A Dual Evaluation Method for Shale Reservoir Fracturing Performance Based on a Cohesion-Friction-Layering Coupling Model

Abstract

This invention relates to a dual evaluation method for fracturing performance of shale reservoirs based on a cohesion-friction-bedding coupling model. It includes: preparing shale samples with different bedding angles; conducting Brazilian splitting experiments, uniaxial compression experiments, and triaxial compression experiments to obtain time-load curves and stress-strain curves for shale at different bedding angles; and constructing a shale brittleness index under uniaxial stress. B un Determine the degradation coefficient of shale brittleness due to confining pressure. B wc Constructing the brittleness index of shale under triaxial stress conditions B tr Shale brittleness index under triaxial stress conditions B tr This invention combines well logging data to predict the distribution of rock brittleness across the entire block and well section, clarifying reservoir compressibility, and integrates with geological modeling software to predict fracture morphology and fracturing effectiveness. This invention solves the problems of inconsistent rock brittleness assessment results and the difficulty in combining them with geophysical data for application in anisotropic shale, which negatively impacts shale reservoir fracturing design and fracturing effectiveness evaluation.

Description

Technical Field

[0001] This invention relates to the field of unconventional reservoir rock mechanics in oil and gas engineering, specifically to a dual evaluation method for the fracturing effect of shale reservoirs based on a cohesion-friction-stratification coupling model. Background Technology

[0002] With the gradual depletion of conventional fossil energy resources, my country's oil and gas exploration and development is accelerating its transformation towards unconventional fields. Shale oil and gas, as an important component of unconventional resources, is characterized by its wide distribution, large reserves, and enormous development potential, and has become a key focus of current oil and gas exploration and development. However, shale reservoirs are characterized by high density, low permeability, and insufficient development of natural fractures, making it difficult to form effective oil and gas migration channels. Therefore, hydraulic fracturing technology is necessary for reservoir stimulation and production enhancement. Currently, hydraulic fracturing has become an indispensable construction measure for shale oil and gas development. Shale reservoirs typically have well-developed bedding, significantly affecting fracture propagation paths and fracturing effects. Therefore, it is necessary to conduct compressibility assessments of the entire well section and block of the shale reservoir before operations. Rock brittleness is a key indicator for reservoir compressibility assessment; therefore, conducting shale brittleness anisotropy assessment can provide a basis for formation compressibility assessment and fracturing effect evaluation. Existing research indicates that mineral content, strain, strength, elastic parameters, and energy evolution are closely related to brittleness. Therefore, brittleness indices based on mineral content, strain, strength, elastic parameters, and energy have been constructed both domestically and internationally. However, these brittleness indices consider relatively singular factors. Shale reservoirs, due to their well-developed bedding, strong anisotropy, and complex energy characteristics and fracture modes of rock failure, require a comprehensive brittleness index that considers multiple factors. This leads to inaccurate evaluation results of existing brittleness indices for shale brittleness and makes it difficult to effectively characterize the anisotropy of shale brittleness. Therefore, there is an urgent need for a brittleness index that can accurately characterize shale brittleness.

[0003] The mineral content-based brittleness assessment method assumes that the brittle mineral content of a rock is related to its brittle fracture, considering only the mineral content of brittle minerals in the rock. However, shale has well-developed bedding, and the mineral content does not change with the direction of bedding, making it difficult to characterize the rock's anisotropy. The strain-based brittleness assessment method uses the stress-strain curve to reflect the rock's brittleness through deformation characteristics. The strength parameter-based brittleness assessment method considers the rock's bearing capacity to construct a brittleness assessment method. The rock strength parameter-based brittleness assessment method assumes that rock brittleness is related to the rock's elastic modulus and Poisson's ratio. Although the above three parameters can characterize the anisotropy of shale, the results are not accurate, and the assessment results are inconsistent and even far apart. Therefore, researchers have proposed a brittleness assessment method based on energy evolution, which is currently a relatively scientific assessment method that can accurately evaluate rock brittleness. However, due to the strong bedding of shale, the experimental curves often fluctuate greatly, making it difficult to accurately obtain the yield point and residual point. This makes the energy-based brittleness assessment method inaccurate for the brittleness assessment results of bedding shale. Summary of the Invention

[0004] The purpose of this invention is to provide a dual evaluation method for shale reservoir fracturing effect based on a cohesion-friction-bedding coupling model. This dual evaluation method for shale reservoir fracturing effect based on a cohesion-friction-bedding coupling model is used to solve the problem that the existing rock brittleness evaluation results are inconsistent and difficult to combine with geophysical data for application in anisotropic shale, which affects the fracturing design and fracturing effect evaluation of shale reservoirs.

