Method for analyzing influence of carbon dioxide complex fracturing fluid on different shale fractures

Abstract

The application discloses a method for analyzing influence of carbon dioxide composite fracturing fluid on different shale breakage, and belongs to the technical field of shale oil and gas resource development, and comprises the following steps: selecting fracturing fluid according to shale types, determining corresponding fracturing fluid injection process, and then performing fracturing experiment; calculating breakage fractal dimension and breakage energy of shale samples, then calculating breakage energy efficiency, and analyzing shale breakage according to the breakage energy efficiency. The application can improve the defects of the prior art, optimizes injection combination and process parameters of CO2 and water-based fracturing fluid, realizes improvement of adaptability of different shale types, and establishes a breakage energy analysis method for composite fracturing fluid design and energy efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of shale oil and gas resource development, and particularly relates to a method for analyzing the influence of a carbon dioxide composite fracturing fluid on different shale fractures. BACKGROUND

[0002] Shale oil and gas resource development is an important direction of modern energy structure transformation. Shale oil and gas resources usually need to form a fracture network through fracturing technology to improve the flow conductivity due to its low permeability and complex reservoir structure. Carbon dioxide (CO2) fracturing technology has shown significant advantages in the fields of shale gas, coalbed methane and tight oil and gas. Carbon dioxide fracturing can significantly reduce the fracture pressure and increase the complexity of the fracture, and can increase the reservoir pressure, reduce the fluid viscosity, extract hydrocarbon substances, improve the oil-water interface characteristics and change the physical and chemical properties of the reservoir after being injected into the reservoir.

[0003] Shale with developed foliation and shale with laminated structure are two most common types of shale. Shale with developed foliation has the characteristics of uniform and continuous bedding, low mechanical strength, and easy to peel along the bedding; shale with laminated structure has the characteristics of complex bedding, alternating deposition of minerals and organic matter, high compressive strength and poor fracture propagation.

[0004] Although the use of carbon dioxide alone can significantly optimize the fracture complexity and improve the energy efficiency, its effect is limited in shale with high clay content or complex bedding. Water-based fracturing fluids (such as guar gum and slickwater) can form a relatively complex fracture network, but their high viscosity and high energy consumption result in a significant reduction in energy efficiency, making it difficult to balance fracture complexity and economy. Shale with developed foliation and shale with laminated structure have significant differences in response to fracturing fluids due to differences in bedding structure and mineral composition. However, existing fracturing technologies are not optimized for shale types, and the contradiction between fracture complexity improvement and energy consumption of shale with laminated structure has not been effectively solved. The differences in viscosity, interfacial tension and influence on mineral structure of different fracturing fluids lack systematic research, resulting in increased uncertainty of design parameters and operation process. Current experiments are mostly conducted under ideal conditions, which fail to completely simulate the actual complex environment of the reservoir, such as high temperature and high pressure conditions, multiphase fluid action and mineral reaction. This difference may lead to inconsistency between experimental results and actual application effects, affecting the promotion and optimization of the technology. Water-based fracturing fluids have problems in long-term maintenance of fracture conductivity. For example, guar gum may cause fracture plugging due to residue deposition, and slickwater may not be able to keep the fracture open for a long time. These shortcomings have an adverse effect on the post-fracturing effect, but there are few solutions to these problems in existing research. Single or composite fracturing fluids are difficult to significantly improve the energy efficiency in low permeability reservoirs, especially in fracture modification that requires long-term action, and the energy utilization rate of existing technologies is still low. SUMMARY

[0005] The technical problem solved by the present application is to provide an analysis method for the influence of carbon dioxide composite fracturing fluid on the fracture of different shales, which can solve the shortcomings of the prior art, optimize the injection combination and process parameters of CO2 and water-based fracturing fluid, improve the adaptability of different shale types, and establish a composite fracturing fluid design and energy efficiency fracture energy analysis method.

[0006] To solve the above technical problems, the technical solutions adopted by the present application are as follows.

