Geomechanical method for quantitatively evaluating well wall stability of fractured formation
By combining seismic data, well logging data, and core experiments, a comprehensive wellbore instability index was calculated, solving the objectivity and accuracy issues in wellbore stability evaluation in existing technologies, and achieving accurate quantification and prediction of wellbore instability risk.
Patent Information
- Application Number
- CN202511472387.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-10-15
- Publication Date
- 2025-11-28
AI Technical Summary
Existing wellbore stability assessment methods fail to effectively combine geostress state, rock strength parameters, and fracture strength parameters, resulting in a lack of objectivity and accuracy in the assessment results, and an inability to accurately predict wellbore instability risks.
By predicting the formation stress state and drilling azimuth using seismic and well logging data, and combining rock strength parameters and fracture orientation measured by core experiments, the comprehensive wellbore instability index is calculated. The degree of wellbore instability is quantified by considering the weights of different parameters.
It enables precise quantitative evaluation of wellbore instability risk, improves the objectivity and accuracy of wellbore stability evaluation, and can identify the degree of wellbore instability in advance, thereby reducing drilling risks.
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Figure CN121024591A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to a geomechanics method for quantitatively evaluating the wellbore stability of fractured formations, and belongs to the technical field of oil and gas exploration and development. BACKGROUND
[0002] In the engineering of oil and gas drilling, the wellbore stability is one of the key problems that restrict the safety and efficiency of drilling. With the exploration and development objects gradually turning to deep and ultra-deep layers and complex structural blocks, the wellbore instability problem is becoming more and more prominent, and the performance forms include wellbore collapse, falling block, shrinkage, sticking, rupture and leakage, etc. These complex wellbore instability behaviors not only lead to the extension of drilling period and the increase of drilling cost, but also may cause blowout and well collapse and other serious engineering accidents, which pose a serious threat to the safe development of oil and gas fields.
[0003] Most of the existing wellbore stability evaluation methods only consider the mechanical properties of intact rock, and ignore the dominant effect of natural fracture structure on wellbore stability. A few methods that consider natural fractures are often independent of intact rock analysis, and lack of quantitative coupling between the two. Different parameters have different contribution degrees to the instability index, and the existing methods generally fail to establish a reasonable weight system, resulting in the lack of objectivity and accuracy of the evaluation results.
[0004] The existing technology still lacks an evaluation method that can comprehensively quantify the in-situ stress state, rock strength parameters, fracture occurrence and fracture strength parameters, and form a comprehensive instability index. SUMMARY
[0005] The purpose of the present application is to provide a geomechanics method for quantitatively evaluating the wellbore stability of fractured formations, which can comprehensively quantify the in-situ stress state, rock strength parameters, fracture occurrence and fracture strength parameters, form a comprehensive instability index evaluation, and improve the objectivity and accuracy of the evaluation results.
[0006] In order to achieve the above purpose, the present application provides a geomechanics method for quantitatively evaluating the wellbore stability of fractured formations, comprising the following steps: Step 1, predicting the in-situ stress state of the formation and the drilling direction through seismic data and logging data; Step 2, selecting a core to perform uniaxial and triaxial compression tests in the laboratory to measure rock strength parameters, including uniaxial compressive strength, uniaxial tensile strength, internal friction angle and cohesion; Step 3, calculating the stress state of the intact rock in the target layer section according to the measured data; Step 4, solving the instability index of the intact rock according to the stress state of the intact rock; Step 5, according to seismic and logging data, including seismic attributes, imaging logging data, fracture occurrence is obtained, including fracture dip and tendency; Step 6, according to the core shear mechanics experiment to determine the fracture strength parameters; Step 7, according to the fracture strength parameters to solve the normal stress and shear stress on the fracture; Step 8, according to the stress state of the fracture to solve the instability index of the rock containing the fracture; Step 9, according to the instability index of the complete rock, the instability index of the rock containing the fracture, and the instability index weight of the complete rock, the instability index weight of the rock containing the fracture, the comprehensive instability index of the well wall is obtained; Step 10, according to the corresponding relationship between the comprehensive instability index of the well wall and the instability degree of the well wall, the instability index of the well wall corresponding to the instability degree of the target interval is determined, including: When the comprehensive instability index of the well wall is greater than or equal to the first preset value and less than or equal to the second preset value, it is determined that the well wall is in a stable state when drilling the target interval; When the comprehensive instability index is greater than the second preset value and less than or equal to the third preset value, it is determined that the well wall is in a slight instability state when drilling the target interval; When the comprehensive instability index is greater than the third preset value and less than or equal to the fourth preset value, it is determined that the well wall is in a moderate instability state when drilling the target interval; When the comprehensive instability index is greater than the fourth preset value and less than or equal to the fifth preset value, it is determined that the well wall is in a serious instability state when drilling the target interval; When the comprehensive instability index is greater than the fifth preset value, the well wall is most likely to be unstable, and it is determined that the well wall is in a limit instability state when drilling the target interval, and according to the adjacent well condition, sidetracking to a new target area; Wherein, the first preset value is less than the second preset value, the second preset value is less than the third preset value, the third preset value is less than the fourth preset value, and the fourth preset value is less than the fifth preset value.
