Method for analyzing influence of carbon dioxide composite fracturing fluid on fracture of different shale

Abstract

The invention discloses a method for analyzing the influence of carbon dioxide composite fracturing fluid on different shale fractures, and belongs to the technical field of shale oil and gas resource development, and the method comprises the following steps: selecting fracturing fluid according to shale types, determining a corresponding fracturing fluid injection process, and then carrying out a fracturing experiment; the fracture fractal dimension and fracture energy of the shale sample are calculated, then the fracture energy efficiency is calculated, and shale fracture is analyzed according to the fracture energy efficiency. According to the method, the defects in the prior art can be overcome, the injection combination and process parameters of CO2 and the water-based fracturing fluid are optimized, adaptability improvement of different shale types is achieved, and a fracturing energy analysis method for composite fracturing fluid design and energy efficiency is established.

Description

Technical Field

[0001] This invention relates to the field of shale oil and gas resource development technology, and in particular to an analytical method for the effect of carbon dioxide composite fracturing fluid on the fracturing of different shale formations. Background Technology

[0002] Shale oil and gas resource development is an important direction for the transformation of modern energy structure. Due to their low permeability and complex reservoir structure, shale oil and gas resources typically require fracturing technology to form a fracture network to improve conductivity. Among these technologies, carbon dioxide (CO2) fracturing has demonstrated significant advantages in the fields of shale gas, coalbed methane, and tight oil and gas. CO2 fracturing can significantly reduce fracture pressure and increase fracture complexity. Furthermore, after injection into the reservoir, it can increase reservoir pressure, reduce fluid viscosity, extract hydrocarbons, improve oil-water interface properties, and alter the physicochemical properties of the reservoir.

[0003] Foliar shale and lamellar shale are the two most common types of shale. Foliar shale is characterized by uniform and continuous bedding, low mechanical strength, but easy stripping along bedding planes; lamellar shale is characterized by complex bedding, alternating deposition of minerals and organic matter, high compressive strength, but poor fracture propagation.

[0004] While carbon dioxide alone can significantly optimize fracture complexity and improve energy efficiency, its effectiveness is limited in shale with high clay content or complex bedding. Water-based fracturing fluids (such as guar gum and slickwater) can form relatively complex fracture networks, but their high viscosity and high energy consumption lead to a significant reduction in energy efficiency, making it difficult to balance fracture complexity and economic efficiency. Bent shale and lamellar shale exhibit significantly different responses to fracturing fluids due to differences in bedding structure and mineral composition. However, existing fracturing technologies are generally not optimized for shale types, failing to effectively address the contradiction between increased fracture complexity and energy consumption in lamellar shale. The lack of systematic research on the viscosity differences, interfacial tension variations, and impacts on mineral structure of different fracturing fluids increases the uncertainty of design parameters and operating procedures. Current experiments are mostly conducted under idealized conditions, failing to fully simulate the actual complex environment of reservoirs, such as high temperature and high pressure conditions, multiphase fluid interactions, and mineral reactions. This discrepancy may lead to inconsistencies between experimental results and practical applications, affecting the promotion and optimization of the technology. Water-based fracturing fluids present challenges in maintaining fracture conductivity over the long term. For example, guar gum can clog fractures due to residue deposition, and slickwater may fail to keep fractures open for extended periods. These drawbacks negatively impact the later stages of fracturing, yet few solutions exist in current research. Single or composite fracturing fluids struggle to significantly improve energy efficiency in low-permeability reservoirs, particularly in fracture stimulation requiring prolonged exposure; current technologies still exhibit low energy utilization rates. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide an analytical method for the impact of carbon dioxide composite fracturing fluid on the fracturing of different shale types. This method can overcome the shortcomings of the prior art, optimize the injection combination and process parameters of CO2 and water-based fracturing fluid, improve the adaptability to different shale types, and establish a fracturing energy analysis method for composite fracturing fluid design and energy efficiency.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows.

[0007] An analytical method for the effect of carbon dioxide composite fracturing fluid on the fracturing of different shale formations includes the following steps: Select the fracturing fluid according to the shale type and determine the corresponding fracturing fluid injection process, and then conduct fracturing experiments; calculate the fractal dimension and fracturing energy of the shale sample, then calculate the fracturing energy efficiency, and analyze the shale fracturing based on the fracturing energy efficiency.

[0008] As a preferred option, if the shale sample is a foliated shale, CO2 fracturing fluid with a CO2 purity ≥ 99.5% should be used.

