CN111980667A - Quantitative evaluation method for influences of anisotropy on shale borehole wall collapse pressure - Google Patents

Abstract

The invention discloses a quantitative evaluation method for the influence of anisotropy on shale wellbore collapse pressure, which includes obtaining parameters such as in-situ stress azimuth coordinates, bedding occurrence, wellbore trajectory and the like in a target well area under the geodetic coordinate system; The stress component around the wellbore of the transversely isotropic formation in the polar coordinates of the borehole; obtain the occurrence of the bedding plane in the geodetic coordinate system, establish the relationship between the bedding plane coordinate system and the geodetic coordinate system, and determine the maximum principal stress around the wellbore The included angle with the normal direction of the bedding plane; three true triaxial strength criteria of MGC, ML and MWC are selected to obtain the strength envelope of the principal stress space of the four criteria; Calculate the maximum and minimum principal stress at different positions on the hour hand, and substitute them into different strength criteria to judge the stability of the bottom hole pressure value. Repeat the above steps to obtain the critical bottom hole pressure at each point around the well, and take the The maximum value is the collapse pressure under the wellbore trajectory.

Description

A quantitative evaluation method for the effect of anisotropy on shale borehole collapse pressure

technical field

The invention relates to the technical field of petroleum engineering, in particular to a quantitative evaluation method for the influence of anisotropy on the collapse pressure of shale wellbore.

Background technique

and Cook在井筒与层理平行的方向上进行了厚壁圆筒试验表明，平行于或接近平行于层理方向的钻井更容易发生严重的井壁失稳；Lu、Zhou、Liu等人提出，泥浆比重越大，裂缝性页岩的孔隙压力越大，有效应力越小，屈服强度越大。目前对页岩地层井壁稳定性的分析一般是基于常规的应力状态，采用Mohr-coulomb准则，忽略了中间主应力的影响。页岩储层处于真三轴应力状态，大量的实验已经表明，中间主应力对强度的影响是显著的且具有范围效应；Singh等人发现三轴准则估计的塑性区范围比二轴准则小约13％～20％；Rahimi等人提出，在井壁稳定分析中修正的Lade准则、WiebolsCook准则和Mogi-Coulomb准则预测的坍塌压力要么过于保守，要么不安全，但假设地层为各向同性，并不适用于页岩，弹性各向异性对井筒稳定性影响的研究很少。During the drilling process, the complex downhole situation caused by the instability of the wellbore and other induced downhole accidents are extremely harmful to oil and gas drilling and production. The problem of the instability of the wellbore has always been a worldwide problem. As an important unconventional resource, shale gas has received more and more attention worldwide, but the wellbore instability caused by shale formations is as high as 75%. Therefore, scholars at home and abroad have carried out a lot of research on wellbore stability, which can be mainly divided into two aspects: stress distribution in the near-wellbore zone and rock strength criteria. Kirsch first proposed a linear elastic wellbore stress model, but it is only applicable when the wellbore points to the main in-situ stress direction; Fairhurst first proposed a linear elastic wellbore stress model applicable to any wellbore; Westergaard first proposed an elastic-plastic wellbore perimeter stress model stress model, but requires too many parameters, which limits its popularization and application; shale is a laterally isotropic material, and its linear elastic parameters are significantly different in the vertical and parallel bedding directions. For the isotropic medium, the influence of elastic anisotropy on the wellbore stability is ignored. Later, Lekhnitskii introduced elastic transverse isotropy for the first time, and obtained the stress component of the shale formation wellbore. The transversely isotropic stress model proposed by him fully considers the lithological characteristics of shale, and is the preferred method for many scholars to analyze the wellbore stability of shale formations. A lot of research has been done. Jaeger first proposed a weak surface strength criterion based on the Coulomb failure theory. The wellbore failure predicted by this strength criterion has two components, namely the rock bulk strength and the bedding or fracture strength, although the shale strength is well explained. Anisotropic, but there is a large error, such as there is a significant difference between the vertical bedding loading strength and the parallel bedding loading strength, and the shale strength is also significantly reduced at low dip angles; Weijermars et al. proposed that with the increase of the elastic modulus , the mud density window of wellbore stability decreases, and the Poisson's ratio has very limited influence on wellbore stability;

