Cement sheath plastic strain accumulation evaluation method under large-scale staged fracturing working condition

Abstract

The application discloses a cement sheath plastic strain accumulation evaluation method under large-scale staged fracturing conditions, and is applied to the technical field of oil and gas well engineering. The existing evaluation methods do not fully consider the influence of temperature on the mechanical properties of cement stone and the plastic strain accumulation law of cement stone under high-temperature environment, and there is still a large difference between the evaluation results and the actual situation. The application prepares cement stone samples maintained under different temperature environments, carries out triaxial compression tests on the cement stone under different temperatures, establishes a functional relationship between the mechanical parameters of the cement stone and the test temperature, carries out triaxial cyclic loading and unloading tests on the cement stone under a specific temperature, obtains the cyclic loading and unloading stress-strain curve of the cement stone, and establishes a cement stone nonlinear elastic-plastic constitutive model considering the temperature effect and the cyclic attenuation characteristics. Based on the constitutive model, numerical simulation of the mechanical response of the cement sheath under large-scale staged fracturing conditions and evaluation of the plastic strain accumulation law of the cement sheath are carried out.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas well engineering technology, and specifically relates to a cement ring plastic strain cumulative evolution evaluation technology. Background Technology

[0002] Tight oil and gas resources (shale gas, shale oil, tight sandstone gas, etc.) have abundant reserves and broad development prospects in my country. These reservoirs are characterized by oil and natural gas resources being located within much smaller pores and fractures compared to conventional reservoirs. After drilling reaches the reservoir, these resources cannot flow into the wellbore on their own to form industrial oil and gas flows, necessitating thorough reservoir stimulation. For these tight oil and gas resources, a relatively mature current method for reservoir stimulation is drilling horizontal wells and performing large-scale staged fracturing. This artificially creates more fractures in the reservoir, increasing the seepage channels for oil and gas flows, thereby effectively extracting the oil and gas resources from the tiny pores and fractures.

[0003] The wellbore cement sheath is a ring-shaped cement stone structure located between the casing and the wellbore. It serves to seal the formation, reinforce the wellbore, and ensure a well-sealed flow channel. During large-scale staged fracturing in unconventional oil and gas wells, the casing pressure undergoes significant and frequent fluctuations. Consequently, the cement sheath experiences dozens of cyclic loading and unloading processes, leading to a continuous accumulation of plastic strain. Plastic strain is the irreversible residual deformation after cyclic loading and unloading. The continuous increase in cement sheath plastic strain will lead to two consequences: firstly, the cement sheath itself may develop macroscopic cracks due to excessive plastic deformation; secondly, the cement sheath may thin due to continuous plastic compression, causing interface bonding failure between the cement sheath, casing, and formation, forming micro-annular gaps. The combined effect of these two factors will cause cement sheath sealing failure, further leading to problems such as wellhead annular pressure, oil and gas leakage, and seriously threatening the long-term safe production of unconventional oil and gas wells. Therefore, evaluating the cumulative evolution of cement sheath plastic strain under large-scale staged fracturing conditions has significant engineering value.

[0004] Currently, there are few evaluation methods specifically designed for the cumulative evolution of cement sheath plastic strain under large-scale segmented fracturing conditions. In reference CN 116698631A (A Method for Predicting Micro-Annular Gap at Cement Sheath Interface in Oil and Gas Wells under Multiple Alternating Loads), steps six and seven determine the stress-strain relationship of cement stone under cyclic triaxial compressive stress. Based on the cyclic triaxial test results of cement stone, a functional relationship between the plastic strain of cement stone and the number of cycles is established to predict the plastic strain of the cement sheath. This functional relationship is based on a test curve under a certain constant compressive stress. However, in actual downhole conditions, the pressure during segmented fracturing operations is constantly changing, and the pressure peaks during each segment are basically different. The stress acting on the cement sheath is also dynamically changing. Therefore, using a fixed functional relationship to predict the plastic strain of the cement sheath in the literature will inevitably introduce significant errors. In other relevant literature (Generation and Evolution of Micro-Annular Gap under Cyclic Loading, Fault Block Oil and Gas Field, 2020, 27(4); Experimental and Numerical Study on the Influence of Cyclic Loading on the Sealing Performance of Cement Rings, Petroleum Machinery, 2021, 49(2)), the cement ring body is set as an ideal elastic-plastic model, and the cement ring at the interface is simulated using Cohesive elements to simulate the plastic deformation of the cement ring near the interface. This type of method does not take into account that the cement stone will still produce nonlinear deformation after yielding, rather than completely entering the ideal plastic state. Moreover, as the number of cyclic loading and unloading increases, the plastic strain of the cement ring generated in a single cycle usually shows a nonlinear decreasing law, while the cumulative plastic strain gradually shows a nonlinear slow increasing trend. However, the results given in the literature show a linear increasing trend, which is quite different from the actual situation and needs to be improved. In addition, none of the relevant literature gives an evaluation method for the cumulative evolution law of the plastic strain of cement rings caused by temperature changes.

