Method for judging integrity of well cementation cement sheath of oil and gas well

Abstract

The invention relates to the technical field of petroleum engineering, and provides an oil and gas well cementation cement sheath integrity evaluation method which comprises the following steps: S1, simulation experiment: simulating underground environment and cement sheath stress changes under various operation conditions through an indoor physical simulation device to obtain reliable experiment data and experiment phenomena; s2, analysis and verification experiment: analyzing and verifying an experiment phenomenon through the mathematical model of the cement sheath; and S3, judgment: forming a cement sheath sealing integrity judgment method with practical significance through organic combination of a mechanical model and physical simulation. Through a method of combining a simulation experiment and mechanical model analysis, the integrity and the packing performance of the cement sheath can be evaluated more accurately, a more comprehensive and objective evaluation basis can be provided by experimental data and phenomenon acquisition and a stress analysis result, the defect of a single basis is avoided, the integrity of the cement sheath is evaluated accurately, and the sealing performance of the cement sheath is improved. And potential well cementation problems can be found in time.

Description

Technical Field

[0001] The invention relates to the technical field of petroleum engineering, in particular to a method for evaluating the integrity of cement rings in oil and gas well cementing. Background Art

[0002] The cement ring of oil and gas well cementing is a hard and brittle sealing medium. Under the action of alternating stress, it is easy to produce cracks or annular gaps in the cement ring, especially in the gas storage reservoir throughput and casing staged fracturing operations, the sealing of the cement ring is seriously damaged. In order to cope with the damage to the cement ring under the action of such external forces, the cementing site often uses a tough or elastic cement (stone) system to buffer the damage and impact caused by stress changes to the cement ring. However, different factors such as ground stress, reservoir pressure, fracturing technology, injection and production conditions, service life, casing centering, well temperature, etc. have different effects on the stress of the cement ring. In order to ensure the integrity of the cement annulus of oil and gas wells, how to accurately give the mechanical property range or index requirements of tough or flexible cement stone is a technical problem that needs to be solved in the cementing field.

[0003] At present, some research has been carried out in cement sheath integrity at home and abroad, mainly focusing on the establishment of cement sheath integrity mathematical models and physical simulation devices. At present, most of the research on the damage and failure mechanism of cement sheath integrity under load is to establish a combined model of casing-cement sheath-stratum or use ANSYS and other software for simulation and deduction, calculate the stress distribution of cement sheath, and indirectly analyze the failure mechanism of cement sheath. Only from a purely theoretical perspective, the correctness of the results cannot be verified, and the fatigue damage of cement sheath cannot be intuitively reflected. It still needs to use simulation devices to verify or guide the accuracy of its mathematical model. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a method for evaluating the integrity of the cement ring of oil and gas well cementing, which solves the problem that the damage mechanism of the cement ring currently analyzed can only be verified from a purely theoretical perspective, and the correctness of the results cannot be verified, and the damage situation of the cement ring fatigue damage cannot be intuitively reflected.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solutions: A method for evaluating the integrity of cement ring in oil and gas wells, comprising the following steps:

[0006] S1. Simulation experiment: Use indoor physical simulation devices to simulate the underground environment and the changes in cement sheath stress under various operating conditions to obtain reliable experimental data and experimental phenomena;

[0007] S2. Analysis and verification experiments: Analyze and verify the experimental phenomena through the mathematical model of cement sheath;

[0008] S3. Evaluation: A method for evaluating the sealing integrity of the cement sheath with practical significance is formed by the organic combination of the mechanical model and physical simulation.

[0009] Preferably, the specific steps of the simulation experiment in step S1 are as follows:

[0010] S1-1. Refer to the commonly used wellbore structure and casing size on site to establish a simulation device for the integrity of oil well cement.

[0011] S1-2. Use casings of different sizes to form an inner and outer cylinder (1 m long), inject cement into the annulus between the inner and outer cylinders to form a cement sheath, and pay attention to controlling the height and density of the cement sheath.

[0012] S1-3. Simulate the effect of the internal pressure change (alternating pressure) of the casing on the cement sheath by adding pressure relief inside the inner cylinder.

[0013] S1-4. Pass nitrogen through the upper gas detection port, dynamically and real-time monitor the sealing state of the cement sheath, and simultaneously conduct flow detection on the other end.

[0014] S1-5. If gas leakage is detected, it proves that "channeling" has occurred in the cement sheath, and it is determined that the integrity of the cement sheath has been damaged.

[0015] S1-6. Then take out the cement sheath for microscopic detection to reveal the microscopic mechanism of the failure of the cement sheath.

[0016] Preferably, the specific steps of the analysis and verification experiment in step S2 are as follows:

[0017] S2-1. Based on the multi-layered thick-walled cylinder theory of elastic mechanics, establish a mechanical model of the cement sheath coupling pressure and temperature.

[0018] S2-2. Set the mechanical property parameters of different cement stones.