and Cook conducted a thick-walled cylinder test in the direction of the wellbore parallel to the bedding, showing that the drilling parallel or nearly parallel to the bedding direction is more prone to serious wellbore instability; Lu, Zhou, Liu et al. proposed, The larger the specific gravity of the mud, the larger the pore pressure of the fractured shale, the smaller the effective stress and the larger the yield strength. The current analysis of wellbore stability in shale formations is generally based on the conventional stress state, using the Mohr-coulomb criterion and ignoring the influence of the intermediate principal stress. Shale reservoirs are in a state of true triaxial stress. A large number of experiments have shown that the effect of intermediate principal stress on strength is significant and has a range effect; Singh et al. found that the plastic zone range estimated by the triaxial criterion is approximately smaller than that of the biaxial criterion. 13% to 20%; Rahimi et al. proposed that the collapse pressure predicted by the modified Lade criterion, WiebolsCook criterion and Mogi-Coulomb criterion in the wellbore stability analysis were either too conservative or unsafe, but assumed that the formation was isotropic and Not applicable to shale, there are few studies on the effect of elastic anisotropy on wellbore stability.

Although the above studies have discussed the influence of many characteristics of shale on wellbore stability, the proposed countermeasures still cannot eliminate the complexity of drilling operations. The wellbore stability model that simply treats shale as an isotropic medium is not accurate, and the stability of shale is affected by many factors. This method combines the intermediate principal stress and elastic anisotropy parameters, and uses the transverse isotropic model to calculate the three-dimensional stress of the borehole wall. The sensitivity of collapse pressure to the degree of anisotropy of elastic parameters was evaluated, a new mechanism of wellbore instability in bedded shale was explored, and the problem of wellbore instability encountered in drilling was minimized.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the deficiencies of the prior art, and provide a quantitative evaluation method for the impact of anisotropy on the collapse pressure of shale wellbore, comprising the following steps:

Step 1: Under the geodetic coordinate system, obtain the in-situ stress azimuth coordinates, bedding occurrence and wellbore trajectory of the target well area; in-situ stress and pore pressure in the target well area; parameters such as angle;

Step 2: Convert the in-situ stress from the geodetic coordinate system to the wellbore rectangular coordinate system, and then convert the stress in the wellbore rectangular coordinate system to the wellbore polar coordinate to obtain the transversely isotropic formation in the wellbore polar coordinate. and convert it into the form of principal stress;

Step 3: Obtain the occurrence of the bedding plane under the geodetic coordinate system, establish the relationship between the bedding plane coordinate system and the geodetic coordinate system, and determine the angle between the maximum principal stress around the well and the normal direction of the bedding plane;

Step 4: Select the three true triaxial strength criteria of MGC, ML and MWC, compare with the MC strength criterion, and obtain the strength envelope of the principal stress space of the four criteria;

Step 5: Use MATLAB software to compile wellbore collapse pressure calculation programs with different strength criteria, given bottom hole pressure Pw, calculate the maximum and minimum principal stresses at different positions counterclockwise along the borehole axis, and substitute them into different strength criteria , judge the stability of the bottom hole pressure value, gradually increase the bottom hole pressure, and use the Newton iteration method to repeat the above steps until the bottom hole pressure when the well wall is stable is the critical bottom hole pressure at this point, and the well After the critical bottom hole pressure at each point in the week, the maximum value is taken as the collapse pressure under the wellbore trajectory.

