Cement sheath integrity evaluation method based on equivalent physical experiment

Abstract

The invention discloses a cement sheath integrity evaluation method based on equivalent physical experiments, comprising: calculating the displacement of an interface of an actual wellbore under the condition of internal and external pressure difference based on the elastic-plastic mechanical model of the cement sheath assembly, calculating the equivalent displacement of an interface of an experimental device using the principle of equal strain of an interface, and then back-calculating the equivalent experimental pressure difference and the equivalent displacement of two interfaces of the experimental device according to the elastic-plastic mechanical model of the cement sheath assembly, and finally calculating the radial displacement of the inner wall of the simulated casing and the radial displacement of the outer wall of the simulated surrounding rock using the equivalent displacement of the one and two interfaces, and comparing and analyzing the test results of the experimental device, defining S i , S o To simulate the radial displacement of the inner wall of the casing and the radial displacement of the outer wall of the surrounding rock, when min{S i ,S o When the value of the cement sheath is greater than or equal to 80%, the experimental result is used as the evaluation basis for the integrity of the underground cement sheath. The invention is easy to operate, and uses an equivalent physical experiment to effectively combine the theoretical research method with the experimental evaluation device, so that the evaluation result of the cement sheath integrity is closer to the actual situation underground.

Description

Technical Field

[0001] The invention belongs to the field of oil and gas well cementing cement ring integrity detection and evaluation, and specifically relates to a cement ring integrity evaluation method based on equivalent physical experiments. Background Art

[0002] In the process of oil and gas exploration and development, cementing can achieve the purpose of supporting casing and isolating oil, gas and water layers in the wellbore, and is one of the key technologies to ensure subsequent safe drilling and long-term exploitation. After the cementing operation is completed, the cement ring hardens in the annular space underground. The hardened cement ring will bear the load of various working conditions underground. After cementing, the casing pressure test, oil test, water injection and later fracturing transformation and other underground operations will cause the inside and outside of the wellbore to bear a large pressure difference, causing the stress state of the cement ring to change. If the stress on the cement ring exceeds its own strength limit, it will cause the cement ring to crack, break and micro-annular gaps on the interface, destroying the mechanical integrity of the cement ring and causing the annular space to fail.

[0003] Domestic and foreign researchers have conducted extensive research on the evaluation and detection technology of the impact of pressure changes on cement ring integrity. The invention patent "A method for designing the minimum thickness of cement ring to ensure the integrity of cement ring" (CN118070437A) is based on the calculation of casing centering and cement ring integrity analysis under different wellbore diameter conditions, and iteratively calculates the minimum allowable thickness of cement ring to ensure the integrity of cement ring. The invention patent "A quantitative evaluation method for cement ring integrity and damage evolution in oil and gas wells" (CN115711120B) is based on fractal theory, image processing technology, and the structural characteristics and failure mechanism of the casing-cement ring-formation combination. The fractal dimension of the casing-cement ring interface morphology after alternating loads, the fractal dimension of the microscopic pore morphology of the cement ring body, the fractal dimension of the particle morphology, the fractal dimension of the crack morphology, and the correlation between them and the macroscopic mechanical properties of the cement ring are used to quantitatively evaluate the damage evolution law of the cement ring integrity. The invention patent "Method and device for analyzing and predicting the integrity of cement sheath" (CN106991235A) uses the stress distribution and displacement distribution of the elastic-plastic zone of the wellbore (Mohr-Coulomb plastic yield criterion) and the stress distribution of the wellbore under the action of a non-uniform geostress field (Airy stress function) to linearly superpose the total stress distribution and total displacement distribution of the wellbore, which are used to analyze and predict the integrity of the cement sheath.