[0019] S2-3. Use the established mechanical model of the cement sheath to calculate the stresses of the cement sheath under different stress conditions and the deformation at the interface.

[0020] S2-4. Analyze the stress distribution of the cement sheath.

[0021] S2-5. According to the stress analysis results, predict and evaluate the integrity and sealing performance of the cement sheath.

[0022] Preferably, the mechanical property parameters of the cement stone in step S2-2 include 7-day compressive strength, 7-day tensile strength, and 7-day elastic modulus.

[0023] Preferably, the analysis of the stress distribution of the cement sheath in step S2-4 includes the change trends of the radial stress and tangential stress.

[0024] Preferably, the specific steps for evaluation in step S3 are as follows:

[0025] S3-1. Using the mechanical theory model of the cement sheath coupling pressure and temperature, calculate and predict the stresses and strains in all directions of the cement sheath;

[0026] S3-2. Using the experimental device to simulate downhole temperature, pressure, and alternation, and analyze and verify by monitoring the fluid leakage rate in real time;

[0027] S3-3. Conduct microscopic inspection on the cement sheath after detection and record the experimental phenomena, and explore and reveal the microscopic mechanism of the failed or non-fluid-leaking cement sheath;

[0028] S3-4. Judge whether the cement sheath is sealed according to the two red line criteria.

[0029] Preferably, the two red lines in step S3-4 are respectively that the tangential stress is greater than 60% of the tensile strength and the radial stress is greater than 50% of the compressive strength.

[0030] Preferably, the tangential stress greater than 60% of the tensile strength is used to evaluate the stress damage degree of the cement sheath, and the radial stress greater than 50% of the compressive strength is used to judge the sealing performance of the cement sheath.

[0031] Preferably, the specific steps for calculating and predicting the stresses and strains in all directions of the cement sheath using the mechanical theory model of the cement sheath coupling pressure and temperature in step S3-1 are as follows:

[0032] S3-11. Based on the elastic mechanics theory and numerical calculation method, establish a mechanical model of the cement sheath coupling pressure and temperature;

[0033] S3-12. According to the model, calculate the stresses and strains in all directions of the cement sheath under different stress and temperature conditions. Through numerical calculation, the stress and strain distribution of the cement sheath under different working conditions can be obtained, so as to predict the mechanical response of the cement sheath;

[0034] S3-13. Verify the accuracy and reliability of the model by comparing the consistency between the calculation results and the experimental data.

[0035] Preferably, the specific steps for analyzing and verifying by using the experimental device to simulate downhole temperature, pressure, and alternation and monitoring the fluid leakage rate in real time in step S3-2 are as follows:

[0036] S3-21. Simulate the downhole temperature, pressure, and alternation working conditions in the experimental device.

[0037] S3-22. Monitor the change of the fluid leakage rate of the cement sheath in real time;

[0038] S3-23. Analyze the changing trend and characteristics of the crossflow rate, compare with the model prediction results, and further verify the accuracy and reliability of the model.

[0039] The present invention provides a method for judging the integrity of the cement sheath in oil and gas wells, having the following beneficial effects:

[0040] 1. By combining simulation experiments and mechanical model analysis, the present invention can more accurately evaluate the integrity and sealing performance of the cement sheath. The obtained experimental data and phenomena, as well as the results of stress analysis, can provide a more comprehensive and objective evaluation basis, avoiding the deficiencies of a single basis.

[0041] 2. According to the stress analysis results of the cement sheath and the two red line criteria, the present invention can guide the optimization of cementing design and operation parameters.

[0042] 3. By judging the integrity and sealing performance of the cement sheath, the present invention can timely discover potential problems and take corresponding measures for repair or improvement, which helps to maintain the integrity of the oil and gas well, reduce the occurrence of leakage and crossflow phenomena, and improve the sealing performance of the wellbore and the safety of long-term operation.

[0043] 4. By accurately evaluating the integrity of the cement sheath, the present invention can timely discover potential cementing problems, avoid downhole accidents caused by the failure of the cement sheath, and timely take necessary repair measures to ensure the cementing quality and the safety of the wellbore. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 is a flow chart of the present invention;

[0045] Figure 2 is a schematic diagram showing the change of crossflow rate in the stress cycle experiment of conventional cement of 10 - 30 MPa and 10 - 40 MPa of the present invention;

[0046] Figure 3 is a trend chart of the change of equivalent permeability of the conventional cement sheath of the present invention;

[0047] Figure 4 is a schematic diagram showing the change of crossflow rate in the stress cycle experiment of conventional cement of 10 - 50 MPa of the present invention;

[0048] Figure 5 is a schematic diagram showing the calculation results of the radial and tangential stresses on the inner wall of the cement sheath of the present invention;

[0049] Figure 6 is a schematic diagram showing the calculation results of the radial and tangential stresses on the inner wall of the cement sheath in the annulus B of the caprock section of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0050] Next, in combination with the accompanying drawings of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0051] Embodiment:

