Weak plane stratum well wall stability simulation and characterization method and application

Abstract

The invention provides a weak plane stratum well wall stability simulation and characterization method and application, and the method comprises the following steps: firstly, preparing a sample with rock mechanical properties and microfracture distribution rules similar to those of a natural weak plane stratum rock core, and carrying out a large-scale true triaxial complex fracture network physical simulation experiment; then, CT scanning is conducted on weak plane stratum rock cores before and after the experiment, and the well wall stability rule and the microfracture influence rule are analyzed according to CT scanning results before and after the experiment. According to the method, CT scanning is carried out on weak plane stratum rock cores before and after a large true triaxial complex fracture network physical simulation experiment, a CT scanning result is analyzed through a mathematical method, a weak plane stratum complex fracture network structure analysis model is established, the weak plane stratum well wall instability degree and the influence rule are quantitatively analyzed, and the weak plane stratum complex fracture network structure analysis model is established. And a basis is provided for on-site weak plane stratum drilling optimization.

Description

Technical Field

[0001] The invention belongs to the field of oil drilling wellbore stability analysis, and specifically relates to a weak surface formation wellbore stability simulation and characterization method and application. Background Art

[0002] Weak-face formations have well-developed microcracks, obvious heterogeneity, large variations in mineral composition and content, and large variations in the brittleness of the formation. During the formation of the complex rock fracture network structure of the weak-face formation, the mineral composition, content, microcracks, and structural weak faces of the weak-face formation rock can have an important impact on the stability of the wellbore, and bifurcation and turning phenomena are prone to occur at the microcracks and mineral composition changes. The parameters of microcracks in the weak-face formation rock (microcrack length, density, opening, orientation), the size and difference of ground stress, and the drilling fluid construction parameters (injection volume, displacement, viscosity) will affect the stability law of the weak-face formation rock wellbore. Therefore, it is of great significance to optimize the drilling construction process parameters and analyze the influence of factors such as microcracks on the stability law of the complex wellbore of the weak-face formation rock.

[0003] Indoor large-scale true triaxial physical simulation experiments can effectively simulate the stability law of the wellbore under the action of different factors. However, the current drilling experiment research objects are mainly artificial samples + prefabricated micro-fracture weak surface mode, or the use of ground outcrop cores, which cannot truly reflect the state of the rock core of the deep weak surface formation, and the simulation of the wellbore stability law is not accurate enough. In view of the fact that it is difficult to accurately evaluate the complex sealing network structure characteristics of the weak surface formation rock in different geological layers underground using conventional experimental methods.

[0004] Chinese patent publication CN113935092A discloses a dual-pore thermoelastic coupling evaluation method for wellbore stability in fractured formations. Based on rocks, it quantifies the relationship between the sealing ratio of fracture size and wellbore collapse-rupture, and reasonably designs a time-correlated safety density window of coupled dynamic temperature disturbance boundary effect before drilling in some fractured formations. However, it does not mention the destruction mechanism and destruction experiment.

[0005] Therefore, it is urgent to propose an effective fracture network characteristic evaluation method for the complex fracture network structure of weak surface formation rocks, so as to accurately describe the microcrack parameters of weak surface formation rocks and quantitatively analyze the influence of microcracks on wellbore collapse, so as to provide basic data and theoretical reference for on-site drilling construction. Summary of the invention

[0006] The purpose of the present invention is to solve the difficulties existing in the above-mentioned prior art, and to provide a method and application for simulating and characterizing the stability of the wellbore in a weak surface formation, to analyze the logging data by mathematical methods, to describe the mechanical parameters of the formation, to calculate the ground stress, to establish a complex fracture network structure analysis model for the weak surface formation, to quantitatively analyze the degree of wellbore instability and the influencing rules of the weak surface formation, and to provide a basis for on-site optimization of drilling in weak surface formations.