[0004] The above methods generally have the following shortcomings: (1) The existing evaluation methods mostly remain at the theoretical analysis level and do not effectively combine the theoretical research results with the experimental research results; (2) Even if there are integrity evaluation methods that combine experimental research with theoretical analysis, they focus more on the influence of the geometric dimensions, mechanical properties and interfacial bonding strength of the cement sheath on the integrity of the cement sheath, and do not clarify the actual stress-strain development law of the cement sheath assembly. Summary of the invention

[0005] The purpose of the present invention is to provide a cement sheath integrity evaluation method based on equivalent physical experiments. The method has reliable principles, convenient operation, and controllable processes. By using an elastoplastic mechanics model of a cement sheath assembly and utilizing equivalent physical experiments, the theoretical research method is effectively combined with an experimental evaluation device, so that the cement sheath integrity evaluation result is closer to the actual situation underground.

[0006] In order to achieve the above technical objectives, the present invention adopts the following technical solutions.

[0007] Firstly, based on the elastic-plastic mechanical model of cement sheath assembly, the displacement of the first interface of the actual wellbore under the condition of internal and external pressure difference is calculated. The equivalent displacement of the first interface of the experimental device is calculated by using the principle of equal strain of the first interface. Then, the equivalent experimental pressure difference and the equivalent displacement of the second interface of the experimental device are calculated based on the elastic-plastic mechanical model of cement sheath assembly. Finally, the radial displacement of the inner wall of the simulated casing and the radial displacement of the outer wall of the simulated surrounding rock are calculated by using the equivalent displacement of the first and second interfaces, and compared with the test results of the experimental device. S is defined. i , S o To simulate the radial displacement of the inner wall of the casing and the radial displacement of the outer wall of the surrounding rock, when min{S i ,S o When the value of the cement ring is ≥80%, the experimental results are used as the evaluation basis for the integrity of the underground cement ring. The present invention combines the theoretical calculation results with the experimental evaluation results to clarify the stress distribution, deformation and plastic zone development of the cement ring assembly, and comprehensively evaluates the sealing integrity of the cement ring according to the air crossflow of the cement ring during the experiment.

[0008] The cement sheath integrity evaluation method based on equivalent physical experiments includes the following steps:

[0009] (1) Based on the elastic-plastic mechanical model of casing-cement sheath-surrounding rock assembly (referred to as cement sheath assembly) (Chu Wei, Shen Jiyun, Yang Yunfei, et al. Calculation of micro-annulus of casing-cement sheath-surrounding rock assembly under continuously changing internal pressure [J]. Petroleum Exploration and Development, 2015, 42(03): 379-385), the displacement of an interface of the actual wellbore under the condition of internal and external pressure difference is calculated. The process is as follows:

[0010] ① When the cement sheath is in a completely elastic state (seeFigure 1 ) is as follows:

[0011]

[0012] Casing outer wall radial displacement Radial displacement of cement sheath inner wall Radial displacement of cement sheath outer wall Radial displacement of the inner wall of the surrounding rock

[0013] Formula (1) is about P 1 , P 2 The binary equations are given by the casing-cement sheath-surrounding rock combination under the casing internal pressure P i and formation confining pressure P o Under the condition of , solving the equations, we can get the stress and displacement of the assembly at each location in the fully elastic state of cement sheath, and the radial displacement u of the outer wall of casing so The actual wellbore-interface displacement u 1实际 ;

[0014] ① When the cement sheath is in an elastic-plastic state (see Figure 2 ) is as follows:

[0015]

[0016] Casing outer wall radial displacement Radial displacement of the inner boundary of the plastic zone of cement sheath Radial displacement of the outer boundary of the plastic zone of cement sheath Radial displacement of the inner boundary of the elastic zone of cement sheath Radial displacement of the outer boundary of the elastic zone of cement sheath Radial displacement of the inner wall of the surrounding rock

[0017] Formula (2) is about P 1 , P p , P 2 、r p and the integral constant K, given that the casing-cement sheath-surrounding rock combination is subjected to the casing internal pressure P i and formation confining pressure P o Under the condition of , solving the equation group, we can obtain the stress and displacement of the assembly under the elastic-plastic state of cement sheath, and the radial displacement u of the outer wall of casing so The actual wellbore-interface displacement u 1实际 ;