[0007] An analysis method for the influence of carbon dioxide composite fracturing fluid on the fracture of different shales, comprising the following steps:

[0008] According to the type of shale, select the fracturing fluid and determine the corresponding fracturing fluid injection process, then perform fracturing experiment; calculate the fracture fractal dimension and fracture energy of the shale sample, then calculate the fracture energy efficiency, and analyze the shale fracture according to the fracture energy efficiency.

[0009] As a preferred, if the shale sample is a shale with developed foliation, CO2 fracturing fluid is selected, and the purity of CO2 is ≥ 99.5%.

[0010] As a preferred, the fracturing experiment process is to vacuum treat the shale sample in a saturated oil state to simulate the initial conditions of the reservoir, inject CO2 fracturing fluid in stages to a target pressure of 40-70 MPa, maintain for more than 60 minutes, release the pressure and analyze the crack morphology.

[0011] As a preferred, if the shale sample is a shale with developed foliation, CO2 fracturing fluid and slick water are selected, the purity of CO2 in the CO2 fracturing fluid is ≥ 99.5%, and the concentration of the slick water is 0.2%-0.5%.

[0012] As a preferred, the fracturing experiment process is to vacuum treat the shale sample in a saturated oil state to simulate the initial conditions of the reservoir, inject CO2 fracturing fluid to a target pressure of 40-70 MPa, maintain for 30 minutes, inject slick water in stages while continuously injecting CO2 fracturing fluid, maintain a stable flow rate of 2-5 m³ / min for 60 minutes, then stop injection, and perform microscopic structure analysis of the crack network.

[0013] As a preferred, the method for calculating the fracture fractal dimension of the shale sample is,

[0014] First, obtain the crack image after shale fracturing through scanning or imaging technology; process the image to convert it into a binary image; sequentially cover a number of square grids of different sizes on the crack image, and count the number of grids containing cracks under each grid size, , is the grid size The number of grids covering the fracture area, D is the fractal dimension of the fracture, by logarithmic transformation, using linear regression fitting With The relationship between the maximum load and the fracture energy, and the relationship between the maximum load and the fracture energy .

[0015] As preferred, the fracture energy includes the elastic deformation energy stored by the shale before fracture and the fracture propagation energy for forming a new fracture surface and driving the fracture propagation, and the fracture energy Wherein, F is the external load (unit: N), d is the loading displacement (unit: m), Is the maximum loading displacement at the time of fracture.

[0016] As preferred, the fracture energy efficiency is defined as The fractal dimension generated per unit fracture energy, .

[0017] The beneficial effects brought by the above technical scheme are that: according to the difference of the bedding structure and mineral distribution of the shale with developed foliation and the shale with laminated structure, different fracturing fluid ratio and process parameters are customized and designed, and the balance between the fracture energy and the fracture complexity is optimized, so that the high fracture complexity is realized while the energy consumption is effectively reduced. The fracturing effect analysis method taking fractal dimension and fracture energy as the core is first proposed, which can scientifically and quantitatively analyze the fracturing fluid performance and guide the design optimization, and fills the blank of lack of unified analysis index in the prior art. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 Is the photo of the GCTS rock mechanics test system.

[0019] Figure 2 Is the photo of the shale sample with developed foliation.

[0020] Figure 3 Is the photo of the shale sample with laminated structure.

[0021] Figure 4 Is the tensile strength test curve of the shale sample with developed foliation.

[0022] Figure 5 Is the tensile strength test curve of the shale sample with laminated structure.

[0023] Figure 6 Is the comparison result of the elastic modulus data of the shale with developed foliation and the shale with laminated structure.

[0024] Figure 7 Is the picture of the sample after fracturing in the fracture fractal dimension calculation process, wherein Figure 7 The red box part of (a) is the prepared enlarged area, Figure 7 (b) is Figure 7(a) is a partial enlarged view of the red box part, Figure 7 (c) is Figure 7 (b) is a partial enlarged view of the red box part.

