A method for predicting the compressive strength of fractured strata under in-situ conditions from the upturned block

Abstract

The present invention provides a method for predicting the compressive strength of a broken formation under in-situ conditions from an upper return block. First, effective blocks are screened from the upper return irregular blocks. Second, a point load test is carried out to obtain the uniaxial compressive strength of the effective blocks. Then, the equivalent diameter of the effective blocks is defined and a relationship curve between the uniaxial compressive strength of the effective blocks and the equivalent diameter of the effective blocks is established. Again, the equivalent diameter of a standard rock core of the broken formation is substituted into the relationship curve to obtain a predicted value of the uniaxial compressive strength of the standard rock core of the broken formation. Further, a correction coefficient for the predicted value of the uniaxial compressive strength of the standard rock core of the broken formation is obtained. On this basis, the compressive strength of the broken formation is determined. Compared with the in-situ drilling sounding method, it can be seen that the prediction accuracy of the method of the present invention exceeds 90%.

Description

Technical Field

[0001] The invention belongs to the field of oil and gas drilling engineering and relates to a method for predicting the compressive strength of a crushed formation under in-situ conditions based on an upturned block. Background Art

[0002] Fractured formations are highly susceptible to borehole collapse during drilling. Accurately identifying the compressive strength of fractured formations under in situ conditions can more accurately design drilling fluid density to prevent wellbore collapse. Currently, evaluating the compressive strength of fractured formations under in situ conditions primarily relies on core strength tests, typically obtained through sidewall coring. However, due to the fractured nature of the formation, obtaining cores through sidewall coring is extremely challenging, resulting in either no cores at all or only a small number of cores. The inability to obtain cores makes strength tests impossible, and the limited number of cores also results in significant variability in the strength test data, making it difficult to accurately evaluate the compressive strength of fractured formations under in situ conditions. In summary, it is currently difficult to accurately evaluate the compressive strength of fractured formations under in situ conditions. Notably, when drilling into fractured formations, a large amount of blocks returned with the drilling fluid originate from the in situ fractured formation. This provides the basis for developing a method to predict the compressive strength of fractured formations under in situ conditions based on these returned blocks.

[0003] Through patent research, there are currently some methods to predict formation mechanical parameters (mechanical parameters include compressive strength), such as a method for predicting complex formation rock mechanical parameters based on intelligent fusion strategy (CN202211183997.X), a finite element modeling method and terminal equipment for formation mechanical parameters based on drilling data (CN202211115101.4), a method for measuring formation rock mechanical parameters (CN201510733748.7), and a system and method for measuring formation rock mechanical parameters based on multiple sensors (CN201910022875.4). However, these methods are only applicable to complete continuous medium formations and not to broken non-continuous medium formations. In particular, for fractured formations, some scholars have proposed a method for determining the rock mechanical parameters of fractured formations (CN202010308912.0). However, this method also has shortcomings: (1) This method does not explain whether the prepared cores come from in-situ fractured formations. Considering that the borehole coring technology for fractured formations is difficult, the possibility of obtaining a large number of cores through borehole coring technology is low. However, if the prepared cores do not come from in-situ fractured formations, then the method formed will also be difficult to evaluate the compressive strength of fractured formations under in-situ conditions; (2) The analysis process of this method is complicated and tedious. First, the damage constitutive equation is obtained by fitting the rock damage variable-strain curve, and then the stress-strain constitutive equation is obtained from the damage constitutive equation. Again, the stress-strain curve is drawn according to the stress-strain constitutive equation. Finally, the rock compressive strength is determined by the characteristics of the stress-strain curve. In addition, this method only indirectly determines the rock compressive strength from the stress-strain curve, rather than directly testing the formation compressive strength. There is an essential difference between the two. In addition, a method for measuring rock mechanical parameters using rock cuttings (201510751268.3) and a device for measuring rock mechanical parameters using rock cuttings (201520883141.2) also proposed methods for measuring the strength of rock cuttings. However, these two patents do not establish a relationship between the strength of rock cuttings and formation strength. In summary, it is imperative to develop a method for predicting the compressive strength of fractured formations under in situ conditions based on the return of the rock mass. Summary of the Invention

