Sleeve shear strength evaluation method

Abstract

The invention discloses a casing shear strength evaluation method which comprises the following steps: under a fracturing working condition, calculating and analyzing a change rule of temperature and pressure in a casing; acquiring a service boundary condition of the casing; on the basis of service boundary conditions, multiple rounds of temperature and pressure alternating tests are conducted on the casing pipe; after the temperature-pressure alternating test, carrying out an anti-shearing performance physical simulation test on the casing pipe, and recording a test numerical value; drawing a load-displacement curve according to the test value, obtaining a maximum shear load value, and recording the maximum shear load value as an actual test value; establishing an anti-shearing strength calculation model of the casing after the anti-shearing performance physical simulation test, and obtaining a model calculation value; and calculating the difference between the actual test value and the model calculation value, and evaluating the accuracy of the actual test value. According to the method, the shear strength of the casing under different fracturing working conditions can be accurately calculated and analyzed under the complex fracturing cyclic load condition, and technical support is provided for casing strength design under different fracturing working conditions.

Description

Technical Field

[0001] This invention belongs to the field of drilling technology, specifically relating to a method for evaluating the shear strength of casing. Background Technology

[0002] In the complex fracturing process of shale oil and gas wells, casing deformation may occur. Existing methods for detecting casing deformation include lead stamping and monitoring with multi-arm calipers, which have revealed asymmetrical radial deformation of the casing, with the casing's central axis shifting radially. Preliminary studies by domestic and international experts suggest that natural fractures / faults are easily activated during volumetric fracturing, leading to casing shear deformation, a reduction in the casing's inner diameter, and the inability to properly run downhole tools. Currently, there is limited stress analysis under casing shear deformation conditions. Existing research mainly uses the Mises stress-yield criterion to determine casing failure, but this criterion is no longer applicable to large casing shear deformation problems. To investigate the casing shear deformation problem caused by natural fracture fault slip, Zeng Yijin et al. established a finite element model of the casing-cement sheath-formation assembly in their paper "Load Analysis and Strength Design of Shear Deformation of Casing under Volumetric Fracturing" (Journal of Science and Engineering, 2021, Vol. 21 (No. 29)). They studied the casing mechanical behavior under displacement load conditions, proposed a casing external load design method combining casing diameter reduction and stress, and suggested using shear load for strength verification. Currently, the main method for evaluating the shear resistance of casing is numerical simulation. This method aims to determine the relationship between external shear load and casing inner diameter deformation, but it lacks practical experimental verification and has certain errors.

[0003] To more accurately evaluate the shear resistance of casing, invention patent CN108279173A proposes a method for evaluating casing shear resistance. This method establishes the relationship between casing shear resistance and wall thickness changes under different shear load conditions, and determines the shear resistance characteristic diagrams of casings of different specifications by varying the shear spacing. However, during shale oil and gas well fracturing, the injection of large-volume fracturing fluid into the wellbore causes a sharp drop in bottom hole temperature. Simultaneously, high pump pressure increases the stress on the casing, exacerbating the risk of casing failure. The pressure and temperature changes within the casing during complex fracturing processes severely test the casing's integrity. Currently, casing string design relies on static strength design methods and does not consider the fatigue effects during subsequent multi-stage complex fracturing processes.

[0004] Therefore, it is necessary to use physical simulation tests to determine the shear resistance performance charts under different temperatures and pressure loads, establish the casing strength attenuation law under multi-stage fracturing, optimize the strength design method, and thus form a set of evaluation methods for casing shear resistance performance under complex fracturing conditions. This is particularly urgent and important to provide technical support for casing selection in the field shale oil and gas well reservoir volume stimulation. Summary of the Invention

[0005] The purpose of this invention is to provide a method for evaluating the shear strength of casing, which solves the problem in the prior art that it is impossible to accurately evaluate the shear strength of casing under different fracturing conditions.

[0006] The technical solution adopted in this invention is a method for evaluating the shear strength of casing, comprising the following steps:

[0007] S1: Under fracturing conditions, calculate and analyze the variation of temperature and pressure inside the casing;

[0008] S2: Obtain the extreme temperature T during casing service. max and the extreme value of pressure load P max This is denoted as the service boundary condition;

[0009] S3: Based on the service boundary conditions, the casing is subjected to multiple rounds of temperature and pressure alternating load tests, which are referred to as temperature and pressure alternating tests.

[0010] S4: After the temperature and pressure alternation test, conduct a physical simulation test on the shear resistance of the bushing and record the test values;

[0011] S5: Plot the load-displacement curve based on the test values, obtain the maximum shear load value, and record it as the actual test value;

[0012] S6: Establish a calculation model for the shear strength of the casing after a physical simulation test of shear resistance performance, and obtain the calculated values ​​from the model;

[0013] S7: Calculate the difference between the actual experimental value and the model calculated value, and evaluate the accuracy of the actual experimental value.