[0018] Where: P i , P o are casing internal pressure and formation confining pressure, MPa, respectively;

[0019] P1 is the contact stress between casing and cement sheath, MPa;

[0020] P p is the elastoplastic interface stress of cement sheath, MPa;

[0021] P 2 is the contact stress between cement sheath and surrounding rock of wellbore, MPa;

[0022] E s 、E c 、E f are the elastic modulus of casing, cement sheath, and wellbore surrounding rock, MPa;

[0023] υ s ,υ c ,υ f are the Poisson's ratios of casing, cement sheath, and wellbore surrounding rock, dimensionless;

[0024] r 1 、r p are the inner radius of casing and the elastic-plastic interface radius of cement sheath, mm;

[0025] r 2 、r 3 、r 4 are the outer radius of casing, cement sheath, and wellbore surrounding rock, mm;

[0026] C is the cohesion of cement paste, MPa;

[0027] is the internal friction angle of cement paste, °;

[0028]

[0029] K is the integration constant, an unknown quantity;

[0030] (2) Based on the principle of equal interface strain, the experimental device (see Figure 3 , Figure 4 ) Equivalent interface displacement u 1装置 :

[0031]

[0032] Where: ε is the interface strain, dimensionless;

[0033] u 1实际 is the actual wellbore-interface displacement, mm;

[0034] u 1装置 is the equivalent interface displacement of the experimental device, mm;

[0035] r 2实际is the actual wellbore casing outer radius, mm;

[0036] r 2装置 is the outer radius of the simulated casing of the experimental device, mm;

[0037] (3) Determine the experimental internal pressure P of the experimental device i实验 and experimental confining pressure P o实验 , the equivalent experimental pressure difference ΔP is the experimental internal pressure P i实验 and experimental confining pressure P o实验 The specific process is as follows:

[0038] From formula (3), we can know that:

[0039] u 1装置 =ε·u 1实际 =ε·u so (4)

[0040] Experimental device equivalent interface displacement u 1装置 Determined by formula (4), u 1装置 The following should be satisfied:

[0041]

[0042] From formula (5), we can know that the experimental internal pressure P i实验 , simulated wellbore equivalent interface contact stress P 1实验 All of them are unknown. To determine the experimental internal pressure and experimental confining pressure of the experimental device, the following steps are required:

[0043] ① Given a specific experimental confining pressure (According to the actual pressure bearing capacity of the device, the experimental confining pressure is 0~50MPa), the experimental internal pressure is Equivalent experimental pressure difference according to Sure;

[0044] ② The geometric parameters and mechanical parameters of the experimental device are introduced into the elastoplastic mechanical model of the cement sheath assembly to determine whether the cement sheath is in a completely elastic or elastoplastic state, and the state of the cement sheath in the experimental device is compared with that of the actual wellbore at P. i , P o If the cement sheath state under the two conditions (completely elastic or elastoplastic state) is the same, then the experimental confining pressure Otherwise, Repeat the above process until the state of the cement sheath in the experimental device is consistent with the actual wellbore at P i , P o The state of cement sheath under the same conditions is consistent, so the experimental confining pressure P is determined o实验 ;

[0045] ③Determine the experimental confining pressure P o实验 After that, a specific experimental internal pressure is given as P o实验 , Bringing it into the elastoplastic mechanical model of cement sheath assembly, the equivalent interface displacement u of the experimental device is calculated. 1 ' 装置 , if u 1 ' 装置 >u 1装置 , then re-take the experimental internal pressure If u 1 ' 装置 <u 1装置 , then re-take the experimental internal pressure Repeat the above process until |u 1 ' 装置 -u 1装置 |≤0.001, thus determining the experimental internal pressure P i实验 ;