[0007] The present invention is achieved through the following technical solutions:

[0008] According to a first aspect of the present invention, a method for simulating and characterizing the wellbore stability of a weak surface formation is provided. First, a sample having rock mechanical properties and microcrack distribution laws similar to those of a natural weak surface formation rock core is prepared, and a large-scale true triaxial complex fracture network physical simulation experiment is carried out. Then, CT scans are performed on the rock core of the weak surface formation before and after the experiment. Based on the CT scanning results before and after the experiment, the wellbore stability laws and the influence laws of microcracks are analyzed.

[0009] A further improvement of the present invention is:

[0010] The method comprises the following steps:

[0011] Step 1, analyzing the mineral composition of the rock core of the natural weak surface formation;

[0012] Step 2, testing the rock mechanical parameters of the rock core of the natural weak surface formation;

[0013] Step 3, testing the maximum horizontal principal stress and the minimum horizontal principal stress of the rock core of the natural weak surface formation, and calibrating their orientations;

[0014] Step 4, CT scanning to test the rock core of the natural weak surface formation;

[0015] Step 5, preparing rock samples for rock physics simulation experiments of weak-surface formations;

[0016] Step 6, using the prepared weak-surface formation rock physics simulation experiment rock sample to carry out a large-scale true triaxial complex fracture network physical simulation experiment;

[0017] Step 7: After the experiment, CT scanning is performed on the rock samples of the weak-plane formation rock physics simulation experiment and the rock cores of the natural weak-plane formation;

[0018] Step 8: Compare the CT scan results before and after the experiment to analyze the stability of the wellbore and the influence of microcracks.

[0019] A further improvement of the present invention is:

[0020] In step 1, the mineral composition of the natural weak-face formation rock core is analyzed, and the specific operations include:

[0021] The mineral composition and content of the natural weak surface strata rocks are analyzed using a mineral component analyzer, the brittleness index of the weak surface strata rocks is calculated, and the complexity of the fracture network formed by the interlacing of weak surfaces is determined.

[0022] A further improvement of the present invention is:

[0023] In step 2, the rock mechanical parameters of the rock core of the natural weak surface formation are tested. The specific operations include:

[0024] The rock mechanics parameters of natural weak surface formation rocks are tested using a rock mechanics parameter triaxial testing device, wherein the rock mechanics parameters include elastic modulus, Poisson's ratio and peak shear strength.

[0025] A further improvement of the present invention is:

[0026] In step 3, the maximum horizontal principal stress and the minimum horizontal principal stress of the rock core of the natural weak surface formation are tested and their orientations are calibrated. The specific operations include:

[0027] The maximum and minimum horizontal principal stresses of the rock core in the natural weak surface formation were tested by differential strain experiment.

[0028] The original orientation of the rock core of the weak-surface formation is determined by paleomagnetic experiments, and the coordinate system of the rock core of the weak-surface formation is established;

[0029] The orientation of ground stress is determined based on the core logging data of weak-face formation rocks.

[0030] A further improvement of the present invention is:

[0031] In step 4, CT scanning is used to test the rock core of the natural weak surface formation. The specific operations include:

[0032] The rock core of the natural weak-surface stratum is placed in an industrial CT scanning device for scanning, and the scanned image is processed to make it clearer and the microcracks more obvious, and then the image is weighted average interpolated, and a standard layer is selected and set as the initial image;

[0033] Obtain the length, density and azimuth of microcracks inside the natural weak surface formation rock, establish a spatial coordinate system XYZ with the center point of the rock sample as the coordinate origin (0, 0, 0), establish a three-dimensional data body, and obtain the distribution law of microcracks;

[0034] Using the maximum horizontal principal stress orientation as a benchmark, calibrate the distribution orientation of micro-cracks in the rock core of the natural weak surface formation;

[0035] Get the coordinates of the beginning and end of the microcrack in the three-dimensional data volume (X i , Y i , Z i ) and (X i+1 , Y i+1 , Z i+1 ), the length and azimuth of the microcrack are obtained according to the coordinates of the beginning and end of the microcrack;

[0036] The microcrack length l is calculated using formula (1):

[0037]

[0038] Combined with the determined maximum horizontal principal stress orientation, the deflection angle between the microcrack and the maximum horizontal geostress orientation is obtained;

[0039] Draw a histogram of the frequency of microcrack length, density, azimuth and deflection angle of the maximum horizontal principal stress direction in the analysis section to determine the angle variation range and average value.