[0025] Figure 8 are the analysis results of different grid size divisions in the calculation process of the fractal dimension of rupture, wherein Figure 8 (a) is the analysis result of 25 grid divisions, Figure 8 (b) is the analysis result of 100 grid divisions, Figure 8 (c) is the analysis result of 225 grid divisions.

[0026] Figure 9 is a comparison chart of the rupture energy and fractal dimension of the shale with developed foliation.

[0027] Figure 10 is a comparison chart of the rupture energy and fractal dimension of the laminated shale. DETAILED DESCRIPTION

[0028] When the abundant clay minerals in shale come into contact with water, they often exhibit significant softening and swelling phenomena, which lead to changes in their mechanical properties. The injection of carbon dioxide (CO2) further increases the complexity and uncertainty of these mechanical property changes. In the study of fracturing technology, the mechanical properties of rock are one of the key parameters that determine the effectiveness of fracturing modification. However, most current CO2 fracturing design schemes typically assume that the mechanical parameters of the reservoir rock remain constant throughout the fracturing process. This simplifying assumption ignores the hydration reactions that occur in shale when it comes into contact with water and the potential impact of CO2 on its mechanical properties. Therefore, this treatment method is difficult to truly reflect the dynamic changes of reservoir rock in actual fracturing engineering, which may lead to insufficient accuracy of the design scheme and introduce additional errors. By improving the study of these dynamic changes, especially the comprehensive analysis of hydration effects and CO2 interactions, the scientificity of fracturing design and engineering effectiveness will be improved.

[0029] The present application carries out laboratory experimental research on Brazilian splitting experiments and uniaxial compression experiments. The experimental devices and instruments mainly include a GCTS rock mechanics test system (see Figure 1 ), a CO2 cooling device, a core holder, etc.

[0030] The shale used in the test is shale with developed foliation (see Figure 2 ) and laminated shale (see Figure 3). The bedding development type shale has typical horizontal or sub-horizontal tabular bedding, which is characterized by thin layer distribution and easy peeling along the bedding plane. The bedding is uniform and continuous. The laminated type shale shows a combination of various laminations, and the texture is relatively complex, which may contain different particle sizes of substances or alternating deposition of organic matter and minerals. To ensure that the initial state of the rock remains consistent, a control group in the saturated oil state is set for each experimental treatment condition. The control group and the fluid treatment group rocks are from the same core segment.

[0031] To restore the changes of the rock in the reservoir, the rock is first saturated with oil before the experiment to simulate the original state of the reservoir. Then, the rock is soaked in different combinations of fracturing fluid, i.e., CO2, CO2+ slick water / gu gel, slick water, and gu gel. In the experiment, the shale sample is dried at 110°C to a constant weight, vacuumed, and injected with simulated oil prepared from Gulong shale oil + kerosene at 110°C and 37 MPa pressure. Then, a uniaxial compression experiment is performed to determine the elastic modulus and Poisson's ratio of the original rock. Subsequently, CO2 (-20°C, 60 MPa) is injected into another core holder, and slick water is continuously injected. After 3 hours, a uniaxial compression experiment is performed again to measure the mechanical parameters after fluid action. A core with a diameter of 50 mm and a thickness of 25 mm is also made to perform a Brazilian split experiment. By repeating the above steps, the mechanical properties of different types of shale before and after the action of different external fluids are tested. Finally, the effects of CO2, CO2+ slick water / gu gel, slick water, gu gel, and shale type on the tensile strength, elastic modulus, and Poisson's ratio of the rock are analyzed.