[0046] (4) The experimental internal pressure P i实验 , experimental confining pressure P o实验 Substitute it into the elastic-plastic mechanical model of the cement sheath assembly to calculate the radial displacement of the inner wall of the surrounding rock of the experimental device, which is the equivalent two-interface displacement u of the experimental device. 2装置 ;

[0047] (5) Using the experimental device to equivalently displace the first and second interfaces u 1装置 、u 2装置 , respectively calculate the radial displacement u of the inner wall of the simulated casing it , simulated radial displacement of the outer wall of surrounding rock u ot , the calculation formula derivation process is as follows:

[0048] During the equivalent experimental pressure difference loading process, both ends of the simulated casing are subject to fixed constraints. Therefore, it can be considered that the main deformation of the simulated casing is radial deformation rather than axial deformation, and the volume strain of the simulated casing itself is very small. Therefore, in order to use the equivalent interface displacement to calculate the radial displacement of the inner wall of the simulated casing and then conduct subsequent research more conveniently, it can be assumed that the volume of the simulated casing per unit length remains unchanged before and after loading. Based on this assumption, we have:

[0049] V 加载前 =V 加载后 (6)

[0050] That is, the cross-sectional area of ​​the casing end face remains unchanged before and after loading, then:

[0051]

[0052] From formula (7), we can see that Then we have:

[0053]

[0054] Similarly, during the equivalent experimental pressure difference loading process, the volume strain of the simulated surrounding rock (steel pipe) is very small. It can be assumed that the volume of the simulated surrounding rock per unit length remains unchanged before and after loading. By the same token, it can be obtained:

[0055]

[0056] Where: r i装置 (i=1, 2, 3, 4) are respectively the simulated casing inner radius, simulated casing outer radius, simulated surrounding rock inner radius and simulated surrounding rock outer radius, u it To simulate the radial displacement of the inner wall of the casing, u ot To simulate the radial displacement of the outer wall of the surrounding rock, u 1装置 is the equivalent interface displacement of the experimental device, u 2装置 is the equivalent displacement of the two interfaces of the experimental device, both in mm;

[0057] (6) Test the radial displacement of the inner wall of the simulated casing and the outer wall of the simulated surrounding rock of the experimental device, compare the test results with the radial displacement of the inner wall of the simulated casing and the radial displacement of the outer wall of the simulated surrounding rock calculated in step (5), and define the similarity S:

[0058]

[0059]

[0060] Where: S i , S o are the similarities of radial displacement of the inner wall of the simulated casing and the outer wall of the simulated surrounding rock, dimensionless; u it 、u ie are respectively the calculated value and the tested value of the radial displacement of the inner wall of the simulated casing, mm; u ot 、u oe are the calculated and tested values ​​of the radial displacement of the outer wall of the simulated surrounding rock, mm;

[0061] (7) When the similarity satisfies min{S i ,S o When}≥80%, it indicates that the calculated results are in good agreement with the test results. The experimental results under this condition can be used as a basis for evaluating the integrity of the downhole cement sheath.

[0062] Furthermore, the basis for determining whether the cement sheath is in a completely elastic or elastoplastic state is: the geometric dimensions, mechanical parameters and boundary loads of the cement sheath assembly are substituted into the mechanical model of the cement sheath in an elastoplastic state for calculation. If the calculation result satisfies The cement sheath enters an elastoplastic state, otherwise the cement sheath is in a completely elastic state.

[0063] Furthermore, the experimental results can be used as a basis for evaluating the integrity of the underground cement sheath, which means that the circumferential stress distribution of the cement sheath of the experimental device, the development of the plastic zone of the cement sheath and the air crossflow rate of the cement sheath during the experiment can be used as a basis for evaluating the integrity of the underground cement sheath (see Table 1).

