Numerical simulation method for integrity of oil-gas well cement sheath system under action of temperature-pressure alternation

Abstract

A numerical simulation method for integrity of an oil and gas well cement sheath system under the action of temperature-pressure alternation is characterized in that an extended finite element method (XFEM), a concrete damage plastic model theory, a Cohesive element method and a bilinear traction separation criterion are used in a finite element simulation method on the basis of an elastic-plastic theory and a plastic damage theory; the failure process of a cement sheath-casing interface under the temperature-pressure alternating action can be simulated, the mechanical behavior of interface failure can be represented, the failure process of a cement sheath body can be simulated, the mechanical behavior of body failure can be represented, and the failure behavior of oil and gas well cement sheath system integrity can be comprehensively analyzed. Real evaluation, prediction and diagnosis of the integrity of the casing-cement sheath-casing combination system under the temperature-pressure alternating action are achieved, a cement slurry system can be optimized, the well cementation quality is improved, and failure of the integrity of the cement sheath is prevented. The method is suitable for the technical field of petroleum and natural gas drilling engineering.

Description

Technical Field

[0001] The invention relates to the technical field of oil and natural gas drilling and production engineering, in particular to a numerical simulation method for the integrity of a cement ring system of an oil and gas well under the action of temperature-pressure alternation. Background Art

[0002] Cementing is one of the most important links in oil and gas well drilling projects. Its main purpose is to isolate the oil layer, gas layer and water layer in the wellbore, protect the casing of the oil and gas well, increase the life of the oil and gas well and increase the oil and gas production. With the development of my country's oil and gas industry, the connotation of wellbore integrity is also developing, and the requirements for wellbore integrity are also higher, including cement sheath integrity.

[0003] In order to stabilize and increase production, various oil fields have successively carried out downhole engineering operations such as water injection, fracturing, acidizing, and plugging. Different operation processes will inevitably change the wellbore conditions. The changes in casing internal pressure, formation surrounding rock pressure, and temperature stress caused by changes in wellbore temperature will change the stress state of the cement sheath, which may cause cracks in the cement sheath body, micro-annulus at the cement sheath-casing interface or cement sheath-formation interface, making the cement sheath's sealing effect ineffective.

[0004] When the integrity of the cement ring fails, its sealing performance will be greatly reduced, which will lead to many complex working conditions, such as: annulus pressure, reduced casing life, and oil, gas and water intercommunication. In order to reduce the problems caused by the failure of cement ring integrity, scholars have analyzed its failure process through theoretical models and finite element analysis. However, there is no systematic simulation method for the finite element analysis of cement ring system integrity failure. To this end, the present invention proposes a numerical simulation method for the integrity of the cement ring system of oil and gas wells under the action of temperature-pressure alternation.

[0005] The purpose of the present invention is to provide a numerical simulation method for the integrity of the cement ring system of oil and gas wells under the action of temperature-pressure alternation, which can not only simulate the failure process of the cement ring-casing interface under the action of temperature-pressure alternation and characterize the mechanical behavior of interface failure, but also simulate the failure process of the cement ring body and characterize the mechanical behavior of body failure, comprehensively reflect the failure behavior of the integrity of the cement ring system of oil and gas wells, realize the real evaluation, prediction and diagnosis of the integrity of the casing-cement ring-formation combination system under the action of temperature-pressure alternation, and help to optimize the cement slurry system, improve the cementing quality, and prevent the failure of cement ring integrity.