[0004] The present invention provides a method for predicting the compressive strength of a broken formation under in-situ conditions based on an upper return block. First, effective blocks are screened from the upper return irregular blocks. Second, a point load test is carried out to obtain the uniaxial compressive strength of the effective blocks. Then, the equivalent diameter of the effective blocks is defined and a relationship curve between the uniaxial compressive strength of the effective blocks and the equivalent diameter of the effective blocks is established. Again, the equivalent diameter of a standard rock core of the broken formation is substituted into the relationship curve to obtain a predicted value of the uniaxial compressive strength of the standard rock core of the broken formation. Further, a correction coefficient of the predicted value of the uniaxial compressive strength of the standard rock core of the broken formation is obtained. On this basis, the compressive strength of the broken formation is determined. Finally, the effectiveness of the proposed method is verified through an in-situ borehole probing test.

[0005] To achieve the above object, the first aspect of the present invention provides a method for predicting the compressive strength of a fractured stratum under in-situ conditions based on an upturned block, wherein the calculation steps are:

[0006] S1. Screening effective blocks from the returned blocks

[0007] Most of the returned blocks are irregular blocks. To select effective blocks from irregular blocks, the following four conditions should be met at the same time:

[0008] (1) The ratio of the longest side to the shortest side of the effective block does not exceed 2; that is, 1≤the ratio of the longest side to the shortest side≤2;

[0009] Preferably, the shortest side of the effective block is not less than 35 mm and the longest side is not less than 45 mm;

[0010] (2) The volume of the effective block should not be less than 30cm 3 ;

[0011] The larger the effective block volume, the better. The larger the effective block volume, the more internal defects it contains, and the closer it is to the geological conditions of the fractured strata under in-situ conditions. Preferably, the effective block volume is obtained by the measuring cup method.

[0012] (3) The effective block should be as close to a sphere as possible, and the approximate ellipticity should not be less than 50%;

[0013] Substitute the longest side and shortest side of the effective block into the following formula to calculate the approximate ellipticity of the effective block:

[0014]

[0015] Where: b is the minor axis of the ellipse, that is, the shortest side of the effective block, mm; a is the major axis of the ellipse, that is, the longest side of the effective block, mm;

[0016] (4) The effective block defect coefficient should not be less than 15%;

[0017] The larger the effective block defect coefficient is, the closer it is to the in-situ geological conditions of the fractured strata. The effective block defect coefficient is defined as D, and its calculation formula is as follows:

[0018]

[0019] Where: V s is the effective block defect volume, obtained by CT scanning of the effective block and combining digitalization method, mm 3 ; V is the effective block volume, obtained by measuring cup method, mm 3 ;

[0020] S2. Conduct point load tests to obtain the uniaxial compressive strength of the effective block

[0021] (1) Determine the optimal loading point position of the effective block

[0022] For the effective block, different loading point positions will produce different uniaxial compressive strengths. The actual state of the effective block in the fractured stratum under in-situ conditions is defined as the most stable state of the effective block. Therefore, the upper and lower ends of the effective block in the most stable state are the optimal loading point positions of the effective block.

[0023] 1) Calculate the density of the effective block

[0024] First, add enough water to submerge the effective block into the measuring cup, and read the volume of the water V1 at this time. Then put the effective block into the water, and read the combined volume of the water and the effective block V2 at this time. The volume V of the effective block is:

[0025] V=V2-V1 (3)

[0026] The effective block is dried, and then the mass m of the effective block is measured using a tray balance;

[0027] The density ρ of the effective block is calculated based on the measured volume and mass of the effective block, as shown in the following formula:

[0028]

[0029] 2) Conduct suspension tests to confirm the most stable potential state of the effective block

[0030] Sodium metatungstate is used to prepare sodium metatungstate aqueous solution. The density of sodium metatungstate is known to be 3.1g / cm 3 Higher than the density of rock, the sodium metatungstate aqueous solution prepared for this purpose can make the effective block in a suspended state. Add 500mL of water to a 1000mL measuring cup and then add (500ρ-500)g of sodium metatungstate to prepare a sodium metatungstate aqueous solution with a density of ρ;

[0031] The effective blocks are sequentially placed in a sodium metatungstate aqueous solution and subjected to slight disturbances. The state in which the effective blocks finally remain stable is the potential most stable state of the effective blocks. Preferably, 5 groups of suspension tests are carried out for each effective block to obtain 5 groups of potential most stable states of the effective blocks respectively.