[0009] As a preferred method, the fracturing experiment involves vacuum treatment of the shale sample in a saturated oil state to simulate the initial reservoir conditions, injecting CO2 fracturing fluid in stages to a target pressure of 40-70 MPa, maintaining it for more than 60 minutes, releasing the pressure, and analyzing the fracture morphology.

[0010] As a preferred option, if the shale sample is a layered shale, CO2 fracturing fluid and slickwater are selected, with CO2 purity ≥ 99.5% in the CO2 fracturing fluid and slickwater concentration of 0.2%-0.5%.

[0011] As a preferred method, the fracturing experiment is as follows: the shale sample is vacuum-treated in a saturated oil state to simulate the initial reservoir conditions, CO2 fracturing fluid is injected to a target pressure of 40-70 MPa and maintained for 30 minutes, and while the CO2 fracturing fluid is continuously injected, slickwater is injected in stages at a stable flow rate of 2-5 m³ / min for 60 minutes, and then the injection is stopped to perform microstructural analysis of the fracture network.

[0012] As a preferred method, the method for calculating the fractal dimension of the fracture of the shale sample is as follows: First, images of the fractured shale rock after hydraulic fracturing are acquired using scanning or imaging techniques. These images are then processed and converted into binary images. Several square grids of different sizes are then sequentially overlaid on the fracture images, and the number of grids containing fractures within each grid size is counted. , In grid size Below, the number of grids covering the crack region, where D is the fractal dimension of the crack, is fitted using linear regression after logarithmic transformation. and The relationship between them .

[0013] Preferably, the fracturing energy includes the elastic deformation energy stored in the shale before fracturing and the fracturing propagation energy used to form new fracture surfaces and drive fracture propagation. Where F is the applied load (unit: N) and d is the applied displacement (unit: m). This represents the maximum loaded displacement at the time of rupture.

[0014] As a preferred option, define fracture energy efficiency. The fractal dimension produced by a unit fracture energy. .

[0015] The beneficial effects of adopting the above technical solution are as follows: Based on the differences in bedding structure and mineral distribution between bedding-developed and lamellar shale, this invention customizes different fracturing fluid ratios and process parameters, optimizing the balance between fracturing energy and fracture complexity, effectively reducing energy consumption while achieving high fracture complexity. This invention is the first to propose a fracturing effect analysis method based on fractal dimension and fracturing energy, which can scientifically quantify fracturing fluid performance and guide design optimization, filling the gap in existing technologies that lack unified analytical indicators. Attached Figure Description

[0016] Figure 1 This is a photo of the GCTS rock mechanics testing system.

[0017] Figure 2 This is a photograph of a foliated shale sample.

[0018] Figure 3 This is a photograph of a layered shale sample.

[0019] Figure 4 This is the tensile strength test curve of a foliated shale sample.

[0020] Figure 5 This is the tensile strength test curve of a layered shale sample.

[0021] Figure 6 This is a comparison of the elastic modulus data of foliated shale and lamellar shale.

[0022] Figure 7 These are images of the post-fracturing specimen during the fractal dimension calculation process. Figure 7 (a) The area within the red box is the area to be enlarged. Figure 7 (b) is Figure 7(a) A magnified view of the area within the red box. Figure 7 (c) is Figure 7 (b) A magnified view of the area within the red box.

[0023] Figure 8 This is the analysis result after different mesh sizes were applied during the calculation of the fractal dimension of the fractured structure. Figure 8 (a) shows the analysis results after dividing the data into 25 grids. Figure 8 (b) shows the analysis results after dividing the data into 100 grids. Figure 8 (c) is the analysis result after dividing the grid into 225 cells.

[0024] Figure 9 This is a comparison diagram of the fracture energy and fractal dimension of foliated shale.

[0025] Figure 10 This is a comparison diagram of the fracture energy and fractal dimension of laminated shale. Detailed Implementation

[0026] The abundant clay minerals in shale often undergo significant softening and expansion upon contact with water, leading to changes in their mechanical properties. The injection of carbon dioxide (CO2) further increases the complexity and uncertainty of these mechanical property changes. In fracturing technology research, the mechanical properties of the rock are one of the key parameters determining the effectiveness of fracturing. However, most current CO2 fracturing designs typically assume that the mechanical parameters of the reservoir rock remain constant throughout the fracturing process. This simplistic assumption ignores the hydration reaction that occurs in shale upon contact with water and the potential impact of CO2 on its mechanical properties. Therefore, this approach fails to accurately reflect the dynamic changes in the reservoir rock during actual fracturing operations, potentially leading to insufficient accuracy in the design and the introduction of additional errors. Improving the study of these dynamic changes, especially the comprehensive analysis of the interaction between hydration effects and CO2, will help enhance the scientific rigor and engineering effectiveness of fracturing design.