[0032] Table 1 Tensile strength experiment scheme

[0033]

[0034] Table 2 Tensile strength experiment data

[0035]

[0036] According to the above table and Figure 4-5According to the data, the tensile strength of the foliation-developed shale is 6.536 MPa, and the tensile strength of the laminated shale is significantly higher than that of the former, reaching 12.0866 MPa. After the foliation-developed shale is treated with slick water and guanidine gum, the tensile strength of the shale decreases significantly, with a decrease of 45.63% to 43.67%, and the effects of the two treatment methods are similar. In contrast, the tensile strength of the rock treated with CO2 also decreases, but the decrease is small, only 12.42%. When the combination of CO2 and slick water or guanidine gum is used for treatment, the decrease in tensile strength of the sample is smaller than that when slick water or guanidine gum is used alone, with a decrease of 16.94% to 21.15%. After the laminated shale is treated with slick water and guanidine gum, the tensile strength of the shale also decreases significantly, with a decrease of 42.21% to 44.22%, and the tensile strength of the rock treated with CO2 also decreases, with a decrease of 29.38%, which is significantly higher than that of the foliation-developed shale. When the combination of CO2 and slick water or guanidine gum is used for treatment, the decrease in tensile strength is smaller than that when slick water or guanidine gum is used alone, with a decrease of 31.00% to 32.51%. This shows that CO2 can slow down the effect of slick water or guanidine gum on the tensile strength of the rock, but the effect is small in numerical value.

[0037] The compressive strength test scheme is shown in the following table. The shale compressive strength test and the fracturing sample are a cylinder with a diameter of 48-54 mm, the height to diameter ratio is 2.0-2.5, the flatness of the bottom should be within 0.02 mm, and the maximum deviation of the axis from the vertical direction should not exceed 1 / 1000 of the diameter. When the sample is made, the size is 50 mm x Φ25 mm, and after preparation, the sample is fixed on the test equipment. Install the uniaxial compression test fixture, and apply a thin layer of lubricant to the upper and lower surfaces of the sample, and place it in the center of the base. Place a rigid spacer between the upper end of the sample and the pressure plate, and install the axial and radial displacement extensometer, and adjust to the initial reading. Adjust the pressure head so that the spacer and the pressure plate are in uniform contact, and ensure that the sample is uniformly stressed. The loading rate is set to 0.1-0.5 MPa per second, and the lower rate is taken for soft rock, until the sample is destroyed.

[0038] Table 3 Compressive strength test scheme

[0039]

[0040] By using different fracturing fluids, i.e. CO2, CO2+ slick water / guanidine gum, slick water, and guanidine gum, on the sample, the following table and Figure 6The initial elastic modulus of the foliation-developed shale is about 21.61 GPa, while that of the laminated shale is 22.77 GPa. Taking the foliation-developed shale as an example, slickwater and guar gum have the most significant impact on the elastic modulus of the rock during the treatment process, resulting in a decrease of about 48.82% and 27.76%, respectively. In contrast, CO2 has a smaller impact on the elastic modulus. When using a combination of CO2 and slickwater or guar gum for treatment, the elastic modulus of the sample decreases by less than when using slickwater or guar gum alone, with a decrease of 13.56% to 24.20%. This indicates that CO2 can slow down the decrease in the elastic modulus of the rock caused by slickwater or guar gum.

[0041] The initial Poisson's ratio of the foliation-developed shale is 0.187, while that of the laminated shale is 0.184. Taking the foliation-developed shale as an example, the impact of external fluids on the Poisson's ratio is relatively small, with slickwater and guar gum causing a slight increase in the Poisson's ratio of the rock, with an increase of 12.83% and 12.85%, respectively. When using a combination of CO2 and slickwater or guar gum for treatment, the Poisson's ratio of the sample increases by less than when using slickwater or guar gum alone, with an increase of 10.70% to 3.21%. This indicates that CO2 can slow down the increase in the Poisson's ratio of the rock caused by slickwater or guar gum.

[0042] Table 4 Experimental data of Poisson's ratio and elastic modulus

[0043]

[0044] Fractal dimension is used to measure the self-similarity and complexity of an object, and is often used to describe the geometric characteristics and complexity of the morphology of cracks. By using fractal dimension, the effects of different types of fracturing fluids on shale fracturing can be further analyzed. In this study, the fractal dimension of the shale sample was calculated using the box counting method. The box counting method is a coverage-based fractal dimension estimation method widely used in fractal analysis of crack morphology. The basic principle is to cover the crack image with a series of square grids of different sizes, and count the number of grids containing cracks under each grid size. According to the relationship between grid size and grid number, the fractal dimension is fitted using the logarithmic rule.