[0040] A further improvement of the present invention is:

[0041] In step 5, rock samples for rock physics simulation experiments on weak-surface formations are prepared. The specific operations include:

[0042] Adjust the proportions of water, quartz sand, lime and additives to prepare cement slurry so that its rock mechanical parameters are similar to those of the rock mechanical parameters of the natural weak-face formation rock core; drill a through hole in the center of the natural weak-face formation rock core, put the prepared cement slurry in a steel mold, and then place the natural weak-face formation rock core in the exact center, and adjust the direction so that the maximum horizontal principal stress orientation of the weak-face formation rock core is parallel to the side, and mark it;

[0043] Then the steel wellbore is cast together in cement slurry, the wellbore perforations are filled with urea in advance, and the surface is treated after constant temperature maintenance.

[0044] A further improvement of the present invention is:

[0045] After the experiment in step 7, CT scanning is performed on the rock samples of the weak surface formation rock physics simulation experiment and the rock cores of the natural weak surface formation. The specific operations include:

[0046] CT scanning was performed on the rock samples of the rock physics simulation experiment of the weak surface formation after the experiment to obtain the three-dimensional data body of the wellbore stability morphology and analyze the influence law of the wellbore stability;

[0047] The natural weak-face formation rock cores were taken from the rock samples of the weak-face formation rock physics simulation experiment. CT scanning was used to obtain a three-dimensional data volume containing microcracks and complex fracture networks, and the influence of microcracks and rock mineral components in the weak-face formation rocks on the wellbore stability characteristics was analyzed.

[0048] A further improvement of the present invention is:

[0049] In step 8, the CT scan results before and after the experiment are compared to analyze the stability of the wellbore and the influence of microcracks. The specific operations include:

[0050] Comparing the CT scan results of the rock cores of the natural weak-surface formations before and after the experiment, micro-cracks and complex fracture networks were identified;

[0051] Using the maximum horizontal principal stress orientation marked in step 5, calibrate the orientation of the complex fracture network after drilling;

[0052] The relationship between complex fracture network and micro-crack azimuth is statistically analyzed to quantitatively evaluate the effect of micro-crack azimuth on wellbore stability.

[0053] Draw a histogram of the length, density, azimuth and frequency of the deflection angle of the maximum horizontal principal stress direction of the complex fracture network in the analysis section to determine the angle variation range and average value;

[0054] By comparing the statistical results of the frequency histogram of the crack deflection angle before and after the experiment, the deflection angle range and the main direction of the deflection between the crack and the maximum principal stress direction before and after drilling were obtained, and the stability law of the wellbore was quantitatively characterized.

[0055] A second aspect of the present invention provides a method for simulating and characterizing the wellbore stability of a weak surface formation and its application in the wellbore stability analysis of oil drilling.

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] The present invention performs CT scanning on rock cores of weak-face formations before and after a large-scale true triaxial complex fracture network physical simulation experiment, and analyzes the CT scanning results by mathematical methods, thereby establishing a complex fracture network structure analysis model for weak-face formations, quantitatively analyzing the degree of wellbore instability and the influence law of weak-face formations, and providing a basis for on-site optimization of weak-face formation drilling. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 is a flow chart of a method for simulating and characterizing wellbore stability in a weak surface formation in an embodiment of the present invention;

[0059] Figure 2 Schematic diagram of the three-dimensional data coordinate system of weak surface strata;

[0060] Figure 3 Schematic diagram of large-scale rock samples in weak-face formations. DETAILED DESCRIPTION

[0061] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0062] The present invention provides a method for simulating and characterizing the wellbore stability of a weak surface formation, prepares a sample with rock mechanical properties and microcrack distribution laws similar to those of a natural weak surface formation rock core, conducts a large-scale true triaxial complex fracture network physical simulation experiment, and performs CT scanning on the rock core of the weak surface formation before and after the experiment. According to the CT scanning results before and after the experiment, the wellbore stability law and the influence law of microcracks are analyzed.