[0027] This invention conducts indoor experimental research on Brazilian splitting and uniaxial compression tests. The experimental apparatus and instruments mainly include the GCTS rock mechanics testing system (see...). Figure 1 CO2 cooling devices, core holders, etc.

[0028] The shale used in the experiment were foliated shale (see...) Figure 2 ) and laminated shale (see Figure 3Shales with well-developed foliation exhibit typical horizontal or sub-horizontal tabular foliation, characterized by thin-layered distribution, easy detachment along bedding planes, and uniform and continuous bedding. Laminated shale, on the other hand, displays a combination of multiple laminations, with relatively complex textures, potentially containing alternating deposits of materials of different grain sizes or organic matter and minerals. To ensure consistency in the initial state of the rocks, a control group under saturated oil conditions was included for each experimental treatment. Both the control and fluid-treated groups were derived from the same core sample.

[0029] To simulate the changes in rocks within the reservoir, the rocks were first treated with saturated oil to mimic the original reservoir state before the experiment. Then, the rocks were immersed in different combinations of fracturing fluids: CO2, CO2 + slickwater / guar gum, slickwater, and guar gum. In the experiment, shale samples were dried to constant weight at 110℃, then vacuumed and injected with a simulated oil mixture of Gulong shale oil and kerosene at 110℃ and 37MPa. Next, a uniaxial compression test was conducted to determine the elastic modulus and Poisson's ratio of the original rock. Subsequently, CO2 (-20℃, 60MPa) was injected into another core holder, followed by slickwater injection. After 3 hours, a uniaxial compression test was performed again to measure the mechanical parameters after fluid action. A 50mm diameter, 25mm thick core was also prepared for the Brazil fracturing test. By repeating the above steps, the mechanical properties of different types of shale before and after the action of different external fluids were tested. Finally, the effects of CO2, CO2 + slickwater / guar gum, slickwater, guar gum, and shale type on the tensile strength, elastic modulus, and Poisson's ratio of the rock were analyzed.

[0030] Table 1 Tensile Strength Test Scheme

[0031] Table 2. Tensile strength test data

[0032] According to the table above and Figure 4-5According to the data, the tensile strength of foliated shale is 6.536 MPa, while the tensile strength of laminated shale is significantly higher, reaching 12.0866 MPa. After treatment with slickwater and guar gum, the tensile strength of foliated shale decreased significantly, by 45.63% to 43.67%, and the effects of these two treatments were similar. In contrast, the tensile strength of the rock treated with CO2 also decreased, but the decrease was smaller, only 12.42%. When CO2 was used in combination with slickwater or guar gum, the decrease in tensile strength was smaller than when slickwater or guar gum was used alone, ranging from 16.94% to 21.15%. After treatment with slickwater and guar gum, the tensile strength of laminated shale also showed a significant decrease, by 42.21% to 44.22%. The tensile strength of the rock treated with CO2 also decreased, by 29.38%, a significantly larger decrease than that of foliated shale. When CO2 is used in combination with slickwater or guar gum, the reduction in tensile strength is less than when slickwater or guar gum is used alone, with a reduction ranging from 31.00% to 32.51%. This indicates that CO2 can mitigate the effect of slickwater or guar gum on the tensile strength of rocks, but its numerical effect is relatively small.

[0033] The compressive strength test procedure is shown in the table below. Shale compressive strength test and fracturing specimens are cylinders with a diameter of 48-54 mm, a height-to-diameter ratio of 2.0-2.5, a bottom flatness within 0.02 mm, and a maximum deviation of the axis from the vertical direction not exceeding 1 / 1000 of the diameter. Specimens are prepared to be 50 mm × Φ25 mm. After preparation, the specimen is fixed on the testing equipment. A uniaxial compression test fixture is installed, and a thin layer of lubricant is applied to the upper and lower bottom surfaces of the specimen, which is then placed in the center of the base. A rigid pad is placed between the upper end of the specimen and the bearing plate, and axial and radial displacement extensometers are installed and adjusted to the initial readings. The bearing head is adjusted to ensure uniform contact between the pad and the bearing plate, ensuring uniform stress on the specimen. The loading rate is set to 0.1-0.5 MPa per second, with a lower rate for soft rock, until the specimen fails.