[0014] The invention is further characterized by:

[0015] The specific steps for calculating and analyzing the changes in temperature and pressure inside the casing are as follows:

[0016] S101: Establish the equations for the rheological properties of fracturing fluid during the fracturing process;

[0017] S102: Calculate the convective heat transfer coefficient between fracturing fluid and casing using the Marshall model;

[0018] S103: Calculation at a well depth of h and radius r n The temperature at that location is denoted as the wellbore temperature field;

[0019] S104: Calculate the pressure gradient of the fracturing fluid inside the wellbore string, denoted as the wellbore pressure field;

[0020] S105: Analyze the distribution patterns of wellbore temperature and pressure fields during multi-stage fracturing.

[0021] The rheological property equation is:

[0022] τ=Kγ n (1)

[0023] Where τ is the shear stress, Pa; K is the consistency coefficient, Pa / s; n is the flow index, n<1 is pseudoplastic fluid, n>1 is expansive fluid, and for fracturing fluid n<1 is pseudoplastic fluid; γ is the shear rate, 1 / s.

[0024] The Marshall model calculation equation is as follows:

[0025]

[0026] Where h is the convective heat transfer coefficient, W / (m²) 2 ·℃); k m The heat transfer coefficient is W / (m²). 2 ·℃); D is the inner diameter of the casing, m; ρ a Fluid density, kg / m³ 3 ;D eff Q is the equivalent diameter of the casing, in meters; Q is the fracturing fluid surface pump displacement, in meters. 3 / min; K is the consistency coefficient, Pa / s; n is the flow index; C m is the specific heat capacity of fracturing fluid, J / (kg·℃).

[0027] The equation for calculating the temperature field in the wellbore is:

[0028]

[0029] Among them, T h Let the well depth be h and the radius be r. n Temperature at that location, °C; T hso r is the temperature at the outer edge of the cement sheath at a well depth of h, in °C. yo U represents the outer diameter of the injection tubing, in meters (m). to U is the overall heat transfer coefficient from the fluid to the cement ring, W / (m·℃); T1 is the fluid temperature inside the tubing; U no Let be the heat transfer coefficient of the cement sheath at a well depth of h, in W / (m·℃).

[0030] The equation for calculating the pressure gradient is:

[0031]

[0032] Where, ρ m Fluid density, kg / m³ 3 v is the fluid velocity in the wellbore, m / s; θ is the well inclination angle; d is the diameter of the injection string, m; f is a dimensionless time function.

[0033] The specific steps for the temperature and pressure alternation test are as follows:

[0034] S301: The bushing is heated and kept at a constant temperature, the heating temperature being the extreme temperature T during the bushing's service life. max The temperature range is from room temperature to 200°C.

[0035] S302: Fill the casing with fracturing fluid or water to fill the internal cavity of the casing. While filling with fracturing fluid or water, stop heating and allow the casing to cool naturally.

[0036] S303: Pressurize and maintain the pressure of the above-mentioned casing. The pressurization pressure is the extreme pressure load P of the casing during its service life. max The range is 0-200 MPa;

[0037] S304: After the pressure holding is completed, the fracturing fluid or water inside the casing is depressurized;

[0038] S305: After depressurization is completed, the casing is reheated for a time equal to the interval between multi-stage fracturing in the field.

[0039] S306: Repeat the above steps, with the number of cycles N ranging from 1 to 100.

[0040] In the physical simulation test, the casing includes, from left to right, the casing end constraint influence area, the casing shear load application area, and the casing end constraint influence area. The total length of the casing is greater than 8D, where D is the outer diameter of the casing. The length of the casing end constraint influence area is l1, and the length of the casing shear load application area is l2.

[0041] An upper shear clamp is installed on one side of the middle of the shear load application zone of the casing. A first lower shear clamp and a second lower shear clamp are symmetrically arranged on opposite sides of the upper shear clamp. The width of the upper shear clamp is l. u The distance between the first lower shearing fixture and the second lower shearing fixture is l. d The installation deviation between the upper shearing fixture and the first and second lower shearing fixtures (l) d / 2-l u / 2) Not greater than 5mm.

[0042] The calculation equation for the shear strength calculation model is as follows:

[0043] σ 剪 =-0.73P′ 2 +91.12P′+0.06n 2 -8.75n+1477.73 (5)

[0044] Where P′ is the percentage value of the internal pressure load, %; and n is the number of temperature and pressure cycles.

[0045] The beneficial effects of this invention are:

[0046] (1) Compared with the traditional casing static load strength analysis method, the casing shear strength evaluation method of this invention introduces experimental simulation of temperature and internal pressure cyclic load conditions. By studying the variation law of casing shear strength under multiple rounds of temperature and pressure changes, a calculation model of casing shear strength variation with temperature, internal pressure and cyclic load is established. This invention can accurately calculate and analyze the casing shear strength under complex fracturing cyclic load conditions, providing technical support for casing strength design under different fracturing conditions.

[0047] (2) The casing shear strength evaluation method of the present invention constructs a comprehensive size simulation test process for temperature and internal pressure cyclic loading conditions, abandoning the traditional small-size material sample method. This design enables the experimental conditions to more accurately simulate the complex environment in real fracturing operations. Therefore, the data and conclusions obtained from the experiment have higher relevance and accuracy to practical applications, and can more realistically reflect the material behavior and performance in the fracturing process.