[0063] The embodiments of the method of the present invention are as follows:

[0064] [Example 1]

[0065] like Figure 1As shown, the method comprises the following steps:

[0066] Step 1, analyzing the mineral composition of the rock core of the natural weak surface formation;

[0067] The mineral composition and content of natural weak surface strata rocks, including quartz, calcite, clay, etc., are analyzed using a mineral component analyzer to calculate the brittleness index of weak surface strata rocks and determine the complexity of the fracture network formed by the interlacing of weak surfaces.

[0068] Step 2, testing the rock mechanical parameters of the rock core of the natural weak surface formation;

[0069] The rock mechanics parameters of natural weak surface formation rocks are tested using a rock mechanics parameter triaxial testing device, wherein the rock mechanics parameters include elastic modulus, Poisson's ratio and peak shear strength.

[0070] Step 3, testing the maximum horizontal principal stress and the minimum horizontal principal stress of the rock core of the natural weak surface formation, and calibrating their orientations;

[0071] Specific operations include:

[0072] The maximum horizontal principal stress and minimum horizontal principal stress of the rock core of the natural weak surface formation are tested by differential strain experiment, the original orientation of the rock core of the weak surface formation is determined by paleomagnetic experiment, and the coordinate system of the rock core of the weak surface formation is established; the orientation of the ground stress is determined according to the well logging data of the rock core of the weak surface formation (including acoustic wave, well diameter, resistivity and other data).

[0073] Step 4, CT scanning to test the rock core of the natural weak surface formation;

[0074] Specific operations include:

[0075] The rock core of the natural weak-surface stratum is placed in an industrial CT scanning device for scanning, and the scanned image is processed to make it clearer and the microcracks more obvious, and then the image is weighted average interpolated, and a standard layer is selected and set as the initial image;

[0076] Obtain the distribution parameters of microcrack length, density, azimuth, etc. inside the natural weak surface formation rock, establish the spatial coordinate system XYZ with the center point of the rock sample as the coordinate origin (0, 0, 0), establish a three-dimensional data body, and obtain the distribution law of microcracks;

[0077] Using the maximum horizontal principal stress orientation as a benchmark, calibrate the distribution orientation of micro-cracks in the rock core of the natural weak surface formation;

[0078] Get the coordinates of the beginning and end of the microcrack in the three-dimensional data volume (X i , Y i , Z i ) and (X i+1 , Yi+1 , Z i+1 ), the length and azimuth of the microcrack are obtained according to the coordinates of the beginning and end of the microcrack;

[0079] The microcrack length l is calculated using formula (1):

[0080]

[0081] Combined with the determined maximum horizontal principal stress orientation, the deflection angle between the microcrack and the maximum horizontal geostress orientation is obtained;

[0082] Draw a histogram of the frequency of microcrack length, density, azimuth and deflection angle of the maximum horizontal principal stress direction in the analysis section to determine the angle variation range and average value.

[0083] Step 5, preparing rock samples for rock physics simulation experiments of weak-surface formations;

[0084] Specific operations include:

[0085] Adjust the proportions of water, quartz sand, lime and additives to prepare cement slurry so that its rock mechanical parameters are similar to those of the rock core of the natural weak-face formation; drill a through hole with a diameter of φ25 mm in the center of the rock core of the natural weak-face formation; place the prepared cement slurry in a 300mm×300mm×300mm regular hexahedron steel mold, place the rock core of the natural weak-face formation in the exact center, and adjust the direction so that the maximum horizontal principal stress orientation of the rock core of the weak-face formation is parallel to the side, and mark it;

[0086] Then the steel wellbore is cast in cement slurry, and the wellbore perforations are filled with urea in advance; it is cured at a constant temperature of 20°C for 28 days, and then the surface of the large-size specimens is treated to make their parallelism and verticality meet the requirements, and then the experiment can be carried out.