[0005] It can be seen that the existing evaluation methods are few and relatively simple, and the evaluation results still differ greatly from the actual situation. The main reason is that the influence of temperature on the mechanical properties of cement stone and the cumulative law of plastic strain of cement stone under high temperature environment have not been fully considered. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes an evaluation method for the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, taking into account temperature effects. This method can effectively evaluate the cumulative plastic strain law of cement sheaths in deep formations under large-scale segmented fracturing conditions in the field.

[0007] The technical solution adopted in this invention is: a method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, comprising:

[0008] S1. Prepare multiple batches of cement stone samples, and cure each batch of cement stone samples under different temperature conditions.

[0009] S2. Conduct triaxial compression tests on cement stone at different temperatures to obtain the mechanical parameters of cement stone samples at different temperatures; based on the stress-strain curves and mechanical parameters of cement stone samples at different temperatures, use scatter fitting to establish the functional relationship between the mechanical parameters of cement stone and the test temperature.

[0010] S3. Conduct a triaxial cyclic loading and unloading test on cement stone at a specific temperature, where the specific temperature is the temperature of the deep formation; obtain the stress-strain curve of cement stone under cyclic loading and unloading through the triaxial cyclic loading and unloading test, and further extract key data points on the curve to obtain the evolution law of plastic strain of cement stone with the number of cycles.

[0011] S4. Establish a nonlinear elastoplastic constitutive model for cement stone that considers temperature effects and cyclic decay characteristics. This constitutive model is divided into an initial linear elastic segment and a subsequent nonlinear elastoplastic segment, with the elastic limit stress as the boundary. The scaling factor of the initial linear elastic segment is obtained based on the functional relationship between the mechanical parameters of cement stone and the test temperature established in step S2.

[0012] The constitutive model for the subsequent nonlinear elastoplastic segment is obtained by nonlinear fitting of the high-temperature triaxial compressive stress-strain curve of cement stone.

[0013] S5. Based on the nonlinear elastoplastic constitutive model of cement stone considering temperature effect and cyclic decay characteristics established in step S4, a three-dimensional geomechanical model of casing-cement sheath-formation is established using a finite element platform.

[0014] S6. Based on the three-dimensional geomechanical model of casing-cement sheath-formation established in step S5, conduct numerical simulation of the mechanical response of cement sheath under large-scale segmented fracturing conditions.

[0015] S7. Based on the numerical simulation results of step S6, evaluate the cumulative law of plastic strain in cement sheath under large-scale segmented fracturing conditions.

[0016] The beneficial effects of this invention are as follows: The evaluation model established by this invention comprehensively considers the influence of temperature on the mechanical properties of cement sheath, the nonlinear characteristics of cement stone during cyclic loading and unloading, and the attenuation effect of plastic strain. It can effectively evaluate the cumulative law of plastic strain of cement sheath in deep formations under large-scale segmented fracturing conditions in the field. Attached Figure Description

[0017] Figure 1 This is a flowchart of the solution of the present invention;

[0018] Figure 2 The curve showing the fitting of the elastic modulus of cement stone with curing and test temperatures;

[0019] Figure 3 Stress-strain curves for cyclic loading and unloading of cement stone;

[0020] Figure 4 The evolution law of plastic strain of cement stone with the number of cycles;

[0021] Figure 5 The plastic strain lapse rate of cement stone;

[0022] Figure 6 For comparison between the constitutive model of the cement ring and the experimental results;

[0023] Figure 7 A three-dimensional geomechanical model of casing-cement sheath-stratum at the field scale;

[0024] Figure 8 This is a plastic strain contour plot of a cement ring.

[0025] Figure 9 The evolution of plastic strain in the cement ring during 27 staged fracturing processes. Detailed Implementation

[0026] To facilitate understanding of the technical content of this invention by those skilled in the art, the following description, in conjunction with the accompanying drawings, further illustrates the invention.