[0034] Table 3 Compressive Strength Test Scheme

[0035] By conducting experiments on samples using different fracturing fluids, namely CO2, CO2 + slickwater / guar gum, slickwater, and guar gum, the stress-strain curves were calculated and processed to obtain the following table and... Figure 6Data shows that the initial elastic modulus of foliated shale is approximately 21.61 GPa, while that of laminated shale is 22.77 GPa. Taking foliated shale as an example, during treatment, slickwater and guar gum had the most significant impact on the rock's elastic modulus, resulting in decreases of approximately 48.82% and 27.76%, respectively. In contrast, CO2 had a smaller impact on the elastic modulus. When CO2 was used in combination with slickwater or guar gum, the decrease in the sample's elastic modulus was smaller than when slickwater or guar gum was used alone, ranging from 13.56% to 24.20%. This indicates that CO2 can mitigate the reduction in the rock's elastic modulus caused by slickwater or guar gum.

[0036] The initial Poisson's ratio for foliated shale is 0.187, while that for laminated shale is 0.184. Taking foliated shale as an example, the influence of external fluids on the Poisson's ratio is relatively small. Slickwater and guar gum slightly increase the Poisson's ratio, by 12.83% and 12.85%, respectively. When CO2 is used in combination with slickwater or guar gum, the increase in the Poisson's ratio is smaller than when slickwater or guar gum is used alone, ranging from 10.70% to 3.21%. This indicates that CO2 can mitigate the increase in the Poisson's ratio caused by slickwater or guar gum.

[0037] Table 4. Experimental data on Poisson's ratio and elastic modulus.

[0038] Fractal dimension measures the self-similarity and complexity of an object, typically used to describe the geometric features and morphological complexity of cracks. It can also be used to further analyze the effects of different types of fracturing fluids on shale fracturing. In this study, the box-counting method was used to calculate the fractal dimension of shale sample fractures. The box-counting method is a covered fractal dimension estimation method widely used in fractal analysis of crack morphology. Its 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 for each grid size. Based on the relationship between grid size and the number of grids, the logarithmic rule is used to fit the fractal dimension.

[0039] First, images of the fractured shale rock after fracturing are acquired using scanning or imaging techniques. To ensure computational accuracy, the images must be clear and able to display fracture details. Next, the images are processed to convert them into binary images for subsequent fracture analysis; the relevant procedures are detailed below. Figure 7-8 .

[0040] The binary image is covered with grids of different scales. Grid size Gradually reduce the size of the mesh and record the number of meshes containing cracks at each mesh coverage. According to the formula of the box counting method , In grid size Below, the number of grids covering the crack region, where D is the fractal dimension of the crack, is fitted using linear regression after logarithmic transformation. and The relationship between them The table below shows the fractal dimension of different shale formations fracturing under different types of fracturing fluids.

[0041] Table 5 Fractal Dimension of Shale Samples

[0042] In foliated shale, using CO2 as a fracturing fluid produces relatively regular cracks that primarily propagate along bedding planes, resulting in significant shear failure and a relatively complex crack structure. Therefore, the fractal dimension is relatively high, leading to better fracturing performance. Guar gum, due to its high viscosity, effectively increases crack complexity, causing cracks to propagate not only along bedding planes but also potentially into the shale interior, resulting in more complex crack morphologies and a higher fractal dimension. Slickwater, with its low viscosity and high permeability, primarily propagates cracks along relatively regular paths, resulting in a relatively high fractal dimension. The combination of CO2 and guar gum simultaneously enhances crack propagation and complexity, allowing cracks to propagate not only along bedding planes but also alternately between different layers, forming more complex crack structures with a fractal dimension of 2.5-2.6. The combination of CO2 and slickwater enhances crack permeability, but due to the lower viscosity of slickwater, the crack complexity is relatively low, with a fractal dimension of 2.3-2.4. After treatment with a combination of water-based fracturing fluid and CO2, the fractal dimension increased by 4%-8.7% compared to using water-based fracturing fluid alone. Overall, after treatment with different fracturing fluids, the fractal dimension of foliated shale was 3.8%-8% higher than that of laminated shale.