[0052] Please refer to the attached Figure 1 , the embodiment of the present invention provides a method for judging the integrity of the cement sheath in an oil and gas well, including the following steps:

[0053] S1. Simulation experiment: Simulate the stress changes of the cement sheath under downhole environment and various operating conditions through an indoor physical simulation device to obtain reliable experimental data and experimental phenomena;

[0054] S2. Analysis and verification experiment: Analyze and verify the experimental phenomena through the mathematical model of the cement sheath;

[0055] S3. Judgment: Form a judgment method for the sealing integrity of the cement sheath with practical significance through the organic combination of the mechanical model and physical simulation. Through the method of combining simulation experiments and mechanical model analysis, the integrity and sealing performance of the cement sheath can be evaluated more accurately. The acquisition of experimental data and phenomena and the results of stress analysis can provide a more comprehensive and objective evaluation basis, avoiding the deficiencies of a single basis.

[0056] The specific steps of the simulation experiment in step S1 are as follows;

[0057] S1-1. Refer to the commonly used wellbore structure and casing size on site to establish a simulation device for the integrity of oil well cement;

[0058] S1-2. Use casings of different sizes to form inner and outer cylinders (1 m long), inject cement between the annulus of the inner and outer cylinders to form a cement sheath, and pay attention to controlling the height and density of the cement sheath; by precisely controlling the proportion and injection process of the cement slurry, the formation process of the cement sheath in the real downhole environment can be simulated, so as to obtain more accurate experimental phenomena and data.

[0059] S1-3. Simulate the effect of the change in the internal pressure of the casing (alternating pressure) on the cement sheath by adding pressure relief inside the inner cylinder; by simulating the change in pressure in the downhole environment, further observe and record the stress changes of the cement sheath under different pressures, providing a data basis for subsequent analysis.

[0060] S1-4. Pass nitrogen gas through the upper gas inspection port to dynamically and real-time monitor the sealing state of the cement sheath, and at the same time conduct flow detection on the other end. By monitoring the inflow and outflow of nitrogen gas, judge the sealing performance of the cement sheath, promptly detect the situation of seal failure, and provide an experimental basis for subsequent evaluation.

[0061] S1-5. If gas leakage is detected, it proves that "channeling" has occurred in the cement sheath, and it is determined that the integrity of the cement sheath has been damaged. By observing and recording the change in gas channeling flow rate, the integrity of the cement sheath can be determined, promptly detecting damage and leakage phenomena, and providing data support for subsequent evaluation and analysis.

[0062] S1-6. Then take out the cement sheath for microscopic detection to reveal the microscopic mechanism of cement sheath failure. By taking out the cement sheath sample and observing it under a microscope, the details and mechanism of cement sheath failure can be deeply understood, providing more comprehensive information for subsequent model verification and evaluation.

[0063] The specific steps of the analysis and verification experiment in step S2 are as follows:

[0064] S2-1. Based on the multi-layered thick-wall cylinder theory of elasticity mechanics, establish a mechanical model of the cement sheath that couples pressure and temperature. Analyze the mechanical response of the cement sheath under different stress and temperature conditions through calculation methods, and verify the consistency between the experimental phenomena and the theoretical model.

[0065] S2-2. Set the mechanical property parameters of different cement stones. By setting the mechanical property parameters of different cement stones, different cement materials used in the actual downhole environment can be simulated, further verifying the reliability and applicability of the experimental results.

[0066] S2-3. Use the established mechanical model of the cement sheath to calculate the various stresses of the cement sheath under different stress conditions and the deformation amount at the interface. Through numerical calculation, the stress distribution of the cement sheath under different stresses can be obtained, thereby further verifying the consistency between the experimental phenomena and the model.

[0067] S2-4. Analyze the stress distribution of the cement sheath. By analyzing the stress distribution, the change law of the cement sheath under different stress conditions can be revealed, providing a theoretical basis for subsequent evaluation.

[0068] S2-5. According to the stress analysis results, predict and evaluate the integrity and sealing performance of the cement sheath. By comparing the stress analysis results with the experimental phenomena, the integrity and sealing performance of the cement sheath can be predicted and evaluated, providing a scientific basis for subsequent evaluation and decision-making. By judging the integrity and sealing performance of the cement sheath, potential problems can be promptly discovered and corresponding measures can be taken for repair or improvement, which helps to maintain the integrity of the oil and gas well, reduce the occurrence of leakage and channeling phenomena, and improve the sealing performance of the wellbore and the safety of long-term operation.

[0069] The mechanical property parameters of the cement stone in step S2-2 include 7-day compressive strength, 7-day tensile strength, and 7-day elastic modulus.

[0070] The analysis of the stress distribution of the cement sheath in step S2-4 includes the changing trends of radial stress and tangential stress.