[0048] (3) The shear strength evaluation method of the casing of the present invention involves the calculation method of shear strength of casing under alternating temperature and pressure load conditions. Based on the temperature and internal pressure values ​​under the field conditions, the specific shear strength calculation value can be given according to the calculation model. This not only significantly shortens the time required for traditional tests, but also greatly reduces the test cost, bringing higher economic and comprehensive benefits to industrial applications. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of the thermo-pressure cycling test device used in the shear strength evaluation method for sleeves of the present invention;

[0050] Figure 2 This is a schematic diagram of the casing shear strength evaluation method of the present invention.

[0051] Figure 3 This is a test diagram of the shear strength of the casing after temperature and pressure load cycling, which is the shear strength evaluation method of the present invention.

[0052] In the figure, 1. Upper plug, 2. Lower plug, 3. Sleeve, 4. Heating pad, 5. Testing machine; 6. Sleeve end constraint influence area, 7. Sleeve shear load application area, 8. Upper shear fixture, 9. First lower shear fixture, 10. Second lower shear fixture. Detailed Implementation

[0053] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0054] The method for evaluating the shear strength of casing includes the following steps:

[0055] S1: Under fracturing conditions, calculate and analyze the variation of temperature and pressure inside the casing;

[0056] S2: Obtain the extreme temperature T during casing service. max and the extreme value of pressure load P max This is denoted as the service boundary condition;

[0057] S3: Based on the service boundary conditions, the casing is subjected to multiple rounds of temperature and pressure alternating load tests, which are referred to as temperature and pressure alternating tests.

[0058] S4: After the temperature and pressure alternation test, conduct a physical simulation test on the shear resistance of the bushing and record the test values;

[0059] S5: Plot the load-displacement curve based on the test values, obtain the maximum shear load value, and record it as the actual test value;

[0060] S6: Establish a calculation model for the shear strength of the casing after a physical simulation test of shear resistance performance, and obtain the calculated values ​​from the model;

[0061] S7: Calculate the difference between the actual experimental value and the model calculated value, and evaluate the accuracy of the actual experimental value.

[0062] Furthermore, the specific steps for calculating and analyzing the changes in temperature and pressure inside the casing are as follows:

[0063] S101: Establish the equations for the rheological properties of fracturing fluid during the fracturing process;

[0064] S102: Calculate the convective heat transfer coefficient between fracturing fluid and casing using the Marshall model;

[0065] S103: Calculation at a well depth of h and radius r n The temperature at that location is denoted as the wellbore temperature field;

[0066] S104: Calculate the pressure gradient of the fracturing fluid inside the wellbore string, denoted as the wellbore pressure field;

[0067] S105: Analyze the distribution patterns of wellbore temperature and pressure fields during multi-stage fracturing.

[0068] Furthermore, the rheological properties of the fracturing fluid during fracturing can be described using Newton's power-law fluid model, and the rheological property equation is as follows:

[0069] τ=Kγ n (1)

[0070] Where τ is the shear stress, Pa; K is the consistency coefficient, Pa / s; n is the flow index, n<1 is pseudoplastic fluid, n>1 is expansive fluid, and for fracturing fluid n<1 is pseudoplastic fluid; γ is the shear rate, 1 / s.

[0071] Furthermore, after a large volume of fracturing fluid is injected into the bottom of the well, due to the high velocity of the fracturing fluid, the flow state is generally turbulent. Therefore, the convective heat transfer coefficient between the fracturing fluid and the casing is calculated using the Marshall model. The Marshall model calculation equation is as follows:

[0072]

[0073] Where h is the convective heat transfer coefficient, W / (m²) 2 ·℃); k m The heat transfer coefficient is W / (m²). 2 ·℃); D is the inner diameter of the casing, m; ρ a Fluid density, kg / m³ 3 ;D eff Q is the equivalent diameter of the casing, in meters; Q is the fracturing fluid surface pump displacement, in meters. 3 / min; K is the consistency coefficient, Pa / s; n is the flow index; C m is the specific heat capacity of fracturing fluid, J / (kg·℃).

[0074] Furthermore, based on the casing-cement sheath-formation wellbore assembly, it can be seen that at a well depth of h and a radius of r... n The equation for calculating the temperature field in the wellbore is:

[0075]

[0076] Among them, T h Let the well depth be h and the radius be r. n Temperature at that location, °C; T hso r is the temperature at the outer edge of the cement sheath at a well depth of h, in °C. yo U represents the outer diameter of the injection tubing, in meters (m). to U is the overall heat transfer coefficient from the fluid to the cement ring, W / (m·℃); T1 is the fluid temperature inside the tubing; U no Let be the heat transfer coefficient of the cement sheath at a well depth of h, in W / (m·℃).