[0006] To achieve the above-mentioned purpose, the present invention discloses a numerical simulation method for the integrity of the cement ring system of oil and gas wells under the action of temperature-pressure alternation, characterized in that the finite element simulation method uses the extended finite element method (XFEM), concrete damage plasticity model, cohesive unit method and bilinear traction-separation criterion on the basis of elastoplastic theory and plastic damage theory. Based on the elastic-plastic theory and plastic damage theory, a three-dimensional mechanical model of the "casing-cement sheath-casing" combination is established; the extended finite element method and concrete damage plasticity model are used to simulate and analyze the cracking, expansion and complete failure process of the cement sheath body, keeping the same number of load alternations, and obtaining the minimum load peak value corresponding to the initiation, expansion and complete failure of the cement sheath crack; keeping the same load peak value, and obtaining the minimum load alternation number corresponding to the initiation, expansion and complete failure of the cement sheath crack; the cohesive unit method and bilinear traction-separation criterion are used to simulate the failure process of cement sheath-casing interface damage initiation and micro-annulus formation, keeping the same number of load alternations, and obtaining the minimum load peak value corresponding to the initiation and micro-annulus formation of the cement sheath-casing interface damage; keeping the same load peak value, and obtaining the minimum load alternation number corresponding to the initiation and micro-annulus formation of the cement sheath-casing interface damage. The mechanism of cracking, expansion and complete failure of the cement sheath body and the mechanism of cement sheath-interface peeling and complete failure under the action of temperature-pressure alternation are studied, and the integrity of the cement sheath system of oil and gas wells under the action of temperature-pressure alternation is evaluated. The specific technical solutions adopted are as follows:

[0007] Step 1: Prepare samples for the mechanical properties test of "casing-cement sheath-casing": Use the on-site cement slurry system to prepare the "casing-cement sheath-casing" assembly under normal temperature and pressure curing conditions, and carry out the mechanical properties test experiment of the "casing-cement sheath-casing" assembly to obtain the cement sheath-casing interface bonding strength t n .

[0008] Step 2: Prepare experimental samples for cement stone mechanical properties testing: Use the on-site cement slurry system to prepare standard cement stone specimens (25 mm in diameter and 50 mm in height) under normal temperature and pressure curing conditions.

[0009] Step 3: Test the mechanical properties of cement paste: (1) According to the Engineering Rock Test Method Standard GB / T50266-2013, measure the uniaxial compression / tensile and triaxial compression stress-strain curves of cement paste; (2) Calculate the uniaxial compressive / tensile strength and triaxial compressive strength of cement paste, elastic modulus E 0 , Poisson's ratio μ 0 (3) Based on the stress-strain curve, the inelastic strain calculation formula for cement paste under compression and the cracking strain calculation formula for cement paste under tension are used to calculate the inelastic strain under compression and the cracking strain under tension respectively. Tensile cracking strain

[0010] The calculation formula of the inelastic strain of cement paste under compression is shown in Equation 1. In the formula, is the inelastic strain under compression; σ c , ε c are the compressive stress and compressive strain in GB50010-2012; is the elastic compressive strain corresponding to the initial elastic modulus; is the plastic strain under compression.

[0011] The calculation formula of tensile cracking strain of cement paste is shown in Equation 2. In the formula, is the tensile cracking strain; t , ε t is the tensile stress and tensile strain in GB50010-2012; is the elastic tensile strain corresponding to the initial elastic modulus; is the plastic strain under tension.

[0012] Step 4: Based on the compressive inelastic strain obtained in step 3 and tensile cracking strain The compressive damage factor d is calculated using the compressive damage factor formula and the tensile damage factor formula respectively. c , tensile damage factor d t .

[0013] The calculation formula of cement paste compressive damage factor is shown in formula 3. In the formula, σ c , ε c They are compressive stress and compressive strain in GB50010-2012 respectively; is the compressive inelastic strain; is the plastic strain under compression.

[0014] The calculation formula of cement paste tensile damage factor is shown in formula 4. In the formula, σ t , ε t They are the tensile stress and tensile strain in GB50010-2012 respectively; is the tensile cracking strain; is the plastic strain under tension.

[0015] Step 5: Based on the elastic-plastic theory and plastic damage theory, ABAQUS finite element software was used to establish two three-dimensional mechanical models of the "casing-cement sheath-casing" combination. Model 1 and Model 2 were used to carry out the simulation analysis of the integrity failure of the cement sheath body and the integrity failure simulation analysis of the cement sheath-casing interface, respectively.

[0016] Step 6: Use C3D8R elements (eight-node linear hexahedral elements with reduced integration) to sweep the model mesh and refine the cement sheath by locally refining the mesh.

[0017] Step 7: Define boundary conditions and loads. Apply full fixed constraints to the outer boundary of the technical casing and apply uniform alternating loads to the inner wall of the production casing.