[0005] The technical solution adopted by this invention to solve its technical problem is as follows: This dual evaluation method for fracturing effect of shale reservoirs based on a cohesion-friction-stratification coupling model includes the following steps: Step 1: Prepare shale samples with different bedding angles; Step 2: Using the shale samples prepared in Step 1, Brazilian splitting test, uniaxial compression and triaxial compression test were carried out to obtain time-load curves and stress-strain curves of shale with different bedding angles. Step 3: Construct the shale brittleness index under uniaxial stress B un : Step 3: Construct the shale brittleness index under uniaxial stress B un : ; In the formula: x Q for Q Dot at x The x-coordinate on the axis x Qmax for xQ The maximum value, x Qmin for x Q The minimum value, f u The internal friction angle determined by uniaxial compressive strength and tensile strength; Step 4: Determine the degradation coefficient of shale brittleness due to confining pressure. B wc ; Step 5: Construct the shale brittleness index under triaxial stress conditions B tr : ; In the formula: C u The cohesion determined by uniaxial compressive strength and tensile strength; s 3 represents the minimum principal stress; C t The triaxial cohesion of shale; f t It is the internal friction angle; Step 6: Obtain the brittleness index of shale under triaxial stress conditions using the method from Step 5. B tr By combining well logging data, the distribution of rock brittleness in the entire block and well section is predicted, the compressibility of the reservoir is clarified, and the fracture morphology and fracturing effect are predicted in conjunction with geological modeling software.

[0006] Step 3 in the above scheme is specifically as follows: 1) Determine the cohesion and internal friction angle of shale based on the load-time curve of the tensile strength test and the stress-strain curve of the uniaxial compression test; 2) The MC criterion and its failure mode function under principal stress are expressed as follows: ; ; MC criteria in pq The representation in the coordinate system is as follows: ; In the formula: C u and f u The cohesion and angle of internal friction determined for uniaxial compressive and tensile strength. p =( s 1 + s 3) q =( s 1 -s 3) s 1 represents the maximum principal stress. s 3 is the minimum principal stress. , ; 3) Plot the two coordinate system representations of the MC criterion in the same coordinate system, and combine the cohesion-friction coupling model. The friction components are then expressed as: ; The cohesive component is represented as: ; 4) The molar intensity envelope equation in parabolic form, which truly reflects the confining pressure effect of shale, is: ; The shear strength curve tangent to the envelope is represented as: ; In the formula: τ is the shear strength, s This represents the normal stress acting on the shear plane; At this point, the cohesive force and the angle of internal friction in the parabolic state are respectively: ; At this point, the tensile strength and compressive strength are respectively: ; 5) Determine the brittleness index of shale under uniaxial conditions. B un : .

[0007] Step 4 in the above scheme is specifically as follows: 1) Obtain the triaxial compressive strength of shale based on the stress-strain curves obtained from the triaxial compression test of shale. s tc ; 2) Based on the trends in the variation of Mohr's circle, cohesion, and internal friction angle under triaxial stress, define... in tc To determine the ultimate triaxial compressive strength of shale, use s tc and in tc The ratio of these values ​​is used as the brittleness degradation coefficient of shale under confining pressure: .

[0008] Step 5 in the above scheme is specifically as follows: 1) Combining the shale brittleness index under uniaxial conditions and the shale brittleness degradation coefficient under confining pressure, the shale triaxial brittleness index of this invention is constructed as follows: Btr : ; 2) The MC criterion is modified to obtain the triaxial cohesion of shale. C t and internal friction angle f t The prediction formula is: ; In the formula: A and B are undetermined coefficients, which are determined by fitting the results of multiple sets of shale triaxial compression tests; 3) By fitting A and B together with the results of multiple sets of shale triaxial compression tests, the triaxial brittleness index of shale was finally obtained. B tr : ; In the formula: s tc To measure the triaxial compressive strength of the rock, C u The cohesion determined by uniaxial compressive strength and tensile strength. s 3 represents the minimum principal stress.