[0045] First, the crack image after shale fracturing is obtained through scanning or imaging technology. In order to ensure the accuracy of the calculation, it is necessary to ensure that the image is clear and can display the details of the crack. Then, the image is processed to convert it into a binary image, which is convenient for subsequent crack analysis. See Figure 7-8 .

[0046] The binary image is covered with grids of different scales. The grid size is gradually reduced, and the number of grids containing cracks is recorded each time the grid is covered , according to the formula of box counting method , is the number of grids covering the fracture area at the grid size , D is the fractal dimension of the fracture, by logarithmic transformation, using linear regression to fit the relationship between and , we get . The following table is the fractal dimension of different shales under the action of different kinds of fracturing fluid.

[0047] Table 5 Fracture fractal dimension of shale samples

[0048]

[0049] In the shale with developed foliation, using CO2 as fracturing fluid can produce relatively regular cracks, mainly along the bedding, and produce obvious shear failure, and the crack structure is relatively complex, so the fractal dimension is relatively high, and the fracturing effect is good. Guan gum liquid can effectively increase the complexity of the crack due to its high viscosity, and the crack not only expands along the bedding, but also may expand into the shale, resulting in more complex crack morphology, so the fractal dimension is high. The viscosity of slick water is low, although the permeability is strong, but the crack propagation mainly follows a relatively regular path, so the fractal dimension is relatively high. The combination of CO2 and guan gum can enhance the expansion and complexity of the crack at the same time, so that the crack not only expands along the bedding, but also alternately propagates between different layers, forming a more complex crack structure, and the fractal dimension reaches 2.5-2.6. The combination of CO2 and slick water enhances the permeability of the crack, but due to the low viscosity of slick water, the complexity of the crack is relatively low, and the fractal dimension is 2.3-2.4. After using water-based fracturing fluid combined with CO2 treatment, the fractal dimension is increased by 4%-8.7% compared with using water-based fracturing fluid alone. Overall, after treatment with different fracturing fluids, the fractal dimension of shale with developed foliation is 3.8%-8% higher than that of shale with laminated structure.

[0050] The external energy absorbed and released by the shale sample during the fracturing process is the key driving factor for the formation and expansion of the crack. The size and distribution of the breakage external energy not only directly affect the formation path and complexity of the crack, but also determine the efficiency and effect of reservoir reconstruction. Based on the experimental results under the action of different fracturing fluids, the breakage external energy of shale is calculated, and the influence of breakage energy on crack complexity is discussed combined with fractal dimension, and then the energy efficiency and applicability of different fracturing fluids are analyzed. The breakage energy mainly includes two parts, the elastic deformation energy stored by the shale before breakage and the crack expansion energy used to form new crack surface and promote crack expansion.

[0051] Through the loading and displacement data in the test, the total breakage energy W can be calculated:

[0052] ,

[0053] where F is the external load (unit: N), d is the loading displacement (unit: m), is the maximum loading displacement at breakage.

[0054] According to the calculation results, the breakage energies of the shale samples with developed bedding and laminations under the action of different fracturing fluids are shown in the following table. The breakage energy after treatment with guar gum is the highest, indicating that the high viscosity of guar gum requires more energy to promote crack propagation. The breakage energy after treatment with CO2 is the lowest, indicating that CO2 effectively reduces the energy required for crack formation by reducing the breakage pressure and increasing the pore permeability. The combined effect of CO2 and water-based fracturing fluid makes the breakage energy between single CO2 and water-based fracturing fluid, showing good energy efficiency, see the following table.