[0064] Table 1 Judgment basis and failure form of cement sheath integrity failure

[0065]

[0066] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention effectively combines theoretical research methods with experimental evaluation devices, so that the cement sheath integrity evaluation results are closer to the actual situation downhole; (2) The evaluation method can not only clarify the influence of the geometric dimensions, mechanical properties and interface bonding strength of the cement sheath itself on the integrity of the cement sheath, but also accurately obtain the stress distribution and deformation of the cement sheath assembly under different load conditions, and then clarify the actual stress-strain development law of the cement sheath assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 Schematic diagram of the casing-cement sheath-surrounding rock assembly of an actual wellbore under the elastic state of the cement sheath.

[0068] Figure 2 It is a schematic diagram of the casing-cement sheath-surrounding rock combination of an actual wellbore under the elastoplastic state of the cement sheath.

[0069] Figure 3 Schematic diagram of the simulated casing-cement sheath-simulated surrounding rock combination of the experimental device under the elastic state of the cement sheath.

[0070] Figure 4 Schematic diagram of the simulated casing-cement sheath-simulated surrounding rock combination of the experimental device under the elastoplastic state of the cement sheath. DETAILED DESCRIPTION

[0071] The present invention is further described below with reference to the accompanying drawings and examples so that those skilled in the art can understand the present invention.

[0072] However, it should be clear that those skilled in the art can make various modifications or additions to the specific embodiments described or replace them in similar ways. It should be understood that other embodiments made by ordinary technicians in the art without creative work or equivalent replacement of some technical features without departing from the spirit of the technical solution of the present invention should be included in the scope of protection requested by the present invention.

[0073] The experimental device comprises an upper kettle cover, a lower kettle cover, a simulated casing, simulated surrounding rock, a confining pressure kettle cover, a center positioning shaft and a heating sleeve, wherein the heating sleeve is installed on the outer wall of the confining pressure kettle cover, the confining pressure kettle cover is sealedly connected with the upper kettle cover and the lower kettle cover by bolts and metal cones, the lower kettle cover is fixed on the bottom base, and the confining pressure kettle cover is provided with a confining pressure exhaust port; there are simulated casing and simulated surrounding rock in the kettle, the annular space between the simulated casing and the simulated surrounding rock is filled with cement slurry, and a simulated casing-cement ring-simulated surrounding rock combination is formed after curing; a through hole matching the size of the center positioning shaft is provided at the center position of the end faces of the upper kettle cover and the lower kettle cover, the center positioning shaft is located in the simulated casing, and the center positioning shaft is provided with three through holes distributed at 120 degrees in space, each through hole is installed with a radial displacement sensor for measuring the radial displacement of the inner wall of the simulated casing, and the outer wall of the simulated surrounding rock is installed with a radial displacement sensor for measuring the radial displacement of the outer wall of the simulated surrounding rock.

[0074] Using the above experimental test device, the evaluation method of cement sheath integrity based on equivalent physical experiment includes the following steps in sequence:

[0075] (1) Based on the elastic-plastic mechanical model of the casing-cement sheath-surrounding rock assembly, the displacement of an interface of the actual wellbore under the condition of internal and external pressure difference is calculated. The actual wellbore geometric dimensions and mechanical parameters are shown in Table 2:

[0076] Table 2 Wellbore geometric dimensions and mechanical parameters

[0077]

[0078] Through calculation, it can be known that the cement sheath is in an elastic-plastic state, and the actual wellbore-interface displacement u 1实际 0.0798mm;

[0079] (2) Based on the principle of equal interface strain, the equivalent interface displacement u of the experimental device is calculated using formula (3): 1装置 The geometric dimensions and mechanical parameters of the experimental device are shown in Table 3:

[0080] Table 3 Geometric dimensions and mechanical parameters of the experimental device

[0081]

[0082] (3) The experimental internal pressure P can be obtained by calculation i实验 The experimental confining pressure P is 45.8MPa. o实验 10MPa;