[0043] The external force energy absorbed and released by shale samples during fracturing is a key driving factor for fracture formation and propagation. The magnitude and distribution of fracturing external force energy directly affect not only the formation path and complexity of fractures but also determine the efficiency and effectiveness of reservoir stimulation. Based on experimental results under different fracturing fluids, this study calculates the fracturing external force energy of shale and explores the influence of fracturing energy on fracture complexity using fractal dimension, thereby analyzing the energy efficiency and applicability of different fracturing fluids. Fracturing energy mainly consists of two parts: the elastic deformation energy stored in the shale before fracturing and the fracture propagation energy used to form new fracture surfaces and drive fracture propagation.

[0044] The total fracture energy W can be calculated using the loading and displacement data from the experiment. , Where F is the applied load (unit: N), and d is the applied displacement (unit: m). This represents the maximum loaded displacement at the time of rupture.

[0045] Based on the calculation results, the fracturing energies of bedding-type and lamellar shale under different fracturing fluids are shown in the table below. Guarniol treatment resulted in the highest fracturing energy, indicating that the high viscosity of guarniol requires more energy to drive fracture propagation. CO2 treatment resulted in the lowest fracturing energy, demonstrating that CO2 effectively reduces the energy required for fracture formation by lowering fracturing pressure and increasing porosity. The combined effect of CO2 and water-based fracturing fluid resulted in fracturing energies between those of CO2 alone and water-based fracturing fluid, exhibiting good energy efficiency (see table below).

[0046] Table 6. Fracturing Energy of Shale Samples

[0047] A comparative analysis of fractal energy and fractal dimension reveals the relationship between fractal dimension and fractal energy under different fracturing fluid treatments, as follows: Figure 9 and Figure 10 As shown in Table 7, data analysis reveals a positive correlation between fractal dimension and fracture energy. Specifically, a higher fractal dimension corresponds to greater fracture energy, indicating that fracture complexity requires more energy for bifurcation and propagation. While CO2 treatment resulted in lower fracture energy, its fractal dimension reached 2.5, demonstrating a significant advantage in fracture complexity. Furthermore, the synergistic effect of the combined fracturing fluid achieved a relative balance between fractal dimension and fracture energy. The combined effect of CO2 and slickwater achieved a balance between fractal dimension (2.6) and fracture energy (19.4 J), ensuring both fracture complexity and effective energy control.

[0048] Table 7 Fractal dimension and fracture energy analysis of shale samples

[0049] To compare the performance of fracturing fluids, fracturing energy efficiency is defined. The fractal dimension produced by a unit fracture energy. ,in, Let W be the fractal dimension of the shale fracture, and W be the fracture energy (in J). The fracture energy efficiency is shown in the table below.

[0050] Table 8. Fracturing energy efficiency of shale samples

[0051] As shown in Table 8, CO2 alone exhibits the highest energy efficiency, reaching 0.203 and 0.152 for the two shale types, respectively, indicating that it achieves high fracture complexity with minimal energy consumption. Guar gum alone has the lowest energy efficiency, suggesting that while its high viscosity increases fracture complexity, it also significantly increases energy consumption. The combined energy efficiency of CO2 and guar gum falls between the two, demonstrating good overall performance. CO2 fracturing fluid exhibits the lowest fracturing energy and a high fractal dimension, resulting in the highest energy efficiency, making it suitable for complex fracture stimulation in low-permeability reservoirs. Although guar gum fracturing fluid offers high fracture complexity, its energy efficiency is relatively low, requiring a balance between energy consumption and effectiveness in its application. The combined effect of CO2 and water-based fracturing fluid achieves a good balance between fracturing energy and fractal dimension, improving energy efficiency by 15.32%–16.52% compared to water-based fracturing fluid alone.

[0052] This patent, based on the differences in bedding structure and mineral distribution between foliated and lamellar shale, customized different fracturing fluid ratios and process parameters. Pure CO2 fracturing fluid is suitable for foliated shale, while a composite fracturing fluid of CO2 and slickwater is suitable for lamellar shale. Through targeted optimization, fracture complexity and compatibility can be significantly improved. By using a combination of CO2 and water-based fracturing fluid, the balance between fracturing energy and fracture complexity is optimized. According to experimental data, the energy efficiency of the CO2 and slickwater combination is 15.32%-16.52% higher than that of using water-based fracturing fluid alone, effectively reducing energy consumption while achieving high fracture complexity. The fractal dimension of the composite fracturing fluid reaches 2.5-2.6, significantly higher than that of using slickwater or guar gum alone (2.2-2.4). After the CO2 and slickwater combination is applied to lamellar shale, the fractal dimension increases by 4%-8.7%, forming a more complex fracture network, which is beneficial for reservoir stimulation and long-term maintenance of conductivity. The decompression effect of CO2, combined with the fracture propagation capacity of water-based fracturing fluids, can overcome the limitations of single fracturing fluids in terms of adaptability to different reservoirs. For foliated shale, pure CO2 fracturing fluid avoids the bedding plane stripping problem caused by water-based fluids; for lamellar shale, the composite fluid achieves higher fracture propagation and complexity. According to experimental data, when using pure CO2, foliated shale achieves an energy efficiency as high as 0.203 and a fracture complexity (fractal dimension) of 2.5, with the lowest energy consumption. For lamellar shale, under the combined action of CO2 and slickwater, the energy efficiency reaches 0.106, and the fracture complexity (fractal dimension) is 2.5, significantly higher than that of slickwater treatment alone (fractal dimension 2.3).

