body and the failure process of the cement ring-sleeve interface.

[0051] Step 6: Use C3D8R elements (eight-node linear hexahedral elements with reduced integrals) to sweep and mesh the model, and refine the cement ring part by local densification seeding.

[0052] Step 7: Define boundary conditions and loads. Apply a fully fixed constraint to the outer boundary of the technical casing and apply a uniform alternating load to the inner wall of the production casing.

[0053] Step 8: Input the stress-strain constitutive relationship of the casing material and define the mechanical properties of the casing material, including elastic modulus of 210 GPa and Poisson's ratio of 0.3.

[0054] Step 9: Based on Model 1 established in Step 5, define the mechanical properties of the cement ring, including plastic damage model parameters, elastic modulus of 5.61 GPa and Poisson's ratio of 0.13. Use the extended finite element method and concrete damage plasticity model to study the crack initiation and crack propagation process of the cement ring body.

[0055] Step 10: The Maxpe damage mode based on energy and linear softening is used to control the damage evolution law of the cement ring body.

[0056] Step 11: Based on Model 2 established in Step 5, add a Cohesive unit layer of type COH3D8 to the cement ring-sleeve interface, and define the mechanical properties of the Cohesive layer, including the interfacial bonding strength of 1.2 MPa.

[0057] Step 12: Select the maximum nominal stress criterion to control the initial damage of the Cohesive element, and select the BK fracture criterion to control the damage evolution process of the Cohesive element.

[0058] Step 13: The maximum nominal stress criterion, BK fracture criterion, and Maxs damage mode based on energy and linear softening are used to control the interface damage evolution and micro-annulus formation mechanism.

[0059] Step Fourteen: Use DAMAGEC (compressive damage value) and DAMAGET (tensile damage value) to characterize the degree of compressive and tensile damage to the cement ring body. DAMAGEC and DAMAGET are between 0 and 1. If DAMAGEC or DAMAGET = 0, the cement ring body has not experienced compressive or tensile damage. If DAMAGEC or DAMAGET > 0 and is close to 1, the cement ring body has reached the compressive or tensile damage limit. Use STATUSXFEM state value (state of xfem element) to characterize the crack initiation and propagation process of the cement ring. STATUSXFEM state value is between 0 and 1. If STATUSXFEM = 0, the cement ring is intact and has no cracks. If 0 < STATUSXFEM < 1, the cement ring is partially cracked and in the propagation stage. If STATUSXFEM = 1, the cement ring is completely cracked.

[0060] Step 15: Use the SDEG state value (stiffness reduction rate) to characterize the damage initiation and micro-annular gap generation process at the cement sheath-casing interface. If SDEG = 0, the interface is intact; if 0 < SDEG < 1, the interface has not generated micro-annular gaps but damage has initiated; if SDEG = 1 and element deletion occurs, the interface generates micro-annular gaps.

[0061] Step 16: Keep the number of cycles at 10, apply loads to the two models, and obtain the minimum load peak value of 50MPa for the cement sheath to crack and the minimum load peak value of 60MPa for the cement sheath-casing interface to peel off. Based on the magnitude of the minimum load peak value, it is determined that the cement sheath body fails first in the "casing-cement sheath-casing" mechanical model.

[0062] Step 17: Based on Model 1 from Step 8, using the amplitude function (in tabular form), loads with different peak values ​​and different number of cycles are applied respectively. Keeping the number of load cycles the same, the minimum peak load for cement sheath crack initiation is obtained as 30 MPa and the corresponding number of cycles is 23. Keeping the peak load value the same, the minimum number of load cycles for cement sheath crack initiation is obtained as 3 and the corresponding peak load value is 80 MPa. Record the crack propagation process of the cement sheath as the peak load value and number of cycles increase, then stop the calculation.

[0063] Step 18: Based on Model 2 from Step 9, using the amplitude function (tabular form), loads with different peak values ​​and different cycle numbers are applied, keeping the load cycle number the same. The minimum peak load of 30 MPa for the initiation of damage and the formation of micro-annular gaps at the cement sheath-casing interface, and the corresponding cycle numbers of 9 and 25, are obtained. Keeping the peak load value the same, the minimum cycle numbers for the initiation of damage and the formation of micro-annular gaps at the cement sheath-casing interface are obtained, which are 1 and 4, respectively, with the corresponding peak load value of 80 MPa. The expansion process of the micro-annular gaps at the cement sheath-casing interface is recorded as the peak load value and cycle number increase, and then the calculation is stopped.