[0007] Furthermore, the seismic and well logging data in Step 1 include seismic profiles / data volumes, natural gamma rays, spontaneous potential, wellbore, formation resistivity, formation density, conventional acoustic waves, imaging data, and shear wave data. Specifically, by analyzing the seismic profiles / data volumes and combining them with pre-stack elastic inversion, key elastic parameters of the formation, such as P-wave impedance, S-wave impedance, and density, are directly obtained through inversion. This allows for the calculation of Young's modulus, Poisson's ratio, and shear modulus—rock physical and mechanical parameters used to assess the strength, brittleness, and stress state of the formation rocks. Natural gamma rays reflect the abundance of radioactive elements (such as potassium, uranium, and thorium) in the formation, used to identify clay content and stratigraphic correlation. Spontaneous potential records the electrochemical potential difference between the drilling mud and formation water inside and outside the wellbore, used to determine permeable layers and... The data includes: the presence of clay layers and mud filtrate intrusion; wellbore diameter curves reflecting wellbore morphology and size variations, used to identify wellbore collapse, narrowing, or enlargement; formation resistivity measurements of the formation's resistance to current, used to distinguish between water-bearing and oil-bearing layers and to aid in fluid property assessment; formation density curves measuring rock bulk density using gamma ray scattering, used for porosity calculation and lithology identification; conventional acoustic measurements of P-wave propagation time difference, reflecting formation porosity, density, and mechanical properties; imaging data acquiring two-dimensional or three-dimensional images of the wellbore using acoustic or resistivity imaging techniques, used to analyze fractures, bedding, joints, and wellbore integrity; and shear wave data revealing rock stiffness, elastic parameters, and anisotropic characteristics through shear wave propagation features.
[0008] Furthermore, the calculation process for the stress state of the intact rock in the target layer in Step 3 is as follows: S3.1 Select a fixed initial depth for the target layer and obtain the geomechanical parameters at this depth; S3.2. Through coordinate transformation, the geostress matrix is transformed to the wellbore coordinate system to form the wellbore boundary conditions in the wellbore coordinate system. The coordinate transformation matrix is as follows: ; ; in, It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the geostress coordinate system; It is the angle between the direction of maximum ground stress and the due north direction; It is the angle between the vertical stress and the normal to the horizontal plane; It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the wellbore coordinate system; It is the well inclination angle; It is the well inclination azimuth; The coordinate transformation formula is: ; in, This is the result after transforming the in-situ stress matrix to the wellbore coordinate system; These are the normal stresses in the x, y, and z directions after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses in the xy plane after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses on the xz plane after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses on the yz plane after the in-situ stress matrix is transformed to the wellbore coordinate system; It is the maximum horizontal ground stress; It is the minimum horizontal ground stress; It is vertical stress; S3.3 Solve the stress state of the surrounding rock of the wellbore using the boundary conditions obtained in the wellbore coordinate system above: ; Right now: ; in, It is the pressure of the liquid column; It is pore pressure; It is the Biot coefficient; It is the borehole radius; It is the distance from a point in the formation to the center of the wellbore; It is Poisson's ratio; It is the circumferential angle of the wellbore; It is the stress state matrix of the rock surrounding the well; These are the expressions for the normal stress state of the surrounding rock in cylindrical coordinates; These are the expressions for the shear stress state of the surrounding rock in cylindrical coordinates.