[0032] 3) Calculate the coordinates of the center of gravity of the effective block when it is in the most stable state

[0033] ① Use 3D laser scanning technology to obtain point cloud data of the effective block's outer contour when it is in its most stable state. Import the point cloud data into Geomagic Studio to establish a 3D geometric model of the effective block's spatial distribution.

[0034] ② Import the three-dimensional geometric model of the effective block space distribution into HyperMesh, use HyperMesh to mesh the three-dimensional geometric model, and extract the volume v of each unit after meshing. i and centroid coordinates (x i ,y i ,z i ), the volume of each unit v i and centroid coordinates (x i ,y i ,z i ) is substituted into the following formula to calculate the effective block center of gravity coordinates (x c ,y c ,z c ):

[0035]

[0036] Where: x c ,y c ,z c is the center of gravity coordinate of the effective block, cm; v i is the volume of the i-th unit, cm 3 ;x i ,y i ,z i is the centroid coordinate of the i-th unit; i is the unit number; n is the total number of units; ρ is the density of the effective block, g / cm 3 ; m is the mass of the effective block, g;

[0037] ③ Repeat steps ① and ② to obtain the center of gravity coordinates (x c1 ,y c1 ,z c1 )、(x c2 ,y c2 ,z c2 )、(x c3 ,y c3,z c3 )、(x c4 ,y c4 ,z c4 )、(x c5 ,y c5 ,z c5 );

[0038] 4) Determine the most stable state of the effective block with the lowest center of gravity as the control condition

[0039] Taking the lowest effective block center of gravity as the control condition, compare and select the effective block center of gravity coordinate z when it is in the five groups of potential most stable states c1 、z c2 、z c3 、z c4 、z c5 The minimum value among , to determine the most stable state of the effective block;

[0040] 5) Obtain the optimal loading point position of the effective block through the most stable state of the effective block

[0041] When the effective block is in the most stable state, that is, when the effective block is in the actual state in the fractured stratum under in-situ conditions, the upper and lower ends of the effective block, that is, the highest point and the lowest point of the effective block are the optimal loading point positions of the effective block;

[0042] (2) Conduct point load tests on effective blocks

[0043] Use a geological hammer to grind the optimal loading point position of the effective block flat so that the effective block can be stably placed on the loading plate, then apply displacement through the loading plate to carry out a point load test, stop loading after the effective block breaks, and finally record the load (force) data during the point load test, the effective block deformation data, and the distance data between the upper and lower loading plates when the effective block breaks; preferably, the number of effective block point load test groups carried out should be no less than 300;

[0044] (3) Obtaining the uniaxial compressive strength of the effective block

[0045] Referring to the provisions of the point load test in the American ASTM D5731 specification, the uniaxial compressive strength of the effective block is defined as the ratio of the load (force) at the time of effective block rupture to the square of the distance between the upper and lower loading plates at the time of effective block rupture, as shown in the following formula:

[0046]

[0047] Where: σ c is the uniaxial compressive strength of the effective block, kN / mm 2 ; P is the load (force) when the effective block breaks, kN; d is the distance between the upper and lower loading plates when the effective block breaks, mm;

[0048] In the present invention, establishing the screening conditions for effective blocks, determining the optimal loading point positions of effective blocks, and conducting a sufficient number of point load test groups are all aimed at avoiding the discreteness of the uniaxial compressive strength test data of effective blocks to the greatest extent.

