Shale reservoir polymer fracturing fluid retention characteristic analysis method and system

Abstract

The invention belongs to but is not limited to the technical field of petroleum engineering, and particularly relates to a shale reservoir polymer fracturing fluid retention characteristic analysis method and system, and the method comprises the following steps: S1, carrying out a displacement experiment by using polymer solutions with different molecular weights and concentrations; s2, the retention condition of polymers in the rock core before and after displacement is observed through a micron CT and scanning electron microscope technology; and S3, analyzing the influence of the polymer solutions with different molecular weights and concentrations on the retention characteristics in the shale reservoir. The invention aims to carry out a displacement experiment on the rock core by using polymer solutions with different molecular weights and concentrations, and then observe and compare the polymer solutions of the rock core before and after displacement by using equipment such as a CT scanner and an electron microscope, so as to obtain the retention and migration conditions of the polymer in the hole seam of the rock core. The retention characteristics of fracturing fluid polymer molecules in a microporous structure and a matrix are studied, so that a correct polymer molecular weight range is provided for reservoir transformation, fracturing fluid reservoir damage is reduced, and the fracturing yield increasing effect is finally improved.

Description

Technical Field

[0001] The present invention belongs to, but is not limited to, the technical field of petroleum engineering, and particularly relates to a method and system for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs. Background Art

[0002] In the development of shale oil and gas, the use of polymer fracturing fluid is one of the key technologies to increase oil and gas production. However, in current polymer fracturing, there is a blind requirement for low application concentration and high viscosity of the fracturing fluid. If such requirements are met, the polymer molecular weight must be extremely high. Especially for current suspensions, the molecular weight can reach tens of millions. However, the larger the polymer molecular weight, the more obvious the retention effect in microporous structures or matrices theoretically, that is, the larger the molecular weight, the shorter the distance it can travel, and it will be retained and accumulated near the fracture mouth, ultimately affecting the post-fracture production and causing greater pollution to the formation, thus restricting the stimulation effect of oil and gas resources.

[0003] In view of the above analysis, the technical problem that urgently needs to be solved in the prior art is that the larger the polymer molecular weight, the more obvious the retention effect in microporous structures or matrices theoretically, that is, the larger the molecular weight, the shorter the distance it can travel, and it will be retained and accumulated near the fracture mouth, ultimately affecting the post-fracture production and causing greater pollution to the formation, thus restricting the stimulation effect of oil and gas resources. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention provides a method and system for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs.

[0005] The present invention is implemented as follows. A method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs includes:

[0006] S1, performing displacement experiments using polymer solutions with different molecular weights and concentrations;

[0007] S2, observing the retention of polymers in the core before and after displacement through micro-CT and scanning electron microscopy (SEM) techniques;

[0008] S3, analyzing the influence of polymer solutions with different molecular weights and concentrations on the retention characteristics in shale reservoirs.

[0009] Furthermore, the polymer solution displacement experiment specifically includes:

[0010] The experimental materials mainly include: polymer powders with different molecular weights. The basic information of the polymers is shown in Table 1. The permeability and porosity of the used shale rock samples are 0.1897×10 -3 μm 2 and 2.653%, respectively.

[0011] The experimental equipment and instruments mainly include: Teledyne SSI ReaXus peristaltic pump, core holder, piston intermediate container, hand-operated confining pressure pump, six-way valve, pressure gauge, etc.

[0012] Furthermore, the specific process of the polymer solution displacement experiment is as follows: When conducting the polymer displacement experiment, first, adjust the temperature of the experimental environment to room temperature to ensure the naturalness and stability of the experimental conditions. Then, connect the pipelines and set the fluid injection rate to 0.2 mL / min. The injection rate is adjusted according to the specific requirements of the experiment and the characteristics of the core to ensure that the polymer solution can effectively displace the fluid in the core. Next, place the core in the core holder, set the initial confining pressure to 10 MPa, and adjust the confining pressure according to the change of the displacement pressure difference. During the experiment, closely monitor the changes in the injection pressure and confining pressure, and at the same time observe the liquid outlet situation at the outlet end of the core holder. When the liquid starts to flow out at the outlet end, record the pressure, flow rate, and time at this moment. Subsequently, clean the core holder and related equipment and replace the polymer solution in the intermediate container to conduct the next group of displacement experiments, repeating the above steps.

[0013] Furthermore, the micro-CT scanning before and after the polymer solution displacement specifically includes:

[0014] Use a Versa XRM-500 type micro-CT scanner with a resolution of 1 μm to perform micro-CT scanning on the shale sample before and after displacement. The test temperature is 20 °C, and the working voltage is 150 KV.

[0015] Furthermore, the specific process of the micro-CT scanning before and after the polymer solution displacement is as follows: First, perform CT scanning on the original dry core to obtain the three-dimensional pore space distribution of the initial core; then, conduct core displacement experiments with different polymer solutions, and perform CT scanning on the core after the polymer solution displacement, and compare the pore distribution of the core before and after the polymer solution displacement to obtain the retention and migration of the polymer inside the rock.

[0016] Furthermore, the SEM scanning results before and after the polymer displacement specifically include:

[0017] The energy spectrometer determines the composition of a sample by analyzing the energy of the characteristic X-rays released by the sample. Since there is a certain functional relationship between the energy of the characteristic X-rays and the atomic number of the atoms in the sample, by measuring the energy of a certain characteristic X-ray, the corresponding element in the sample can be identified. Analyze the rock sample through a scanning electron microscope energy spectrometer to obtain the elemental composition of different substances in the rock sample.