[0071] The specific steps for evaluation in step S3 are as follows:

[0072] S3-1. Using the mechanical theory model of the cement sheath that couples pressure and temperature, calculate and predict the stresses and strains in all directions of the cement sheath;

[0073] S3-2. Using the experimental device to simulate downhole temperature, pressure, and alternating conditions, and analyze and verify by monitoring the channeling flow rate in real time; By using the experimental device to simulate downhole temperature, pressure, and alternating conditions, and simultaneously monitor the channeling flow rate in the cement sheath in real time. Through the analysis and verification of the channeling flow rate, the plugging effect and stability of the cement sheath in the actual downhole environment can be understood. This experimental verification can provide direct observation and quantitative evaluation of the performance of the cement sheath to determine whether it meets the requirements.

[0074] S3-3. Conduct microscopic inspection on the cement sheath after detection and record the experimental phenomena, and explore and reveal the microscopic mechanism of the cement sheath that fails or does not experience channeling; Further understand the reasons for the failure of the cement sheath and provide reference for subsequent evaluation and improvement. Conduct microscopic inspection on the detected cement sheath and record the experimental phenomena, especially for the cement sheath that fails or does not experience channeling, conduct detailed observation and analysis. Through such microscopic inspection and mechanism exploration, the reasons for the failure of the cement sheath can be deeply understood, including possible structural damage, pore leakage, or other incompleteness.

[0075] S3-4. Judge whether the cement sheath is sealed according to the two red line criteria. According to the established two red line criteria, judge the stress analysis results of the cement sheath. According to the calculation results of tangential stress and radial stress, judge whether the cement sheath reaches or exceeds the thresholds of the two red lines. Determine the sealing performance and integrity of the cement sheath. According to the judgment results of the two red line criteria, evaluate the sealing performance and integrity of the cement sheath and judge whether it meets the requirements. By based on the stress analysis results of the cement sheath and the two red line criteria, it can guide the optimization of cementing design and operation parameters.

[0076] The two red lines in step S3-4 are accurately that the tangential stress is greater than 60% of the tensile strength and the radial stress is greater than 50% of the compressive strength respectively.

[0077] The tangential stress greater than 60% of the tensile strength is used to evaluate the stress damage degree of the cement sheath, and the radial stress greater than 50% of the compressive strength is used to judge the sealing performance of the cement sheath.

[0078] In step S3-1, the specific steps for calculating and predicting the stresses and strains in all directions of the cement sheath by using the mechanical theory model of the cement sheath that couples pressure and temperature are as follows:

[0079] S3-11. Based on the theory of elasticity and numerical calculation methods, establish a mechanical model of the cement sheath that couples pressure and temperature, which can fully consider the influence of complex mechanical and thermal conditions in the downhole environment on the cement sheath.

[0080] S3-12. According to the model, calculate the stresses and strains in all directions of the cement sheath under different stress and temperature conditions. Through numerical calculation, the stress and strain distribution of the cement sheath under different working conditions can be obtained, thereby predicting the mechanical response of the cement sheath.

[0081] S3-13. Verify the accuracy and reliability of the model by comparing the consistency between the calculation results and the experimental data. By comparing with the experimental results, the applicability of the model can be evaluated, and the model parameters can be further optimized to improve the accuracy of the judgment. By accurately evaluating the integrity of the cement sheath, potential cementing problems can be discovered in a timely manner, the downhole accidents caused by the failure of the cement sheath can be avoided, and necessary repair measures can be taken in a timely manner to ensure the cementing quality and the safety of the wellbore.

[0082] The specific steps for simulating the downhole temperature, pressure, and alternation by using the experimental device in step S3-2, and analyzing and verifying by real-time monitoring of the channeling flow rate are as follows:

[0083] S3-21. Simulate the working conditions of downhole temperature, pressure, and alternation in the experimental device.

[0084] S3-22. Real-time monitor the change of the channeling flow rate of the cement sheath.

[0085] S3-23. Analyze the change trend and characteristics of the channeling flow rate, compare with the model prediction results, and further verify the accuracy and reliability of the model. By analyzing the channeling flow rate data, the sealing performance of the cement sheath can be evaluated and compared with the results predicted by the model to further verify the accuracy and reliability of the model.

[0086] Verification experiment:

[0087] In order to verify whether the mechanical model in the above method for judging the integrity of the cement sheath in oil and gas wells is accurate and reliable, analyze the integrity of the cement sheath under different working conditions, explore the failure matrix and microscopic mechanism of the cement sheath, and establish two red line criteria, the integrity experiments of the cement sheath under low stress and low temperature environment and the integrity experiments of the cement sheath under high stress and high temperature environment are respectively carried out by using the above mechanical model and physical simulation device, including the simulation of the alternating load cycle and the exploration of the bearing limit pressure difference of the cement sheath.