Further, the in-situ stress is converted from the geodetic coordinate system to the wellbore rectangular coordinate system, and then the stress in the wellbore rectangular coordinate system is converted into the wellbore polar coordinate, so as to obtain the horizontal direction in the wellbore polar coordinate. The stress components around the wellbore of the isotropic formation are converted into the form of principal stress, including the following processes:

The stress component around the borehole in the Cartesian coordinate system is as follows:

表示仅取括号中结果的实部，σ_x,σ_y,σ_z,τ_xy,τ_xz,τ_yz为总应力分量，下标i表示由地应力引起的分量，下标a表示由弹性各向异性引起的分量；In the formula,

Indicates that only the real part of the result in parentheses is taken, σ _x , σ _y , σ _z , τ _xy , τ _xz , τ _yz are the total stress components, the subscript i represents the component caused by the ground stress, and the subscript a represents the elastic components due to anisotropy;

The stress components around the wellbore in the wellbore rectangular coordinate system are converted into the wellbore polar coordinates, as shown in the following formula:

The stress components around the borehole in the borehole polar coordinate system are converted into the form of principal stress, as follows:

Further, the occurrence state of the stratum bedding plane is obtained under the geodetic coordinate system, the relationship between the bedding plane coordinate system and the geodetic coordinate system is established, and the angle between the maximum principal stress around the well and the normal direction of the bedding plane is determined, including: Follow the steps below:

The angle between the maximum principal stress around the well and the normal to the bedding plane is as follows:

In the formula, n is the direction vector of the normal direction of the bedding plane in the geodetic coordinate system, N is the direction vector of the maximum principal stress around the well in the geodetic coordinate system, and its expressions are as follows:

n=[cosα _bp sinβ _bp , sinα _bp sinβ _bp , cosβ _bp ]

in,

γ=0.5arctan[2τ _θz /(σ _θ -σ _z )]

where α _bp and β _bp are the bedding tendency and bedding dip angle, respectively; α _b and β _b are the wellbore tendency and bedding dip angle, respectively; γ is the angle between the maximum principal stress and the _wellbore axis Ze.

Further, the step 5 specifically includes the following steps:

1. Set the variation range of the borehole circumference angle to 0° to 180°, and the given increment is 2°; 0°～360°, the interval is 2°.

2. Set the bottom hole pressure to increase from 0 to x, and the interval is 0.01MPa, substitute it into the wellbore rectangular coordinate system and convert the stress component around the well into the polar coordinate formula of the wellbore, and calculate the surrounding stress at each point around the well. , and respectively transformed into the principal stress of well Wednesday;

3. Substitute the principal stress around the well into different strength criteria, repeat step 2 until the critical condition of wellbore stability is reached, and the bottom hole pressure at this time is the critical value for maintaining wellbore stability;

4. According to step 3, the critical value of bottom hole pressure for maintaining the stability of the wellbore at each point from 0° to 360° around the wellbore is obtained, and the maximum value is taken as the collapse pressure of the wellbore under the condition of the wellbore trajectory.

The beneficial effects of the invention are as follows: the invention solves the problems that the MC criterion ignores the influence of the intermediate principal stress on the strength and the isotropic stress model around the well ignores the influence of the elastic anisotropy, resulting in serious block loss and caving in the wellbore during drilling, Three true triaxial criteria considering the intermediate principal stress are used, combined with the elastic anisotropy parameters of the transverse isotropic model, to quantitatively evaluate the influence of shale borehole wall collapse pressure.

Description of drawings

Figure 1 is the spatial distribution diagram of the in-situ stress azimuth and borehole trajectory in the target well area under the geodetic coordinate system.

Figure 2 is a schematic diagram of the angle between the wellbore axis and the normal to the bedding plane of the reservoir.

Figure 3 shows the occurrence of the reservoir bedding plane in the target well area under the geodetic coordinate system.

Figure 4 shows the sensitivity of the four strength criteria to intermediate stress in the principal stress space.

Figure 5 is a comparison and verification diagram of the calculation results of the isotropic and anisotropic models.

