Method for testing fatigue factor of tensile strength of cement stone

Abstract

This invention discloses a method for testing the fatigue factor of cement stone tensile strength. The method involves applying tensile force to cement stone specimens according to a cementing annulus circumferential stress alternation scheme to simulate circumferential stress changes, and testing the cement stone tensile strength P under i cycles of internal casing pressure alternation. ij ; Calculate different pressures P j Cement stone tensile strength fatigue factor λ under different number of rounds i ij The circumferential stress variation value Δσ tj The X-axis represents the number of alternating pressure cycles i within the casing, the Y-axis represents the cement stone tensile strength fatigue factor λ. ij Using the Z-axis as the reference, a three-dimensional surface plot is drawn, and the solution model is obtained after fitting. This method provides technical support for accurately evaluating the true tensile strength of cement stone and the integrity of the cement sheath after fracturing under low confining pressure conditions, and improves the reliability of low confining pressure fracturing construction.

Description

Technical Field

[0001] This invention relates to the field of oil drilling and cementing technology, and in particular to a method for testing the fatigue factor of cement stone tensile strength. Background Technology

[0002] Wellbore integrity is a crucial indicator affecting cementing quality. Under low confining pressure fracturing conditions, tensile failure of the cement sheath is the primary cause of wellbore integrity failure in oil and gas wells. The tensile strength of the cement stone, as an important mechanical parameter for evaluating cement sheath tensile failure, is currently mainly evaluated using single-cycle strength testing. However, before tensile failure occurs during fracturing, the cement stone undergoes fatigue after multiple fracturing cycles, leading to a decrease in strength. Therefore, using single-cycle strength testing is inaccurate and cannot accurately simulate the actual situation of the cement sheath after multiple fracturing cycles. Summary of the Invention

[0003] The purpose of this invention is to provide a method for testing the fatigue factor of cement stone tensile strength.

[0004] Therefore, the technical solution of the present invention is as follows:

[0005] A method for testing the fatigue factor of cement stone tensile strength includes the following steps:

[0006] 1) Obtain data on the casing wall thickness, cement sheath thickness, elastic modulus and Poisson's ratio of the casing, cement stone and formation in the fracturing section. Use the numerical simulation software Ansys to calculate the circumferential stress σ of the cement stone when the wellbore pressure is P0 under the initial fracturing conditions. t0 ;

[0007] 2) The pressure inside the wellbore during fracturing was calculated using the numerical simulation software Ansys, and the pressure was P. j The circumferential stress σ of the cement ring at that time tj , where j = 1, 2, 3, 4, 5…, and P1≠P2≠P3≠P4≠P5≠…;

[0008] 3) Determine the pressure alternation scheme inside the well casing during well section fracturing, specifically:

[0009] The pressure inside the casing is between P0 and P1. j Within the range, press P0→P j →P0→P j →P0→P j →...the pattern alternates cyclically for n rounds;

[0010] 4) Calculate the corresponding alternating stress scheme of the cement annulus and the circumferential stress change value Δσ based on the casing pressure alternation scheme. tj ;

[0011] 5) Select cement stone specimens and test the initial working tensile strength U0 of the cement stone on a tensile strength testing machine. Separately, select cement stone specimens and apply tensile force according to the alternating circumferential stress scheme of the cementing annulus to simulate circumferential stress changes. Test the tensile strength P of the cement stone under the number of alternating pressure cycles i within the casing. ij ;

[0012] 6) Calculate the fatigue factor λ of cement stone tensile strength under different rounds i. ij =P ij / U0, plot the fatigue factor λ of cement stone tensile strength ij The curve of the change with the number of alternating cycles i is obtained, and the fatigue factor λ of the cement stone tensile strength is obtained by fitting the curve. ij The relationship between the number of alternating cycles i of the pressure inside the casing and the formula for this relationship.

[0013] 7) Change the wellbore pressure during fracturing to P j Repeat steps 2)-6) until the preset requirements are met;

[0014] 8) The circumferential stress variation value Δσ tj The X-axis represents the number of alternating pressure cycles i within the casing, the Y-axis represents the cement stone tensile strength fatigue factor λ. ij Using the Z-axis as the plotting axis, a three-dimensional surface plot was generated, and the fatigue factor λ of the cement stone tensile strength was obtained through fitting. ij Δσ as a function of circumferential stress tj A model relating the number of alternating pressure cycles i inside the casing is used to solve for the fatigue factor of the tensile strength of cement stone.