[0088] The experiment includes four steps:

[0089] a. Using the mechanical theoretical model of the cement sheath that couples pressure and temperature, calculate and predict the stresses and strains in all directions of the cement sheath;

[0090] b. Using the experimental device to simulate downhole temperature, pressure, and alternation, and analyze and verify by monitoring the fluid loss rate in real time;

[0091] c. Conduct microscopic inspection on the cement sheath after testing and record the experimental phenomena, and explore and reveal the microscopic mechanisms of the failed or non-fluid-loss cement sheath;

[0092] d. Summarize and analyze to obtain two red-line criteria, which are the key judgments for predicting and analyzing whether the cement sheath is sealed.

[0093] Experimental content:

[0094] ① Cement sheath integrity experiment under low stress and low temperature environment: Conducted an indoor integrity simulation experiment at 45°C. To capture the experimental phenomena more intuitively, conventional cement was used for alternation experiment indoors (Dalian G-grade + 0.44 w / c, density 1.90 g / cm3).

[0095] ② Conducted an indoor full-size integrity simulation experiment at 90°C, using ductile cement for alternation experiment (Dalian G-grade + 0.6% DSJ + 4% toughening agent, density 1.90 g / cm3). Alternation simulations of 10 - 30 MPa, 10 - 40 MPa, 10 - 50 MPa, and 10 - 60 MPa were each conducted for 50 cycles.

[0096] Experimental phenomena found:

[0097] ① Cement sheath integrity experiment under low stress and low temperature environment:

[0098] a. For the alternation simulation of 10 - 40 MPa for 50 cycles, the fluid loss rate did not change significantly;

[0099] b. When cycling at 10 - 50 MPa to the 23rd cycle, the annulus pressure was abnormal, and at the same time, the fluid loss rate increased significantly and the filtrate was alkaline (Ph = 12). It is speculated that there was a breakthrough in the cement annulus;

[0100] c. There were obvious differences in the equivalent permeability of the cement sheath during the process of loading and unloading the internal casing pressure, inferring that there were also annulus gaps at the interface of the cement sheath;

[0101] d. When disassembling the experiment, the casing was easily pushed out, indicating that the interfacial bonding strength of the cement was very weak;

[0102] e. The presence of red tracer remaining on the outer wall of the casing also corroborated the existence of annulus gaps.

[0103] ②A full-scale integrity simulation experiment was carried out at 90°C indoors:

[0104] a. The equivalent permeability of the cement sheath decreased from the initial 0.01 mD to 0.002 mD and tended to be stable. There was no obvious change in the equivalent permeability during the loading and pressure relief processes, indicating good sealing performance at the interface.

[0105] b. There was no trace of the tracer on the ejected casing, indicating that no annulus was generated. It can be concluded that the toughness cement sheath has good sealing performance under such huff and puff conditions.

[0106] Mechanical models were used to process all the experimental phenomena generated in the above experiments.

[0107] Experimental findings and predictions show that:

[0108] ①In the cement sheath integrity experiment under low stress and low temperature conditions, it was found that: a. When the inner casing circulation pressure exceeded 40 MPa, the tangential stress on the cement sheath reached 60% of the ultimate compressive strength of the cement stone. It was speculated that circumferential cracks had occurred in the cement sheath at this time because the sound of the cement sheath splitting could be clearly heard during the experiment. However, the circumferential cracks did not affect the longitudinal or axial gas channeling volume, so the macroscopic performance and gas channeling volume were not obvious at this time, and the circumferential cracks had little impact on the wellbore sealing performance. b. When the inner casing circulation pressure approached 50 MPa, the radial stress on the cement sheath reached 50% of the ultimate compressive strength of the cement stone. It could be concluded that radial cracks had occurred in the cement sheath at this time and even expanded into axial cracks. At this time, the macroscopic performance was more obvious, with large fluctuations in the equivalent permeability and an increase in the gas channeling volume. c. As the alternating cycle progressed, the fatigue strain of the cement sheath continued to accumulate, and the cement sheath had irreversible deformation. When the deformation reached 0.1 mm, micro-annuli would be generated, and the macroscopic performance at this time was even more obvious, including sudden changes in the equivalent permeability, seal failure, and debonding at the first interface.

[0109] ②The prediction of the 90°C indoor full-scale integrity simulation experiment shows that: a. When the internal circulation pressure in the casing varies from 10 to 50 MPa, the radial stress on the cement sheath is less than 30% of the compressive strength (20.346 MPa), and the tangential stress is also less than 60% of the tensile strength (2.712 MPa). The microcracks inside the cement stone basically remain unchanged. Therefore, within 150 cycles when the internal circulation pressure in the casing varies within the range of 10 to 50 MPa, the equivalent permeability of the cement sheath will not change significantly; b. When the internal circulation pressure in the casing varies from 10 to 60 MPa and the tangential stress on the cement sheath is greater than 60% of the tensile strength (1.674 MPa), circumferential cracks begin to expand and connect. Therefore, it is very likely that the circumferential rupture of the cement sheath will occur first (occasionally, the sound of the cement sheath splitting can be heard during the experiment), but this rupture will not affect the detection of the axial permeability of the cement sheath. Therefore, the equivalent permeability of the cement sheath remains unchanged; the radial stress on the cement sheath is much less than 50% of the compressive strength (33.91 MPa). Therefore, the cement sheath will not generate radial cracks or annulus gaps.