[0077] Furthermore, the pressure gradient of the fracturing fluid within the wellbore string is calculated based on the mass conservation equation and the momentum equation. The equation for calculating the pressure gradient is as follows:

[0078]

[0079] Where, ρ m Fluid density, kg / m³ 3 v is the fluid velocity in the wellbore, m / s; θ is the well inclination angle; d is the diameter of the injection string, m; f is a dimensionless time function.

[0080] Equations (3) and (4) can be used to calculate and analyze the distribution patterns of the wellbore temperature and pressure fields during multi-stage fracturing.

[0081] Furthermore, during the extraction of shale gas wells and oil and gas wells, the casing is subjected to frequent and severe temperature-pressure alternating loads. This may cause micro-deformation of the casing or reduce its strength, thereby affecting the integrity and safety of the wellbore. Therefore, it is necessary to conduct temperature and pressure alternating tests on the casing, such as... Figure 1 As shown, firstly, a plug 1 is welded to one end of the sleeve 3, and a lower plug 2 is welded to the other end. The outer wall of the sleeve 3 is covered with a heating pad 4. A pressure punching hole is provided on the lower plug 2. The lower plug 2 is connected to a testing machine 5 through the pressure punching hole. Through the testing machine 5, multiple rounds of temperature and internal pressure alternating load tests are conducted on the sleeve 3. The specific steps of the temperature and pressure alternating test are as follows:

[0082] S301: Activate the heating pad covering the outer wall of the bushing to heat the bushing and maintain the temperature for 30-40 minutes. The heating temperature is the extreme temperature T that the bushing operates at. max The temperature range is from room temperature to 200°C.

[0083] S302: Fill the casing with fracturing fluid or water to fill the internal cavity of the casing. While filling with fracturing fluid or water, turn off the heating pad switch and the casing will begin to cool down naturally.

[0084] S303: Pressurize and maintain the pressure of the above-mentioned casing for a holding time of t, where t is the holding time under the fracturing conditions during on-site oil and gas well construction. The pressurization pressure is the extreme pressure load P of the casing during its service life. max The range is 0-200 MPa;

[0085] S304: After the pressure holding is completed, the fracturing fluid or water inside the casing is depressurized;

[0086] S305: After depressurization is completed, start the heating pad to heat the casing. The heating time is equal to the interval between the multi-stage fracturing in the field.

[0087] S306: Repeat the above steps, with the number of cycles N ranging from 1 to 100.

[0088] Complete the extreme temperature T inside the casing max and the extreme value of pressure load P max After the cyclic treatment, remove the heating pad on the outer wall of the sleeve and allow the sleeve to cool naturally to room temperature.

[0089] Furthermore, after the temperature and pressure alternation test, a physical simulation test is conducted, such as... Figure 2As shown, in the physical simulation test, the sleeve 3 includes, from left to right, the sleeve end constraint influence area 6, the sleeve shear load application area 7, and the sleeve end constraint influence area 6. The total length of the sleeve 3 is greater than 8D, where D is the outer diameter of the sleeve. The length of the sleeve end constraint influence area 6 is l1, and the length of the sleeve shear load application area 7 is l2.

[0090] Furthermore, an upper shear clamp 8 is provided on one side of the middle of the casing shear load application area 7, and a first lower shear clamp 9 and a second lower shear clamp 10 are symmetrically provided on opposite sides of the upper shear clamp 8. The width of the upper shear clamp 8 is l. u The distance between the first lower shearing fixture 9 and the second lower shearing fixture 10 is l. d The installation deviation between the upper shearing fixture 8 and the first lower shearing fixture 9 and the second lower shearing fixture 10 (l) d / 2-l u / 2) The shearing die is displacement controlled and the data during the shearing test of the sleeve is recorded and the load-displacement curve is plotted.

[0091] Furthermore, an experimental method was used to establish a shear strength model for the casing under alternating temperature and pressure loads. The calculation equation for the shear strength model is as follows:

[0092] σ 剪 =-0.73P′ 2 +91.12P′+0.06n 2 -8.75n+1477.73 (5)

[0093] Where P′ is the percentage value of the internal pressure load, %; and n is the number of temperature and pressure cycles.

[0094] Example 1

[0095] The method for evaluating the shear strength of casing includes the following steps:

[0096] S1: Under fracturing conditions, calculate and analyze the variation of temperature and pressure inside the casing;

[0097] S2: Obtain the extreme temperature T during casing service. max and the extreme value of pressure load P max This is denoted as the service boundary condition;

[0098] S3: Based on the service boundary conditions, the casing is subjected to multiple rounds of temperature and pressure alternating load tests, which are referred to as temperature and pressure alternating tests.

[0099] S4: After the temperature and pressure alternation test, conduct a physical simulation test on the shear resistance of the bushing and record the test values;

[0100] S5: Plot the load-displacement curve based on the test values, obtain the maximum shear load value, and record it as the actual test value;

[0101] S6: Establish a calculation model for the shear strength of the casing after a physical simulation test of shear resistance performance, and obtain the calculated values ​​from the model;

[0102] S7: Calculate the difference between the actual experimental value and the model calculated value, and evaluate the accuracy of the actual experimental value.