[0009] Furthermore, the calculation process for the normal stress and shear stress on the crack in Step 7 is as follows: S7.1. Based on the coordinate transformation formula, the stress state in the wellbore coordinate system is transformed to the fracture surface. The fracture coordinate transformation matrix is: ; ; in, It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the crack coordinate system; It is prone to cracking; It is the angle of the crack; It is the coordinate transformation matrix used when transforming from a spatial rectangular coordinate system to a cylindrical coordinate system; S7.2 The stress state matrix of the crack is: ; The normal stress and shear stress on the crack are respectively: ; ; in, It is the stress state matrix of the crack; These are the normal stresses in the x, y, and z directions, respectively, in the crack coordinate system; These are the shear stresses on the xy plane in the crack coordinate system; These are the shear stresses on the xz plane in the crack coordinate system; These are the shear stresses on the yz plane in the crack coordinate system; It is the normal stress along the normal direction on the crack surface; It is the shear stress on the crack surface.
[0010] Furthermore, the calculation processes for the instability index of intact rock and the instability index of fractured rock in Steps 4 and 8 are as follows: (1) The formula for calculating the instability index of intact rock is derived based on the Mogi-Coulomb failure criterion as follows: ; in: ; ; in, It is octahedral shear stress; These are the first, second, and third principal stresses, which are obtained by solving... The eigenvalues of the matrix are obtained; It is the cohesive force of the rock; It is the internal friction angle of the rock; (2) The calculation formula for the instability index of fractured rocks is derived based on the Jaeger weak surface failure criterion as follows: ; in, It is the cohesive force of the crack; It is the internal friction angle of the crack.
[0011] Furthermore, in Step 9, the weights of the instability index for intact rock and the instability index for fractured rock are determined based on the predicted probability of fracture development in the target formation; or based on previous experience from nearby wells in the area where the target formation is located; or they are initially set to the same value and then adjusted in real time during drilling based on the actual performance of wellbore instability. If the actual damage to the wellbore is greater than the instability state corresponding to the calculated comprehensive instability index, the weight of the instability index for fractured rock is increased while the weight of the instability index for intact rock is decreased; conversely, the weight of the instability index for fractured rock is decreased while the weight of the instability index for intact rock is increased. Based on the instability indices of intact rock and fractured rock, along with their respective weights, a linear weighted sum method is used to obtain the comprehensive instability index of the target formation. The specific process is as follows: multiply the instability index of intact rock by its weighted value to obtain a first value; multiply the instability index of fractured rock by its weighted value to obtain a second value; and add the first and second values to obtain the comprehensive instability index of the target formation. Through this algorithm, a comprehensive instability index can be derived from the instability indices of intact rock and fractured rock. This comprehensive instability index is a specific numerical value that quantifies the degree of wellbore instability in different formations during drilling, facilitating more precise subsequent engineering responses.
[0012] This invention acquires geostress state, rock strength parameters, fracture orientation, and fracture strength parameters based on seismic data, well logging data, and rock mechanics experimental data. Based on these parameters, it determines the stress state of intact rock and the instability index of fractured rock in the target formation. Then, it solves for the instability index of both intact and fractured rock using the stress state. Considering the weights of these parameters, it obtains a comprehensive instability index for the target formation. Finally, based on the correspondence between the calculated comprehensive wellbore instability index and the degree of instability, it determines the degree of wellbore instability in the target formation corresponding to the instability index. This invention quantifies wellbore instability problems in oil and gas drilling by using the instability indices of intact rock and fractured rock in the target formation, along with their corresponding weights. It then accurately identifies the degree of wellbore instability under actual drilling fluid density based on the quantified indices. This achieves a comprehensive quantification of geostress state, rock strength parameters, fracture orientation, and fracture strength parameters, forming a comprehensive evaluation of instability indices, thus improving the objectivity and accuracy of the evaluation results. Attached Figure Description
[0013] Figure 1 This is a flowchart illustrating the present invention; Figure 2 This is a schematic diagram illustrating the degree of instability based on the comprehensive instability index in an embodiment of the present invention; Figure 3 This is a schematic curve of the instability index results at different depths in an embodiment of the present invention. Detailed Implementation
[0014] The invention will now be further described with reference to the accompanying drawings.