[0055] Table 6 Breakage energy of shale samples

[0056]

[0057] Comparative analysis of breakage energy and fractal dimension, fractal dimension and breakage energy under the action of different fracturing fluids are shown in Figure 9 and Figure 10 and Table 7. Analysis of the data shows that the fractal dimension and breakage energy are positively correlated, that is, the higher the fractal dimension, the greater the breakage energy, indicating that more energy is required for crack complexity for branching and expansion. At the same time, although the breakage energy of CO2 treatment is low, its fractal dimension reaches 2.5, indicating that it has a significant advantage in crack complexity. In addition, the synergistic effect of the composite fracturing fluid makes the fractal dimension and breakage energy reach a relatively balanced effect. The combined effect of CO2 and slickwater reaches a balance between fractal dimension (2.6) and breakage energy (19.4 J), which not only ensures the complexity of the crack, but also better controls the energy consumption.

[0058] Table 7 Analysis of fractal dimension and breakage energy of shale samples

[0059]

[0060] In order to compare the performance of the fracturing fluid, define the breakage energy efficiency as the fractal dimension generated per unit breakage energy, where, is the fractal dimension of shale breakage, W is the breakage energy (unit: J). The breakage energy efficiency is shown in the following table.

[0061] Table 8 Breakage energy efficiency of shale samples

[0062]

[0063] From Table 8, it can be seen that the energy efficiency of CO2 alone is the highest, reaching 0.203 and 0.152 for two kinds of shale respectively, indicating that it achieves high fracture complexity with the lowest energy consumption. The energy efficiency of guanidine gum alone is the lowest, indicating that although its high viscosity increases fracture complexity, it significantly increases energy consumption. The energy efficiency of the combination of CO2 and guanidine gum is between the two, showing good comprehensive performance. CO2 fracturing fluid shows the lowest breakdown energy and higher fractal dimension, the highest energy efficiency, and is suitable for complex fracture reconstruction of low permeability reservoirs. Although guanidine gum fracturing fluid has higher fracture complexity, its energy efficiency is lower, and its application needs to balance energy consumption and effect. The combination of CO2 and water-based fracturing fluid achieves a good balance between breakdown energy and fractal dimension, and its energy efficiency is increased by 15.32%-16.52% compared with water-based fracturing fluid alone.

[0064] The present patent customizes different fracturing fluid ratios and process parameters according to the differences in bedding structure and mineral distribution of the foliation developed shale and laminated shale. The foliation developed shale is suitable for pure CO2 fracturing fluid, and the laminated shale is suitable for the combination of CO2 and slickwater fracturing fluid. Through targeted optimization, the fracture complexity and adaptability can be significantly improved. By using CO2 and water-based fracturing fluid in combination, the balance between breakdown energy and fracture complexity is optimized. According to the test data, the energy efficiency of the combination of CO2 and slickwater is increased by 15.32%-16.52% compared with the use of water-based fracturing fluid alone, which effectively reduces energy consumption while achieving high fracture complexity. The fractal dimension of the composite fracturing fluid reaches 2.5-2.6, which is significantly higher than that of slickwater or guanidine gum alone (2.2-2.4). After the combination of CO2 and slickwater, the fractal dimension of the laminated shale is increased by 4%-8.7%, forming a more complex fracture network, which is beneficial to reservoir reconstruction and long-term maintenance of conductivity. The combination of the depressurization effect of CO2 and the fracture propagation ability of water-based fracturing fluid can overcome the limitations of single fracturing fluid in different reservoirs. For foliation developed shale, pure CO2 fracturing fluid avoids the problem of bedding surface peeling caused by water-based fluid. For laminated shale, the combination of fluids achieves higher fracture extendibility and complexity. According to the experimental data, for foliation developed shale, the energy efficiency of pure CO2 is as high as 0.203, the fracture complexity (fractal dimension) reaches 2.5, and the energy consumption is the lowest. For laminated shale, the energy efficiency of the combination of CO2 and slickwater reaches 0.106, the fracture complexity (fractal dimension) is 2.5, which is significantly higher than that of single slickwater treatment (fractal dimension 2.3).