[0027] This invention provides a method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects. Figure 1 As shown, it includes the following steps:

[0028] (1) Prepare cement stone samples and cure them under different temperature conditions; wherein, the cement slurry formula for preparing cement stone is selected as needed, and the cement slurry formula used for field cementing can be selected, or the cement slurry formula newly developed in the laboratory can be selected; wherein, considering that the mechanical properties of cement stone are greatly affected by temperature, the present invention should set no less than 3 temperature points for curing, and the highest temperature should not be lower than the temperature of the deep formation being studied, and the other temperature points can be set at equal intervals between the normal temperature and the highest temperature; wherein, the high temperature curing time should not be less than 3 days, and the total curing time should not be less than 28 days, so as to ensure that the cement slurry is fully hydrated and hardened into cement stone.

[0029] (2) Conduct triaxial compression tests of cement stone at different temperatures; wherein, the triaxial compression tests of cement stone are conducted on a high-temperature triaxial mechanical testing machine with heating function; wherein, the temperature of the triaxial compression test is kept consistent with the temperature during sample curing, so as to simulate the cement slurry solidifying and hardening into cement stone at a constant stratum temperature, and undergoing deformation and failure under load at this specific temperature; wherein, through the triaxial compression test, the stress-strain curves, compressive strength, elastic modulus and other mechanical parameters of cement stone samples at different temperatures are obtained; wherein, through the obtained mechanical parameters of cement stone at different temperatures, the functional relationship between the mechanical parameters of cement stone and the test temperature can be established by using scatter fitting.

[0030] (3) Conduct triaxial cyclic loading and unloading tests on cement stone at specific temperatures; the test temperature should preferably be set to a higher temperature, close to the temperature of the deep formation, so as to reasonably reflect the cumulative plastic strain of cement stone under the high temperature environment in the deep formation; in the cyclic loading and unloading test, the upper limit stress should preferably be no less than 50% of the triaxial compressive strength, and the lower limit stress should be as low as possible, so that the cement stone can change within a larger stress range and fully reflect its cumulative plastic strain evolution law; in the cyclic loading and unloading test, the number of cycles should be set with reference to the number of stages of large-scale segmented fracturing in the field, and should preferably be no less than the number of stages of segmented fracturing; through the triaxial cyclic loading and unloading test, the stress-strain curve of cement stone can be obtained, and the key data points on the curve can be extracted to obtain the evolution law of plastic strain of cement stone with the number of cycles.

[0031] The temperature of deep formations can be measured during drilling or estimated based on the geothermal gradient. Assuming an average surface air temperature of 15℃ and a geothermal gradient of 3℃ / 100 meters, the temperature of a formation at a depth of 4000 meters would be approximately 15 + 3 × 4000 / 100 = 135℃. The setting should be close to the deep formation temperature; this temperature setting can be the same as the formation temperature or an integer value close to it. For example, if the formation temperature is 127℃, this temperature could be set to 125℃ or 130℃.

[0032] The lower limit stress can be set to a value slightly greater than zero, such as 1 to 2 kN. The main purpose is to ensure that the specimen does not detach from the pressure head of the testing machine and remains under pressure.

[0033] The minimum and maximum stress values ​​remain constant in each cycle. The stress increases uniformly from the minimum to the maximum value and then decreases uniformly back to the minimum value. Here, "a larger range of stress variation" means that the difference between the maximum and minimum values ​​is relatively large, that is, the amplitude of stress variation is relatively large.

[0034] (4) Establish a nonlinear elastoplastic constitutive model of cement stone that considers temperature effects and cyclic decay characteristics; wherein, the constitutive model is divided into an initial linear elastic segment and a subsequent nonlinear elastoplastic segment with the elastic limit stress as the boundary; wherein, in the linear elastic segment of the model, the stress-strain of cement stone is linearly related, and the proportionality coefficient is the elastic modulus of cement stone. Based on the functional relationship between the mechanical parameters of cement stone and the test temperature established in step (2), the elastic modulus is calculated and determined by the actual formation temperature; wherein, in the nonlinear elastoplastic segment of the model, the cement stone constitutive model in the elastoplastic stage can be obtained by nonlinear fitting of the high-temperature triaxial compressive stress-strain curve of cement stone; wherein, the cement stone constitutive model in the elastoplastic stage includes a plastic strain decay coefficient, and the setting of the decay coefficient is based on the nonlinear decreasing law of the plastic residual strain generated under each single loading and unloading cycle measured in step (3) with the number of cycles.