[0015] like Figure 1 As shown, a geomechanical method for quantitatively evaluating wellbore stability in fractured formations includes the following steps: Step 1: Predict the formation stress state and drilling location using seismic and well logging data; Step 2: Select rock cores and conduct uniaxial and triaxial compression tests in the laboratory to measure rock strength parameters, including uniaxial compressive strength, uniaxial tensile strength, internal friction angle and cohesion; Step 3: Calculate the stress state of the intact rock in the target layer based on the measured data; Step 4: Determine the instability index of the intact rock based on its stress state; Step 5: Based on seismic and well logging data, including seismic attributes and imaging well logging data, obtain the fracture orientation, including the fracture dip angle and dip direction; Step 6: Determine the strength parameters of the cracks based on core shear mechanics experiments; Step 7: Solve for the normal stress and shear stress on the crack based on the crack strength parameters; Step 8: Solve for the instability index of the rock containing the cracks based on the stress state of the cracks; Step 9: Based on the instability index of intact rock, the instability index of fractured rock, and the weights of the instability index of intact rock and the instability index of fractured rock, obtain the comprehensive wellbore instability index. Step 10. Based on the correspondence between the comprehensive wellbore instability index and the degree of wellbore instability, determine the degree of wellbore instability of the target layer corresponding to the wellbore instability index, including: When the overall wellbore instability index is greater than or equal to the first preset value and less than or equal to the second preset value, the wellbore is determined to be in a stable state during drilling of the target section. When the comprehensive instability index is greater than the second preset value and less than or equal to the third preset value, the wellbore is determined to be in a state of slight instability during drilling of the target section. When the comprehensive instability index is greater than the third preset value and less than or equal to the fourth preset value, the wellbore is determined to be in a moderately unstable state during drilling of the target section. When the comprehensive instability index is greater than the fourth preset value and less than or equal to the fifth preset value, the wellbore is determined to be in a state of severe instability during drilling of the target section. When the comprehensive instability index is greater than the fifth preset value, the well wall is most prone to instability. When drilling the target section, the well wall is in an extreme instability state. Based on the situation of adjacent wells, sidetrack to the new target area. Among them, the first preset value is less than the second preset value, the second preset value is less than the third preset value, the third preset value is less than the fourth preset value, and the fourth preset value is less than the fifth preset value.
[0016] Example: Step 1: Based on seismic data, well logging data, and rock mechanics tests, calculate the geomechanical parameters for different depths. These parameters include: geostress state, rock strength parameters, drilling azimuth, fracture orientation, and fracture strength parameters. Here, the seismic and well logging data can be collected from geological engineering data or obtained directly downhole using seismic exploration and well logging instruments. Seismic and well logging data include seismic profiles / data volumes, natural gamma rays, spontaneous potential (SP), wellbore diameter, formation resistivity, formation density, conventional acoustic waves, imaging data, and shear wave data. Specifically, by analyzing seismic profiles / data volumes and combining them with pre-stack elastic inversion, key elastic parameters of the formation, such as P-wave impedance, S-wave impedance, and density, are directly obtained. These parameters are then used to calculate Young's modulus, Poisson's ratio, and shear modulus, which are used to assess the strength, brittleness, and stress state of the formation rocks. Natural gamma rays reflect the abundance of radioactive elements in the formation and are used to identify clay content and stratigraphic correlation. Spontaneous potential (SP) records the electrochemical potential difference between drilling mud and formation water inside and outside the wellbore, used to determine permeable and clayey layers. Mud filtrate intrusion; caliper curves reflect changes in wellbore morphology and size, used to identify wellbore collapse, narrowing, or enlargement issues; formation resistivity measures the resistance of the formation to electric current, used to distinguish between water-bearing and oil- and gas-bearing layers, and to assist in determining fluid properties; formation density curves rely on gamma-ray scattering to measure the bulk density of rocks, used for porosity calculation and lithology identification; conventional acoustic wave measurements measure the time difference of P-wave propagation, reflecting formation porosity, density, and mechanical properties; imaging data acquires two-dimensional or three-dimensional images of the wellbore through acoustic or resistivity imaging techniques, used to analyze fractures, bedding, joints, and wellbore integrity; shear wave data reveals the stiffness, elastic parameters, and anisotropic characteristics of rocks through shear wave propagation characteristics.