[0018] Step 8: Input the stress-strain constitutive relationship of the casing material and define the mechanical properties of the casing material.

[0019] Step 9: Based on the model 1 established in step 5, define the plastic damage model parameters, elastic modulus and Poisson’s ratio of the cement sheath, and use the extended finite element method and concrete damage plasticity model to study the crack initiation and crack propagation process of the cement sheath body.

[0020] Step 10: Use the Maxpe damage model based on energy and linear softening to control the damage evolution law of the cement sheath, and use the XFEM type crack to characterize the crack of the cement sheath. The displacement u(x) of any node of the crack is approximated by the formula Among them, u i is the node displacement; a j , b kl is the node reinforcement variable; N i 、N j 、N k is the node shape function; I is the set of all nodes in the discrete domain; J is the set of unit nodes where the crack completely penetrates.

[0021] Step 11: Based on the model 2 established in step 5, add a cohesive unit layer of COH3D8 unit type at the cement sheath-casing interface, and define the mechanical properties of the cohesive layer, including the interface bonding strength. Step 12: Select the maximum nominal stress criterion Control the initial damage of the Cohesive unit. is the critical stress in the normal direction of the Cohesive Element, i.e., the interface bonding strength (MPa); is the first and second tangential critical stress of the unit (MPa); <> is the Macaulay bracket, indicating that the interface is not damaged under pure compressive stress state. Select BK fracture criterion Control the damage evolution process of the Cohesive unit. Among them, is the normal critical strain energy release rate at cracking (J / m 2 ); is the first and second tangential critical strain energy release rate at cracking (J / m 2 );G n , G s , G t is the strain energy release rate in the crack normal and two tangential directions (J / m 2 ), β is the material parameter.

[0022] Step 13: Use the maximum nominal stress criterion, BK fracture criterion and Maxs damage mode based on energy and linear softening to control the interface damage evolution and micro-annulus formation mechanism.

[0023] Step 14: Use DAMAGEC (compressive damage value) and DAMAGET (tensile damage value) to characterize the compressive damage degree and tensile damage degree of the cement sheath body. DAMAGEC and DAMAGET are between 0 and 1. If DAMAGEC or DAMAGET = 0, the cement sheath body does not suffer from compressive or tensile damage. If DAMAGEC or DAMAGET > 0 and close to 1, the cement sheath body reaches the compressive or tensile damage limit.

[0024] Step 15: Use the STATUSXFEM state value (the state of the XFEM unit) to characterize the initiation and expansion process of the cement sheath crack. The STATUSXFEM state value is between 0 and 1. If STATUSXFEM=0, the cement sheath is intact and has no cracks. If 0<STATUSXFEM<1, the cement sheath partially cracks and expands. If STATUSXFEM=1, the cement sheath is completely cracked.

[0025] Step 16: Use the SDEG state value (stiffness decrease rate) to characterize the process of damage initiation and micro-annulus generation at the cement sheath-casing interface. If SDEG = 0, the interface is intact. If 0 < SDEG < 1, no micro-annulus is generated at the interface but damage is initiated. If SDEG = 1 and unit deletion occurs, micro-annulus is generated at the interface.

[0026] Step 17: Apply loads to the two models to obtain the minimum load peak for cement sheath cracking and the minimum load peak for cement sheath-casing interface peeling. According to the size of the minimum load peak, determine the type of failure that occurs first in the "casing-cement sheath-casing" mechanical model. If the cement sheath body fails first, proceed to step 18; if the cement sheath-casing interface fails first, proceed to step 19.

[0027] Step 18: Based on model 1 in step 8, use the amplitude function (tabular form) to load loads with different peak values ​​and different number of cycles respectively, keep the number of load cycles the same, obtain the minimum load peak value corresponding to the initiation of cement sheath cracks, keep the load peak value the same, obtain the minimum load alternation number corresponding to the initiation of cement sheath cracks, and record the expansion process of cement sheath cracks as the load peak value increases and the number of cycles increases, and stop the calculation.