[0053] This invention proposes a process method for staged injection of CO2 and water-based fracturing fluid, optimizing the injection sequence, pressure range, and flow rate parameters to ensure the operability and efficiency of the process.

[0054] This invention is the first to propose a fracturing effect analysis method based on fractal dimension and fracturing energy, which can scientifically quantify the performance of fracturing fluid and guide design optimization, filling the gap in the lack of unified analysis indicators in the existing technology.

[0055] Compared to existing single fracturing fluids, the composite fracturing fluid of this invention achieves more efficient energy utilization and more complex fracture morphologies, and is suitable for various reservoir environments. Compared to existing empirical design methods, this invention proposes an optimization scheme based on experimental data and establishes an analytical model through fractal dimension and fracture energy, resulting in a more scientific and precise technical design.

[0056] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0057] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for analyzing the effects of carbon dioxide composite fracturing fluid on the fracturing of different shale formations, characterized in that... Includes the following steps: Select the fracturing fluid according to the shale type and determine the corresponding fracturing fluid injection process, and then conduct fracturing experiments; calculate the fractal dimension and fracturing energy of the shale sample, then calculate the fracturing energy efficiency, and analyze the shale fracturing based on the fracturing energy efficiency.

2. The method for analyzing the effect of carbon dioxide composite fracturing fluid on the fracturing of different shale formations according to claim 1, characterized in that: If the shale sample is a foliated shale, use CO2 fracturing fluid with a CO2 purity ≥ 99.5%.

3. The method for analyzing the effect of carbon dioxide composite fracturing fluid on the fracturing of different shale formations according to claim 2, characterized in that: The fracturing experiment involved vacuum treatment of the shale sample in a saturated oil state to simulate the initial reservoir conditions. CO2 fracturing fluid was injected in stages to a target pressure of 40-70 MPa, maintained for more than 60 minutes, and then the pressure was released and the fracture morphology was analyzed.

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

5. The method for analyzing the effect of carbon dioxide composite fracturing fluid on the fracturing of different shale formations according to claim 4, characterized in that: The fracturing experiment involved vacuum treatment of the shale sample in a saturated oil state to simulate the initial reservoir conditions. CO2 fracturing fluid was injected to a target pressure of 40-70 MPa and maintained for 30 minutes. While continuously injecting CO2 fracturing fluid, slickwater was injected in stages at a stable flow rate of 2-5 m³ / min for 60 minutes. Then, the injection was stopped, and the microstructure of the fracture network was analyzed.

6. The method for analyzing the effect of carbon dioxide composite fracturing fluid on the fracturing of different shale formations according to claim 1, characterized in that: The method for calculating the fractal dimension of fracture in shale samples is as follows: First, images of the fractured shale rock after hydraulic fracturing are acquired using scanning or imaging techniques. These images are then processed and converted into binary images. Several square grids of different sizes are then sequentially overlaid on the fracture images, and the number of grids containing fractures within each grid size is counted. , In grid size Below, the number of grids covering the crack region, where D is the fractal dimension of the crack, is fitted using linear regression after logarithmic transformation. and The relationship between them .

7. The method for analyzing the effect of carbon dioxide composite fracturing fluid on the fracturing of different shale formations according to claim 6, characterized in that: Fracturing energy includes the elastic deformation energy stored in the shale before fracturing and the fracture propagation energy used to form new fracture surfaces and drive fracture extension. Where F is the external load, d To load displacement, This represents the maximum loaded displacement at the time of rupture.

8. The method for analyzing the effect of carbon dioxide composite fracturing fluid on the fracturing of different shale formations according to claim 7, characterized in that: Define fracture energy efficiency The fractal dimension produced by a unit fracture energy. .

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