Method for evaluating the development of natural microfractures in shale using focused ion beam-scanning electron microscopy

Abstract

The application provides a method for evaluating shale natural micro-fracture development by using a focused ion beam-scanning electron microscope, comprising the following steps: obtaining a rock sample suitable for scanning electron microscope observation in size; performing mechanical-ion polishing treatment on multiple surfaces of the sample and numbering the surfaces respectively; cutting an arbitrary position on a certain surface of the sample at equal intervals by using a FIB, and shooting to count the number of fractures on different sections; repeating the FIB cutting and fracture density calculation; performing point projection of the fracture surface density-section sequence of each section on a two-dimensional rectangular coordinate system, and connecting by using a smooth curve; and calculating the surface density of natural fractures. The method for evaluating shale natural micro-fracture development by using a focused ion beam-scanning electron microscope solves the problem of insufficient micro-fracture characterization scale of a rock mechanics parameter calculation method by using scanning electron microscope observation and statistics experiments, and avoids the problem of large transfer error of a mineral component parameter calculation method.

Description

Technical Field

[0001] The present invention relates to the fields of material science, engineering geology and oil and gas exploration technology, and in particular to a method for evaluating the development of natural micro-fractures in shale using a focused ion beam-scanning electron microscope. Background Art

[0002] In recent years, the rapid growth of shale oil and gas production has triggered a global research boom. Among them, the study of shale reservoir properties is an important direction in rock oil and gas storage and development. Shale mainly develops nano-micron pores. Under natural conditions, the connectivity of these pores is extremely poor, which makes shale oil and gas have no natural production capacity. It needs artificial fracture modification to obtain commercial oil and gas flow. The effect of artificial fracture creation is largely affected by the degree of development of natural microcracks in shale. Therefore, the density of natural microcracks in shale is an important factor affecting shale oil and gas production capacity. Natural microcracks in shale reservoirs are mainly nano- to sub-centimeter-scale. At present, the research methods for evaluating the density of natural microcracks in rocks mainly include three categories: (1) fracture statistics method, (2) calculation method based on mineral component parameters and (3) calculation method based on rock mechanics parameters. The fracture statistics method is to observe the rock with the naked eye, optical microscope and scanning electron microscope, count the number of fractures, and then calculate the fracture density. Differences in the mechanical properties of different minerals will lead to differences in the ability of shale to generate fractures. The mineral component parameter calculation method uses the content of minerals in the shale to calculate the mechanical properties of the shale, and further calculates the ability of the shale to generate fractures through mechanical property parameters (such as Poisson's ratio and Young's modulus, etc.), and then infers the density of natural microcracks in the shale. The rock mechanics parameter calculation method and the mineral component parameter calculation method have the same principle. The difference between the two lies in the way the mechanical parameters are obtained. The mechanical parameters used in the rock mechanics parameter calculation method come from rock mechanics experiments, while the mechanical parameters used in the mineral component parameter calculation method are calculated based on mineral component data. Compared with the two, the rock mechanics parameter calculation method has a higher experimental cost, but it has higher accuracy due to the reduction of data transmission errors.