[0028] Step 19: Based on model 1 in step 9, use the amplitude function (tabular form) to load loads with different peak values ​​and different number of cycles respectively, keep the number of load cycles the same, and obtain the minimum load peak value for the initiation of damage at the cement sheath-casing interface and the generation of micro-annulus. Keep the load peak value the same, and obtain the minimum number of cycles for the initiation of damage at the cement sheath-casing interface and the generation of micro-annulus. Record the expansion process of the micro-annulus at the cement sheath-casing interface as the load peak value increases and the number of cycles increases, and stop the calculation.

[0029] Step 20: Determine the environmental loads for failure of the cement sheath body and cement sheath-casing interface 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 failure of the integrity of the cement sheath body and the minimum number of cycles required for failure of the integrity of the interface under different loads, and record the expansion process of the cement sheath crack and the development process of the micro-annulus as the load increases and the number of cycles increases.

[0030] Step 21: Compare the finite element simulation results with the full-scale experimental results to verify and improve the three-dimensional mechanical model of the "casing-cement sheath-casing" assembly.

[0031] The present invention has the following advantages:

[0032] On the basis of elastoplastic theory and plastic damage theory, the extended finite element method (XFEM), concrete damage plasticity model, cohesive unit method and bilinear traction-separation criterion were used to provide a systematic method to simulate the failure process of the cement sheath body and the failure process of the cement sheath-casing interface in the casing-cement sheath-casing combination under alternating loads, providing new methods and basis for indoor integrity evaluation research, cement slurry system evaluation and improvement research, and even taking targeted measures on site. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is the technical roadmap of the present invention.

[0034] Figure 2 It is a schematic diagram of the bilinear pull-separation criterion.

[0035] Figure 3 To compare and verify the simulation results with the experimental results. DETAILED DESCRIPTION

[0036] The present invention is described in detail below in conjunction with embodiments.

[0037] The present invention proposes a numerical simulation method for the integrity of a cement sheath system in an oil and gas well under the action of temperature-pressure alternation, and the method mainly comprises the following steps:

[0038] Step 1: Taking ordinary cement ring with an outer diameter of 152.5mm and a wall thickness of 12.75mm, P110 production casing with an outer diameter of 127.0mm and a wall thickness of 12.14mm and P110 technical casing with an outer diameter of 177.8mm and a wall thickness of 12.65mm as examples, prepare experimental samples for carrying out mechanical properties test of "casing-cement ring-casing" assembly: adopt the cement slurry system of an oil field in western my country, prepare "casing-cement ring-casing" assembly under normal temperature and pressure curing conditions, and carry out mechanical properties test experiment of "casing-cement ring-casing" assembly, and obtain the cement ring-casing interface bonding strength of 1.2MPa.

[0039] Step 2: Prepare experimental samples for testing the mechanical properties of cement paste: Based on GB 19139-2012 standard, a cement slurry system of an oil field in western my country was used to prepare standard cement paste specimens (25 mm in diameter and 50 mm in height) under normal temperature and pressure curing conditions.

[0040] Step 3: Test the mechanical properties of cement paste: According to the Standard for Engineering Rock Mass Test Methods GB / T 50266-2013, the uniaxial compression / tensile and triaxial compression stress-strain curves of cement paste were measured; (2) The uniaxial compressive strength of cement paste was calculated to be 20.1 MPa, the uniaxial tensile strength was 0.91 MPa, the triaxial compressive strength was 49.12 MPa, the elastic modulus was 5.61 GPa, and the Poisson's ratio was 0.13; (3) Based on the stress-strain curve, the compressive inelastic strain calculation formula of cement paste and the tensile cracking strain calculation formula were used to calculate the compressive inelastic strain respectively. Tensile cracking strain

[0041] Step 4: Based on the compressive inelastic strain obtained in step 3 and tensile cracking strain The compressive damage factor d is calculated using the compressive damage factor calculation formula (Formula 3) and the tensile damage factor calculation formula (Formula 4) respectively. c , tensile damage factor d t .

[0042] Step 5: Based on the elastic-plastic theory and plastic damage theory, ABAQUS finite element software is used to establish the three-dimensional mechanical model 1 and model 2 of the "casing-cement sheath-casing" combination, which are used to simulate the failure process of the cement sheath body and the failure process of the cement sheath-casing interface, respectively.