[0087] Step 6, using the prepared weak-surface formation rock physics simulation experiment rock sample to carry out a large-scale true triaxial complex fracture network physical simulation experiment;

[0088] Using the similarity principle, the experimental parameters for drilling construction in the weak-face formation rock complex fracture network in the laboratory were determined;

[0089] According to the rock mechanical parameters, ground stress, brittleness index and microcrack distribution characteristics of natural weak-face formation rocks, a large-scale true triaxial physical simulation experiment drilling construction plan is designed, including drilling fluid viscosity, drilling fluid displacement, drilling fluid type, drilling fluid injection volume, etc. Tracer materials are added to the drilling fluid to facilitate the analysis of the stability law of the weak-face formation rock wellbore;

[0090] According to the set experimental plan, a large-scale true triaxial physical simulation experiment is carried out.

[0091] Step 7: After the experiment, CT scanning is performed on the rock samples of the weak-plane formation rock physics simulation experiment and the rock cores of the natural weak-plane formation;

[0092] Specific operations include:

[0093] CT scanning was performed on the rock samples of the rock physics simulation experiment of the weak surface formation after the experiment to obtain the three-dimensional data body of the wellbore stability morphology and analyze the influence law of the wellbore stability;

[0094] Take the natural weak-face formation rock core from the weak-face formation rock physics simulation experiment rock sample, use CT scanning to obtain the three-dimensional data volume containing microcracks and complex fracture networks, and analyze the influence of microcracks and rock mineral components of weak-face formation rocks on the stability characteristics of the wellbore;

[0095] Step 8, comparing the CT scanning results before and after the experiment, analyzing the stability law of the wellbore and the influence law of microcracks;

[0096] Specific operations include:

[0097] Comparing the CT scan results of the rock cores of the natural weak-surface formations before and after the test, micro-cracks and complex fracture networks were identified;

[0098] Use the maximum horizontal principal stress azimuth marked in step 5 to calibrate the azimuth of the complex fracture network after drilling; statistically analyze the relationship between the complex fracture network and the azimuth of microcracks, and quantitatively evaluate the influence of the azimuth of microcracks on the stability of the wellbore; draw a histogram of the frequency of the deflection angle of the complex fracture network length, density, azimuth and maximum horizontal principal stress direction in the analysis section, and determine the angle variation range and average value;

[0099] By comparing the statistical results of the frequency histogram of the crack deflection angle before and after the experiment, the deflection angle range and the main direction of the deflection between the crack and the maximum principal stress direction before and after drilling were obtained, and the stability law of the wellbore was quantitatively characterized.

[0100] The embodiments of the method of the present invention are as follows:

[0101] [Example 2]

[0102] The method of the present invention is used to simulate and characterize the wellbore stability of weak-surface strata rocks in a certain area, and specifically includes the following steps:

[0103] (1) Analysis and content determination of rock mineral components in natural weak surface formations;

[0104] Mineral component analysis showed that the natural weak surface stratum rocks contain quartz, potassium feldspar, dolomite, clay minerals, etc., with proportions of 58%, 14%, 19%, and 9%, respectively. The brittle mineral content is 91%, and the non-brittle mineral content is 9%. When preparing cement mortar, the mass ratio of cement sand to clay is adjusted to 9:1 to make their brittleness as similar as possible.

[0105] (2) Prepare samples with rock mechanical properties similar to those of natural weak surface formation rocks;

[0106] The rock mechanical parameters such as elastic modulus, Poisson's ratio, peak shear strength, etc. of natural weak surface formation rocks were tested and analyzed. By adjusting the content and proportion of water-lime-quartz sand-clay-additives, the prepared rock samples were made to have similar rock mechanical properties to those of weak surface formation rocks. The weak surface formation rocks were taken from 3300m of the formation, and the analysis showed that their elastic modulus was 29GPa, Poisson's ratio was 0.21, and peak shear strength was 276MPa. The lime used to prepare artificial samples was PC52.5R composite silicate cement, quartz sand of 40-80 mesh was used, montmorillonite was used as clay, and filtrate reducer was used as additive.