[0017] Step 2: Calculate the stress state of intact rock and the stress state of rock with cracks respectively: The calculation process for the stress state of the intact rock in the target layer is as follows: (1) Select a fixed initial depth in the target layer and obtain the geomechanical parameters at this depth; (2) By transforming the coordinates, the geostress matrix is transformed to the wellbore coordinate system to form the wellbore boundary conditions in the wellbore coordinate system. The coordinate transformation matrix is as follows: ; ; in, It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the geostress coordinate system; It is the angle between the direction of maximum ground stress and the due north direction; It is the angle between the vertical stress and the normal to the horizontal plane; It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the wellbore coordinate system; It is the well inclination angle; It is the well inclination azimuth; The coordinate transformation formula is: ; in, This is the result after transforming the in-situ stress matrix to the wellbore coordinate system; These are the normal stresses in the x, y, and z directions after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses in the xy plane after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses on the xz plane after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses on the yz plane after the in-situ stress matrix is transformed to the wellbore coordinate system; It is the maximum horizontal ground stress; It is the minimum horizontal ground stress; It is vertical stress; (3) Solve the stress state of the surrounding rock of the wellbore using the boundary conditions in the wellbore coordinate system obtained above: ; Right now: ; in, It is the pressure of the liquid column; It is pore pressure; It is the Biot coefficient; It is the borehole radius; It is the distance from a point in the formation to the center of the wellbore; It is Poisson's ratio; It is the circumferential angle of the wellbore; It is the stress state matrix of the rock surrounding the well; These are the expressions for the normal stress state of the surrounding rock in cylindrical coordinates; These are the expressions for the shear stress state of the surrounding rock in cylindrical coordinates.
[0018] The calculation process for normal stress and shear stress on the crack is as follows: (1) The stress state in the wellbore coordinate system is transformed to the fracture surface according to the coordinate transformation formula. The fracture coordinate transformation matrix is: ; ; in, It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the crack coordinate system; It is prone to cracking; It is the angle of the crack; It is the coordinate transformation matrix used when transforming from a spatial rectangular coordinate system to a cylindrical coordinate system.
[0019] (2) The stress state matrix of the crack is: ; The normal stress and shear stress on the crack are respectively: ; ; in, It is the stress state matrix of the crack; These are the normal stresses in the x, y, and z directions, respectively, in the crack coordinate system; These are the shear stresses on the xy plane in the crack coordinate system; These are the shear stresses on the xz plane in the crack coordinate system; These are the shear stresses on the yz plane in the crack coordinate system; It is the normal stress along the normal direction on the crack surface; It is the shear stress on the crack surface.
[0020] Step 3: Calculate the instability index for intact rock and the instability index for rock with fractures respectively: (1) The formula for calculating the instability index of intact rock is derived based on the Mogi-Coulomb failure criterion as follows: ; in: ; ; in, It is octahedral shear stress; These are the first, second, and third principal stresses, which are obtained by solving... The eigenvalues of the matrix are obtained; It is the cohesive force of the rock; It is the internal friction angle of the rock; (2) The calculation formula for the instability index of fractured rocks is derived based on the Jaeger weak surface failure criterion as follows: ; in, It is the cohesive force of the crack; It is the internal friction angle of the crack.
[0021] Step 4: Select the rock mechanics parameters for the next depth, and repeat steps 2 and 3 to obtain the instability index of intact rock and the instability index of fractured rock in the target layer.
[0022] Step 5: Based on the instability index of intact rock, the instability index of fractured rock, and the weight values of the instability index of intact rock and the instability index of fractured rock, obtain a comprehensive quantitative evaluation index of the instability degree of the target layer. First, determine the weight values for the instability indices of intact rock and fractured rock. Specifically, after determining the instability indices for intact rock and fractured rock, it is necessary to assign corresponding weights to them. In other words, determine the degree of influence of intact rock and fractures on wellbore instability in different formations.
[0023] Generally, the above weights can be expressed as percentages or decimals, where the decimal can be greater than 0 and less than 1, or in other forms. In practice, the weights of the instability index for intact rock and the instability index for fractured rock can be determined based on the predicted fracture development probability of the target formation; they can be determined based on previous well experience in the area where the target formation is located; or they can be initially set to the same value and then adjusted in real-time during drilling based on the actual performance of wellbore instability: if the actual wellbore instability exceeds the instability state corresponding to the calculated comprehensive instability index, then increase the weight of the instability index for fractured rock and decrease the weight of the instability index for intact rock; conversely, decrease the weight of the instability index for fractured rock and increase the weight of the instability index for intact rock.