[0015] Furthermore, in step 4),

[0016] When the pressure inside the casing is between P0 and P... j Within the range, press P0→P j →P0→P j →P0→P j →…When the mode is cyclically alternating for n rounds, the alternating scheme for the circumferential stress of the cementing ring is as follows:

[0017] According to σ t0 →σ t1 →σ t0 →σ t1 →σ t0 →σ t1 →…The pattern alternates cyclically for n rounds.

[0018] Furthermore, the circumferential stress variation value Δσ tj The calculation method is Δσ tj =σ tj -σ t0 .

[0019] Furthermore, the method for preparing the cement specimen is as follows: prepare cement slurry according to the formula of the cement slurry system for the fracturing section, and cure it at a curing temperature T0 for 48 hours.

[0020] Furthermore, by using a variational relationship model to study the fatigue factor λ of the tensile strength of cement paste... ij When solving the problem, calculate the circumferential stress variation Δσ under alternating pressure conditions. t And determine the number of casing pressure alternation cycles i during the fracturing process, and assign Δσ t Substituting i into the model yields the corresponding cement stone tensile strength fatigue factor λ. ij .

[0021] Compared with existing technologies, this method for testing the tensile strength fatigue factor of cement stone provides a new approach to calculating the tensile strength fatigue factor of cement sheath. It provides technical support for accurately evaluating the true tensile strength of cement stone and the integrity of cement sheath after fracturing under low confining pressure conditions, and improves the reliability of low confining pressure fracturing operations. Attached Figure Description

[0022] Figure 1 The fatigue factor λ i1 A graph showing the variation of the number of alternating pressure cycles (i) within the casing.

[0023] Figure 2 The fatigue factor λ ij A graph showing the variation of the number of alternating pressure cycles (i) within the casing.

[0024] Figure 3 This is a three-dimensional curve showing the variation of fatigue factor with circumferential stress and the number of cycles of internal pressure alternation. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0026] Unless the context clearly indicates otherwise. It should also be understood that the term "and / or" as used in one or more embodiments of this application refers to, and many specific details are set forth in the following description to provide a full understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of this application; therefore, this application is not limited to the specific embodiments disclosed below.

[0027] The terminology used in one or more embodiments of this application is for the purpose of describing particular embodiments only and is not intended to limit the scope of one or more embodiments of this application. The singular forms “a,” “the,” and “the” used in one or more embodiments of this application and in the appended claims are also intended to include the plural forms, encompassing any or all possible combinations of one or more associated listed items.

[0028] It should be understood that although the terms first, second, etc., may be used to describe various information in one or more embodiments of this application, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first may also be referred to as second without departing from the scope of one or more embodiments of this application, and similarly, second may also be referred to as first. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to a determination."

[0029] Example:

[0030] Step 1: Analyze and determine the wellbore pressure P0 = 35 MPa under the initial operating conditions (after cementing curing and before fracturing) of a fracturing section with a temperature of 90℃. The wellbore pressure during fracturing is P1 = 105 MPa. During fracturing, the casing pressure cyclically alternates between P0 and P1 for 5 cycles in the pattern 35 MPa → 105 MPa → 35 MPa → 105 MPa → 35 MPa → 105 MPa → …. Additionally, obtain data on the casing wall thickness, cement sheath thickness, elastic modulus and Poisson's ratio of the casing, cement stone, and formation for this fracturing section. Use the numerical simulation software Ansys to calculate the circumferential stress σ of the cement stone under the initial fracturing operating conditions with a wellbore pressure of 35 MPa. t0 = -2.44MPa (negative value represents circumferential compressive stress); prepare cement slurry according to the formula of cement slurry system of fracturing section, cure at curing temperature of 90℃ for 48h, after curing to form cement stone specimen, apply tensile force to it on tensile strength testing machine, and test the tensile strength of cement stone under this condition U0 = 3.09MPa.

[0031] Step 2: Based on the alternating pressure scheme inside the casing of this well, the circumferential stress σ of the cement sheath is calculated using the numerical simulation software Ansys when the pressure inside the casing is P1 = 105 MPa during the fracturing process. t1 =1.16MPa (positive value represents circumferential tensile stress), calculate the circumferential stress variation Δσ t1 =σ t1 -σ t0=3.6MPa, then the circumferential stress of the cement annulus during the fracturing process, when the casing pressure cyclically alternates for 5 cycles in the pattern of 35MPa→105MPa→35MPa→105MPa→35MPa→105MPa→…, can be calculated as follows:

[0032] The cycle repeats for 5 rounds, changing from -2.44MPa to 1.16MPa, from -2.44MPa to 1.16MPa, from -2.44MPa to 1.16MPa, and so on.