[0110] Experiment summary:

[0111] ①Summary of the cement sheath integrity experiment under low stress and low temperature environment: Two red lines are obtained. The first red line is that the tangential stress is greater than 60% of the tensile strength. Although this red line has little impact on the wellbore sealing performance, it does cause stress damage to the cement stone; the second red line is that the radial stress is greater than 50% of the compressive strength. This red line is the bottom line and the most critical criterion for ensuring the wellbore sealing performance. Based on these two red lines, the integrity of the cement sheath can be predicted and analyzed through the stress analysis of the cement sheath.

[0112] ②Summary of the 90°C indoor full-scale integrity simulation experiment: Based on the two red lines, it can be inferred that the cement sheath can maintain good integrity and isolation performance under such huff and puff conditions. The tangential stress on the cement sheath touches the first red line, but the radial stress does not touch the second red line. The cement sheath can maintain good sealing performance and will not occur channeling, which is consistent with the experimental results; at the same time, the reliability and guidance of the mechanical model are also verified.

[0113] Experimental data:

[0114] ①Experimental simulation of conventional cement under 10 - 30 MPa and 10 - 40 stress cycles

[0115] The formation pressure is kept constant at 20 ± 1 MPa, the annulus pressure is set at 20 ± 1 MPa, and the internal pressure in the wellbore circulation is applied with 10 - 30 MPa and 10 - 40 MPa before and after. Each cycle is 30 - 40 minutes, and it is cycled to 100 cycles. The change in the flow rate of the cement sheath (1 m long and cross-sectional area 0.044 m²) is monitored in real time (please refer to the attachment Figure 2)。Convert the real-time fluid loss rate into the equivalent permeability of the cement sheath and plot it as a curve, as shown in the appendix Figure 3 As shown. The equivalent permeability of the cement sheath gradually increases from the initial 0.1 mD to 0.16 mD, and then drops to 0.01 mD and stabilizes in the later stage.

[0116] Experimental results: The regular change of the internal casing pressure within this range will slightly reduce the permeability of the cement sheath, indicating that the cement sheath is very likely to have a small amount of plastic deformation at this stage, and the pores of the cement stone are compressed; and there is no obvious change in the permeability during the loading and pressure relief processes, indicating that the fluid loss rate only represents the permeability of the cement sheath body, and the integrity at the interface is good. Therefore, it can be inferred that the cement sheath can maintain good integrity and sealing performance under such alternating working conditions.

[0117] ② Experimental simulation of the stress cycle of conventional cement at 10 - 50 MPa

[0118] On the basis of the above stress cycle test, continue to apply the internal casing circulation pressure of 10 - 50 MPa (as shown in Figure 4 ). When the 23rd cycle is reached, the fluid loss rate changes significantly, and the annulus pressure of the cement sheath drops very rapidly. As can be seen from Figure 3 , the equivalent permeability of the cement sheath increases from 0.01 mD to a maximum of 0.24 mD, and there are obvious changes in the permeability of the cement sheath during the loading and pressure relief processes of the internal casing pressure, indicating that there may also be micro-annular gaps at the interface of the cement sheath. Therefore, it can be determined that the sealing performance of the cement sheath is severely damaged under such alternating working conditions.

[0119] ③ Analysis of the influence of alternating stress on the annulus sealing performance of cementing

[0120] According to the mechanical model analysis, the radial stress and tangential stress on the inner wall of the cement sheath are obtained, as shown in the appendix Figure 5 As shown. Combining appendix Figure 3 and appendix Figure 5 it can be found that when the internal casing circulation pressure changes from 10 to 40 MPa, the radial stress (25.685 MPa) on the cement sheath is less than 50% of the compressive strength and the tangential stress (1.474 MPa) is also less than 60% of the tensile strength. The micro-cracks inside the cement stone start to expand relatively stably and basically no new micro-cracks are generated. Therefore, when the internal casing circulation pressure changes within the range of 10 - 40 MPa for 100 cycles, the equivalent permeability of the cement sheath does not change significantly.

[0121] Only when the pressure in the inner loop varies from 10 to 50 MPa, and the tangential stress (1.674 MPa) on the cement sheath is greater than 60% of the tensile strength, the first red line is touched, and the circumferential cracks begin to expand and connect. Therefore, it is very likely that the circumferential rupture of the cement sheath will occur first. With the cycle of the alternating period and the accumulation of the fatigue strain of the cement stone, the radial stress on the cement sheath gradually becomes greater than 50% of the compressive strength, touching the second red line. The micro-cracks inside the cement stone begin to expand rapidly and the internal system of the cement stone becomes extremely unstable. Macroscopically, it is manifested as an obvious change in the equivalent permeability of the cement sheath. At the same time, the radial stress will cause radial cracks or micro-annular gaps in the cement sheath, and the tangential stress will cause circumferential cracks in the cement sheath. Eventually, the cement sheath will show a cracked state.