[0083] (4) The experimental internal pressure P i实验 , experimental confining pressure P o实验Substituting it into the elastic-plastic mechanical model of the cement sheath assembly, the radial displacement of the inner wall of the surrounding rock of the wellbore is calculated, that is, the equivalent two-interface displacement u of the experimental device 2装置 0.00566mm;

[0084] (5) Using the experimental device to equivalently displace the first and second interfaces u 1装置 、u 2装置 , the radial displacement of the simulated casing inner wall and the radial displacement of the simulated surrounding rock outer wall are calculated by formula (8) and formula (9) to be 0.04365mm and 0.00532mm respectively;

[0085] (6) The radial displacement of the simulated casing inner wall and the radial displacement of the simulated surrounding rock outer wall of the test device are 0.04779 mm and 0.00629 mm respectively. According to formula (10) and formula (11), the similarity S of the simulated casing inner wall radial displacement can be obtained. i , similarity S of radial displacement of outer wall of simulated surrounding rock o 91.33% and 84.58% respectively;

[0086] (7) Similarity satisfies min{S i ,S o}≥80%, indicating that the calculated results are in good agreement with the test results. The experimental results under this condition can be used as the basis for evaluating the integrity of the underground cement sheath. The failure form of the cement sheath integrity is shown in Table 4:

[0087] Table 4 Judgment of cement sheath failure form

[0088]