[0064] Step 19: Experimentally determine the environmental loads for cement sheath failure and cement sheath-casing interface failure: Apply alternating temperature / pressure loads to the inside of the casing-cement sheath-casing assembly, taking alternating pressures of 0~30MPa~0, 0~40MPa~0, 0~50MPa~0, 0~60MPa~0, 0~70MPa~0, and 0~80MPa~0 as examples. Record the minimum peak load of 30MPa for cement sheath failure and the corresponding number of cycles of 25, the minimum number of cycles of 5 for cement sheath failure and the corresponding peak load of 80MPa, the minimum peak load of 30MPa for micro-annular gap formation at the cement sheath-casing interface and the corresponding number of cycles of 30, and the minimum number of cycles of 5 for micro-annular gap formation at the interface and the corresponding peak load of 80MPa. Record the propagation process of cement sheath cracks and the development process of micro-annular gaps as the peak load increases or the number of cycles increases.

[0065] Step 20: The finite element simulation results were compared with the full-scale experimental results to verify the accuracy and reliability of the three-dimensional mechanical model of the "sleeve-cement ring-sleeve" assembly. Figure 3 As shown.

Claims

1. A numerical simulation method for the integrity of an oil and gas well cement sheath system under alternating temperature and pressure, characterized in that, The numerical simulation method for the integrity of the cement sheath system in oil and gas wells under alternating temperature and pressure mainly includes the following steps: Step 1: Preparation of test samples for mechanical property testing of the "casing-cement ring-casing": Using an on-site cement grout system, the "casing-cement ring-casing" assembly was prepared under normal temperature and pressure curing conditions. Mechanical property testing experiments were then conducted on the "casing-cement ring-casing" assembly to obtain the cement ring-casing interface bonding strength t. n ; Step 2: Preparation of test samples for testing the mechanical properties of cement stone: Using an on-site cement slurry system, standard cement stone samples with a diameter of 25 mm and a height of 50 mm were prepared under normal temperature and pressure curing conditions. Step 3: Test the mechanical properties of cement stone: (1) Determine the uniaxial compression / tension and triaxial compression stress-strain curves of cement stone according to the "Standard for Test Methods of Engineering Rock Mass" GB / T 50266-2013; (2) Calculate the uniaxial compressive / tensile strength, triaxial compressive strength, elastic modulus E0, and Poisson's ratio μ0 of cement stone; (3) Based on the stress-strain curves, calculate the compressive inelastic strain using the formula (1) for calculating the inelastic strain under compression and the formula (2) for calculating the cracking strain under tension of cement stone. Tensile crack strain In the formula, For compressive inelastic strain; σ c ε c Compressive stress and compressive strain are defined in GB50010—2012. This represents the elastic compressive strain corresponding to the initial elastic modulus. This represents the plastic strain under compression. In the formula, For tensile cracking strain; σ t ε t The tensile stress and tensile strain are specified in GB50010—2012. This represents the elastic tensile strain corresponding to the initial elastic modulus. This represents the plastic strain under tension. Step 4: Based on the compressive inelastic strain obtained in Step 3 and tensile crack strain The compressive damage factor d is calculated using the compressive damage factor calculation formula (3) and the tensile damage factor calculation formula (4), respectively. c Tensile damage factor d t ; In the formula, σ c ε c These are the compressive stress and compressive strain as defined in GB50010—2012. It is an inelastic strain under compression; This represents the plastic strain under compression. In the formula, σ t ε t These are the tensile stress and tensile strain specified in GB50010—2012, respectively. For tensile cracking strain; This represents the plastic strain under tension. Step 5: Based on the elastic-plastic theory and the plastic damage theory, use ABAQUS finite element software to establish two three-dimensional mechanical models of the "casing-cement ring-casing" assembly. Model 1 and Model 2 are used to carry out simulation analysis of cement ring body integrity failure and cement ring-casing interface integrity failure, respectively. Step 6: Use C3D8R elements, that is, eight-node linear hexahedral elements with reduced integrals, to sweep and divide the model mesh, and refine the cement ring part by local densification seeding. Step 7: Define boundary conditions and loads. Apply a fully fixed constraint to the outer boundary of the technical casing and apply a uniform alternating load to the inner wall of the production casing. Step 8: Input the stress-strain constitutive relationship of the casing material and define the mechanical properties of the casing material; Step 9: Based on Model 1 established in Step 5, define the plastic damage model