[0049] S3. Establish the effective block uniaxial compressive strength σ c With equivalent diameter D e The relationship curve

[0050] (1) Calculate the equivalent diameter D of the effective block e

[0051] Taking the volume equality as the control condition, the equivalent diameter D of the effective block is defined as e is the diameter of a sphere of equal volume, and the calculation formula is as follows:

[0052]

[0053] Where: V e is the volume of the effective block, mm 3 ;D e is the equivalent diameter of the effective block, mm;

[0054] (2) Obtain the uniaxial compressive strength σ of all effective blocks c Test data and calculate the equivalent diameter D of all effective blocks e On this basis, the effective block uniaxial compressive strength σ is plotted c and equivalent diameter D e The scatter plot of the effective block uniaxial compressive strength σ is established by fitting the scatter plot. c and equivalent diameter D e Relationship curve: σ c =f(D e );

[0055] The larger the size of the effective block, the more internal defects there are, and the corresponding effective block uniaxial compressive strength σ c The smaller it is (there is a size effect), so the established relationship curve σ c =f(D e ) is a decreasing function, that is, the uniaxial compressive strength σ of the effective block c With equivalent diameter D e decreases with the increase of

[0056] S4. According to the relationship curve σ c =f(D e ) Calculate the predicted value of uniaxial compressive strength of standard core in fractured formation

[0057] The compressive strength of the formation is generally reflected by the uniaxial compressive strength of the standard core. Different formations have different standard core sizes. For continuous medium formations, the standard core is generally a standard cylindrical rock sample with a diameter of 25 mm and a height of 50 mm. For broken formations, the standard core is generally a standard cylindrical rock sample with a diameter of 50 mm and a height of 100 mm. After calculating the equivalent diameter D of the standard core of the broken formation es Then, substitute it into the effective block uniaxial compressive strength σ c and equivalent diameter D e The relationship curve σ c =f(D e ), the predicted value of uniaxial compressive strength of standard core of fractured formation can be obtained

[0058] S5. Obtain the predicted value of uniaxial compressive strength of standard core in fractured formation Correction factor

[0059] When using the relationship curve σ c =f(D e ) to obtain the predicted value of uniaxial compressive strength of standard core of fractured formation After that, it needs to be corrected. Considering that it is difficult to coring or the number of coring is small in the fractured non-continuous medium formation, a large number of corings are taken from the continuous medium formation and a standard core of the continuous medium formation is made to carry out uniaxial compressive strength test to obtain the measured average value. Calculate the equivalent diameter of the standard core of the continuous medium formation and substitute it into the effective block uniaxial compressive strength σ c and equivalent diameter D e The relationship curve σ c =f(D e ) to obtain the predicted value of uniaxial compressive strength of the standard core of the continuous medium formation Define η as the predicted value of uniaxial compressive strength of standard core in fractured formations The correction coefficient is as follows:

[0060]

[0061] Where: η is the correction coefficient; is the measured average value of uniaxial compressive strength of a large number of standard cores in continuous medium formations, MPa; is the predicted value of uniaxial compressive strength of standard core of continuous medium formation, MPa;

[0062] S6. Prediction of compressive strength of fractured formations

[0063] Through steps S1 to S5, the compressive strength of the crushed formation σ c As described in the following formula:

[0064]

[0065] S7. Verify the accuracy of the above calculation method

[0066] The measured value of the compressive strength of the fractured stratum was obtained through in-situ drilling penetration test and compared with the predicted value of the above method. The error was within 10%, indicating that the above method can meet the requirements of on-site engineering.

[0067] Compared with the prior art, the present invention has the following beneficial effects:

[0068] 1. The present invention defines the most stable state of an effective block, and forms a method for determining the optimal loading point position of the effective block when conducting a point load test through the most stable state. The effective block is an irregular block;

[0069] 2. The present invention establishes a relationship curve between the uniaxial compressive strength of an effective block and the equivalent diameter of an effective block. Substituting the equivalent diameter of a standard core into the relationship curve can obtain the uniaxial compressive strength of the standard core;

[0070] 3. The present invention proposes a method for predicting the compressive strength of fractured strata under in-situ conditions based on the upturned block. Compared with the in-situ drilling sounding method, it can be seen that the prediction accuracy of the method of the present invention exceeds 90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 The present invention is a schematic diagram of the calculation process of the method;

[0072] Figure 2 is a relationship curve between the effective block defect coefficient and the effective block volume in Example 1 of the present invention;

[0073] Figure 3 Schematic diagram of mesh division of the three-dimensional geometric model of the spatial distribution of effective block No. 172 in Example 1 of the present invention;

[0074] Figure 4 This is a relationship curve between the uniaxial compressive strength of the effective block and the equivalent diameter of the effective block in Example 1 of the present invention. DETAILED DESCRIPTION

[0075] The details of the present invention can be more clearly understood by referring to the accompanying drawings and the description of the specific embodiments of the present invention. However, the specific embodiments of the present invention described herein are only for the purpose of explaining the present invention and are not to be construed as limiting the present invention in any way. Based on the teachings of the present invention, a skilled person can conceive of any possible variations based on the present invention, and such variations should be considered to fall within the scope of the present invention.