[0122] Judgment experiment:

[0123] According to the geological parameters, casing parameters, and mechanical parameters of the cement sheath of a certain well as shown in Table 1, the alternating cycle pressure (10 - 70 MPa) and the wellbore structure characteristics of this well are determined. Using the evaluation method introduced in the present invention, the sealing performance judgment and the performance index requirements for the long-term sealing integrity of the cement sheath are given for different annuli respectively.

[0124] Table 1 - Basic parameters of the formation, casing, and cement stone under the condition of 120 °C

[0125]

[0126]

[0127] Note: The cement slurry is Dalian G - grade + 12% latex + 25% quartz sand + 4% toughening agent, and the density is 1.90 g / cm3.

[0128] ① Judging the integrity of annulus B in the caprock section based on integrity

[0129] This annulus is between the double - layer casings. After conversion, it is obtained that the cement sheath of annulus B is equivalent to bearing a confining pressure of 12 - 13 MPa. When the pressure in the inner loop varies from 10 to 50 MPa, the radial stress on the cement sheath is less than 30% of the compressive strength (18.054 MPa) and does not touch the red line; and the tangential stress is also less than 60% of the tensile strength (2.208 MPa) and does not touch the red line, as shown in the appendix Figure 6 shown. It can be inferred that the cement sheath can maintain good sealing performance under such alternating working conditions. Secondly, using this evaluation method, the specific mechanical property parameters can be calculated as a reference for the performance index of the cement used in well cementing (as shown in Table 2). From the calculation results, it can be seen that this annulus has relatively high requirements for the elastic modulus and compressive strength of the cement stone, and the influence of alternation on the sealing performance of this annulus should be fully considered.

[0130] Table 2 - Reference Indexes of Cement Stone Properties with Different Strengths and Elastic Moduli in the Annulus of the Capping Section B

[0131]

[0132] Note: Simulated confining pressure is 13 MPa, the inner casing circulation pressure alternates within the range of 10 - 50 MPa for 50 cycles, the casing centrality is 100%, and the wellbore temperature change is 20℃.

[0133] ② Performance Requirements for the Annulus of Reservoir Section B Considering Integrity

[0134] Since the annulus of Section B is in a concentrated area with stress changes, temperature changes, and eccentricity changes, it is the most critical well section for the sealing integrity of the cement sheath. Therefore, the sealing performance of the narrow - side cement sheath should be taken as the core of the analysis. When the inner casing circulation pressure varies from 10 to 50 MPa, the alternating cycle is 50 groups, and the wellbore temperature change is 30℃. The following table gives the index requirements for casing centralities of 75% and 50% respectively (as shown in Table 3). By comparison, the thinner the cement sheath, the more demanding the mechanical property requirements are.

[0135] Table 3 - Reference Indexes of Cement Stone Properties with Different Strengths and Elastic Moduli in the Annulus of Reservoir Section B

[0136]

[0137]

[0138] Note: Simulated in - situ stress is 40 MPa, the inner casing circulation pressure alternates within the range of 10 - 50 MPa for 50 cycles, the casing centrality is 50%, and the wellbore temperature change is 30℃.

[0139] ③ Performance Requirements for the Annulus under Extreme High - Pressure Conditions Considering Integrity

[0140] As shown in the appendix Figure 6 When the inner casing circulation pressure varies from 10 to 60 MPa, when the tangential stress on the cement sheath is greater than 60% of the tensile strength, circumferential cracks begin to expand and connect. Therefore, it is very likely to cause circumferential rupture of the cement sheath first, but this rupture will not affect the axial sealing of the cement sheath. Even when the inner casing circulation pressure increases to 70 MPa, it will not have a great impact on the overall sealing of the cement sheath. Secondly, when the inner casing circulation pressure varies from 10 to 70 MPa, the radial stress on the cement sheath is far less than 50% of the compressive strength. Therefore, the cement sheath will not produce radial cracks or annulus gaps. In summary, it can be inferred that the cement sheath can maintain good integrity and sealing performance under this extreme high - pressure working condition.

[0141] Although embodiments of the present invention have been shown and described, those of ordinary skill in the art will appreciate that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for evaluating the integrity of a cement sheath in an oil and gas well, characterized in that, It includes the following steps: S1. Simulation experiment: Use an indoor physical simulation device to simulate the stress changes of the cement sheath under downhole environments and various operating conditions, and obtain reliable experimental data and experimental phenomena; S2. Analysis and verification experiment: Analyze and verify the experimental phenomena through the mathematical model of the cement sheath; S3. Evaluation: Combine the mechanical model and physical simulation organically to form a method for evaluating the sealing integrity of the cement sheath with practical significance.