[0103] Furthermore, the specific steps for calculating and analyzing the changes in temperature and pressure inside the casing are as follows:

[0104] S101: Establish the equations for the rheological properties of fracturing fluid during the fracturing process;

[0105] S102: Calculate the convective heat transfer coefficient between fracturing fluid and casing using the Marshall model;

[0106] S103: Calculation at a well depth of h and radius r n The temperature at that location is denoted as the wellbore temperature field;

[0107] S104: Calculate the pressure gradient of the fracturing fluid inside the wellbore string, denoted as the wellbore pressure field;

[0108] S105: Analyze the distribution patterns of wellbore temperature and pressure fields during multi-stage fracturing.

[0109] Furthermore, the rheological properties of the fracturing fluid during fracturing can be described using Newton's power-law fluid model, and the rheological property equation is as follows:

[0110] τ=Kγ n (1)

[0111] Where τ is the shear stress, Pa; K is the consistency coefficient, Pa / s; n is the flow index, n<1 is pseudoplastic fluid, n>1 is expansive fluid, and for fracturing fluid n<1 is pseudoplastic fluid; γ is the shear rate, 1 / s.

[0112] Furthermore, after a large volume of fracturing fluid is injected into the bottom of the well, due to the high velocity of the fracturing fluid, the flow state is generally turbulent. Therefore, the convective heat transfer coefficient between the fracturing fluid and the casing is calculated using the Marshall model. The Marshall model calculation equation is as follows:

[0113]

[0114] Where h is the convective heat transfer coefficient, W / (m²) 2 ·℃); k m The heat transfer coefficient is W / (m²). 2 ·℃); D is the inner diameter of the casing, m; ρa Fluid density, kg / m³ 3 ;D eff Q is the equivalent diameter of the casing, in meters; Q is the fracturing fluid surface pump displacement, in meters. 3 / min; K is the consistency coefficient, Pa / s; n is the flow index; C m is the specific heat capacity of fracturing fluid, J / (kg·℃).

[0115] Furthermore, based on the casing-cement sheath-formation wellbore assembly, it can be seen that at a well depth of h and a radius of r... n The equation for calculating the temperature field in the wellbore is:

[0116]

[0117] Among them, T h Let the well depth be h and the radius be r. n Temperature at that location, °C; T hso r is the temperature at the outer edge of the cement sheath at a well depth of h, in °C. yo U represents the outer diameter of the injection tubing, in meters (m). to U is the overall heat transfer coefficient from the fluid to the cement ring, W / (m·℃); T1 is the fluid temperature inside the tubing; U no Let be the heat transfer coefficient of the cement sheath at a well depth of h, in W / (m·℃).

[0118] Furthermore, the pressure gradient of the fracturing fluid within the wellbore string is calculated based on the mass conservation equation and the momentum equation. The equation for calculating the pressure gradient is as follows:

[0119]

[0120] Where, ρ m Fluid density, kg / m³ 3 v is the fluid velocity in the wellbore, m / s; θ is the well inclination angle; d is the diameter of the injection string, m; f is a dimensionless time function.

[0121] Equations (3) and (4) can be used to calculate and analyze the distribution patterns of the wellbore temperature and pressure fields during multi-stage fracturing.

[0122] Example 2

[0123] Based on Example 1, a temperature-pressure alternating test was conducted on the casing. During the extraction of shale gas wells and oil and gas wells, the casing is subjected to frequent and severe temperature-pressure alternating loads. This may cause micro-deformation of the casing or reduce its strength, thereby affecting the integrity and safety of the wellbore. Therefore, a temperature-pressure alternating test is necessary for the casing. Figure 1As shown, firstly, a plug 1 is welded to one end of the sleeve 3, and a lower plug 2 is welded to the other end. The outer wall of the sleeve 3 is covered with a heating pad 4. A pressure punching hole is provided on the lower plug 2. The lower plug 2 is connected to a testing machine 5 through the pressure punching hole. Through the testing machine 5, multiple rounds of temperature and internal pressure alternating load tests are conducted on the sleeve 3. The specific steps of the temperature and pressure alternating test are as follows:

[0124] S301: Activate the heating pad covering the outer wall of the bushing to heat the bushing and maintain the temperature for 30-40 minutes. The heating temperature is the extreme temperature T that the bushing operates at. max The temperature range is from room temperature to 200°C.

[0125] S302: Fill the casing with fracturing fluid or water to fill the internal cavity of the casing. While filling with fracturing fluid or water, turn off the heating pad switch and the casing will begin to cool down naturally.

[0126] S303: Pressurize and maintain the pressure of the above-mentioned casing for a holding time of t, where t is the holding time under the fracturing conditions during on-site oil and gas well construction. The pressurization pressure is the extreme pressure load P of the casing during its service life. max The range is 0-200 MPa;

[0127] S304: After the pressure holding is completed, the fracturing fluid or water inside the casing is depressurized;

[0128] S305: After depressurization is completed, start the heating pad to heat the casing. The heating time is equal to the interval between the multi-stage fracturing in the field.