Figure 6 is the collapse pressure sensitivity analysis based on the MC criterion.

Figure 7 is the collapse pressure sensitivity analysis based on the MGC criterion.

Figure 8 is a collapse pressure sensitivity analysis based on the ML criterion.

Figure 9 is a collapse pressure sensitivity analysis based on the MWC criterion.

Detailed ways

The technical solutions of the present invention are further described in detail below with reference to the accompanying drawings, but the protection scope of the present invention is not limited to the following.

As shown in Figure 1, the object of the present invention is achieved through the following technical solutions: a quantitative evaluation method for the influence of anisotropy on the collapse pressure of shale wellbore, which comprises the following steps:

Step 1: Investigate on-site drilling and logging data, and obtain spatial parameters such as in-situ stress orientation, bedding occurrence, and borehole trajectory in the target well area under the geodetic coordinate system; Describe the spatial distribution of in-situ stress and borehole trajectory, as shown in Figure 1;

Step 2: Obtain the in situ stress and pore pressure of the target well area by investigating on-site drilling, well logging and other data, and obtain strength parameters such as cohesion and internal friction angle of the rock body and fracture plane in the mining area through laboratory experiments;

Step 3: Convert the in-situ stress from the geodetic coordinate system to the wellbore rectangular coordinate system, and then convert the stress in the wellbore rectangular coordinate system to the wellbore polar coordinate to obtain the transversely isotropic formation in the wellbore polar coordinate. and convert it into the form of principal stress;

Among them, the stress component around the borehole in the Cartesian coordinate system is shown in formula (1):

表示仅取括号中结果的实部，σ_x,σ_y,σ_z,τ_xy,τ_xz,τ_yz为总应力分量，下标i表示由地应力引起的分量，下标a表示由弹性各向异性引起的分量。In the formula,

Indicates that only the real part of the result in parentheses is taken, σ _x , σ _y , σ _z , τ _xy , τ _xz , τ _yz are the total stress components, the subscript i represents the component caused by the ground stress, and the subscript a represents the elastic Component due to anisotropy.

The stress components around the wellbore in the wellbore rectangular coordinate system are converted into the wellbore polar coordinates, as shown in equation (2):

The stress component around the borehole in the borehole polar coordinate system is converted into the form of principal stress, as shown in formula (3):

Step 4: Obtain the occurrence of the formation bedding plane in the geodetic coordinate system, and establish the relationship between the bedding plane coordinate system and the geodetic coordinate system. The angle between the borehole axis and the normal direction of the reservoir bedding plane is shown in Figure 2. The occurrence of the bedding plane in the geodetic coordinate system is shown in Figure 3. Finally, the angle between the maximum principal stress around the well and the normal to the bedding plane is determined, as shown in formula (4):

In the formula, n is the direction vector of the normal direction of the bedding plane in the geodetic coordinate system, N is the direction vector of the maximum principal stress around the well in the geodetic coordinate system, and its expressions are shown in equations (5) and (6) respectively :

n=[cosα _bp sinβ _bp , sinα _bp sinβ _bp , cosβ _bp ] (5)

in,

γ=0.5arctan[2τ _θz /(σ _θ -σ _z )] (7)

where α _bp and β _bp are the bedding tendency and dip angle, respectively, °; α _b and β _b are the wellbore dip and dip angle, respectively, °; γ is the angle between the maximum principal stress and the _borehole axis Ze, °.

Step 5. Select three true triaxial strength criteria, MGC, ML and MWC, which have been proved to be suitable for wellbore stability analysis. The sensitivity of the intermediate principal stress is shown in Fig. 4;

Mohr-Coulomb criterion, as shown in equation (8):

Mogi-Coulomb criterion, as shown in equations (9) and (10):

τ _oct = a+bσ _m,2 (9)

Among them, τ _oct is the shear stress, MPa; σ _m,2 is the average normal stress, MPa; a and b are the yield parameters of the MGC criterion, which can be obtained from c _o and φ _o , as shown in formula (11):

Modified Lade criterion, as shown in equations (12), (13) and (14):

I″ ₁ ³ /I″ ₃ =η+27 (12)

where S is the parameter of the cohesion function.