[0107] (3) Testing the magnitude and orientation of horizontal stress in natural weak surface formations;

[0108] The differential strain experiment was used to obtain the magnitude of horizontal geostress in the rock strata of the weak surface, and the paleomagnetic method was used to obtain the orientation of horizontal geostress in the strata. The maximum horizontal geostress was obtained to be 62MPa, the minimum horizontal principal stress was 53MPa, and the orientation was NE36°.

[0109] (4) CT scanning test of rock cores of natural weak-surface formations before the experiment;

[0110] The core of the natural weak-surface stratum rock is placed in an industrial CT scanning device to obtain the distribution parameters of the microcrack length, density, orientation, etc. inside the weak-surface stratum rock. The center point of the rock sample is used as the coordinate origin (0, 0, 0) to establish the spatial coordinate system XYZ, establish a three-dimensional data body, and obtain the distribution law of microcracks, such as Figure 2 shown.

[0111] Based on the maximum horizontal principal stress orientation, the distribution orientation of micro-cracks in the rock core of the natural weak surface formation is calibrated.

[0112] by Figure 2 As an example, the No. 1 crack in the CT image is identified based on the two pixel points (X i , Y i , Z i ), (X i+1 , Y i+1 , Z i+1 ) is:

[0113]

[0114] By obtaining the coordinates of the beginning and end of the microcrack in the three-dimensional data volume, the length of the microcrack l can be obtained. The corresponding coordinates of the beginning and end pixels of the crack are (3, 4, 5) and (6, 7, 8). The length of the crack is 5.196 mm. Assuming the orientation of the microcrack is α, then:

[0115]

[0116] It can be found that the microcrack azimuth is α = 45°;

[0117] Combined with the determined maximum horizontal principal stress orientation, the deflection angle y between the microcrack and the maximum horizontal geostress orientation is 81°;

[0118] Draw a frequency histogram of the microcrack length, density, azimuth angle α and the deflection angle y in the direction of the maximum horizontal principal stress in the analysis section to determine the angle variation range and average value.

[0119] (5) Prepare rock samples for rock physics simulation experiments in weak-surface formations;

[0120] Adjust the proportion of water-quartz sand-lime-additives to prepare cement slurry so that its rock mechanical parameters are similar to those of weak-face formation rock mechanical parameters; drill a through hole with a diameter of φ25mm in the center of the natural weak-face formation rock core; place the prepared cement slurry in a 300mm×300mm×300mm regular hexahedron steel mold, then place the natural weak-face formation rock core in the center, and adjust the direction so that the maximum horizontal principal stress orientation of the weak-face formation rock core is parallel to the side, and mark it; then cast the steel wellbore in cement mortar together, and fill the wellbore perforations with urea in advance; maintain at a constant temperature of 20℃ for 28d, and then treat the surface of the large-sized rock sample to make its parallelism and verticality meet the requirements, such as Figure 3 shown.

[0121] (6) Conduct large-scale true triaxial physical simulation experiments on wellbore stability in weak-surface formations;

[0122] According to the experimental purpose and based on the principle of similarity, an experimental plan was formulated; tracer materials were added to the drilling fluid, and experiments were carried out to test and analyze the expansion characteristics of fracture networks in rock layers of weak surface formations.

[0123] (7) After the experiment, the rock samples of the weak-face formation rock physics simulation experiment and the rock cores of the weak-face formation were CT scanned and analyzed.

[0124] CT scanning was performed on the rock samples of the rock physics simulation experiment of the weak surface formation after the experiment to obtain the three-dimensional data body of the wellbore stability morphology and analyze the wellbore stability law;

[0125] The rock core of the internal weak surface formation was taken out and CT scan was performed to obtain a three-dimensional data volume containing microcracks and complex fracture networks, and the stability law of the wellbore and the influence law of microcracks were analyzed.

[0126] By comparing the rock core scanning results of weak-face formations before and after the experiment, microcracks and complex fracture networks were identified.

[0127] The orientation of the complex fracture network after drilling is calibrated by using the maximum horizontal principal stress orientation of the weak-surface formation rock core marked when preparing the large-scale physical simulation experiment rock sample in step (5);

[0128] The relationship between complex fracture network and microcrack azimuth α is statistically analyzed to quantitatively evaluate the effect of microcrack azimuth on wellbore stability.