[0129] S306: Repeat the above steps, with the number of cycles N ranging from 1 to 100.

[0130] Complete the extreme temperature T inside the casing max and the extreme value of pressure load P max After the cyclic treatment, remove the heating pad on the outer wall of the sleeve and allow the sleeve to cool naturally to room temperature.

[0131] Example 3

[0132] Based on Example 2, after the temperature and pressure alternation test, a physical simulation test was conducted, such as... Figure 2 As shown, in the physical simulation test, the sleeve 3 includes, from left to right, the sleeve end constraint influence area 6, the sleeve shear load application area 7, and the sleeve end constraint influence area 6. The total length of the sleeve 3 is greater than 8D, where D is the outer diameter of the sleeve. The length of the sleeve end constraint influence area 6 is l1, and the length of the sleeve shear load application area 7 is l2.

[0133] Furthermore, an upper shear clamp 8 is provided on one side of the middle of the casing shear load application area 7, and a first lower shear clamp 9 and a second lower shear clamp 10 are symmetrically provided on opposite sides of the upper shear clamp 8. The width of the upper shear clamp 8 is l.u The distance between the first lower shearing fixture 9 and the second lower shearing fixture 10 is l. d The installation deviation between the upper shearing fixture 8 and the first lower shearing fixture 9 and the second lower shearing fixture 10 (l) d / 2-l u / 2) The shearing die is displacement controlled and the data during the shearing test of the sleeve is recorded and the load-displacement curve is plotted.

[0134] Example 4

[0135] Based on Example 3, an experimental method was used to establish a shear strength model for the casing under alternating temperature and pressure loads. The calculation equation for the shear strength model is as follows:

[0136] σ 剪 =-0.73P′ 2 +91.12P′+0.06n 2 -8.75n+1477.73 (5)

[0137] Where P′ is the percentage value of the internal pressure load, %; and n is the number of temperature and pressure cycles.

[0138] Example 5

[0139] Based on Example 4, the temperature and internal pressure variation of shale oil well SY2 under fracturing conditions were calculated, the temperature field and pressure field of the wellbore under multi-stage complex fracturing conditions were analyzed, and the service boundary conditions of the casing under fracturing conditions were determined.

[0140] The rheological properties of fracturing fluid during fracturing can be described by Newton's power-law fluid, and the rheological property equation is as follows:

[0141] τ=Kγ n (1)

[0142] Where τ is the shear stress (Pa); K is the consistency coefficient (Pa / s); n is the flow index, where n < 1 indicates a pseudoplastic fluid, and n > 1 indicates a dilatant fluid; for fracturing fluid, n < 1 indicates a pseudoplastic fluid; and γ is the shear rate (1 / s). Specifically, K = 1.5 Pa / s, n = 0.55, and γ = 4000 s. -1 .

[0143] The convective heat transfer coefficient between the fracturing fluid and the casing can be calculated using the Marshall model;

[0144]

[0145] Where: h is the convective heat transfer coefficient, W / (m²) 2 ·℃); k mThe heat transfer coefficient is W / (m²). 2 ·℃); D is the inner diameter of the casing, m; ρ a Fluid density, kg / m³ 3 ;D eff Q is the equivalent diameter of the casing, in meters; Q is the fracturing fluid surface pump displacement, in meters. 3 / min; C m Specific heat capacity of fracturing fluid, J / (kg·℃). Specifically, kJ / (kg·℃). m =30W / (m 2 ·℃), D=118.62mm, ρ a =1100kg / m 3 D eff =139.7mm, Q=950m 3 / min, C m = 810 J / (kg·℃), and the calculated h is 1.78 W / (m²). 2 ·℃).

[0146] Furthermore, based on the casing-cement sheath-formation wellbore assembly, it can be seen that at a well depth of h and a radius of r... n The equation for calculating the temperature field in the wellbore is:

[0147]

[0148] Among them, T h Let the well depth be h and the radius be r. n Temperature at that location, °C; T hso r is the temperature at the outer edge of the cement sheath at a well depth of h, in °C. yo U represents the outer diameter of the injection tubing, in meters (m). to U is the overall heat transfer coefficient from the fluid to the cement ring, W / (m·℃); T1 is the fluid temperature inside the tubing; U no Let be the heat transfer coefficient of the cement sheath at a well depth of h, in W / (m·℃).

[0149] Furthermore, the pressure gradient of the fracturing fluid within the wellbore string is calculated based on the mass conservation equation and the momentum equation. The equation for calculating the pressure gradient is as follows:

[0150]

[0151] Where, ρ m Fluid density, kg / m³ 3 v is the fluid velocity in the wellbore, m / s; θ is the well inclination angle; d is the diameter of the injection string, m; f is a dimensionless time function.

[0152] Equations (3) and (4) can be used to calculate and analyze the distribution of temperature and pressure fields in the wellbore during multi-stage fracturing. Combined with software calculations, the temperature and internal pressure fields of shale oil well SY2 under fracturing conditions change with well depth. The temperature change of the casing under fracturing conditions is ≤100℃, the internal pressure change is ≤100MPa, and the number of cycles is ≤50. These are the boundary conditions for casing service under fracturing conditions.