[0035] The elastic limit of cement stone is determined based on its triaxial compressive stress-strain curve. The initial stage of the stress-strain curve is approximately linear. After the stress exceeds a certain value, the curve bends and enters the nonlinear stage. The stress value corresponding to the transition point from linear to nonlinear is the elastic limit stress.

[0036] (5) Establish a three-dimensional geomechanical model of casing-cement sheath-formation; the model can be established based on large general-purpose finite element platforms such as ABAQUS and ANSYS; the model involves the size parameters of casing, cement sheath and formation; the model involves the mechanical parameters of casing, cement sheath and formation; the mechanical parameters of cement sheath are based on the temperature corresponding to the burial depth of formation, and adopt the nonlinear elastoplastic constitutive model of cement stone that considers temperature effect and has cyclic decay characteristics given in step (4).

[0037] (6) Conduct numerical simulation of the mechanical response of the cement sheath under large-scale segmented fracturing conditions; before numerical simulation, based on the formation depth, the formation part of the three-dimensional geomechanical model of casing-cement sheath-formation established in step (5) needs to be subjected to triaxial initial stress to simulate the effect of the in-situ stress field, and hydrostatic pressure is applied to the inner wall of the casing; based on the large-scale segmented fracturing construction data, the number of cyclic loading and unloading and the peak value of the ground construction pressure for each cycle need to be set; in the simulation calculation, the ground construction pressure is superimposed on the hydrostatic pressure on the inner wall of the casing. For each segment of fracturing construction, the ground construction pressure is superimposed on the inner wall of the casing and then unloaded. The number of cyclic loading and unloading is consistent with the number of fracturing segments in the field to realize the simulation of the mechanical response of the cement sheath under large-scale segmented fracturing conditions. Through numerical simulation, the stress and deformation of the cement sheath at different fracturing construction stages can be obtained, and specifically stored and presented in the form of stress field, deformation field and plastic deformation field.

[0038] (7) Evaluate the cumulative law of plastic strain of cement sheath under large-scale segmented fracturing conditions. Specifically: Based on the constitutive model obtained in step (4), then perform the geomechanical modeling in step (5), and then carry out the numerical calculation in step (6). The numerical calculation adopts the relatively conventional finite element method, which can generally be realized with the help of general finite element calculation software. The calculation results of stress field and plastic strain field of cement sheath in different stages of simulation can be obtained. Among them, by extracting the numerical calculation results, the overall plastic strain cloud map of cement sheath in each segment of fracturing process can be obtained, the variation law of plastic strain of cement sheath from the inside to the outside, the maximum and minimum values ​​of plastic strain and their locations can be obtained. Among them, by further extracting the plastic strain value under each fracturing cycle, the cumulative evolution of plastic strain of cement sheath in the entire fracturing cycle process can be obtained. This invention establishes a pattern, thereby enabling the analysis and evaluation of the cumulative evolution of plastic strain in the cement sheath under large-scale segmented fracturing conditions. For example, the maximum value and location of the plastic strain in the cement sheath after each fracturing segment are obtained. Then, the maximum values ​​calculated from each fracturing segment are summed to obtain the final cumulative value of the plastic strain. This is the cumulative evolution of the plastic strain in the cement sheath under large-scale segmented fracturing conditions. By further combining some discrimination criteria, the state of the cement sheath at this time can be determined, whether it has been damaged or has formed an interfacial micro-annulus, thus evaluating the sealing integrity of the cement sheath. The main purpose of this invention is to solve how to calculate the cumulative value of the plastic strain in the cement sheath relatively reasonably and accurately. The discrimination criteria and evaluation are not the focus of this invention and will not be described in detail here.

[0039] Example

[0040] In this embodiment, the method proposed in this invention will be used to evaluate the cumulative evolution of plastic strain in the cement sheath under large-scale staged fracturing conditions in a deep shale gas well. The specific implementation process is as follows:

[0041] (1) Preparation of cement stone samples and curing under different temperature conditions. Grade G oil well cement was used to prepare cement stone samples. The specific formula of the cement slurry was: 100% pure cement + 4% water loss reducer + 40% water + 0.25% defoamer (mass fraction). The mixed cement slurry was poured into a cylindrical mold, and then placed in a high-temperature and high-pressure curing autoclave for 3 days. Afterwards, it was removed from the mold and placed in a normal temperature and pressure water bath for curing for 28 days. Cement stone samples were prepared in 4 batches, with the same formula for each batch. The curing temperature and pressure in the high-temperature and high-pressure autoclave were set to 25℃-20MPa, 90℃-20MPa, 115℃-20MPa, and 140℃-20MPa, respectively. After curing, the cement stone samples were cut and polished to obtain cylindrical standard specimens with a diameter of 50mm and a height of 100mm, which were used for subsequent tests.