[0003] Through the analysis of the experimental principles, it can be found that the three methods have certain deficiencies in evaluating the density of natural microcracks in shale: (1) The rationality of the results of the fracture statistics method for evaluating the density of natural microcracks is unclear. The development of natural microcracks in shale is affected by the properties of the shale itself and the external environment. The fractures studied by the fracture statistics method are the product of the combined influence of natural conditions and artificial induction. However, in actual operation, it is often impossible to effectively remove the influence of induced fractures. Therefore, it is impossible to perform effective calculation and analysis on two variables or even multiple variables in the evaluation system, resulting in the loss of rationality of the calculation results. (2) The calculation method based on mineral component parameters has large calculation errors. The core of this method is to calculate and analyze the rock mechanical parameters of shale based on the properties of shale itself. In addition to being affected by the content of mineral components, the properties of shale itself are also affected by the source of minerals, mineral combination forms, and tectonic environment. However, the influence of these factors cannot be represented based on the mineral component parameters, so the calculated rock mechanical parameters have large errors, which ultimately leads to large errors in the evaluation of the density of microcracks. In addition, the calculation method based on mineral component parameters also has the problem of unclear rationality of the results. (3) The characterization scale of the calculation method based on rock mechanical parameters is insufficient. The basic data of this method comes from rock mechanics experiments. The scale of cracks that can be characterized by this type of experiment is mainly affected by the signal acquisition method. Currently, the commonly used signal acquisition methods are pressure sensing and acoustic wave signal acquisition. The pressure sensing method requires the internal structure of the rock to produce obvious fracture displacement to collect crack information, and the minimum scale for characterizing cracks is at the centimeter level. Acoustic wave signal acquisition is divided into two types: active sound source and passive sound source. The passive sound source is to mark the crack location and calculate the crack scale by collecting sound waves when the internal structure of the rock is broken. Active sound sources generally use ultrasonic waves as the sound source. The time difference between different reflected signals transmitted to the probe can be used to detect internal structural defects. The minimum scale for characterizing cracks using acoustic wave signals is more than ten microns, which cannot characterize the nano-micron-scale crack scale developed in shale.

[0004] In the Chinese patent application with application number CN201610152262.9, a shale reservoir fracture evaluation method is involved, which includes the following technical steps: (a) in a certain oil and gas exploration and development block, based on geological conditions and research objectives, select wells with characterization significance, and perform coring operations and ground stress tests on the shale reservoir section; (b) perform electron microscope scanning on the core to confirm the fracture development section, describe and record the fracture development and occurrence; (c) perform triaxial stress rock strain experiments on the core to obtain stress-strain curves; (d) perform resistivity logging and acoustic wave logging. (e) performing HBTV logging or microresistivity scanning imaging logging on the fracture development section determined in step (d); (f) performing VSP logging or cross-well seismic logging on the fracture development section determined in step (d); and (g) establishing a single well profile model based on all the above data to comprehensively evaluate the fracture development.

[0005] The Chinese patent application with application number CN201610151759.9 involves a single-well modeling method for geological evaluation of shale gas reservoirs. This method sequentially conducts conventional logging, imaging logging, and VSP logging to make a complete and comprehensive logging interpretation. The reservoir obtained from the logging interpretation is then cored on the well wall. The cores are then subjected to electron microscopy scanning observation, porosity measurement, and rock strain experiments. Combined with the logging interpretation, modeling software is used to establish a fracture development model for a single-well profile.

[0006] The Chinese patent application with application number CN201610152171.5 involves a three-dimensional fracture modeling method. Conventional logging and imaging logging are performed in sequence to exclude sections with no developed fractures and retain sections with developed fractures. VSP logging and coring are then performed on the retained sections. The cores are then subjected to electron microscopy scanning, porosity measurement, and rock strain experiments. Combined with logging interpretation, modeling software is used to establish a fracture development model for a single well profile. A three-dimensional fracture development model of the formation is then established from the models of multiple single wells in the same block.

[0007] The above existing technologies are significantly different from the present invention and fail to solve the technical problem we want to solve. Therefore, we have invented a new method for evaluating the development of natural microfractures in shale using focused ion beam-scanning electron microscopy. Summary of the Invention

[0008] The purpose of the present invention is to provide a method for evaluating the development of natural microcracks in shale using a focused ion beam-scanning electron microscope, which uses a scanning electron microscope to observe statistical experiments and solve the problem that the rock mechanics parameter calculation method is insufficient in the scale of microcrack characterization.

[0009] The object of the present invention can be achieved by the following technical measures: a method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope, the method comprising:

[0010] Step 1: Obtain a rock sample of a size suitable for SEM observation;

[0011] Step 2: Perform mechanical-ion polishing on multiple surfaces of the sample and number them separately;

[0012] Step 3: Use FIB to make evenly spaced cuts at random locations on a surface of the sample, and take photos to count the number of cracks on different sections;

[0013] Step 4: Repeat FIB cutting and crack density calculation;

[0014] Step 5: Project the crack surface density-section sequence of each section in the two-dimensional rectangular coordinate system and connect them with smooth curves;

[0015] Step 6: Calculate the surface density of natural fractures.