[0153] Example 6

[0154] Based on Example 5, multiple rounds of temperature and internal pressure alternating load tests, i.e., temperature-pressure alternating tests, were conducted. Based on Example 5, the extreme temperature T of the casing under complex fracturing conditions was calculated. max Take 100℃ and the extreme value of the pressure load P max The pressure is set at 100 MPa. The sleeve 3 is made of Q125 steel with an outer diameter of 139.7 mm and a wall thickness of 10.54 mm. The specific steps of the temperature and pressure alternating test in this embodiment are as follows:

[0155] S301: Activate the heating pad covering the outer wall of the bushing to heat the bushing and maintain the temperature for 40 minutes. The heating temperature is the extreme temperature T of the bushing during service. max Specifically, 100℃;

[0156] S302: Fill the casing with fracturing fluid or water to fill the internal cavity of the casing. While filling with fracturing fluid or water, turn off the heating pad switch and the casing will begin to cool down naturally.

[0157] S303: Pressurize and maintain the pressure on the above-mentioned casing. The pressurization pressure is the extreme pressure load P of the casing during its service life. max Specifically, 100 MPa, at the extreme pressure load P max The pressure holding time is 3 hours at 100MPa;

[0158] S304: After the pressure holding is completed, the fracturing fluid or water inside the casing is depressurized;

[0159] S305: After depressurization is completed, activate the heating pad to heat the casing. The heating time is equal to the interval between multi-stage fracturing operations in the field, t. 间 =90min;

[0160] S306: Repeat the above steps and conduct 50 cycles of the experiment.

[0161] Complete the extreme temperature T inside the casing max and the extreme value of pressure load P max After the cyclic treatment, remove the heating pad on the outer wall of the sleeve and allow the sleeve to cool naturally to room temperature.

[0162] Example 7

[0163] Based on Example 6, a physical simulation test of the shear resistance performance of the casing was conducted. The casing end constraint influence range l1 = 400 mm, the casing shear load application range l2 = 400 mm, the total length of the casing was 1120 mm, and the distance between the first lower shear clamp and the second lower shear clamp was l. d The width of the upper shearing fixture is 80mm. u The installation deviation between the upper shearing fixture and the first and second lower shearing fixtures is 74mm. d / 2-l u / 2) is 3mm, and the extreme value of the cyclic temperature T max At 100℃, 55% of the initial internal pressure strength (i.e., after 50 cycles of internal pressure load), the shear strength of the casing is as follows: Figure 3 As shown, the maximum shear load is 4059 kN.

[0164] The shear strength F of the casing under different temperature and pressure load conditions was obtained through physical simulation tests. s Specifically, the initial internal pressure resistance (i.e., internal pressure load) of the casing was tested at a temperature of 100℃ and internal pressures of 55%, 80%, and 90%, with the initial internal pressure resistance (i.e., internal pressure load) P0 being 180 MPa. The number of test cycles was 20, 30, 40, and 50. The shear strength of the casing after the test under different temperature and pressure loads is shown in Table 1.

[0165] Table 1. Test values ​​of shear strength of casing

[0166]

[0167] By observing and analyzing experimental data, especially how shear strength changes with temperature, pressure, and cycle number, and preliminarily determining the model type for predicting shear strength changes with variables, classifying it as linear, logarithmic, or exponential, a calculation model for shear strength with temperature, pressure, and cycle number is established. Based on the data in Table 1, the calculation model for the shear strength of the Q125 casing is as follows:

[0168] σ 剪 =-0.73P′ 2 +91.12P′+0.06n 2 -8.75n+1477.73 (5)

[0169] Where P′ is the percentage value of the internal pressure load, %; and n is the number of temperature and pressure cycles.

[0170] Based on the aforementioned calculation model, the shear strength of the casing can be calculated at a temperature of 100℃, with the cyclic internal pressure strength being the initial internal pressure resistance (i.e., an internal pressure load of 55%-90%), and the number of cycles ranging from 20 to 50. Statistical analysis shows that the average accuracy between the actual experimental values ​​and the model calculations is 99%. This calculation model can reduce testing costs, save time, and provide technical support for the strength design of casings.

Claims

1. A method for evaluating the shear strength of casing, characterized in that, Includes the following steps: S1: Under fracturing conditions, calculate and analyze the variation of temperature and pressure inside the casing; S2: Obtain the extreme temperature T during the service life of the casing. max and the extreme value of pressure load P max This is denoted as the service boundary condition; S3: Based on the service boundary conditions, the casing is subjected to multiple rounds of temperature and pressure alternating load tests, which are referred to as temperature and pressure alternating tests. S4: After the temperature and pressure alternation test, conduct a physical simulation test on the shear resistance of the bushing and record the test values; S5: Plot the load-displacement curve based on the test values, obtain the maximum shear load value, and record it as the actual test value; S6: Establish a calculation model for the shear strength of the casing after a physical simulation test of shear resistance performance, and obtain the calculated values ​​from the model; S7: Calculate the difference between the actual experimental value and the model calculated value, and evaluate the accuracy of the actual experimental value.