[0042] (2) Triaxial compression tests were conducted on cement stone at different temperatures. Triaxial compression tests were performed on four batches of cement stone samples. The test temperature and confining pressure were consistent with the temperature and pressure during curing in the high-temperature and high-pressure curing autoclave. For example, for the first batch of samples, the curing temperature and confining pressure were 25℃-20MPa, and the corresponding temperature and confining pressure during the triaxial compression test were 25℃ and 20MPa. For the second batch of samples, the curing temperature and confining pressure were 90℃-20MPa, and the corresponding temperature and confining pressure during the triaxial compression test were 90℃ and 20MPa, and so on. Through the tests, the stress-strain curves, compressive strength, elastic modulus, and other mechanical parameters of the cement stone samples at different temperatures were obtained. Figure 2 As shown, a functional relationship between the elastic modulus of cement paste and curing and testing temperatures was established through linear fitting of multiple data sets:

[0043] E(T) = -0.03166T + 8.832

[0044] In the formula: E(T) is the elastic modulus of cement stone, GPa; T is the temperature, °C.

[0045] (3) Conduct triaxial cyclic loading and unloading tests on cement stone at specific temperatures. Select the remaining cement stone samples from the second batch (curing temperature and pressure: 90℃-20MPa) and conduct high-temperature triaxial cyclic loading and unloading tests. The test temperature and confining pressure are set to 90℃ and 20MPa, respectively. Using the compressive strength measured in the previous triaxial compression test as a benchmark, the upper limit stress of the cyclic load is set to 70% of the compressive strength, and the lower limit stress is set to a small value (1.5kN) to ensure the sample does not detach from the testing machine indenter and remains under pressure. The number of cyclic loading and unloading cycles is set to 30. Throughout the test, axial pressure control is used, and both loading and unloading rates are set to 0.5kN / s. Through the triaxial cyclic loading and unloading test, the following results were obtained: Figure 3 The stress-strain curves of the cement stone under cyclic loading and unloading shown can be further extracted to obtain data such as... Figure 4 The evolution of plastic strain in cement stone with the number of cycles is shown below. Figure 4 It can be seen that the plastic strain of cement stone increases rapidly in the early stage, and slows down in the middle and late stages, with a significant decay effect in the increment of plastic strain generated by each cycle. Plastic strain is the irreversible residual deformation after loading and unloading. The second batch of cement stone samples was selected here because a larger number of samples were prepared in the second batch, with some used for triaxial compression and others for cyclic loading and unloading.

[0046] (4) Establish a nonlinear elastoplastic constitutive model for cement stone that considers temperature effects and cyclic decay characteristics. When the compressive stress on the cement stone does not exceed the elastic limit stress, the cement stone is in the linear elastic stage, and its constitutive model can be expressed by the following equation.

[0047]

[0048] In the formula: σ (i) The stress for the i-th loading cycle is given in MPa; σ e (i) Let ε be the elastic limit stress of the i-th loading cycle, in MPa; (i) For the strain of the i-th loading cycle, %; ε p (i-1) denoted as , representing the residual strain generated during the (i-1)th loading cycle, in %; T represents the formation temperature, in ℃; E(T) represents the elastic modulus of cement stone at different formation temperatures, obtained by fitting test data of cement stone at different temperatures, in GPa.

[0049] When the compressive stress on the cement stone exceeds the elastic limit stress, the cement stone will begin to undergo plastic deformation. By nonlinearly fitting the high-temperature triaxial compressive stress-strain curve of the cement stone, the constitutive model of the cement stone in the elastoplastic stage can be obtained.

[0050] In the formula: σ (i) — Compressive strength during the i-th loading cycle, in MPa; — Residual strain generated in the i-th loading cycle, %; α(i) — Plastic strain attenuation coefficient in the i-th loading cycle, dimensionless.