[0033] Step 3: Prepare cement slurry according to the fracturing section cement slurry system formula, and cure it at a curing temperature of 90℃ for 48 hours. After the cement stone specimen is formed, apply tensile force on a tensile strength testing machine according to the alternating circumferential tensile stress scheme of cementing annulus (5 cycles of alternating stress in the pattern of -2.44MPa→1.16MPa→-2.44MPa→1.16MPa→-2.44MPa→1.16MPa→…) to simulate the circumferential stress change law, and test the tensile strength P of cement stone under different numbers of alternating cycles i (1≤i≤5). i1 As shown in Table 1 below.

[0034] Step 4: Calculate the cement stone tensile strength fatigue factor λ under different number of alternating pressure cycles i within the casing. i1 =P i1 / U0(1≤i≤5), plot the fatigue factor λ of cement stone tensile strength. i1 The curve of the change in pressure alternation cycle i within the casing is obtained, and the fatigue factor λ of the cement stone tensile strength is obtained by fitting the curve. i1 The relationship between the number of alternating pressure cycles (i) inside the casing and the fitted curve is as follows: Figure 1 As shown;

[0035] The fitted variation relationship is λ i1 =0.0021i 2 -0.0859i+1.0025, λ i1 The fatigue factor representing the tensile strength of cement stone under the pressure P1 inside the casing, where i represents the number of pressure alternation cycles.

[0036] Step 5: Adjust the internal pressure P of the casing j For P2 = 95 MPa, P3 = 85 MPa, P4 = 75 MPa, and P5 = 65 MPa, the pressure inside the casing during fracturing is between P0 and P1. j Within the range, press P0→P j →P0→P j →P0→P j →… The mode is cyclically alternating for 5 rounds; steps 2 to 4 are repeated to obtain the cement stone tensile strength fatigue factor λ under different casing pressure alternation schemes. ijThe curve of the change with the number of alternating cycles i is obtained, and the fatigue factor λ of the cement stone tensile strength is obtained by fitting the curve. ij The relationship between the number of pressure alternation cycles *i* during fracturing and the fitted curve is as follows: Figure 2 As shown.

[0037] The relationship between the fitted curve and the change is as follows, where λ i2 , λ i3 , λ i4 , λ i5 These represent the cement stone tensile strength fatigue factors under casing pressures of P2, P3, P4, and P5, respectively, where i represents the number of casing pressure alternation cycles during fracturing, expressed as follows:

[0038] λ i2 =0.002i 2 -0.0806i+1.0045 (1)

[0039] λ i3 =0.0013i 2 -0.0703i+0.9999 (2)

[0040] λ i4 =0.0012i 2 -0.066i+1.0001 (3)

[0041] λ i5 =0.0011i 2 -0.0626i+1.0001 (4)

[0042] Step 6: Using the circumferential stress change value Δσ tj The number of alternating pressure cycles i within the casing is along the X and Y axes, and the fatigue factor λ represents the tensile strength of the cement stone. ij =P ij With / U0 as the Z-axis, plot the 3D surface, such as... Figure 3 As shown, the fatigue factor λ for the tensile strength of cement stone was obtained through fitting. ij Δσ as a function of circumferential stress tj The relationship model between the number of alternating cycles i of the casing pressure and the following:

[0043] λ ij =0.99128+0.078Δσ tj -0.07894i-0.0012Δσ tj 2+0.00139i 2 +0.00302Δσ tj i.

[0044] Step 7: For a new fractured well, calculate the circumferential stress variation Δσ under pressure alternation conditions. t =2.32, based on the circumferential stress variation value Δσ t Given the number of alternating pressure cycles i=2 within the casing during fracturing, the fatigue factor λ of the cement stone tensile strength can be obtained from the chart. i =0.8678, providing technical support for accurately evaluating the true tensile strength of cement stone after compression fracturing and the integrity of the cement ring.

[0045] The preferred embodiments disclosed above are merely illustrative of this application. The optional embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the embodiments of this application. These embodiments are selected and specifically described in this application to better explain the principles and practical applications of the embodiments of this application, thereby enabling those skilled in the art to better understand and utilize this application. This application is limited only by the claims and their full scope and equivalents.