2. The method for evaluating the integrity of a cement sheath in an oil and gas well according to claim 1, characterized in that, The specific steps of the simulation experiment in step S1 are as follows; S1-1. Refer to the commonly used wellbore structure and casing size on site, and establish a simulation device for the integrity of oil well cement; S1-2. Use casings of different sizes to form inner and outer cylinders (1 m long), inject cement between the annulus of the inner and outer cylinders to form a cement sheath, and pay attention to controlling the height and density of the cement sheath; S1-3. Simulate the action of the internal pressure change (alternating pressure) of the casing on the cement sheath by adding pressure relief inside the inner cylinder; S1-4. Pass nitrogen through the upper gas detection port to dynamically and real-time monitor the sealing state of the cement sheath, and at the same time perform flow detection on the other end; S1-5. If gas breakthrough is detected, it proves that "channeling" has occurred in the cement sheath, and it is determined that the integrity of the cement sheath has been damaged; S1-6. Then take out the cement sheath for microscopic detection to reveal the microscopic mechanism of the failure of the cement sheath.

3. A method for evaluating the integrity of the cement sheath in an oil and gas well according to claim 1, characterized in that, The specific steps of the analysis and verification experiment in step S2 are as follows: S2-1. Based on the theory of multi-layer thick-walled cylinders in elasticity mechanics, establish a mechanical model of the cement sheath coupling pressure and temperature; S2-2. Set the mechanical property parameters of different cement stones; S2-3. Use the established mechanical model of the cement sheath to calculate the stresses and deformations at the interface of the cement sheath under different stress conditions; S2-4. Analyze the stress distribution of the cement sheath; S2-5. According to the stress analysis results, predict and evaluate the integrity and sealing performance of the cement sheath.

4. A method for evaluating the integrity of a cement sheath in an oil and gas well according to claim 3, characterized in that, The mechanical property parameters of the cement stone in step S2-2 include 7-day compressive strength, 7-day tensile strength, and 7-day elastic modulus.

5. The method for evaluating the integrity of the cement sheath in an oil and gas well according to claim 3, characterized in that, The analysis of the stress distribution of the cement sheath in step S2-4 includes the change trends of radial stress and tangential stress.

6. The method for evaluating the integrity of the cement sheath in an oil and gas well according to claim 1, characterized in that, The specific steps of the evaluation in step S3 are as follows: S3-1. Use the mechanical theory model of the cement sheath coupling pressure and temperature to calculate and predict the stresses and strains in all directions of the cement sheath; S3-2. Use the experimental device to simulate downhole temperature, pressure, and alternation, and analyze and verify by real-time monitoring of the channeling flow rate; S3-3. Conduct microscopic detection on the cement sheath after detection and record the experimental phenomena, and explore and reveal the microscopic mechanism of the failed or non-channeled cement sheath; S3-4. Judge whether the cement sheath is sealed according to the two red line criteria.

7. A method for evaluating the integrity of a cement sheath in an oil and gas well according to claim 6, wherein The two red lines in step S3-4 are accurately the tangential stress is greater than 60% of the tensile strength and the radial stress is greater than 50% of the compressive strength respectively.

8. A method for evaluating the integrity of a cement sheath in an oil and gas well according to claim 7, characterized in that, The tangential stress greater than 60% of the tensile strength is used to evaluate the stress damage degree of the cement sheath, and the radial stress greater than 50% of the compressive strength is used to judge the sealing performance of the cement sheath.

9. A method for evaluating the integrity of a cement sheath in an oil and gas well according to claim 6, characterized in that, The specific steps of using the mechanical theory model of the cement sheath coupling pressure and temperature to calculate and predict the stresses and strains in all directions of the cement sheath in step S3-1 are as follows: S3-11. Based on the theory of elasticity and numerical calculation methods, establish a mechanical model of the cement sheath coupling pressure and temperature; S3-12. According to the model, calculate the stresses and strains in all directions of the cement sheath under different stress and temperature conditions. Through numerical calculation, the stress and strain distribution of the cement sheath under different working conditions can be obtained, so as to predict the mechanical response of the cement sheath; S3-13. Verify the accuracy and reliability of the model by comparing the consistency between the calculation results and the experimental data.

10. A method for evaluating the integrity of a cement sheath in an oil and gas well according to claim 6, characterized in that, The specific steps for simulating downhole temperature, pressure, and alternation using the experimental device and analyzing and verifying the channeling flow rate in real time in step S3-2 are as follows: S3-21. Simulate the downhole temperature, pressure, and alternation working conditions in the experimental device; S3-22. Monitor the change of the channeling flow rate of the cement sheath in real time; S3-23. Analyze the change trend and characteristics of the channeling flow rate, compare with the model prediction results, and further verify the accuracy and reliability of the model.

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