[0009] Beneficial effects: 1. The novel brittleness index established by this invention based on the model of cohesion-friction-layer coupling takes into account both inherent brittleness and apparent brittleness, and has sufficient scientific basis and physical connotation. 2. The dual evaluation method for shale brittleness index established in this invention can not only reflect the brittleness anisotropy of shale at different bedding angles under uniaxial stress, but also quantitatively evaluate the change of brittleness anisotropy with confining pressure. 3. The brittleness index established by this invention has stronger applicability in the field. The key parameters, cohesion, internal friction angle and compressive strength, can be inverted using well logging data without the need for extensive mechanical testing. 4. The shale brittleness index established in this invention is more sensitive to intrinsic lithological characteristics and external stress characteristics. At the same time, it has broad prospects for integration with geophysical data. Combined with geophysical data, it can be used to evaluate the entire block of shale reservoirs in the whole well section. It has important guiding significance for the identification of sweet spots, wellbore stability evaluation and fracturing effect evaluation in the shale oil and gas development process.

[0010] 5. The cohesion-friction force in the new brittleness index proposed in this invention closely matches the energy evolution characteristics during rock failure, providing a sound scientific basis and physical connotation. Furthermore, its coupling with bedding allows for a more comprehensive consideration of the energy evolution and fracture modes of the rock throughout the entire process from initiation to failure. It can accurately characterize the brittle anisotropy of shale and can be combined with geophysical data to conduct brittleness evaluation of the entire block, providing a scientific basis for shale reservoir fracturing design and fracturing effect evaluation. Attached Figure Description

[0011] Figure 1 A flowchart illustrating the establishment of a dual evaluation method for fracturing effects in shale reservoirs based on a cohesion-friction-stratification coupling model; Figure 2 Methods for preparing shale mechanical test specimens with different bedding angles.

[0012] Figure 3 The stress-strain curves of shale under different confining pressures when θ=30° are shown. Figure 4 The evolution of cohesive and internal friction components during rock failure and the evolution of elastic energy and dissipated energy in the energy model are shown in (a) for the evolution of cohesive and internal friction and (b) for the energy evolution. Figure 5 The evolution of cohesion and friction components under different fracture characteristics of rocks is shown in (a) for low brittleness rocks and (b) for high brittleness rocks. Figure 6 To illustrate the cohesion-friction characteristics of different brittle rocks and their correlation with rock fracture modes; Figure 7 The trends of Mohr's circle, cohesion, and friction angle under triaxial stress are shown. Figure 8 The brittleness and fracture modes of shale with different bedding angles under uniaxial stress conditions; Figure 9 The trend of shale brittleness with confining pressure under different bedding angles; Figure 10 The brittleness index established for this invention is compared with the anisotropic brittleness of shale under uniaxial conditions calculated by five existing brittleness indices. Figure 11 The brittleness index established in this invention is compared with five existing brittleness indices to evaluate brittleness under triaxial stress, where (a) is a bedding angle of 0° and (b) is a bedding angle of 90°. Detailed Implementation

[0013] The present invention will be further described below with reference to the accompanying drawings: Combination Figure 1-Figure 11As shown, this dual evaluation method for shale reservoir fracturing effect based on a cohesion-friction-bedding coupling model includes the following steps: Step 1: Prepare shale samples with different bedding angles. Methods for preparing shale samples with different bedding angles, such as Figure 2 The specific dimensions of the shale samples were F25mm*D50mm and F50mm*D25mm, and the samples were finely processed and polished according to the methods recommended by ISRM to avoid errors.

[0014] Step 2: For the shale samples prepared in Step 1, Brazilian splitting test, uniaxial compression and triaxial compression test were carried out to obtain time-load curves and stress-strain curves of shale with different bedding angles.

[0015] Uniaxial and triaxial compression tests were conducted on F25mm*D50mm specimens using a rock mechanics testing system, and Brazilian splitting tests were performed on F50mm*D25mm specimens. The loading method was axial displacement control. The specimens were initially preloaded at a loading rate of 10mm / min, and the loading rate was reset to 0.06mm / min when the contact force reached 1kN. Three tests were performed on shale samples at each bedding angle to avoid experimental errors.