[0024] This embodiment provides a determination method for reference, which calculates the probability of fracture development in the target layer based on rock mechanics theory, and uses this probability as the weight Y of the instability index of fractured rocks: ; in: ; ; In the formula: It is the weight of the instability index of fractured rocks; It is the weight of the instability index of intact rock.
[0025] Considering that fractures in fractured formations occur before drilling, the in-situ stress field is used in the weight Y calculation, distinct from the stress redistribution caused by drilling. ; ; in: ; ; ; Where max represents the maximum value among the three; median represents the median value among the three; and min represents the minimum value among the three.
[0026] It should be noted that if all weights are set to the same value beforehand, the sum of all weights will be 1. This helps to control the numerical range of the comprehensive instability index. By controlling the numerical range of the comprehensive instability index within a certain range, it is possible to clearly classify the corresponding degree of instability for different comprehensive instability indices, thereby enabling a more precise identification of the degree of formation instability.
[0027] Then, based on the instability indices of intact rock and fractured rock, as well as the weights of the instability indices of intact rock and fractured rock, a linear weighted sum method is used to obtain the comprehensive instability index of the target formation. Specifically, the instability index of intact rock is multiplied by its weighted value to obtain a first value, and the instability index of fractured rock is multiplied by its weighted value to obtain a second value. These first and second values are then added together to obtain the comprehensive instability index of the target formation. Through this algorithm, a comprehensive instability index can be derived from the instability indices of intact rock and fractured rock. This comprehensive instability index is a specific numerical value that quantifies the degree of wellbore instability in different formations during drilling, facilitating more precise subsequent engineering responses.
[0028] Step Six: Based on the correlation between the calculated comprehensive instability index and the degree of instability, determine the degree of wellbore instability of the target section corresponding to the instability index. For example... Figure 2 The figure shows the degree of instability as classified according to the comprehensive instability index in this embodiment.
[0029] In practical implementation, the degree of wellbore instability can be divided into five categories: Category 1, Category 2, Category 3, Category 4, and Category 5. Each category corresponds to a comprehensive instability index range. For example, in Category 1, the first preset value... Comprehensive instability indicators Second preset value; in the second type of state, the second preset value < comprehensive instability index Third preset value; in the third type of state, the third preset value < comprehensive instability index The fourth preset value; in the fourth type of state, the fourth preset value < the comprehensive instability index < the fifth preset value; in the fifth type of state, the comprehensive instability index > the fifth preset value.
[0030] Among them, the first preset value < the second preset value < the third preset value < the fourth preset value < the fifth preset value.
[0031] Furthermore, the degree of instability corresponding to each type of state is determined as follows: (1) For the first type of state, that is, when the comprehensive instability index is greater than or equal to the first preset value and less than or equal to the second preset value, the state of the well wall during drilling of the target section is determined to be a stable state; (2) For the second type of state, that is, when the comprehensive instability index is greater than the second preset value and less than or equal to the third preset value, the state of the well wall during drilling of the target section is determined to be a slightly unstable state; (3) For the third type of state, that is, when the comprehensive instability index is greater than the third preset value and less than or equal to the fourth preset value, the state of the well wall during drilling of the target section is determined to be a moderate instability state. (4) For the fourth type of state, that is, when the comprehensive instability index is greater than the fourth preset value and less than or equal to the fifth preset value, the state of the well wall during drilling of the target section is determined to be a severely unstable state; (5) For the fifth type of state, that is, when the comprehensive instability index is greater than the fifth preset value, the state of the well wall when drilling the target section is determined to be the ultimate instability state.
[0032] In practical applications, the embodiments of this application do not specifically limit the magnitudes of the first, second, third, fourth, and fifth preset values. In this embodiment, the first preset value is set to 0, the second preset value is set to 1, the third preset value is set to 1.2, the fourth preset value is set to 1.5, and the fifth preset value is set to 2. The specific data of the first, second, third, fourth, and fifth preset values can also be adjusted according to actual circumstances.
[0033] like Figure 3 The figure shown is a schematic curve of the instability index results provided in this embodiment at different depths.