[0043] Step 6: Use C3D8R elements (eight-node linear hexahedral elements with reduced integration) to sweep the model mesh and refine the cement sheath by locally refining the mesh.

[0044] Step 7: Define boundary conditions and loads. Apply full fixed constraints to the outer boundary of the technical casing and apply uniform alternating loads to the inner wall of the production casing.

[0045] Step 8: Input the stress-strain constitutive relationship of the casing material and define the mechanical properties of the casing material, including the elastic modulus of 210 GPa and the Poisson's ratio of 0.3.

[0046] Step 9: Based on Model 1 established in Step 5, define the mechanical properties of the cement sheath, including the plastic damage model parameters, elastic modulus of 5.61 GPa, and Poisson’s ratio of 0.13, and use the extended finite element method and concrete damage plasticity model to study the crack initiation and crack propagation processes of the cement sheath body.

[0047] Step 10: Use the Maxpe damage model based on energy and linear softening to control the damage evolution law of the cement sheath body.

[0048] Step 11: Based on Model 2 established in Step 5, add a Cohesive unit layer of COH3D8 unit type at the cement sheath-casing interface, and define the mechanical properties of the Cohesive layer, including the interface bonding strength of 1.2 MPa.

[0049] Step 12: Use the maximum nominal stress criterion to control the initial damage of the Cohesive unit, and use the BK fracture criterion to control the damage evolution process of the Cohesive unit.

[0050] Step 13: Use the maximum nominal stress criterion, BK fracture criterion and Maxs damage mode based on energy and linear softening to control the interface damage evolution and micro-annulus formation mechanism.

[0051] Step 14: Use DAMAGEC (compressive damage value) and DAMAGET (tensile damage value) to characterize the compressive damage degree and tensile damage degree of the cement sheath body. DAMAGEC and DAMAGET are between 0 and 1. If DAMAGEC or DAMAGET = 0, the cement sheath body does not suffer from compressive or tensile damage. If DAMAGEC or DAMAGET > 0 and close to 1, the cement sheath body reaches the compressive or tensile damage limit.

[0052] Step 15: Use the STATUSXFEM state value (the state of the xfem unit) to characterize the initiation and expansion process of the cement sheath crack. The STATUSXFEM state value is between 0 and 1. If STATUSXFEM=0, the cement sheath is intact and has no cracks. If 0<STATUSXFEM<1, the cement sheath partially cracks and is in the expansion stage. If STATUSXFEM=1, the cement sheath is completely cracked.

[0053] Step 16: Use the SDEG state value (stiffness decrease rate) to characterize the process of damage initiation and micro-annulus generation at the cement sheath-casing interface. If SDEG = 0, the interface is intact. If 0 < SDEG < 1, no micro-annulus is generated at the interface but damage is initiated. If SDEG = 1 and unit deletion occurs, micro-annulus is generated at the interface.

[0054] Step 17: Keep the number of cycles at 10 times, apply loads to the two models, and obtain the minimum load peak value of 50MPa for cement sheath cracking and the minimum load peak value of 60MPa for cement sheath-casing interface peeling. According to the size of the minimum load peak value, it is judged that the cement sheath body failure occurs first in the "casing-cement sheath-casing" mechanical model.

[0055] Step 18: Based on model 1 in step 8, use the amplitude function (table form) to load different peak values ​​and different number of cycles, keep the number of load cycles the same, and obtain the minimum load peak value of 30MPa and the corresponding number of cycles 23 for the initiation of cement sheath cracks. Keep the load peak value the same, and obtain the minimum load cycle number of 3 times and the corresponding load peak value 80MPa for the initiation of cement sheath cracks. Record the expansion process of cement sheath cracks as the load peak value and the number of cycles increase, and stop the calculation.

[0056] Step 19: Based on model 2 in step 9, use the amplitude function (tabular form) to load loads with different peak values ​​and different number of cycles respectively, keep the number of load cycles the same, and obtain the minimum load peak value of 30MPa and the corresponding number of cycles of 9 and 25 times for the initiation of damage and the generation of micro-annulus at the cement sheath-casing interface. Keep the load peak value the same, and obtain the minimum number of cycles for the initiation of damage and the generation of micro-annulus at the cement sheath-casing interface, which are 1 and 4 times respectively, and the corresponding load peak value of 80MPa. At the same time, record the expansion process of the micro-annulus at the cement sheath-casing interface as the load peak value and the number of cycles increase, and stop the calculation.