[0016] Step 3: Construct the shale brittleness index under uniaxial stress B un .

[0017] Based on experimental data, mechanical parameters such as tensile strength and uniaxial compressive strength of shale at different bedding angles were calculated. Furthermore, the cohesion and internal friction angle of shale at different bedding angles were calculated. The anisotropy of the mechanical properties of bedding shale was clarified, and a shale brittleness index under uniaxial stress was constructed. B un .

[0018] 1) The failure function of the MC criterion (formula (1)) under principal stress can be expressed as formula (2): (1) (2) In the formula: C u and f u The cohesion and internal friction angle determined for uniaxial compressive strength and tensile strength.

[0019] 2) The MC criterion in pq The representation in the coordinate system is as follows: (3) In the formula: p=( s 1 +s 3) q =( s 1 -s 3) s 1 and s 3 represent the maximum and minimum principal stresses, respectively; , .

[0020] 3) Plot the two coordinate system representations of the MC criterion on the same coordinate system, where... By combining the cohesion-friction-layer coupling model with the MC criterion, the following relationship was found: At this point, the frictional force component can be expressed as: (4) cohesive components It can be represented as: (5) 4) To more realistically reflect the confining pressure effect of rocks, this invention derives the equation of the molar intensity envelope in parabolic function form as follows: (6) The shear strength curve tangent to the envelope can be represented as: (7) Therefore, the cohesive force and the angle of internal friction in the parabolic state are respectively: (8) At this point, the tensile strength and compressive strength are: (9) 5) Constructing the shale brittleness index under uniaxial stress according to the present invention. B un : Typically, the compression-to-tension ratio of rock materials is between 5 and 50, therefore x Q There are maximum and minimum values. Combining the variation law of ΔR with brittleness, the shale brittleness index under uniaxial stress in this invention... B un It can be represented as: (10).

[0021] Step 4: Determine the degradation coefficient of shale brittleness due to confining pressure. B wc .

[0022] Based on data obtained from triaxial compression experiments, the evolution of triaxial compressive strength, cohesion, and internal friction angle of layered shale under confining pressure was obtained, and the degradation coefficient of rock brittleness due to confining pressure was calculated. B wc : 1) Use s tc and in tc The ratio of these values ​​is used as the degradation factor of confining pressure on shale brittleness: (11).

[0023] Step 5: Construct the shale brittleness index under triaxial stress conditions B tr .

[0024] The shale brittleness index under uniaxial stress conditions constructed in step 3 was used to... B un The degradation coefficient of rock brittleness due to confining pressure calculated in step 4 B wc The triaxial brittleness index constructed in this invention is obtained by combining these methods. B tr : 1) The product of the uniaxial brittleness index and the degradation coefficient is the shale brittleness index under triaxial stress conditions constructed in this invention: (12) 2) By modifying the MC criterion, the prediction formulas for the triaxial cohesion and internal friction angle of shale are obtained as follows: (13) In the formula: A and B are undetermined coefficients, which are determined by fitting the results of multiple sets of shale triaxial compression tests; 3) The more widely applicable triaxial brittleness index for shale is obtained as: (14).

[0025] Step 6: Use the shale brittleness index Btr obtained in Step 5 under triaxial stress conditions to conduct a dual evaluation of the fracturing effect of shale reservoirs. The shale brittleness index Btr under triaxial stress conditions, combined with well logging data, is used to predict the distribution of rock brittleness across the entire block and well section, clarifying reservoir compressibility (rocks with higher brittleness have better compressibility). Furthermore, it can be linked with geological modeling software to predict fracture morphology and fracturing effect.