[0016] The purpose of the present invention can also be achieved by the following technical measures:

[0017] In step 1, a cutting machine is used to obtain rock samples of a size suitable for SEM observation from intact shale core samples.

[0018] Step 1 also includes using coarse sandpaper to smooth the surface of the rock sample and then gluing it to the scanning electron microscope sample stage using conductive glue.

[0019] In step 2, fine sandpaper is used to finely grind all surfaces except the bottom surface in contact with the stage. The number of surfaces depends on the shape of the sample and specific research needs. Argon ion polishing is used to polish the surfaces to be studied and they are numbered as surface 1, surface 2... surface m.

[0020] In step 3, the sample is placed in a scanning electron microscope (SEM) equipped with a focused ion beam (FIB), and the FIB is used to perform multiple equally spaced cuts on random positions of surface 1 of the sample. The cutting depth is the same as the inward extension length of the sample crack. After each cut, a photo of the cross section is scanned and taken, and the SEM photos of surface 1 obtained are recorded as F11, F12, ..., F1n in the order of the cross sections.

[0021] In step 3, the minimum cutting pitch is 5 nm and the total cutting depth is at least 10 times the longest crack length.

[0022] In step 4, repeat step 3 to cut and photograph the remaining surfaces, count the number of cracks in each section, and calculate the crack surface density as well as the average, standard deviation, and standard error of the surface density.

[0023] In step 5, the change of fracture types from the shale surface to the shale interior is as follows: induced fractures as the main type—induced fractures plus natural fractures—natural fractures as the main type—natural fractures. Three types of change information may be collected through the discrete points counted in steps 3 and 4: (1) induced fractures as the main type—induced fractures plus natural fractures—natural fractures as the main type—natural fractures, (2) induced fractures as the main type—induced fractures plus natural fractures—natural fractures, and (3) induced fractures as the main type—natural fractures as the main type—natural fractures.

[0024] In step 5, the statistics of natural fractures first need to determine a valid set of natural fracture statistics data. Therefore, in the process of collecting natural fractures in each section, as long as the fractures on the section are mainly natural fractures or all are natural fractures, the fracture data counted can be considered as valid data.

[0025] In step 6, the point with the maximum slope of the curve in the figure obtained in step 5 is calculated, and this point is used as the separation point of statistically effective natural microcracks. The vertical coordinates of the points from this point in the positive direction of the X-axis are the effective natural microcrack development density. The vertical coordinates of these points are accumulated and the average value is calculated to obtain the surface density of natural cracks.

[0026] This invention proposes a method for evaluating the density of natural microfractures in shale. The method mainly includes two aspects: (1) using focused ion beam (FIB) cutting combined with mathematical statistical methods to remove the interference of induced fractures in the counting of natural fractures; and (2) using statistical methods to evaluate the density of natural fractures in shale. The method of evaluating the density of natural microfractures in shale using focused ion beam-scanning electron microscopy in this invention adopts statistical methods as the basis for evaluating the density of natural microfractures in shale. It proposes a solution to the problem that the statistical method cannot distinguish between induced fractures and natural fractures. It uses scanning electron microscopy observation and statistical experiments to solve the problem that the rock mechanical parameter calculation method is insufficient in characterizing microfractures. The proposed method avoids the problem of large transmission errors in the calculation method based on mineral component parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart of a specific embodiment of the method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope according to the present invention;

[0028] Figure 2This is a schematic diagram of performing equally spaced cutting at random positions on a surface of a sample using FIB as described in step 3 in a specific embodiment of the present invention;

[0029] Figure 3 A schematic diagram of projecting the crack surface density-cross-section sequence described in step 5 in a two-dimensional rectangular coordinate system in a specific embodiment of the present invention;