[0129] Draw a frequency histogram of the length, density, azimuth angle α and the deflection angle y of the maximum horizontal principal stress direction of the complex fracture network in the analysis section to determine the angle variation range and average value.

[0130] By comparing the statistical results of the fracture deflection angle frequency histograms before and after the experiment, the deflection angle range and the main direction of deflection between the fracture and the maximum principal stress direction before and after drilling can be obtained.

[0131] (8) Change the triaxial stress and difference, drilling operation parameters (drilling fluid viscosity, displacement, injection volume), etc., and analyze the influence of the mineral composition of the weak surface formation rock and microcracks on the wellbore stability.

[0132] The above technical solution is only one implementation mode of the present invention. For those skilled in the art, it is easy to make various types of improvements or modifications based on the principles disclosed in the present invention, and it is not limited to the technical solution described in the above specific embodiments of the present invention. Therefore, the above description is only preferred and does not have a restrictive meaning.

Claims

1. A method for simulating and characterizing the stability of a weak-face formation wellbore, characterized in that: Firstly, samples with rock mechanical properties and microcrack distribution laws similar to those of natural weak surface formation rock cores were prepared, and a large-scale true triaxial complex fracture network physical simulation experiment was carried out. Then, CT scanning was performed on the rock cores of weak surface formations before and after the experiment. According to the CT scanning results before and after the experiment, the wellbore stability law and the influence law of microcracks were analyzed.

2. The method for simulating and characterizing the wellbore stability of a weak surface formation according to claim 1, characterized in that: The method comprises the following steps: Step 1, analyzing the mineral composition of the rock core of the natural weak surface formation; Step 2, testing the rock mechanical parameters of the rock core of the natural weak surface formation; Step 3, testing the maximum horizontal principal stress and the minimum horizontal principal stress of the rock core of the natural weak surface formation, and calibrating their orientations; Step 4, CT scanning to test the rock core of the natural weak surface formation; Step 5, preparing rock samples for rock physics simulation experiments of weak-surface formations; Step 6, using the prepared weak-surface formation rock physics simulation experiment rock sample to carry out a large-scale true triaxial complex fracture network physical simulation experiment; Step 7: After the experiment, CT scanning is performed on the rock samples of the weak-plane formation rock physics simulation experiment and the rock cores of the natural weak-plane formation; Step 8: Compare the CT scan results before and after the experiment to analyze the stability of the wellbore and the influence of microcracks.

3. The method for simulating and characterizing the wellbore stability of a weak surface formation according to claim 2, characterized in that: In step 1, the mineral composition of the natural weak-face formation rock core is analyzed, and the specific operations include: The mineral composition and content of the natural weak surface strata rocks are analyzed using a mineral component analyzer, the brittleness index of the weak surface strata rocks is calculated, and the complexity of the fracture network formed by the interlacing of weak surfaces is determined.

4. The method for simulating and characterizing the wellbore stability of a weak surface formation according to claim 2, characterized in that: In step 2, the rock mechanical parameters of the rock core of the natural weak surface formation are tested. The specific operations include: The rock mechanics parameters of natural weak surface formation rocks are tested using a rock mechanics parameter triaxial testing device, wherein the rock mechanics parameters include elastic modulus, Poisson's ratio and peak shear strength.

5. The method for simulating and characterizing the wellbore stability of a weak surface formation according to claim 2, characterized in that: In step 3, the maximum horizontal principal stress and the minimum horizontal principal stress of the rock core of the natural weak surface formation are tested and their orientations are calibrated. The specific operations include: The maximum and minimum horizontal principal stresses of the rock core in the natural weak surface formation were tested by differential strain experiment. The original orientation of the rock core of the weak-surface formation is determined by paleomagnetic experiments, and the coordinate system of the rock core of the weak-surface formation is established; The orientation of ground stress is determined based on the core logging data of weak-face formation rocks.