[0057] Step 20: Determine the environmental loads for failure of the cement sheath body and failure of the cement sheath-casing interface through experiments: 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 load peak of 30MPa for failure of the cement sheath body and the corresponding number of cycles of 25 times, the minimum number of cycles for failure of the cement sheath body of 5 times and the corresponding load peak of 80MPa, the minimum load peak of 30MPa for micro-annulus at the cement sheath-casing interface and the corresponding number of cycles of 30 times, the minimum number of cycles for micro-annulus at the interface of 5 times and the corresponding load peak of 80MPa, and record the expansion process of cement sheath cracks and the development process of micro-annulus as the load peak increases or the number of cycles increases.

[0058] Step 21: Compare the finite element simulation results with the full-scale experimental results to verify the accuracy and reliability of the three-dimensional mechanical model of the "casing-cement sheath-casing" combination, such as Figure 3 shown.

Claims

1. A numerical simulation method for the integrity of cement sheath system in oil and gas wells under temperature-pressure alternation, characterized in that: The method for numerically simulating the integrity of the cement sheath system of oil and gas wells under the action of temperature-pressure alternation mainly comprises the following steps: Step 1: Prepare samples for the mechanical properties test of "casing-cement sheath-casing": Use the on-site cement slurry system to prepare the "casing-cement sheath-casing" assembly under normal temperature and pressure curing conditions, and conduct the mechanical properties test experiment of the "casing-cement sheath-casing" assembly to obtain the cement sheath-casing interface bonding strength t n ; Step 2: Prepare experimental samples for testing the mechanical properties of cement paste: Use the on-site cement slurry system to prepare standard cement paste specimens (25 mm in diameter and 50 mm in height) under normal temperature and pressure curing conditions; Step 3: Test the mechanical properties of cement paste: (1) According to the Engineering Rock Test Method Standard GB / T 50266-2013, measure the uniaxial compression / tensile and triaxial compression stress-strain curves of cement paste; (2) Calculate the uniaxial compressive / tensile strength and triaxial compressive strength, elastic modulus E0, and Poisson's ratio μ0 of cement paste; (3) Based on the stress-strain curve, calculate the compressive inelastic strain of cement paste using the compressive inelastic strain calculation formula (1) and the tensile cracking strain formula (2) Tensile cracking strain In the formula, is the inelastic strain under compression; σ c , ε c are the compressive stress and compressive strain in GB50010-2012; is the elastic compressive strain corresponding to the initial elastic modulus; is the plastic strain under compression; In the formula, is the tensile cracking strain; t , ε t is the tensile stress and tensile strain in GB50010-2012; is the elastic tensile strain corresponding to the initial elastic modulus; is the plastic strain under tension; Step 4: Based on the compressive inelastic strain obtained in step 3 and tensile cracking 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 They are compressive stress and compressive strain in GB50010-2012 respectively; is the compressive inelastic strain; is the plastic strain under compression; In the formula, σ t , ε t They are the tensile stress and tensile strain in GB50010-2012 respectively; is the tensile inelastic strain; is the plastic strain under tension; Step 5: Based on the elastic-plastic theory and plastic damage theory, ABAQUS finite element software is used to establish the three-dimensional mechanical model 1 and model 2 of the "casing-cement sheath-casing" combination. Model 1 and model 2 are used to carry out the simulation analysis of the integrity failure of the cement sheath body and the integrity failure simulation analysis of the cement sheath-casing interface respectively; Step 6: Use C3D8R elements (eight-node linear hexahedral elements with reduced integration) to sweep the model mesh and refine the cement sheath by locally refining the mesh. Step 7: Define boundary conditions and loads. Apply full fixed constraints to the outer boundary of the technical casing and apply uniform alternating loads 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 the model 1 established in step 5, define the plastic damage model parameters, elastic modulus and Poisson's ratio of the cement sheath, and use the extended finite element method and concrete damage plasticity model to study the crack initiation and crack propagation process of the cement sheath body; Step 10: The Maxpe damage model based