[0030] Figure 4 Schematic diagram of a random position on a surface selected by FIB in specific embodiment 1 of the present invention;

[0031] Figure 5 This is a schematic diagram of using FIB to perform equally spaced cutting at a random position on a surface of a sample in specific embodiment 1 of the present invention;

[0032] Figure 6 Schematic diagram of the projection of the crack surface density-cross-section sequence in a two-dimensional rectangular coordinate system in specific embodiment 1 of the present invention;

[0033] Figure 7 Schematic diagram of the projection of the crack surface density-cross-section sequence in a two-dimensional rectangular coordinate system in specific embodiment 2 of the present invention;

[0034] Figure 8 This is a schematic diagram of using FIB to perform equally spaced cutting at a random position on a surface of a sample in specific embodiment 3 of the present invention;

[0035] Figure 9 Schematic diagram of the projection of the crack surface density-cross-section sequence in a two-dimensional rectangular coordinate system in specific embodiment 3 of the present invention DETAILED DESCRIPTION

[0036] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0037] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations and / or combinations thereof.

[0038] like Figure 1 As shown, Figure 1 This is a flow chart of the method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope. The method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope includes:

[0039] Step 1: Obtain a rock sample of a size suitable for SEM observation;

[0040] A cutting machine was used to obtain rock samples of suitable size for SEM observation from intact shale core samples. The surface of the samples was smoothed with coarse sandpaper and then adhered to the SEM sample stage using conductive adhesive.

[0041] Step 2: Use argon ion polishing to perform mechanical-ion polishing on multiple surfaces of the sample and number them respectively;

[0042] Finely grind all surfaces except the bottom surface in contact with the stage using fine sandpaper. The number of surfaces depends on the sample shape and specific research needs. Polish the surfaces in question using argon ion polishing and number them as surface 1, surface 2, and surface m.

[0043] Step 3: Use FIB to make evenly spaced cuts at random locations on a surface of the sample, and take photos to count the number of cracks on different sections;

[0044] The sample was placed in a scanning electron microscope (SEM) equipped with a focused ion beam (FIB), and the FIB was used to perform multiple equal-spaced cuts on random positions of the sample surface 1 ( Figure 2 ), with the cutting depth equal to the inward extension of the sample crack, the minimum cutting spacing being 5 nm, and the total cutting depth being at least 10 times the longest crack length. Scan and photograph the cross section after each cut. SEM images of surface 1 are recorded in order of cross-section as F11, F12, …, F1n.

[0045] Step 4: Repeat step 3 to cut and photograph the remaining surfaces, count the number of cracks in each section, and calculate the crack surface density as well as the average, standard deviation, and standard error of the surface density.

[0046] Repeat step 3 for the remaining surfaces, cutting and photographing them. Label the SEM images of the second surface F21, F22, ..., F2n, and similarly, label the SEM images of the mth surface Fm1, Fm2, ..., Fmn. Count the number of cracks in each cross-section based on the SEM images, and calculate the crack surface density, as well as the mean, standard deviation, and standard error (Table 1).

[0047] Table 1 Calculated crack surface density and the average, standard deviation and standard error statistics of surface density

[0048]

[0049] Step 5: Project the crack surface density-section sequence of each section in the two-dimensional rectangular coordinate system and connect them with smooth curves.

[0050] The crack surface density-section order of each section is projected in a two-dimensional rectangular coordinate system and connected with a smooth curve ( Figure 3 ). The natural fractures inside the shale can be considered to be uniformly developed, while the induced fractures are mainly developed on the shale surface which is easily damaged. The change of fracture types from the shale surface to the shale interior is as follows: induced fractures as the main type - induced + natural fractures - natural fractures as the main type - natural fractures. Therefore, the discrete points counted by steps 3 and 4 may collect three types of change information: (1) induced fractures as the main type - induced + natural fractures - natural fractures as the main type - natural fractures, (2) induced fractures as the main type - induced + natural fractures - natural fractures, (3) induced fractures as the main type - natural fractures as the main type - natural fractures. To count natural fractures, it is first necessary to determine a valid set of natural fracture statistical data. Therefore, in the process of collecting natural fractures in each section, as long as the fractures on the section are mainly natural fractures or all are natural fractures, the fracture data counted can be considered as valid data.