[0065] The present invention proposes a process method of injecting CO2 and water-based fracturing fluid in stages, optimizes the injection sequence, pressure range and flow rate parameters, and ensures the operability and efficiency of the process.

[0066] The fracturing effect analysis method taking fractal dimension and breaking energy as cores is first proposed, the fracturing fluid performance can be quantified scientifically and the design optimization is guided, and the blank of lacking unified analysis index in the prior art is filled.

[0067] Compared with the existing single fracturing fluid, the composite fracturing fluid of the present application realizes more efficient energy utilization and more complex fracture morphology, and is suitable for various reservoir environments. Compared with the existing empirical design method, the present application proposes an optimization scheme based on experimental data, and establishes an analysis model through fractal dimension and breaking energy, and the technical design is more scientific and accurate.

[0068] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore cannot be understood as a limitation of the present application.

[0069] The basic principles and main features of the present application and the advantages of the present application have been shown and described above. It should be understood by those skilled in the art that the present application is not limited by the above examples, the above examples and descriptions in the specification are only to illustrate the principles of the present application, various changes and improvements can be made to the present application without departing from the spirit and scope of the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for analyzing the effect of carbon dioxide composite fracturing fluid on different shale fractures, characterized in that The method comprises the following steps: According to the type of shale, select the fracturing fluid and determine the corresponding fracturing fluid injection process, then carry out fracturing experiment; calculate the fracturing fractal dimension and fracturing energy of shale sample, then calculate the fracturing energy efficiency, and analyze the shale fracturing according to the fracturing energy efficiency; The method for calculating the fracturing fractal dimension of the shale sample is as follows: firstly, the crack image after the shale fracturing is obtained through scanning or imaging technology; the image is processed to be converted into a binary image; a plurality of square grids with different sizes are sequentially overlaid on the crack image, and the number of grids containing cracks under each grid size is counted, , is the number of grids covering the crack area under the grid size , D is the fractal dimension of the crack, by logarithmic transformation, the relationship between and is fitted by linear regression to obtain ; The fracture energy includes the elastic deformation energy stored in the shale before the fracture and the fracture propagation energy for forming a new fracture surface and pushing the fracture to expand, and the fracture energy where F is the external load and d is the loading displacement, is the maximum loading displacement at the time of fracture; Defining the energy efficiency of breakage the fractal dimension generated for the unit breakage energy, .

2. The method for analyzing the influence of the carbon dioxide complex fracturing fluid on different shale ruptures according to claim 1, characterized in that: If the shale sample is a shale with developed foliation, CO2 fracturing fluid is selected, and the purity of CO2 is ≥ 99.5%.

3. The method of claim 2, wherein the carbon dioxide complex fracturing fluid is analyzed for its effect on different shale fractures. The fracturing experiment process is that the shale sample is vacuum treated in the saturated oil state to simulate the initial condition of the reservoir, CO2 fracturing fluid is injected in stages to the target pressure of 40-70 MPa, and the pressure is kept for more than 60 minutes, the pressure is released, and the crack morphology is analyzed.

4. The method for analyzing the effect of carbon dioxide composite fracturing fluid on different shale ruptures according to claim 1, characterized in that: If the shale sample is a laminated shale, CO2 fracturing fluid and slick water are selected, the purity of CO2 in the CO2 fracturing fluid is ≥ 99.5%, and the concentration of the slick water is 0.2%-0.5%.

5. The method for analyzing the influence of the carbon dioxide complex fracturing fluid on different shale ruptures according to claim 4, characterized in that: The fracturing experiment process is that the shale sample is vacuum treated in the saturated oil state to simulate the initial condition of the reservoir, CO2 fracturing fluid is injected to the target pressure of 40-70 MPa, and the pressure is kept for 30 minutes, the slick water is injected in stages under the condition of continuous injection of CO2 fracturing fluid, the stable flow rate of 2-5 m³ / min is kept, and the injection is stopped after 60 minutes, and the microstructure analysis of the crack network is carried out.

Images