Modified Wiebols-Cook model, as shown in equation (15):

Among them, J ₁ is the average effective hydrostatic pressure, MPa; J ₂ is the second stress deflection invariant, MPa; A, B, C are the strength parameters, which can be determined by conventional triaxial compression experiments, such as formula (16) and formula (17) shows:

Step 6. Use MATLAB software to compile wellbore collapse pressure calculation programs with different strength criteria, given bottom hole pressure Pw, calculate the maximum and minimum principal stresses at different positions counterclockwise along the wellbore axis, and substitute them into different strength criteria , judge the stability of the bottom hole pressure value, gradually increase the bottom hole pressure, and use the Newton iteration method to repeat the above steps until the bottom hole pressure when the well wall is stable is the critical bottom hole pressure at this point, and the well After the critical bottom hole pressure at each point in the week, the maximum value is taken as the collapse pressure under the wellbore trajectory;

Step 6 in the above scheme is specifically:

1. Set the variation range of the borehole circumference angle to 0° to 180°, and the given increment is 2°; °～360°, the interval is 2°.

2. Set the bottom hole pressure to increase from 0 to x, and the interval is 0.01MPa. Substitute it into formula (2), calculate the circumferential stress at each point around the well, and convert it into the principal stress of the well Wednesday; where X is the critical value of bottom hole pressure;

3. Substitute the principal stress around the well into different strength criteria, repeat step 2 until the critical condition of wellbore stability is reached, and the bottom hole pressure at this time is the critical value for maintaining wellbore stability;

4. Step 3 is taken to obtain the critical value of bottom hole pressure for maintaining the stability of the wellbore at each point from 0° to 360° around the wellbore, and the maximum value is taken as the wellbore collapse pressure under the condition of the wellbore trajectory.

Step 7. Verify the accuracy of the calculation results of the transversely isotropic borehole wall collapse pressure program compiled by this method. The commonly used Mohr-Coulomb criterion is used as the criterion, and both the elastic modulus and Poisson's ratio anisotropy are set to 1 , the model degenerates to isotropic, and compares with the calculation results based on the isotropic surrounding stress model;

Step 8. Using the MC criterion, the MGC criterion, the ML criterion and the MWC criterion, respectively, analyze the sensitivity of the collapse pressure of the horizontal well drilled in the direction of the maximum horizontal principal stress to the influence of elastic anisotropy. The isotropic wellbore stability model is used to obtain the collapse pressure under different anisotropy of elastic parameters.

Example

Take a typical strike-slip stratigraphic mechanism shale reservoir as an example:

(1) Calculation process

S1. Investigate on-site drilling, logging and formation testing data, and obtain bedding with an angle of 85° between the azimuth of the horizontal maximum in-situ stress and the true north direction, a reservoir development inclination of 85°, and a dip angle of 23.5°;

S2. Using the field drilling and logging data, the depth of the drilled formation is 3937.29m, the maximum horizontal in-situ stress is 81.11 MPa, the minimum horizontal in-situ stress is 58.66 MPa, the vertical in-situ stress is 74.09 MPa, and the formation pore pressure is 39.47 MPa MPa;

S3. Use laboratory experiments and methods to obtain the required material parameters in the rock strength criterion in the target well area. In the strength criterion, the cohesion of the bulk and bedding planes are 21.4MPa and 18.1MPa, respectively, and the internal friction angles of the bulk and bedding planes are 28.3° and 23.8°; the elastic modulus of rock is 25.47GPa, and the Poisson's ratio is 0.193GPa;