Claims
1. A geomechanical method for quantitatively evaluating wellbore stability in fractured formations, characterized in that, Includes the following steps: Step 1: Predict the formation stress state and drilling location using seismic and well logging data; Step 2: Select rock cores and conduct uniaxial and triaxial compression tests in the laboratory to measure rock strength parameters, including uniaxial compressive strength, uniaxial tensile strength, internal friction angle and cohesion. Step 3: Calculate the stress state of the intact rock in the target layer based on the measured data; Step 4: Determine the instability index of the intact rock based on its stress state; Step 5: Based on seismic and well logging data, including seismic attributes and imaging well logging data, obtain the fracture orientation, including the fracture dip angle and dip direction; Step 6: Determine the strength parameters of the cracks based on core shear mechanics experiments; Step 7: Solve for the normal stress and shear stress on the crack based on the crack strength parameters; Step 8: Solve for the instability index of the rock containing the cracks based on the stress state of the cracks; Step 9: Based on the instability index of intact rock, the instability index of fractured rock, and the weights of the instability index of intact rock and the instability index of fractured rock, obtain the comprehensive wellbore instability index. Step 10. Based on the correspondence between the comprehensive wellbore instability index and the degree of wellbore instability, determine the degree of wellbore instability of the target layer corresponding to the wellbore instability index, including: When the overall wellbore instability index is greater than or equal to the first preset value and less than or equal to the second preset value, the wellbore is determined to be in a stable state during drilling of the target section. When the comprehensive instability index is greater than the second preset value and less than or equal to the third preset value, the wellbore is determined to be in a state of slight instability during drilling of the target section. When the comprehensive instability index is greater than the third preset value and less than or equal to the fourth preset value, the wellbore is determined to be in a moderately unstable state during drilling of the target section. When the comprehensive instability index is greater than the fourth preset value and less than or equal to the fifth preset value, the wellbore is determined to be in a state of severe instability during drilling of the target section. When the comprehensive instability index is greater than the fifth preset value, the well wall is most prone to instability. When drilling the target section, the well wall is in an extreme instability state. Based on the situation of adjacent wells, sidetrack to the new target area. Among them, the first preset value is less than the second preset value, the second preset value is less than the third preset value, the third preset value is less than the fourth preset value, and the fourth preset value is less than the fifth preset value.
2. The geomechanical method for quantitatively evaluating wellbore stability in fractured formations according to claim 1, characterized in that, The seismic and well logging data in Step 1 include seismic profiles / data volumes, natural gamma rays, spontaneous potential (SP), wellbore diameter, formation resistivity, formation density, conventional acoustic waves, imaging data, and shear wave data. Specifically, by analyzing the seismic profiles / data volumes and combining them with pre-stack elastic inversion, key elastic parameters of the formation, such as P-wave impedance, S-wave impedance, and density, are directly obtained through inversion. Young's modulus, Poisson's ratio, and shear modulus are then calculated to determine rock physical and mechanical parameters, used to assess the strength, brittleness, and stress state of the formation rocks. Natural gamma rays reflect the abundance of radioactive elements in the formation, used to identify clay content and stratigraphic correlation. Spontaneous potential (SP) records the electrochemical potential difference between the drilling mud and formation water inside and outside the wellbore, used to determine the presence of permeable and clay layers. The data includes information on mud filtrate intrusion; wellbore diameter curves reflect changes in wellbore morphology and size, used to identify wellbore collapse, narrowing, or enlargement; formation resistivity measures the resistance of the formation to current, used to distinguish between water-bearing and oil- and gas-bearing layers, and to assist in determining fluid properties; formation density curves rely on gamma-ray scattering to measure the bulk density of rocks, used for porosity calculation and lithology identification; conventional acoustic wave measurements measure the time difference of P-wave propagation, reflecting formation porosity, density, and mechanical properties; imaging data obtains two-dimensional or three-dimensional images of the wellbore through acoustic or resistivity imaging techniques, used to analyze fractures, bedding, joints, and wellbore integrity; and shear wave data reveals the stiffness, elastic parameters, and anisotropic characteristics of rocks through shear wave propagation characteristics.
3. The geomechanical method for quantitatively evaluating wellbore stability in fractured formations according to claim 1, characterized in that, The calculation process for the stress state of the intact rock in the target layer in Step 3 is as follows: S3.1 Select a fixed initial depth for the target layer and obtain the geomechanical parameters at this depth; S3.