2. The method for evaluating the shear strength of a casing according to claim 1, characterized in that, The specific steps for calculating and analyzing the changes in temperature and pressure inside the casing are as follows: S101: Establish the equations for the rheological properties of fracturing fluid during the fracturing process; S102: Calculate the convective heat transfer coefficient between fracturing fluid and casing using the Marshall model; S103: Calculation at a well depth of h and radius r n The temperature at that location is denoted as the wellbore temperature field; S104: Calculate the pressure gradient of the fracturing fluid inside the wellbore string, denoted as the wellbore pressure field; S105: Analyze the distribution patterns of wellbore temperature and pressure fields during multi-stage fracturing.

3. The method for evaluating the shear strength of a casing according to claim 2, characterized in that, The rheological property equation is as follows: τ=Kγ n (1) Where τ is the shear stress, Pa; K is the consistency coefficient, Pa / s; n is the flow index, n<1 is pseudoplastic fluid, n>1 is expansive fluid, and for fracturing fluid n<1 is pseudoplastic fluid; γ is the shear rate, 1 / s.

4. The method for evaluating the shear strength of a casing according to claim 2, characterized in that, The Marshall model calculation equation is as follows: Where h is the convective heat transfer coefficient, W / (m²) 2 ·℃); k m The heat transfer coefficient is W / (m²). 2 ·℃); D is the inner diameter of the casing, m; ρ a Fluid density, kg / m³ 3 ;D eff Q is the equivalent diameter of the casing, in meters; Q is the fracturing fluid surface pump displacement, in meters. 3 / min; K is the consistency coefficient, Pa / s; n is the flow index; C m is the specific heat capacity of fracturing fluid, J / (kg·℃).

5. The method for evaluating the shear strength of a casing according to claim 2, characterized in that, The equation for calculating the temperature field in the wellbore is as follows: Among them, T h Let the well depth be h and the radius be r. n Temperature at that location, °C; T hso r is the temperature at the outer edge of the cement sheath at a well depth of h, in °C. yo U represents the outer diameter of the injection tubing, in meters (m). to U is the overall heat transfer coefficient from the fluid to the cement ring, W / (m·℃); T1 is the fluid temperature inside the tubing; U no Let be the heat transfer coefficient of the cement sheath at a well depth of h, in W / (m·℃).

6. The method for evaluating the shear strength of a casing according to claim 2, characterized in that, The equation for calculating the pressure gradient is: Where, ρ m Fluid density, kg / m³ 3 v is the fluid velocity in the wellbore, m / s; θ is the well inclination angle; d is the diameter of the injection string, m; f is a dimensionless time function.

7. The method for evaluating the shear strength of a casing according to claim 1, characterized in that, The specific steps of the temperature and pressure alternation test are as follows: S301: The bushing is heated and kept at a constant temperature, the heating temperature being the extreme temperature T during the bushing's service life. max The temperature range is from room temperature to 200°C. S302: Fill the casing with fracturing fluid or water to fill the internal cavity of the casing. While filling with fracturing fluid or water, stop heating and allow the casing to cool naturally. S303: Pressurize and maintain the pressure of the above-mentioned casing. The pressurization pressure is the extreme pressure load P of the casing during its service life. max The range is 0-200 MPa; S304: After the pressure holding is completed, the fracturing fluid or water inside the casing is depressurized; S305: After depressurization is completed, the casing is reheated for a time equal to the interval between multi-stage fracturing in the field. S306: Repeat the above steps, with the number of cycles N ranging from 1 to 100.

8. The method for evaluating the shear strength of a casing according to claim 1, characterized in that, In the physical simulation test, the sleeve (3) includes, from left to right, the sleeve end constraint influence area (6), the sleeve shear load application area (7), and the sleeve end constraint influence area (6). The total length of the sleeve (3) is greater than 8D, where D is the outer diameter of the sleeve. The length of the sleeve end constraint influence area (6) is l1, and the length of the sleeve shear load application area (7) is l2.

9. The method for evaluating the shear strength of a casing according to claim 8, characterized in that, An upper shear clamp (8) is provided on one side of the middle of the sleeve shear load application area (7). A first lower shear clamp (9) and a second lower shear clamp (10) are symmetrically provided on opposite sides of the upper shear clamp (8). The width of the upper shear clamp (8) is l. u The distance between the first lower shearing fixture (9) and the second lower shearing fixture (10) is l. d The installation deviation (l) between the upper shearing fixture (8) and the first lower shearing fixture (9) and the second lower shearing fixture (10) d / 2-l u / 2) Not greater than 5mm.

10. The method for evaluating the shear strength of a casing according to claim 1, characterized in that, The calculation equation for the shear strength calculation model is as follows: s 剪 =-0.73P′ 2 +91.12P′+0.06n 2 -8.75n+1477.73 (5) Where P′ is the percentage value of the internal pressure load, %; and n is the number of temperature and pressure cycles.

Images