6. The method for simulating and characterizing the wellbore stability of a weak surface formation according to claim 2, characterized in that: In step 4, CT scanning is used to test the rock core of the natural weak surface formation. The specific operations include: The rock core of the natural weak-surface stratum is placed in an industrial CT scanning device for scanning, and the scanned image is processed to make it clearer and the microcracks more obvious, and then the image is weighted average interpolated, and a standard layer is selected and set as the initial image; Obtain the length, density and azimuth of microcracks inside the natural weak surface formation rock, establish a spatial coordinate system XYZ with the center point of the rock sample as the coordinate origin (0, 0, 0), establish a three-dimensional data body, and obtain the distribution law of microcracks; Using the maximum horizontal principal stress orientation as a benchmark, calibrate the distribution orientation of micro-cracks in the rock core of the natural weak surface formation; Get the coordinates of the beginning and end of the microcrack in the three-dimensional data volume (X i , Y i , Z i ) and (X i+1 , Y i+1 , Z i+1 ), the length and azimuth of the microcrack are obtained according to the coordinates of the beginning and end of the microcrack; The microcrack length l is calculated using formula (1): Combined with the determined maximum horizontal principal stress orientation, the deflection angle between the microcrack and the maximum horizontal geostress orientation is obtained; Draw a histogram of the frequency of microcrack length, density, azimuth and deflection angle of the maximum horizontal principal stress direction in the analysis section to determine the angle variation range and average value.

7. The method for simulating and characterizing the wellbore stability of a weak surface formation according to claim 4, characterized in that: In step 5, rock samples for rock physics simulation experiments on weak-surface formations are prepared. The specific operations include: Adjust the proportions of water, quartz sand, lime and additives to prepare cement slurry so that its rock mechanical parameters are similar to those of the rock mechanical parameters of the natural weak-face formation rock core; drill a through hole in the center of the natural weak-face formation rock core, put the prepared cement slurry in a steel mold, and then place the natural weak-face formation rock core in the exact center, and adjust the direction so that the maximum horizontal principal stress orientation of the weak-face formation rock core is parallel to the side, and mark it; Then the steel wellbore is cast together in cement slurry, the wellbore perforations are filled with urea in advance, and the surface is treated after constant temperature maintenance.

8. The method for simulating and characterizing the wellbore stability of a weak surface formation according to claim 6, characterized in that: After the experiment in step 7, CT scanning is performed on the rock samples of the weak surface formation rock physics simulation experiment and the rock cores of the natural weak surface formation. The specific operations include: CT scanning was performed on the rock samples of the rock physics simulation experiment of the weak surface formation after the experiment to obtain the three-dimensional data body of the wellbore stability morphology and analyze the influence law of the wellbore stability; The natural weak-face formation rock cores were taken from the rock samples of the weak-face formation rock physics simulation experiment. CT scanning was used to obtain a three-dimensional data volume containing microcracks and complex fracture networks, and the influence of microcracks and rock mineral components in the weak-face formation rocks on the wellbore stability characteristics was analyzed.

9. The method for simulating and characterizing the wellbore stability of a weak surface formation according to claim 8, characterized in that: In step 8, the CT scan results before and after the experiment are compared to analyze the stability of the wellbore and the influence of microcracks. The specific operations include: Comparing the CT scan results of the rock cores of the natural weak surface formations before and after the experiment, micro-cracks and complex fracture networks were identified; Using the maximum horizontal principal stress orientation marked in step 5, calibrate the orientation of the complex fracture network after drilling; The relationship between complex fracture network and micro-crack azimuth is statistically analyzed to quantitatively evaluate the effect of micro-crack azimuth on wellbore stability. Draw a histogram of the length, density, azimuth and frequency of the deflection angle of the maximum horizontal principal stress direction of the complex fracture network in the analysis section to determine the angle variation range and average value; By comparing the statistical results of the frequency histogram of the crack deflection angle before and after the experiment, the deflection angle range and the main direction of the deflection between the crack and the maximum principal stress direction before and after drilling were obtained, and the stability law of the wellbore was quantitatively characterized.

10. Application of the weak surface formation wellbore stability simulation and characterization method according to any one of claims 1 to 9 in wellbore stability analysis of oil drilling.

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