[0076] Example 1:

[0077] This embodiment provides a method for predicting the compressive strength of a fractured formation under in-situ conditions from an upper return block. The method predicts the compressive strength of a fractured formation (5700m-6000m) under in-situ conditions in the WQ2 well of the LS block. The calculation flow diagram is shown in FIG. Figure 1 .

[0078] S1. When drilling into the fractured strata in Well WQ2, a total of 552 irregular blocks were returned. According to the screening conditions (1) to (4) for valid blocks, 301 valid blocks were selected from the returned blocks. According to formula (2), the defect coefficients of all valid blocks were obtained, see Figure 2 ,from Figure 2 It can be seen that the effective block defect coefficient increases with the increase of the effective block volume;

[0079] S2. Perform point load tests on the effective block. Taking the effective block No. 172 as an example, five groups of potential most stable states of the effective block No. 172 were obtained by conducting five groups of suspension tests. Taking the third group of potential most stable states as an example, a three-dimensional geometric model of the spatial distribution of the effective block No. 172 was established using Geomagic Studio. The three-dimensional geometric model was meshed using HyperMesh. Figure 3 According to formula (5), the center of gravity heights of the effective block No. 172 in the five most stable states are 35.7mm, 37.4mm, 32.1mm, 39.4mm, and 38.3mm respectively. Taking the lowest center of gravity of the effective block as the control condition, the third group of the most stable state with a center of gravity of 32.1mm is determined to be the most stable state of the effective block No. 172. Based on this, the optimal loading point position of the effective block No. 172 is obtained and a point load test is carried out. The uniaxial compressive strength of the effective block No. 172 is obtained based on formula (6);

[0080] S3. Repeat the point load test steps of effective block No. 172 to obtain the uniaxial compressive strength test data of all effective blocks, and calculate the equivalent diameters of all effective blocks according to formula (7). On this basis, draw a scatter plot of the uniaxial compressive strength and equivalent diameter of all effective blocks, and fit the scatter plot to establish the relationship curve σ between the uniaxial compressive strength and equivalent diameter of all effective blocks. c =56.6×e -7.45De ,See Figure 4 ;

[0081] S4. Based on engineering experience, the standard core of the fractured formation drilled in Well WQ2 is a standard cylindrical rock sample with a diameter of 50 mm and a height of 100 mm. Its equivalent diameter is 36 mm. Substituting σ c =86.6×e -11.44De The predicted uniaxial compressive strength of the standard core of the fractured formation is

[0082] S5. Since the fractured formation drilled by Well WQ2 is difficult to coring in large quantities, the correction coefficient η in formula (8) is obtained based on Well WQ1, an adjacent well of Well WQ2. Well WQ1 is a continuous medium formation within the depth range of 5700-6000 m, from which a large number of cores can be cored. Based on engineering experience, the standard core of the continuous medium formation drilled by Well WQ1 is a standard cylindrical rock sample with a diameter of 25 mm and a height of 50 mm. 100 standard cylindrical rock samples with a diameter of 25 mm and a height of 50 mm were cored from the formation and an experiment was carried out to obtain the measured average value of the uniaxial compressive strength of the standard core of the continuous medium formation. According to formula (7), the equivalent diameter of a standard core with a diameter of 25 mm and a height of 50 mm is calculated to be 23 mm. Substituting σ c =86.6×e -11.44De The predicted uniaxial compressive strength of the standard core of the continuous medium formation is According to formula (8), the correction coefficient η = 0.79;

[0083] S6. Substitute the correction coefficient η=0.79 into formula (9) to obtain the predicted compressive strength value σ of the fractured formation c =45.38MPa;

[0084] S7. An in-situ drilling penetration test was conducted on the fractured formation of Well WQ2, and the measured compressive strength of the fractured formation was 41.87 MPa. When compared with the predicted value of the method proposed in the present invention, the error between the two was 8.38%, which is less than 10%, indicating that the method proposed in the present invention can meet the requirements of field engineering.