S4. According to the reservoir geomechanical parameters and rock strength parameters obtained in the above steps S1-S3, set the variation range of the wellbore circumference angle to 0 to 180°, and a given increment of 2°; set the variation range of the wellbore inclination to be 0°～90°, interval of 5°; set wellbore azimuth variation range from 0° to 360°, interval of 2°; substituting the above model to calculate the collapse pressure of any trajectory wellbore in the 3937.29m depth reservoir in this block;

S5. Input the above geological and rock strength parameters into the model of this method, and calculate the variation law of the collapse pressure with the inclination of the wellbore when the wellbore dip angle is 80°. The results are consistent with the calculation results of the isotropic model, indicating that the calculation results of the program in this work are reliable.

S6. Using the MC criterion, the MGC criterion, the ML criterion and the MWC criterion, respectively, to analyze the sensitivity of the collapse pressure of the horizontal well drilled in the direction of the maximum horizontal principal stress to the influence of elastic anisotropy, assuming that the elastic modulus and Poisson's ratio are different The variation range of the anisotropy ratio is 0.25-3, and the interval is 0.25. The collapse pressures obtained for four criteria with different elastic parameter anisotropy degrees are shown in Figures 6-9.

(2) Comparative analysis

According to Fig. 6 to Fig. 9 as a whole, the prediction results using the 2D or 3D strength criterion show that the Poisson's ratio anisotropy has no significant effect on the collapse pressure, but when E _h /E _v is high, the collapse pressure has no significant effect on the Poisson's ratio anisotropy. The sensitivity of the loose ratio is enhanced; while the degree of elastic modulus anisotropy has a significant effect on the collapse pressure, especially in the case of E _h /E _v < 1, small changes in the degree of elastic modulus anisotropy can lead to collapse Significant fluctuations in pressure. In addition, comparing Fig. 6 and Fig. 7 to Fig. 9, the isotropic point collapse pressure obtained by using the MC criterion is much lower than the collapse pressure when the intermediate principal stress is not considered when the three 3D criteria are MGC, ML and MWC. At the middle principal stress, the collapse pressure will increase significantly; at low _vh / _vv , the 2D criterion predicts that the collapse pressure will continue to decrease with the increase of E _h /E _v , while the 3D criterion predicts that the collapse pressure will increase with E _h . The increase of /E _v shows a trend of first decreasing to the lowest value and then increasing, especially in the MGC prediction results, the trend is more obvious. At the same time, the collapse pressure obtained by this criterion is also highly sensitive to changes in elastic anisotropy on ML and MWC guidelines.

The foregoing are only preferred embodiments of the present invention, and it should be understood that the present invention is not limited to the forms disclosed herein, and should not be construed as an exclusion of other embodiments, but may be used in various other combinations, modifications, and environments, and Modifications can be made within the scope of the concepts described herein, from the above teachings or from skill or knowledge in the relevant field. However, modifications and changes made by those skilled in the art do not depart from the spirit and scope of the present invention, and should all fall within the protection scope of the appended claims of the present invention.

Claims (4)