[0026] Figure 4Figure a shows the cohesive-friction coupling model. This model believes that the process of rock from the beginning to the failure process is a process of weakening cohesive characteristics and strengthening frictional characteristics. The process of rock from the beginning of deformation to the appearance of damage and finally the failure of the sample is essentially the result of the interaction between cohesive force and frictional force. Figure 4 b shows that the energy evolution of the entire rock failure process is divided into elastic strain energy and dissipated energy; Figure 4 Comparing the cohesion-friction coupling model with the strain evolution model reveals that the mobilization of cohesion and friction components closely matches the evolution patterns of elastic energy and dissipated energy. Therefore, it can be concluded that the mobilization of cohesion affects the accumulation of elastic energy, while the mobilization of friction affects energy dissipation; the more brittle the rock, the smaller the mobilization of the friction component during its deformation and failure process, and the smaller the energy dissipation.

[0027] like Figure 5 The combined Mohr-Coulomb criterion and cohesion-friction coupling model shown are used to investigate the influence of cohesion and internal friction angle modulation on fracture modes in different brittle rock materials. The MC criterion and its failure function under principal stress are expressed as follows: (1) (2) In the formula: C u and f u The cohesion and internal friction angle are determined by the uniaxial compressive strength and tensile strength, respectively.

[0028] MC criteria in pq Represented in coordinate system as: (3) In the formula: p =( s 1 +s 3) q =( s 1 -s 3) s 1 and s 3 represent the maximum and minimum principal stresses, respectively; , .

[0029] like Figure 5 As shown, formulas (1) and (3) are plotted in the same coordinate system. OB Represents the uniaxial compressive strength of shale s uc ,exist pq In the coordinate system, KS=HB= s ucTherefore, HB corresponds to the peak intensity in the stress-strain curve of the cohesion-friction-layer coupling model, i.e., KS=HB= s uc ;exist s - In the τ coordinate system, ET represents the shear strength corresponding to shale failure. q f The combined model of cohesion-friction-layering coupling and the MC criterion show that MS=ET= q f .therefore q f The shear strength and frictional force components during shale failure can be expressed as: (4) The cohesive component can be expressed as: (5) like Figure 6 As shown, the circumcircle of the Mohr stress circle corresponding to the uniaxial tensile and compressive strengths of shale is used as the auxiliary Mohr circle. Q The point is the center of the auxiliary Mohr's circle. Ideally, the compressive and tensile strengths of plastic materials are almost the same. Figure 6 a), at this time Q and O They almost overlap. As the internal friction angle increases, the difference in compressive and tensile strength between the materials increases, which makes... Q Move to the right. And when f u When the angle approaches 90°, this corresponds to the Mohr circle characteristic of an ideal brittle material. Q Point and G The values ​​are almost identical. Based on this, it can be determined that ΔR is a key parameter affecting the brittle evolution of rocks. Therefore, in order to more realistically reflect the confining pressure effect of shale, this invention derives the equation of the molar strength envelope in parabolic function form as follows: (6) The shear strength curve tangent to the envelope is represented as: (7) Therefore, the cohesive force and the angle of internal friction in the parabolic state are expressed as follows: (8) Therefore, the tensile strength and compressive strength are: (9) The compressive-tensile strength ratio of rock materials is between 5 and 50; therefore x Q There are maximum and minimum values. Based on the variation of ΔR with brittleness, this invention constructs a brittleness index under no confining pressure conditions. Bun It can be represented as: (10) like Figure 7 As shown in the figure. The thick solid line represents the ultimate shear strength curve of an ideal brittle material, and the long dashed curve represents the molar shear strength curve. This invention defines... s tc and in tc These represent the measured triaxial compressive strength and the ultimate triaxial compressive strength of the rock, respectively. Comparison revealed that the effect of confining pressure on brittleness is essentially to shift the Mohr circle towards the zero point. Therefore, this invention defines a degradation coefficient of confining pressure on shale brittleness. B wc for: (12) The MC criterion was modified based on previous experimental data to obtain triaxial cohesion. C t and triaxial internal friction angle f t The prediction formula is: (13) In the formula: A and B These are coefficients to be determined.