Claims

1. A method for predicting the compressive strength of a fractured stratum under in-situ conditions from an upturned block, characterized in that: The specific steps include: S1. Screening valid blocks from the returned blocks Most of the returned blocks are irregular blocks. To select effective blocks from irregular blocks, the following four conditions should be met at the same time: (1) The ratio of the longest side to the shortest side of the effective block does not exceed 2; (2) The volume of the effective block should not be less than 30cm 3 ; (3) The effective block should be as close to a sphere as possible, and the approximate ellipticity should not be less than 50%; (4) The effective block defect coefficient should not be less than 15%; S2. Conduct point load tests to obtain the uniaxial compressive strength of the effective block (1) Determine the optimal loading point position of the effective block For an effective block, the actual state of the effective block in the fractured stratum under in-situ conditions is defined as the most stable state of the effective block. Therefore, the upper and lower ends of the effective block when it is in the most stable state are the optimal loading point positions of the effective block. (2) Conduct point load tests on effective blocks Record the load (force) data, effective block deformation data, and the distance data between the upper and lower loading plates when the effective block breaks during the point load test; (3) Obtaining the uniaxial compressive strength of the effective block ; S3. Establish effective block uniaxial compressive strength and equivalent diameter D e The relationship curve Get the uniaxial compressive strength of all effective blocks Test data and calculate the equivalent diameter D of all effective blocks e On this basis, the effective block uniaxial compressive strength is plotted and equivalent diameter D e The scatter plot of the effective block uniaxial compressive strength is established by fitting the scatter plot. and equivalent diameter D e Relationship curve: ; S4. According to the relationship curve Calculation of the predicted value of uniaxial compressive strength of standard core in fractured formations ; After calculating the equivalent diameter D of the standard core of the fractured formation es Then, substitute it into the relationship curve The predicted value of uniaxial compressive strength of standard core of fractured formation can be obtained ; S5. Obtain the predicted uniaxial compressive strength of standard cores in fractured formations Correction factor ; S6. Prediction of compressive strength of fractured formations Through steps S1 to S5, the compressive strength of the crushed formation As described in the following formula: ； S7. Verify the accuracy of the above calculation method The measured compressive strength of the fractured strata was obtained by in-situ drilling penetration test and compared with the predicted value by the above method. The error was within 10%, indicating that the above method can meet the requirements of the field project. In step S1, the shortest side of the effective block is not less than 35 mm and the longest side is not less than 45 mm; The effective block volume was obtained by the measuring cup method; Substitute the longest side and shortest side of the effective block into the following formula to calculate the approximate ellipticity of the effective block: ； Where: b is the minor axis of the ellipse, that is, the shortest side of the effective block, mm; a is the major axis of the ellipse, that is, the longest side of the effective block, mm; The effective block defect coefficient is defined as D, and its calculation formula is as follows: ； Where: V s is the effective block defect volume, obtained by CT scanning of the effective block and combining digital methods, mm 3 ; V is the effective block volume, obtained by measuring cup method, mm 3 ; In step S3, the equivalent diameter D of the effective block is defined with the volume equal as the control condition. e is the diameter of a sphere of equal volume, and the calculation formula is as follows: ； Where: V e is the volume of the effective block, mm 3 ;D e is the equivalent diameter of the effective block, mm; In step S5, a large number of cores are taken from the continuous medium formation and a standard core of the continuous medium formation is prepared to carry out a uniaxial compressive strength test to obtain the measured average value. , calculate the equivalent diameter of the standard core of the continuous medium formation and substitute it into the effective block uniaxial compressive strength and equivalent diameter D e The relationship curve The predicted value of uniaxial compressive strength of the standard core of the continuous medium formation is obtained ,definition is the predicted value of uniaxial compressive strength of standard core in fractured formation The correction coefficient is as follows: ； Where: is the correction factor; is the measured average value of uniaxial compressive strength of a large number of standard cores in continuous medium formations, MPa; is the predicted value of uniaxial compressive strength of standard core in continuous medium formation, MPa.