[0051] Step 6: Calculate the point with the maximum slope of the curve in the figure obtained in step 5, and use this point as the separation point of statistically effective natural microcracks. The vertical coordinate of the point from this point to the positive direction of the X-axis is the effective natural microcrack development density. The vertical coordinates of these points are accumulated and the average value is obtained to obtain the surface density of natural cracks.

[0052] Calculated Figure 3 At the point where the slope of the midline curve is maximum, the cross-sections collected from this point in the positive direction of the X-axis can be considered to be primarily or entirely natural fractures. By accumulating the ordinates of the points in the positive direction of the X-axis from this point and averaging them, we can obtain the natural fracture density or the approximate natural microcrack density a.

[0053] The following are several specific embodiments of the present invention.

[0054] Example 1:

[0055] In the specific embodiment 1 of the present invention, a shale core of the Longmaxi Formation of the Lower Silurian in the Sichuan Basin was selected as the research object, and the sample was processed as follows:

[0056] Step 1: Use a cutter to cut a 5cm x 5cm x 5cm cube of rock from the intact drill core. Grind the surface of the sample flat with coarse sandpaper and then adhere it to the SEM sample stage with conductive adhesive.

[0057] Step 2: Use fine sandpaper to finely grind all surfaces except the bottom surface, and polish them using argon ion polishing, and number them as surface 1, surface 2, surface 3, surface 4, and surface 5 ( Figure 4 ).

[0058] Step 3: Place the sample in a scanning electron microscope (SEM) equipped with a focused ion beam (FIB), and use the FIB to perform multiple equal-spaced cuts on random positions on surface 1 of the sample ( Figure 5 ), the cutting surface is a square with a length of 10 μm and a cutting area of ​​100 μm 2 The cutting pitch was 20 nm and the total cutting depth was 100 nm. After each cut, a scan was taken of the cross section. The SEM photos of surface 1 were recorded as F11, F12, F13, F14, and F15 in the order of the cross sections.

[0059] Step 4: Repeat step 3 to cut and photograph the remaining four surfaces. Record the SEM images of the second surface as F21, F22, F23, F24, and F25. Record the SEM images of the third surface as F31, F32, F33, F34, and F35. Record the SEM images of the fourth surface as F41, F42, F43, F44, and F45. Record the SEM images of the fifth surface as F51, F52, F53, F54, and F55. Count the number of cracks in each cross section based on the SEM images, and calculate the mean, standard deviation, and standard error of the crack density for each cross section (Table 2).

[0060] Table 2 Parameters of fracture surface density of Longmaxi Formation shale

[0061]

[0062] Step 5: Project the statistical fracture density scatter data with standard error for each section of the Longmaxi Formation shale and connect them with a smooth curve ( Figure 6 ).

[0063] Step 6: Determine the maximum slope point of the corresponding curve of the Longmaxi Formation shale, use the maximum slope point of the curve as the separation point, and determine that the effective sections for the statistical density of fractures in the Longmaxi Formation shale are F13, F14, F15, F23, F24, F25, F33, F34, F35, F43, F44, F45, F53, F54 and F55. Based on the statistics of the number of fractures in the effective sections, calculate the fracture density a of the natural fractures in the Longmaxi Formation shale. L Calculate a L 11.1 / 10 2 μm 2 .

[0064] Example 2:

[0065] In the specific embodiment 2 of the present invention, a shale core of the Taiyuan Formation of the Lower Permian in the Ordos Basin was selected as the research object, and the sample was processed as follows:

[0066] Step 1: Use a cutter to cut a 5cm x 5cm x 5cm cube of rock from the intact drill core. Grind the surface of the sample flat with coarse sandpaper and then adhere it to the SEM sample stage with conductive adhesive.