[0046] Table 1 Tensile strength of cement stone under the first alternating scheme

[0047] Different casing internal pressure alternation cycles i <![CDATA[Tensile strength P of set cement i1 / MPa]]> 1 2.84 2 2.59 3 2.36 4 2.14 5 1.93

[0048] Table 2 Fatigue factors under different cycles of alternating pressure inside the casing (i)

[0049] Different casing internal pressure alternation cycles i Fatigue factor λi1=Pi1 / U0 1 0.9191 2 0.8382 3 0.7638 4 0.6926 5 0.6246

[0050] Table 3 Fatigue factors λij under different cycles of alternating pressure inside the casing i

[0051]

Claims

1. A method for testing the fatigue factor of cement stone tensile strength, characterized in that, Includes the following steps: 1) Obtain data on the casing wall thickness, cement sheath thickness, elastic modulus and Poisson's ratio of the casing, cement stone and formation in the fracturing section. Use the numerical simulation software Ansys to calculate the circumferential stress σ of the cement stone when the wellbore pressure is P0 under the initial fracturing conditions. t0 ; 2) The pressure inside the wellbore during fracturing was calculated using the numerical simulation software Ansys, and the pressure was P. j The circumferential stress σ of the cement ring at that time tj Where j = 1, 2, 3, 4, 5…, and P1 ≠ P2 ≠ P3 ≠ P4 ≠ P5 ≠…; 3) Determine the pressure alternation scheme inside the well casing during well section fracturing, specifically: The pressure inside the casing is between P0 and P1. j Within the range, press P0→P j →P0→P j →P0→P j →.....The pattern alternates cyclically for n rounds; 4) Calculate the corresponding alternating stress scheme of the cement annulus and the circumferential stress variation value Δσ based on the casing pressure alternation scheme. tj ; 5) Select cement stone specimens and test the initial working tensile strength U0 of the cement stone on a tensile strength testing machine. Separately, select cement stone specimens and apply tensile force according to the alternating circumferential stress scheme of the cementing annulus to simulate circumferential stress changes. Test the tensile strength P of the cement stone under the number of alternating pressure cycles i within the casing. ij ; 6) Calculate the fatigue factor λ of cement stone tensile strength under different rounds i. ij = P ij / U0, plot the fatigue factor λ of cement stone tensile strength ij The curve of the change with the number of alternating cycles i is obtained, and the fatigue factor λ of the cement stone tensile strength is obtained by fitting the curve. ij The relationship between the number of alternating cycles i of the pressure inside the casing and the formula for this relationship. 7) Change the wellbore pressure during fracturing to P j Repeat steps 2) to 6) until the preset requirements are met; 8) The circumferential stress variation value Δσ tj The X-axis represents the number of alternating pressure cycles i within the casing, the Y-axis represents the cement stone tensile strength fatigue factor λ. ij Using the Z-axis as the plotting axis, a three-dimensional surface plot was generated, and the fatigue factor λ of the cement stone tensile strength was obtained through fitting. ij The value of Δσ as a function of circumferential stress tj A model showing the relationship between the pressure change and the number of alternating cycles i within the casing was used to solve for the fatigue factor of the tensile strength of cement stone.

2. The method for testing the fatigue factor of cement stone tensile strength according to claim 1, characterized in that, In step 4), When the pressure inside the casing is between P0 and P... j Within the range, press P0→P j →P0→P j →P0→P j → When the mode is cyclically alternating for n rounds, the alternating stress scheme for the circumferential stress of the cement ring is as follows: According to σ t0 →σ t1 →σ t0 →σ t1 →σ t0 →σ t1 →.....The pattern alternates cyclically for n rounds.

3. The method for testing the fatigue factor of cement stone tensile strength according to claim 2, characterized in that, The circumferential stress variation value Δσ tj The calculation method is Δσ tj =σ tj -σ t0 .

4. The method for testing the fatigue factor of cement stone tensile strength according to claim 3, characterized in that, The method for preparing the cement stone specimen is as follows: prepare cement slurry according to the formula of the cement slurry system of the fracturing section, and cure it at the curing temperature T0 for 48 hours.

5. The method for testing the fatigue factor of cement stone tensile strength according to any one of claims 1-4, characterized in that, The fatigue factor λ of cement stone tensile strength was analyzed using a variation relationship model. ij When solving the problem, calculate the circumferential stress variation Δσ under alternating pressure conditions. t And determine the number of casing pressure alternation cycles i during the fracturing process, and assign Δσ t Substituting i into the model yields the corresponding cement stone tensile strength fatigue factor λ. ij .

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