2. The method of predicting the compressive strength of a fractured stratum under in-situ conditions from an upturned block according to claim 1, characterized in that: In step S2 (1), the specific steps of determining the optimal loading point position of the effective block include: 1) Calculate the density of the effective block ; 2) Conduct suspension tests to confirm the most stable potential state of the effective block 3) Calculate the coordinates of the center of gravity of the effective block when it is in the most stable state Establish a three-dimensional geometric model of the effective block space distribution, mesh the three-dimensional geometric model, and extract the volume of each unit after meshing and centroid coordinates ( , , ), and then calculate the effective block center of gravity coordinates ( , , ): 4) Taking the lowest center of gravity of the effective block as the control condition, determine the most stable state of the effective block; 5) Obtain the optimal loading point position of the effective block through the most stable state of the effective block.

3. The method of predicting the compressive strength of a fractured stratum under in-situ conditions from an upper return block according to claim 2, characterized in that: In step S2(1) 2), the specific steps of conducting a suspension test to confirm the potential most stable state of the effective block include: Sodium metatungstate is used to prepare sodium metatungstate aqueous solution. The density of sodium metatungstate is known to be 3.1g / cm 3 Higher than the density of rock, the sodium metatungstate aqueous solution prepared for this purpose can make the effective block in a suspended state. Add 500mL of water to a 1000mL measuring cup and then add (500ρ-500)g of sodium metatungstate to prepare a density of Aqueous solution of sodium metatungstate; The effective blocks are placed in a sodium metatungstate aqueous solution in turn and subjected to slight disturbances. The state in which the effective blocks finally maintain stability is the potential most stable state of the effective blocks.

4. The method of predicting the compressive strength of a fractured stratum under in-situ conditions from an upturned block according to claim 2, characterized in that: In step S2(1) 3), the specific steps of calculating the coordinates of the center of gravity of the effective block when it is in the potential most stable state include: ① Use 3D laser scanning technology to obtain point cloud data of the effective block's outer contour when it is in its most stable state. Import the point cloud data into Geomagic Studio to establish a 3D geometric model of the effective block's spatial distribution. ② Import the 3D geometric model of the effective block space distribution into HyperMesh, use HyperMesh to mesh the 3D geometric model, and extract the volume of each unit after meshing. and centroid coordinates ( , , ), the volume of each unit and centroid coordinates ( , , ) is substituted into the following formula to calculate the effective block center of gravity coordinates ( , , ): ； Where: , , is the center of gravity coordinate of the effective block, cm; v i is the volume of the i-th unit, cm 3 ; , , is the centroid coordinate of the i-th unit; i is the unit number; n is the total number of units; ρ is the density of the effective block, g / cm 3 ; m is the mass of the effective block, g.

5. The method of predicting the compressive strength of a fractured stratum under in-situ conditions from an upturned block according to claim 2, characterized in that: In step S2, 5 groups of suspension tests are carried out for each effective block to obtain 5 groups of potential most stable states and corresponding center of gravity coordinates of the effective block ( , , )、( , , )、( , , )、( , , )、( , , ); compare and select the center of gravity coordinates of the effective block in the five groups of potential most stable states 、 、 、 、 The minimum value among them is used to determine the most stable state of the effective block.

6. The method of predicting the compressive strength of a fractured stratum under in-situ conditions from an upturned block according to claim 1, characterized in that: In step S2 (2), a geological hammer is used to grind the optimal loading point position of the effective block flat so that the effective block can be stably placed on the loading plate, and then a point load test is carried out by applying displacement through the loading plate. After the effective block is broken, the loading is stopped and the data is recorded; The number of effective block point load test groups carried out should be no less than 300.

7. The method of predicting the compressive strength of a fractured stratum under in-situ conditions from an upturned block according to claim 1, characterized in that: In step (3) of step S2, referring to the provisions on point load test in the American ASTM D5731 specification, the uniaxial compressive strength of the effective block is defined as the ratio of the load (force) at the time of effective block rupture to the square of the distance between the upper and lower loading plates at the time of effective block rupture, as shown in the following formula: ； Where: is the uniaxial compressive strength of the effective block, kN / mm 2 ; P is the load (force) when the effective block breaks, kN; d is the distance between the upper and lower loading plates when the effective block breaks, mm.

Images