Claims

1. The cement sheath integrity evaluation method based on equivalent physical experiment includes the following steps in sequence: (1) Based on the elastic-plastic mechanical model of the casing-cement sheath-surrounding rock assembly, the casing-cement sheath-surrounding rock assembly is referred to as the cement sheath assembly. The displacement of an interface under the internal and external pressure difference of the actual wellbore is calculated as follows: The mechanical model when the cement sheath is in a completely elastic state is as follows: （1） Casing outer wall radial displacement ; Radial displacement of cement sheath inner wall ; Radial displacement of cement sheath outer wall ; Radial displacement of the inner wall of the surrounding rock ; It is known that the casing-cement sheath-surrounding rock combination is subject to casing internal pressure and formation confining pressure Under the condition of , the equation group is solved to obtain the radial displacement of the outer wall of the casing under the fully elastic state of the cement sheath: , which is the actual wellbore-interface displacement ; The mechanical model of the cement sheath in the elastic-plastic state is as follows: （2） Casing outer wall radial displacement ; Radial displacement of the inner boundary of the plastic zone of cement sheath ; Radial displacement of the outer boundary of the plastic zone of cement sheath ; Radial displacement of the inner boundary of the elastic zone of cement sheath ; Radial displacement of the outer boundary of the elastic zone of cement sheath ; Radial displacement of the inner wall of the surrounding rock ; It is known that the casing-cement sheath-surrounding rock combination is subject to casing internal pressure and formation confining pressure Under the condition of , the equation group is solved to obtain the radial displacement of the outer wall of the casing under the elastic-plastic state of the cement sheath. , which is the actual wellbore-interface displacement ; Where: , are casing internal pressure and formation confining pressure, MPa, respectively; is the contact stress between casing and cement sheath, MPa; is the elastoplastic interface stress of cement sheath, MPa; is the contact stress between cement sheath and surrounding rock of wellbore, MPa; , , are the elastic modulus of casing, cement sheath, and wellbore surrounding rock, MPa; , , are the Poisson's ratios of casing, cement sheath, and wellbore surrounding rock, dimensionless; , are the inner radius of casing and the elastic-plastic interface radius of cement sheath, mm; , , are the outer radius of casing, cement sheath, and wellbore surrounding rock, mm; is the cohesion of cement paste, MPa; is the internal friction angle of cement paste, ; ； =1； is the integration constant, unknown quantity; (2) Based on the principle of equal interface strain, the equivalent interface displacement of the experimental device is calculated using formula (3): : （3） Where: is the interface strain, dimensionless; is the actual wellbore-interface displacement, mm; is the equivalent interface displacement of the experimental device, mm; is the actual wellbore casing outer radius, mm; is the outer radius of the simulated casing of the experimental device, mm; (3) Determine the experimental internal pressure of the experimental device Experimental confining pressure , equivalent experimental pressure difference The experimental internal pressure Experimental confining pressure The specific process is as follows: Given a specific experimental confining pressure The experimental internal pressure is , equivalent experimental pressure difference , according to Sure; Will , The geometric parameters and mechanical parameters of the experimental device are brought into the elastoplastic mechanical model of the cement sheath assembly to determine whether the cement sheath is in a completely elastic or elastoplastic state, and the state of the cement sheath in the experimental device is compared with that in the actual wellbore. , If the cement sheath state under the two conditions is the same, then the experimental confining pressure , otherwise let Repeat the above process until the state of the cement sheath in the experimental device is consistent with the actual wellbore , The state of the cement sheath is consistent under the conditions, so as to determine the experimental confining pressure ; Determine the experimental confining pressure After that, a specific experimental internal pressure is given as ,Will , The equivalent interface displacement of the experimental device is calculated by bringing it into the elastoplastic mechanical model of the cement sheath assembly. ,like , then re-take the experimental internal pressure ,like , then re-take the experimental internal pressure , repeat the above process until , thereby determining the experimental internal pressure ; (4) Increase the internal pressure of the test , Experimental confining pressure Substitute it into the elastic-plastic mechanical model of the cement sheath assembly to calculate the radial displacement of the inner wall of the surrounding rock of the experimental device, which is the equivalent two-interface displacement of the experimental device. ; (5) Using the experimental device to calculate the equivalent displacement of the first and second interfaces , , respectively calculate the radial displacement of the inner wall of the simulated casing , simulate the radial displacement of the outer wall of the surrounding rock : （8） （9）； Where: , , , They are respectively the simulated casing inner radius, the simulated casing outer radius, the simulated surrounding rock inner radius, and the simulated surrounding rock outer radius. To simulate the radial displacement of the inner wall of the casing, To simulate the radial displacement of the outer wall of the surrounding rock, is the equivalent interface displacement of the experimental device, is the equivalent displacement of the two interfaces of the experimental device, both in mm; (6) Test the radial displacement of the simulated casing inner wall and the simulated surrounding rock outer wall of the experimental device, and compare the test results with the radial displacement of the simulated casing inner wall and the simulated surrounding rock outer wall calculated in step (5): （10） （11）； Where: , They are the similarity of radial displacement of the inner wall of the simulated casing and the outer wall of the simulated surrounding rock, dimensionless; , are the calculated and tested values ​​of the radial displacement of the inner wall of the simulated casing, mm; , are the calculated and tested values ​​of the radial displacement of the outer wall of the simulated surrounding rock, mm; (7) If satisfied , indicating that the calculated results are in good agreement with the test results, and the experimental results under this condition can be used as a basis for evaluating the integrity of the downhole cement sheath.

2. The cement sheath integrity evaluation method based on equivalent physical experiment according to claim 1, characterized in that: The basis for determining whether the cement sheath is in a completely elastic or elastoplastic state is: the geometric dimensions, mechanical parameters and boundary loads of the cement sheath assembly are substituted into the mechanical model of the cement sheath in an elastoplastic state for calculation. If the calculation result satisfies , the cement sheath enters an elastoplastic state, otherwise the cement sheath is in a completely elastic state.

3. The cement sheath integrity evaluation method based on equivalent physical experiment according to claim 1, characterized in that: The experimental results can be used as a basis for evaluating the integrity of the underground cement ring, which means that the integrity of the underground cement ring is evaluated based on the circumferential stress distribution of the cement ring of the experimental device, the development of the plastic zone of the cement ring and the air crossflow rate of the cement ring during the experiment.

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