[0030] Based on the results of multiple sets of triaxial compression experiments, A and B By fitting the data, a more widely applicable triaxial brittleness index for shale can be obtained. B tr : 14). Example

[0031] The dual evaluation method for fracturing performance of shale reservoirs based on a cohesion-friction-layer coupling model is as follows: Step 1: Select shale cores with well-developed bedding and prepare uniaxial and triaxial compression specimens with dimensions F25mm*D50mm, and Brazilian splitting test specimens with F50mm*D25mm (0°, 15°, 30°, 45°, 60°, 75°, 90°). Figure 2 ; Step 2: Mechanical tests were conducted on shale at different bedding angles using a rock mechanics testing system. The loading method was axial displacement control. Initially, the specimen was pre-loaded at a loading rate of 10 mm / min. When the loading platform just contacted the specimen and the contact force was 1 kN, the loading rate was changed to 0.06 mm / min. To ensure the accuracy of the experimental data, three tests were conducted on shale at each bedding angle. The mechanical test results are as follows: Figure 3 The stress-strain curves of shale under different confining pressures when θ=30° are shown. Step 3: Construct the shale brittleness index under uniaxial stress conditions by combining the results of the uniaxial compression test and the Brazilian splitting test in Step 2. B un ; Step 4: Based on the triaxial compression test results in Step 2, derive the degradation coefficient of confining pressure on shale brittleness. B wc Combined with the shale brittleness index under uniaxial conditions constructed in step 3 B un The shale brittleness index under triaxial stress conditions constructed in this invention is obtained; the parameters are fitted using triaxial compression test results to obtain a more widely applicable shale brittleness index under triaxial stress conditions. B tr ; Table 1. Triaxial compression test results of shale with different bedding angles and brittleness evaluation results of this invention.

[0032] The above examples demonstrate that: Brittleness index and fracture mode of shale samples with different bedding angles under uniaxial stress, as follows: Figure 8 As shown, the shale brittleness index increases from 0° to 60°. B un As the brittleness index continues to decrease, the corresponding failure mode gradually changes from tensile-shear composite fracture to single-sided shear fracture, and the number of macroscopic cracks gradually decreases; when the bedding angle further increases to 90°, the shale brittleness index... B un As the fracture intensity increases, the corresponding shale fracture mode gradually changes from single-sided shear failure to multi-line longitudinal splitting failure.

[0033] like Figure 9 As shown, the brittleness of shale at different bedding angles gradually decreases with increasing confining pressure. Comparing the brittleness of shale under different confining pressures, it was found that the anisotropy of shale under uniaxial stress conditions is the greatest, indicating that the bedding direction has the greatest impact on the brittleness of shale under no confining pressure conditions. The brittleness index constructed in this invention and the brittleness of shale at different bedding angles under uniaxial conditions calculated by five existing brittleness indices are plotted in the same coordinate system (e.g., ...). Figure 10)Discover The bedding angle hardly changes with increasing stratification angle, and therefore cannot reflect the anisotropy of shale brittleness. The evaluation results show little variation with bedding angle, making it difficult to reflect the change in shale brittleness with bedding angle. and The evaluation results show poor agreement with the shale fracture model, especially when the bedding angle is between 0° and 60°. Constructed with the present invention B un The evaluation results were similar, but when the bedding angle exceeded 60°... and B un The evaluation results are the opposite. Based on the above evaluation results, it was found that none of the existing brittleness evaluation indices can accurately characterize the anisotropy of shale brittleness. Similarly, the trends of shale brittleness with confining pressure calculated from different bedding angles using different brittleness indices were plotted in the same coordinate system ( Figure 11 Taking bedding angles of 0° and 90° as examples, it was found that the existing five brittleness indices are not ideal for evaluating bedding shale under confining pressure, and some brittleness index evaluation results even contradict objective laws.

[0034] Step 5: Utilize the calculated shale brittleness index under uniaxial stress conditions B un Shale brittleness index under triaxial stress conditions B tr The compressibility evaluation of shale reservoirs under uniaxial conditions shows that the compressibility of shale reservoirs with varying bedding angles follows the same trend as reservoir brittleness. As the bedding angle increases from 0° to 60°, reservoir compressibility gradually decreases, with a decrease in the number and complexity of fractures. When the bedding angle further increases to 90°, reservoir compressibility gradually increases, with an increase in the number and complexity of fractures. Under triaxial stress conditions, reservoir compressibility gradually decreases with increasing confining pressure. Rock compressibility is similar across different bedding angles, with the best compressibility observed at a bedding angle of 90°, which also produces the most complex fractures.