on energy and linear softening is used to control the damage evolution law of the cement sheath body. The displacement u(x) of any crack node is calculated using the approximate formula (5); In the formula, u i is the node displacement; a j , b kl is the node reinforcement variable; N i 、N j 、N k is the node shape function; I is the set of all nodes in the discrete domain; J is the set of nodes where the crack completely penetrates the unit; Step 11: Based on the model 2 established in step 5, add a cohesive unit layer of COH3D8 unit type at the cement sheath-casing interface, and define the mechanical properties of the cohesive layer, including the interface bonding strength; Step 12: Use the maximum nominal stress criterion to control the initial damage of the Cohesive unit, and use the BK fracture criterion to control the damage evolution process of the Cohesive unit; Step 13: Use the maximum nominal stress criterion, BK fracture criterion and Maxs damage mode based on energy and linear softening to control the interface damage evolution and micro-annulus formation mechanism; Step 14: Use DAMAGEC (compressive damage value) and DAMAGET (tensile damage value) to characterize the compressive damage degree and tensile damage degree of the cement sheath body. DAMAGEC and DAMAGET are between 0 and 1. If DAMAGEC or DAMAGET = 0, the cement sheath body does not suffer from compressive or tensile damage. If DAMAGEC or DAMAGET > 0 and close to 1, the cement sheath body reaches the compressive or tensile damage limit. Step 15: Use the STATUSXFEM state value to characterize the crack initiation and expansion process of the cement sheath. The STATUSXFEM state value is between 0 and 1. If STATUSXFEM=0, the cement sheath is intact and has no cracks. If 0<STATUSXFEM<1, the cement sheath partially cracks and expands. If STATUSXFEM=1, the cement sheath is completely cracked. Step 16: Use the SDEG state value (stiffness reduction rate) to characterize the cement sheath-casing interface damage initiation and micro-annulus generation process. If SDEG = 0, the interface is intact. If 0 < SDEG < 1, no micro-annulus is generated on the interface but damage is initiated. If SDEG = 1 and unit deletion occurs, micro-annulus is generated on the interface. Step 17: Apply loads to the two models to obtain the minimum load peak value for cement sheath cracking and the minimum load peak value for cement sheath-casing interface peeling, and determine the type of failure that occurs first in the "casing-cement sheath-casing" mechanical model according to the size of the minimum load value peak value. If the integrity failure of the cement sheath body occurs first, proceed to step 18; if the integrity failure of the cement sheath-casing interface occurs first, proceed to step 19; Step 18: Based on the model 1 in step 8, use the amplitude function (table form) to load different peak values ​​and different number of cycles respectively, keep the number of load cycles the same, obtain the minimum load peak value corresponding to the initiation of cement sheath cracks, keep the load peak value the same, obtain the minimum load alternation number corresponding to the initiation of cement sheath cracks, and record the expansion process of cement sheath cracks as the load peak value increases and the number of cycles increases, and stop the calculation; Step 19: Based on the model 2 in step 9, use the amplitude function (table form) to load different peak values ​​and different number of cycles respectively, keep the number of load cycles the same, obtain the minimum load peak value for the initiation of cement sheath-casing interface damage and the generation of micro-annulus, keep the load peak value the same, obtain the minimum number of cycles for the initiation of cement sheath-casing interface damage and the generation of micro-annulus, and record the expansion process of the micro-annulus at the cement sheath-casing interface as the load peak value increases and the number of cycles increases, and stop the calculation; Step 20: Determine the environmental loads for failure of the cement sheath and the cement sheath-casing interface by experiment: apply alternating temperature / pressure loads to the casing-cement sheath-casing assembly, record the minimum number of cycles required for failure of the integrity of the cement sheath and the minimum number of cycles required for failure of the integrity of the interface under different loads, and record the expansion process of the cement sheath crack and the development process of the micro-annulus as the load increases and the number of cycles increases; Step 21: Compare the finite element simulation results with the experimental results to verify and improve the three-dimensional mechanical model of the "casing-cement sheath-casing" combination.

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