parameters, elastic modulus and Poisson's ratio of the cement ring, and use the extended finite element method and concrete damage plastic model to study the crack initiation and crack propagation process of the cement ring body. Step 10: The Maxpe damage mode based on energy and linear softening is used to control the damage evolution law of the cement ring body. The displacement u(x) of any node of the crack is calculated by approximation formula (5). In the formula, u i It is the nodal displacement; a j b kl It is a node-strengthening variable; N i N j N k I is the nodal shape function; J is the set of all nodes in the discrete domain; and J is the set of nodes through which the crack completely penetrates the element. Step 11: Based on Model 2 established in Step 5, add a Cohesive unit layer of type COH3D8 to the cement ring-sleeve interface, and define the mechanical properties of the Cohesive layer, including the interfacial bonding strength. Step 12: Select the maximum nominal stress criterion to control the initial damage of the Cohesive element, and select the BK fracture criterion to control the damage evolution process of the Cohesive element; Step 13: The maximum nominal stress criterion, BK fracture criterion, and Maxs damage mode based on energy and linear softening are used to control the interface damage evolution and micro-annulus formation mechanism. Step Fourteen: The compressive damage value DAMAGEC and the tensile damage value DAMAGET are used to characterize the degree of compressive and tensile damage to the cement ring body. DAMAGEC and DAMAGET are between 0 and 1. If DAMAGEC or DAMAGET = 0, the cement ring body has not experienced compressive or tensile damage. If DAMAGEC or DAMAGET > 0 and is close to 1, the cement ring body has reached the compressive or tensile damage limit. The STATUSXFEM state value, i.e., the state of the xfem element, is used to characterize the crack initiation and propagation process of the cement ring. The STATUSXFEM state value is between 0 and 1. If STATUSXFEM = 0, the cement ring is intact and has no cracks. If 0 < STATUSXFEM < 1, the cement ring is partially initiating and propagating cracks. If STATUSXFEM = 1, the cement ring is completely cracked. Step 15: Use the SDEG state value, i.e. stiffness reduction rate, to characterize the damage initiation and micro-annular gap generation process at the cement sheath-casing interface. If SDEG = 0, the interface is intact; if 0 < SDEG < 1, the interface has not generated micro-annular gaps but damage has initiated; if SDEG = 1 and element deletion occurs, the interface generates micro-annular gaps. Step 16: Apply loads to the two models to obtain the minimum load peak value for cement sheath crack initiation and the minimum load peak value for cement sheath-sleeve interface peeling. Determine the type of failure that occurs first in the "sleeve-cement sheath-sleeve" mechanical model based on the magnitude of the minimum load peak value. If the cement sheath body integrity fails first, proceed to Step 17. If the cement sheath-sleeve interface integrity fails first, proceed to Step 18. Step 17: Based on Model 1 in Step 8, use the tabular amplitude function to apply loads with different peak values ​​and different number of cycles, keeping the number of load cycles the same, to obtain the minimum peak load corresponding to the initiation of the cement sheath crack. Keeping the peak load the same, obtain the minimum number of load alternations corresponding to the initiation of the cement sheath crack, and record the propagation process of the cement sheath crack as the peak load increases and the number of cycles increases, then stop the calculation. Step 18: Based on Model 2 in Step 9, using the tabular amplitude function, loads with different peak values ​​and different number of cycles are applied respectively, keeping the number of load cycles the same, to obtain the minimum peak load for the initiation of damage and the generation of micro-annular gaps at the cement sheath-casing interface. Keeping the peak load value the same, the minimum number of cycles for the initiation of damage and the generation of micro-annular gaps at the cement sheath-casing interface is obtained, and the expansion process of micro-annular gaps at the cement sheath-casing interface is recorded as the peak load value increases and the number of cycles increases, and the calculation is stopped. Step 19: Determine the environmental load for cement sheath body failure and cement sheath-casing interface failure through experiments: Apply alternating temperature / pressure loads to the inside of the casing-cement sheath-casing assembly, and record the minimum number of cycles required for cement sheath body integrity failure and interface integrity failure under different loads. Also record the propagation process of cement sheath cracks and the development process of micro-annular gaps as the load and number of cycles increase. Step 20: Compare the finite element simulation results with the experimental results to verify and improve the three-dimensional mechanical model of the "casing-cement ring-casing" assembly.

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