[0035] The embodiments described above are merely illustrative of specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A dual evaluation method for fracturing effect in shale reservoirs based on a cohesion-friction-stratification coupling model, characterized in that... Includes the following steps: Step 1: Prepare shale samples with different bedding angles; Step 2: Using the shale samples prepared in Step 1, Brazilian splitting test, uniaxial compression and triaxial compression test were carried out to obtain time-load curves and stress-strain curves of shale with different bedding angles. Step 3: Construct the shale brittleness index under uniaxial stress B un : ； In the formula: x Q for Q Dot at x The x-coordinate on the axis x Qmax for x Q The maximum value, x Qmin for x Q The minimum value, φ u The internal friction angle determined by uniaxial compressive strength and tensile strength; Step 4: Determine the degradation coefficient of shale brittleness due to confining pressure. B wc ; Step 5: Construct the shale brittleness index under triaxial stress conditions B tr : ； In the formula: C u The cohesion determined by uniaxial compressive strength and tensile strength; σ 3 represents the minimum principal stress; C t The triaxial cohesion of shale; φ t It is the internal friction angle; Step 6: Obtain the brittleness index of shale under triaxial stress conditions using the method from Step 5. B tr By combining well logging data, the distribution of rock brittleness in the entire block and well section is predicted, the compressibility of the reservoir is clarified, and the fracture morphology and fracturing effect are predicted in conjunction with geological modeling software.

2. The dual evaluation method for shale reservoir fracturing effect based on the cohesion-friction-stratification coupling model according to claim 1, characterized in that: Step 3 specifically involves: 1) Determine the cohesion and internal friction angle of shale based on the load-time curve of the tensile strength test and the stress-strain curve of the uniaxial compression test; 2) The MC criterion and its failure mode function under principal stress are expressed as follows: ； ； MC criteria in pq The representation in the coordinate system is as follows: ； In the formula: C u and φ u The cohesion and angle of internal friction determined for uniaxial compressive and tensile strength. p =( σ 1 +σ 3) q =( σ 1 -σ 3) σ 1 represents the maximum principal stress. σ 3 is the minimum principal stress. , ; 3) Plot the two coordinate system representations of the MC criterion in the same coordinate system, and combine the cohesion-friction coupling model. The friction components are then expressed as: ； The cohesive component is represented as: ； 4) The molar intensity envelope equation in parabolic form, which truly reflects the confining pressure effect of shale, is: ； The shear strength curve tangent to the envelope is represented as: ； In the formula: τ is the shear strength, σ This represents the normal stress acting on the shear plane; At this point, the cohesive force and the angle of internal friction in the parabolic state are respectively: ； At this point, the tensile strength and compressive strength are respectively: ； 5) Determine the brittleness index of shale under uniaxial conditions. B un : 。 3. The dual evaluation method for shale reservoir fracturing effect based on the cohesion-friction-stratification coupling model according to claim 2, characterized in that: Step 4 specifically involves: 1) Obtain the triaxial compressive strength of shale based on the stress-strain curves obtained from the triaxial compression test of shale. σ tc ; 2) Based on the trends in the variation of Mohr's circle, cohesion, and internal friction angle under triaxial stress, define... σ′ tc To determine the ultimate triaxial compressive strength of shale, use σ tc and σ′ tc The ratio of these values ​​is used as the brittleness degradation coefficient of shale under confining pressure: 。 4. The dual evaluation method for fracturing effect of shale reservoirs based on the cohesion-friction-stratification coupling model as described in claim 3, characterized in that: Step 5 specifically involves: 1) Combining the shale brittleness index under uniaxial conditions and the shale brittleness degradation coefficient under confining pressure, the shale triaxial brittleness index of this invention is constructed as follows: B tr : ； 2) The MC criterion is modified to obtain the triaxial cohesion of shale. C t and internal friction angle φ t The prediction formula is: ； In the formula: A and B are undetermined coefficients, which are determined by fitting the results of multiple sets of shale triaxial compression tests; 3) By fitting A and B together with the results of multiple sets of shale triaxial compression tests, the triaxial brittleness index of shale was finally obtained. B tr : ； In the formula: σ tc To measure the triaxial compressive strength of the rock, C u The cohesion determined by uniaxial compressive strength and tensile strength. σ 3 represents the minimum principal stress.

Images