[0067] Step 2: Use fine sandpaper to finely grind all surfaces except the bottom surface, and polish them using argon ion polishing, and number them as surface 1, surface 2, surface 3, surface 4, and surface 5 ( Figure 4 ).

[0068] Step 3: Place the sample in a scanning electron microscope (SEM) equipped with a focused ion beam (FIB), and use the FIB to perform multiple equal-spaced cuts on random positions on surface 1 of the sample ( Figure 5 ), the cutting surface is a square with a length of 10 μm and a cutting area of ​​100 μm 2 The cutting pitch was 20 nm and the total cutting depth was 100 nm. After each cut, a scan was taken of the cross section. The SEM photos of surface 1 were recorded as F11, F12, F13, F14, and F15 in the order of the cross sections.

[0069] Step 4: Repeat step 3 to cut and photograph the remaining four surfaces. Record the SEM images of the second surface as F21, F22, F23, F24, and F25. Record the SEM images of the third surface as F31, F32, F33, F34, and F35. Record the SEM images of the fourth surface as F41, F42, F43, F44, and F45. Record the SEM images of the fifth surface as F51, F52, F53, F54, and F55. Count the number of cracks in each cross section based on the SEM images, and calculate the mean, standard deviation, and standard error of the crack density for each cross section (Table 3).

[0070] Table 3. Taiyuan Formation shale fracture surface density parameters

[0071]

[0072] Step 5: Project the statistical fracture density scatter data with standard error for each section of the Taiyuan Formation shale and connect them with a smooth curve ( Figure 7 ).

[0073] Step 6: Determine the maximum slope point of the corresponding curve of the Taiyuan Formation shale, use the maximum slope point of the curve as the separation point, and determine that the effective statistical sections of the fracture density of the Taiyuan Formation shale are F13, F14, F15, F23, F24, F25, F33, F34, F35, F43, F44, F45, F53, F54 and F55. Based on the statistics of the number of fractures in the effective sections, calculate the fracture density a of the natural fractures in the Taiyuan Formation shale T Calculate aT 8.13 / 10 2 μm 2 .

[0074] Example 3:

[0075] In the specific embodiment 3 of the present invention, a shale core of the Shanxi Formation of the Lower Permian in the Ordos Basin was selected as the research object, and the sample was processed as follows:

[0076] Step 1: Use a cutter to cut a 3cm*3cm*4cm cube of rock from the intact drill core. Grind the surface of the sample flat with coarse sandpaper and then adhere it to the SEM sample stage with conductive adhesive.

[0077] Step 2: Use fine sandpaper to finely grind all surfaces except the bottom surface, and polish them using argon ion polishing, and number them as surface 1, surface 2, surface 3 and surface 4 ( Figure 8 ).

[0078] Step 3: Place the sample in a scanning electron microscope (SEM) equipped with a focused ion beam (FIB). Use the FIB to perform multiple equally spaced cuts on random positions of surface 1 of the sample. The cut surface is a square with a length of 10 μm and a cut area of ​​100 μm. 2 The cutting pitch was 20 nm, and the total cutting depth was approximately 140 nm. After each cut, a scan was taken of the cross section. The SEM images of surface 1 were labeled F11, F12, F13, F14, F15, F16, and F17, respectively, in the order of the cross sections.

[0079] Step 4: Repeat step 3 to cut and photograph the remaining three surfaces. The SEM images of the second surface are labeled F21, F22, F23, F24, F25, F26, and F27. The SEM images of the third surface are labeled F31, F32, F33, F34, F35, F36, and F37. The SEM images of the fourth surface are labeled F41, F42, F43, F44, F45, F46, and F47. Count the number of cracks in each cross section based on the SEM images, and calculate the mean, standard deviation, and standard error of the crack density for each cross section (Table 4).

[0080] Table 4. Parameters of fracture surface density of Shanxi Formation shale

[0081]

[0082] Step 5: Project the statistical fracture density scatter data with standard error for each cross section of the Shanxi Formation shale and connect them with a smooth curve ( Figure 9 ).