[0051] The experimental observations in the previous step showed that the plastic residual strain generated by the cyclic loading and unloading of cement stone first increased rapidly with the increase of the number of loading cycles, and then increased slowly. The increment of plastic strain generated by each cycle had a significant decay effect. Therefore, the plastic residual strain of a single loading and unloading cycle was set to increase with the number of cycles as follows: Figure 5 The nonlinear law shown decreases and is integrated into the above elastoplastic constitutive model in the form of a decay coefficient α(i).

[0052] The attenuation coefficient α(i) is determined as follows: based on the observed plastic strain accumulation law during the cyclic loading and unloading process, the plastic strain generated by a single cyclic loading and unloading decreases rapidly in the first few cycles and then remains at a low level. The attenuation coefficient α(i) is set with reference to this change law. Taking the plastic strain generated in the first cycle as the benchmark, the attenuation coefficients for the first to the 30th cycles are set to 1, 0.95, 0.85, 0.70, 0.50, 0.25, 0.20, 0.15, 0.10, 0.09, 0.08, 0.07, 0.06, 0.06, 0.06, 0.06, ..., 0.06.

[0053] like Figure 6As shown, stress-strain curves were plotted based on the proposed constitutive model and compared with the measured stress-strain curves of cement stone in the laboratory. The two curves showed good agreement, verifying the effectiveness of the constitutive model.

[0054] (5) Establish a three-dimensional geomechanical model of the casing-cement sheath-formation. Using the ABAQUS large-scale general-purpose finite element analysis platform, a three-dimensional geomechanical model of the casing-cement sheath-formation was established based on field engineering and geological parameters. Specifically, a three-dimensional model was established for the horizontal section of the shale gas well, such as... Figure 7 As shown, the overall dimensions of the model are 4000mm × 4000mm × 2000mm; the outer diameter of the production casing is 139.7mm, and the wall thickness is 12.34mm; the outer diameter of the cement sheath is 227mm. The casing is made of P110 steel grade, using a linear elastic constitutive model with an elastic modulus of 210GPa, Poisson's ratio of 0.3, and a yield strength of 758MPa; the formation uses a linear elastic constitutive model with an elastic modulus of 25GPa and a Poisson's ratio of 0.2, based on reference values ​​from previous shale mechanics tests; the horizontal section of the well is approximately 3650m deep, and the formation temperature is approximately 120℃. The cement sheath uses a nonlinear elastoplastic constitutive model of cement stone with cyclic decay characteristics, based on the model given in the previous step. At this temperature, the elastic modulus of the cement sheath is 5.033GPa.

[0055] (6) Numerical simulation of the mechanical response of the cement sheath under large-scale segmented fracturing conditions was conducted. The horizontal section of the well was buried at a depth of 3650m. Triaxial initial stress was applied to the formation to simulate the effect of the in-situ stress field. The maximum and minimum vertical and horizontal in-situ stresses were 85, 80, and 75 MPa, respectively. A hydrostatic column pressure was applied to the inner wall of the casing, and the hydrostatic column pressure inside the casing was set to 36.5 MPa. According to the large-scale segmented fracturing construction data, a total of 27 fracturing operations were completed in this well. The peak construction pressure of each segment was distributed between 84.5 and 109.7 MPa (the highest construction pressures of each segment were 109.7, 106.5, 106.2, 106.6, 97.67, 98.2, 103.5, 98.7, 108.1, 106.3, 97.8, 103.9, 101.1, 92.7, 100.8, 98.9 MPa, respectively). The simulation calculations used pressures of 9.8, 95.8, 88.5, 86.3, 84.5, 91.7, 96.7, 91.3, 94.5, 95.7, 91.8, and 97.4 MPa. The ground construction pressure was superimposed on the hydrostatic column pressure on the inner wall of the casing. For each fracturing operation, the ground construction pressure was superimposed on the inner wall of the casing, and then removed. This process was repeated 27 times to simulate the mechanical response of the cement sheath under large-scale segmented fracturing conditions.