1. a quantitative evaluation method of anisotropy to the impact of shale borehole wall collapse pressure, is characterized in that, comprises the steps: Step 1: Under the geodetic coordinate system, obtain the in-situ stress azimuth coordinates, bedding occurrence and wellbore trajectory of the target well area; in-situ stress and pore pressure in the target well area; parameters such as angle; Step 2: Convert the in-situ stress from the geodetic coordinate system to the wellbore rectangular coordinate system, and then convert the stress in the wellbore rectangular coordinate system to the wellbore polar coordinate to obtain the transversely isotropic formation in the wellbore polar coordinate. and convert it into the form of principal stress; Step 3: Obtain the occurrence of the bedding plane under the geodetic coordinate system, establish the relationship between the bedding plane coordinate system and the geodetic coordinate system, and determine the angle between the maximum principal stress around the well and the normal direction of the bedding plane; Step 4: Select the three true triaxial strength criteria of MGC, ML and MWC, compare with the MC strength criterion, and obtain the strength envelope of the principal stress space of the four criteria; Step 5: Use MATLAB software to compile wellbore collapse pressure calculation programs with different strength criteria, given bottom hole pressure Pw, calculate the maximum and minimum principal stresses at different positions counterclockwise along the wellbore axis, and substitute them into the three true three In the shaft strength criterion, the stability of the bottom hole pressure value is judged, and the bottom hole pressure is gradually increased. Using the Newton iteration method, the above steps are repeated until the bottom hole pressure when the well wall is stable is the critical bottom hole pressure at this point. , after obtaining the critical bottom hole pressure at each point around the wellbore, the maximum value is taken as the collapse pressure under the wellbore trajectory. 2. a kind of quantitative evaluation method of the influence of anisotropy on shale borehole wall collapse pressure according to claim 1, is characterized in that, described in-situ stress is converted from geodetic coordinate system to wellbore Cartesian coordinate system, Then, the stress in the wellbore rectangular coordinate system is converted into the wellbore polar coordinates, and the stress component around the wellbore of the transversely isotropic formation in the wellbore polar coordinates is obtained, and it is converted into the form of principal stress, including the following process: The stress component around the borehole in the Cartesian coordinate system is as follows:

In the formula,

Indicates that only the real part of the result in parentheses is taken, σ _x , σ _y , σ _z , τ _xy , τ _xz , τ _yz are the total stress components, the subscript i represents the component caused by the ground stress, and the subscript a represents the elastic components due to anisotropy; The stress components around the wellbore in the wellbore rectangular coordinate system are converted into the wellbore polar coordinates, as shown in the following formula:

The stress components around the borehole in the borehole polar coordinate system are converted into the form of principal stress, as follows:

3. The quantitative evaluation method for the influence of anisotropy on the collapse pressure of shale wellbore according to claim 1, characterized in that, under the geodetic coordinate system, the occurrence of the bedding plane of the stratum is obtained, and the bedding is established. The relationship between the surface coordinate system and the geodetic coordinate system is used to determine the angle between the maximum principal stress around the well and the normal to the bedding plane, including the following steps: The angle between the maximum principal stress around the well and the normal to the bedding plane is as follows:

In the formula, n is the direction vector of the normal direction of the bedding plane in the geodetic coordinate system, N is the direction vector of the maximum principal stress around the well in the geodetic coordinate system, and its expressions are as follows: n=[cosα _bp sinβ _bp , sinα _bp sinβ _bp , cosβ _bp ]

in, γ=0.5arctan[2τ _θz /(σ _θ -σ _z )] where α _bp and β _bp are the bedding tendency and bedding dip angle, respectively; α _b and β _b are the wellbore tendency and bedding dip angle, respectively; γ is the angle between the maximum principal stress and the _wellbore axis Ze. 4. The quantitative evaluation method for the influence of anisotropy on shale borehole wall collapse pressure according to claim 1, wherein the step 5 specifically comprises the following steps: 1. Set the variation range of the wellbore circumference angle to 0° to 180°, and the given increment is 2°; 0°～360°, the interval is 2°. 2. Set the bottom hole pressure to increase from 0 to x, with an interval of 0.01MPa, substitute it into the wellbore rectangular coordinate system and convert the stress component around the well into the polar coordinate formula of the wellbore, and calculate the stress around the well at each point around the well. , and respectively transformed into the principal stress of well Wednesday; 3. Substitute the principal stress around the well into different strength criteria, repeat step 2 until the critical condition of wellbore stability is reached, and the bottom hole pressure at this time is the critical value for maintaining wellbore stability; 4. According to step 3, obtain the critical value of the bottom hole pressure for maintaining the stability of the wellbore at each point from 0° to 360° around the wellbore, and take the maximum value as the wellbore collapse pressure under the condition of the wellbore trajectory.

Images