2. Through coordinate transformation, the geostress matrix is transformed to the wellbore coordinate system to form the wellbore boundary conditions in the wellbore coordinate system. The coordinate transformation matrix is as follows: ； ； in, It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the geostress coordinate system; It is the angle between the direction of maximum ground stress and the due north direction; It is the angle between the vertical stress and the normal to the horizontal plane; It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the wellbore coordinate system; It is the well inclination angle; It is the azimuth of the well inclination; The coordinate transformation formula is: ； in, This is the result after transforming the in-situ stress matrix to the wellbore coordinate system; These are the normal stresses in the x, y, and z directions after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses in the xy plane after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses on the xz plane after the in-situ stress matrix is transformed to the wellbore coordinate system; These are the shear stresses on the yz plane after the in-situ stress matrix is transformed to the wellbore coordinate system; It is the maximum horizontal ground stress; It is the minimum horizontal ground stress; It is vertical stress; S3.3 Solve the stress state of the surrounding rock of the wellbore using the boundary conditions obtained in the wellbore coordinate system above: ； Right now: ; in, It is the pressure of the liquid column; It is pore pressure; It is the Biot coefficient; It is the borehole radius; It is the distance from a point in the formation to the center of the wellbore; It is Poisson's ratio; It is the circumferential angle of the well shaft; It is the stress state matrix of the rock surrounding the well; These are the expressions for the normal stress state of the surrounding rock in cylindrical coordinates; These are the expressions for the shear stress state of the surrounding rock in cylindrical coordinates.
4. The geomechanical method for quantitatively evaluating wellbore stability in fractured formations according to claim 3, characterized in that, The calculation process for the normal stress and shear stress on the crack in Step 7 is as follows: S7.
1. Based on the coordinate transformation formula, the stress state in the wellbore coordinate system is transformed to the fracture surface. The fracture coordinate transformation matrix is: ； ； in, It is the coordinate transformation matrix used when transforming from the geodetic coordinate system to the crack coordinate system; It is prone to cracking; It is the angle of the crack; It is the coordinate transformation matrix used when transforming from a spatial rectangular coordinate system to a cylindrical coordinate system; S7.2 The stress state matrix of the crack is: ； The normal stress and shear stress on the crack are respectively: ； ； in, It is the stress state matrix of the crack; These are the normal stresses in the x, y, and z directions, respectively, in the crack coordinate system; These are the shear stresses on the xy plane in the crack coordinate system; These are the shear stresses on the xz plane in the crack coordinate system; These are the shear stresses on the yz plane in the crack coordinate system; It is the normal stress along the normal direction on the crack surface; It is the shear stress on the crack surface.
5. The geomechanical method for quantitatively evaluating wellbore stability in fractured formations according to claim 4, characterized in that, The calculation processes for the instability index of intact rock and the instability index of fractured rock in Steps 4 and 8 are as follows: (1) The formula for calculating the instability index of intact rock is derived based on the Mogi-Coulomb failure criterion as follows: ； in: ； ； in, It is octahedral shear stress; These are the first, second, and third principal stresses, which are obtained by solving... The eigenvalues of the matrix are obtained; It is the cohesive force of the rock; It is the internal friction angle of the rock; (2) The calculation formula for the instability index of fractured rocks is derived based on the Jaeger weak surface failure criterion as follows: ； in, It is the cohesive force of the crack; It is the internal friction angle of the crack.
6. The geomechanical method for quantitatively evaluating wellbore stability in fractured formations according to claim 1, characterized in that, In Step 9, the weights of the instability index for intact rock and the instability index for fractured rock are determined based on the predicted fracture development probability of the target formation; or based on previous experience from nearby wells in the area where the target formation is located; or they are initially set to the same value and then adjusted in real time during drilling based on the actual performance of wellbore instability. If the actual damage to the wellbore is greater than the instability state corresponding to the calculated comprehensive instability index, the weight of the instability index for fractured rock is increased while the weight of the instability index for intact rock is decreased; conversely, the weight of the instability index for fractured rock is decreased while the weight of the instability index for intact rock is increased. Based on the instability indices of intact rock and fractured rock, as well as the weights of the instability indices of intact rock and fractured rock, the specific process of obtaining the comprehensive instability index of the target layer through linear weighted summation is as follows: multiply the instability index of intact rock by its weight to obtain the first value; multiply the instability index of fractured rock by its weight to obtain the second value; and add the first and second values to obtain the comprehensive instability index of the target layer.