[0056] (7) Evaluation of the cumulative plastic strain law of the cement sheath under large-scale segmented fracturing conditions. Numerical calculations show that after the first stage of fracturing, plastic strain is generated inside the cement sheath. The plastic strain decreases from the inside to the outside, with the largest plastic strain on the inner side of the cement sheath, reaching 1.406 × 10⁻⁶. -3 The damage on the outside is relatively small, such as Figure 8 As shown, it is only 0.1297×10 -3 As the staged fracturing operation progressed, the plastic strain within the cement sheath accumulated continuously. The increase was rapid in the early stages of the staged fracturing operation, slowing down in the middle and later stages. After 27 stages of fracturing, as... Figure 9 As shown, the final cumulative plastic strain on the inner side of the cement ring is 13.18 × 10⁻⁶. -3 The cumulative variation law of plastic strain of cement sheath under large-scale segmented fracturing conditions obtained by the method of this invention has good similarity with the overall law of laboratory tests, indicating the rationality and effectiveness of the method.

[0057] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the scope of the claims of the invention.

Claims

1. A method for evaluating the cumulative evolution of plastic strain in a cement sheath under large-scale segmented fracturing conditions, considering temperature effects, characterized in that... include: S1. Prepare multiple batches of cement stone samples, and cure each batch of cement stone samples under different temperature conditions. S2. Conduct triaxial compression tests on cement stone at different temperatures to obtain the mechanical parameters of cement stone samples at different temperatures; based on the stress-strain curves and mechanical parameters of cement stone samples at different temperatures, use scatter fitting to establish the functional relationship between the mechanical parameters of cement stone and the test temperature. S3. Conduct a triaxial cyclic loading and unloading test on cement stone at a specific temperature, where the specific temperature is the temperature of the deep formation; obtain the stress-strain curve of cement stone under cyclic loading and unloading through the triaxial cyclic loading and unloading test, and further extract key data points on the curve to obtain the evolution law of plastic strain of cement stone with the number of cycles. S4. Establish a nonlinear elastoplastic constitutive model for cement stone that considers temperature effects and cyclic decay characteristics. This constitutive model is divided into an initial linear elastic segment and a subsequent nonlinear elastoplastic segment, with the elastic limit stress as the boundary. The scaling factor of the initial linear elastic segment is obtained based on the functional relationship between the mechanical parameters of cement stone and the test temperature established in step S2. The constitutive model for the subsequent nonlinear elastoplastic segment is obtained by nonlinear fitting of the high-temperature triaxial compressive stress-strain curve of cement stone. S5. Based on the nonlinear elastoplastic constitutive model of cement stone considering temperature effect and cyclic decay characteristics established in step S4, a three-dimensional geomechanical model of casing-cement sheath-formation is established using a finite element platform. S6. Based on the three-dimensional geomechanical model of casing-cement sheath-formation established in step S5, conduct numerical simulation of the mechanical response of cement sheath under large-scale segmented fracturing conditions. S7. Based on the numerical simulation results of step S6, evaluate the cumulative law of plastic strain in cement sheath under large-scale segmented fracturing conditions.

2. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 1, is characterized in that... In step S1, the different temperature environments corresponding to the maintenance include at least three temperature points, and the highest temperature is not lower than the temperature of the deep stratum being studied. The remaining temperature points are set at equal intervals between the normal temperature and the highest temperature.

3. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 1, is characterized in that... In step S2, the temperature of the triaxial compression test is kept consistent with the temperature during specimen curing.

4. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 3, is characterized in that... The mechanical parameters of cement stone include: compressive strength and elastic modulus.

5. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 1, is characterized in that... In step S3, during the cyclic loading and unloading test, the upper limit stress shall not be less than 50% of the compressive strength measured by the triaxial compression test at the specific temperature in step S2, and the lower limit stress shall be in the range of 1 to 2 kN. When stress is applied, the stress changes at a constant rate from the lower limit to the upper limit; when stress is unloaded, the stress changes at a constant rate from the upper limit to the lower limit.

6. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 5, is characterized in that... The number of cycles should not be less than the number of stages in the staged fracturing.

7. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 1, is characterized in that... In step S4, the elastic limit stress of the cement stone is determined based on the triaxial compressive stress-strain curve of the cement stone.

8. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 7, is characterized in that... The scaling factor of the initial linear elastic segment is the elastic modulus.

9. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 8, is characterized in that... The constitutive model of the nonlinear elastoplastic segment includes a plastic strain attenuation coefficient, which is set based on the nonlinear decreasing law of the plastic residual strain generated under each loading and unloading cycle measured in step S3 with the number of cycles.

10. The method for evaluating the cumulative evolution of plastic strain in cement sheaths under large-scale segmented fracturing conditions, considering temperature effects, as described in claim 1, is characterized in that... The numerical simulation results of step S6 include: the maximum value and location of the plastic strain of the cement sheath after each fracturing operation.

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