[0083] Step 6: Determine the point with the maximum slope in the corresponding curve of the Shanxi Formation shale, and use the point with the maximum slope as the separation point to determine that the effective sections for the statistical density of fractures in the Shanxi Formation shale are F13, F14, F15, F16, F17, F23, F24, F25, F26, F27, F33, F34, F35, F36, F37, F43, F44, F45, F46, and F47. Based on the statistics of the number of fractures in the effective sections, the fracture density of natural fractures in the Shanxi Formation shale a is calculated. sh Calculate a sh 9.6 / 10 2 μm 2 ,.

[0084] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art may modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features therein. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

[0085] Except for the technical features described in the specification, all other technical features are known technologies to those skilled in the art.

Claims

1. A method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope, characterized in that: The method for evaluating the development of natural microfractures in shale using focused ion beam-scanning electron microscopy includes: Step 1: Obtain a rock sample of a size suitable for SEM observation; Step 2: Perform mechanical-ion polishing on multiple surfaces of the sample and number them separately; Step 3: Use FIB to make evenly spaced cuts at random locations on a surface of the sample, and take photos to count the number of cracks on different sections; Step 4: Repeat FIB cutting and crack density calculation; Step 5: Project the crack surface density-section sequence of each section in the two-dimensional rectangular coordinate system and connect them with smooth curves; Step 6: Calculate the surface density of natural fractures; In step 3, the sample is placed in a scanning electron microscope (SEM) equipped with a focused ion beam (FIB). The FIB is used to perform multiple equally spaced cuts on random locations on surface 1 of the sample, with the cutting depth being the same as the inward extension length of the sample crack. After each cut, a photograph of the cross section is taken, and the SEM photographs of surface 1 are recorded as F11, F12, ..., F1n in the order of the cross sections. In step 4, step 3 is repeated to cut and photograph the remaining surfaces. The number of cracks in each cross section is counted, and the crack surface density, as well as the average value, standard deviation, and standard error of the surface density, are calculated. In step 5, the change of fracture types from the shale surface to the shale interior is as follows: mainly induced fractures - induced + natural fractures - mainly natural fractures - natural fractures. Three types of change information may be collected through the discrete points counted in steps 3 and 4: a. mainly induced fractures - induced + natural fractures - mainly natural fractures - natural fractures, b. mainly induced fractures - induced + natural fractures - natural fractures, c. mainly induced fractures - mainly natural fractures - natural fractures. In step 5, the statistics of natural fractures first need to determine a valid set of natural fracture statistics data. Therefore, in the process of collecting natural fractures on each section, as long as the fractures on the section are mainly natural fractures or all are natural fractures, the fracture data counted can be considered as valid data. In step 6, the point with the maximum slope of the curve in the figure obtained in step 5 is calculated, and this point is used as the separation point of statistically effective natural microcracks. The vertical coordinates of the points from this point in the positive direction of the X-axis are the effective natural microcrack development density. The vertical coordinates of these points are accumulated and the average value is calculated to obtain the surface density of natural cracks.

2. The method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope according to claim 1, characterized in that: In step 1, a cutting machine is used to obtain rock samples of a size suitable for SEM observation from intact shale core samples.

3. The method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope according to claim 2, characterized in that: Step 1 also includes using coarse sandpaper to smooth the surface of the rock sample and then gluing it to the scanning electron microscope sample stage using conductive glue.

4. The method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope according to claim 1, characterized in that: In step 2, fine sandpaper is used to finely grind all surfaces except the bottom surface in contact with the stage. The number of surfaces depends on the shape of the sample and specific research needs. Argon ion polishing is used to polish the surfaces to be studied and they are numbered as surface 1, surface 2... surface m.

5. The method for evaluating the development of natural microfractures in shale using a focused ion beam-scanning electron microscope according to claim 1, characterized in that: In step 3, the minimum cutting pitch is 5 nm and the total cutting depth is at least 10 times the longest crack length.

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