[0018] Another object of the present invention is to provide a shale reservoir polymer fracturing fluid retention characteristic analysis system for implementing the shale reservoir polymer fracturing fluid retention characteristic analysis method, including:

[0019] A displacement experiment module for conducting displacement experiments using polymer solutions with different molecular weights and concentrations;

[0020] An electron microscope scanning module for observing the retention of polymers in the core before and after displacement through micro-CT and scanning electron microscope (SEM) techniques;

[0021] A retention characteristic analysis module for analyzing the influence of polymer solutions with different molecular weights and concentrations on the retention characteristics in shale reservoirs.

[0022] Another object of the present invention is to provide a computer device. The computer device includes a memory and a processor. When a computer program stored in the memory is executed by the processor, the processor executes the steps of the method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs.

[0023] Another object of the present invention is to provide a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, the processor executes the steps of the method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs.

[0024] Another object of the present invention is to provide an information data processing terminal, and the information data processing terminal includes the system for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs.

[0025] Combined with the above technical solutions and the solved technical problems, the advantages and positive effects of the technical solution to be protected by the present invention are:

[0026] First, the present invention aims to systematically study the displacement behavior of polymer solutions with different molecular weights (17.54 million, 26.92 million, 12.75 million, 25.55 million) and concentrations (0.1% - 0.5%) on shale cores. Combining with micro-CT scanner (Versa XRM-500) and field emission scanning electron microscope (Gemini SEM500) technologies, the retention and migration characteristics of polymers in core pores and fractures are revealed from macro and micro multi-scales. The experimental results show that when the molecular weight of the polymer is lower than 17.54 million and the concentration is low, the solution can effectively enter the core and be evenly distributed, with a weak retention effect; while when the molecular weight exceeds 17.54 million or the concentration is high, the polymer significantly accumulates near the fracture entrance, resulting in the reduction of pore space and hindering the flow of oil and gas. The research further clarifies that the molecular weight is the main controlling factor for the retention behavior, followed by the concentration, and quantifies the critical molecular weight threshold (17.54 million). Based on this, this study proposes the molecular weight range for optimizing the fracturing fluid formula. By reducing the use ratio of high molecular weight polymers, the retention and aggregation in the microporous structure of the reservoir are reduced, thereby alleviating reservoir damage and enhancing the fracturing stimulation effect. This achievement provides a scientific basis for the precise design of polymer fracturing fluids in the transformation of shale oil reservoirs and promotes the progress of unconventional oil and gas development technologies.

[0027] Second, the present invention optimizes the polymer fracturing fluid formula, reduces its retention and aggregation in the shale oil reservoir, reduces reservoir damage, and improves the fracturing stimulation effect. This can increase oil and gas production and recovery efficiency, bringing economic benefits to oil and gas production enterprises. At the same time, this technical solution helps to promote the progress of shale oil development technologies, enhance the competitiveness and market share of enterprises in the industry. Its successful application will produce a significant demonstration effect, promote the exchange and cooperation of related technologies, drive the development of the industrial chain, further expand market opportunities, and bring more commercial value to related enterprises.

[0028] In the development of shale oil, the molecular weight and concentration of polymer fracturing fluids have a significant impact on their retention behavior in the reservoir. However, existing research lacks systematic studies on their effects at the microscale and has not formed a consistent conclusion. Through multi-scale analysis, this study clarifies the effects of polymer molecular weight and concentration on the retention behavior and solves this technical problem.

[0029] There are some technical biases in the current research on polymer fracturing fluids. For example, blindly pursuing low concentration and high viscosity of fracturing fluids leads to extremely high molecular weights of polymers, resulting in retention and pollution problems in the reservoir. Through experiments and technical analysis, this study reveals the comprehensive effects of polymer molecular weight and concentration on the retention behavior, determines the appropriate molecular weight range, and overcomes this technical bias. Description of the Drawings

[0030] Figure 1It is the flow chart of the polymer fracturing fluid core displacement experiment provided by the embodiment of the present invention;

[0031] Figure 2 It is the schematic diagram of the CT scanner provided by the embodiment of the present invention;

[0032] Figure 3 It is the CT scan image of the original rock sample provided by the embodiment of the present invention;

[0033] Figure 4 It is the CT scan image of the polymer with a molecular weight of 12.75 million, a concentration of 0.50%, and a viscosity of 604.879 mPa·s provided by the embodiment of the present invention;

[0034] Figure 5 It is the CT scan image of the polymer with a molecular weight of 17.54 million, a concentration of 0.1%, and a viscosity of 33.579 mPa provided by the embodiment of the present invention;

[0035] Figure 6 It is the CT scan image of the injection end of the polymer with a molecular weight of 25.55 million, a concentration of 0.4%, and a viscosity of 884.281 mPa·s provided by the embodiment of the present invention;

[0036] Figure 7 It is the CT scan image of the outlet end of the polymer with a molecular weight of 25.55 million, a concentration of 0.4%, and a viscosity of 884.281 mPa·s provided by the embodiment of the present invention;

[0037] Figure 8 It is the CT scan image of the injection end of the polymer with a molecular weight of 26.92 million, a concentration of 0.2%, and a viscosity of 262.362 mPa·s provided by the embodiment of the present invention;

[0038] Figure 9 It is the CT scan image of the outlet end of the polymer with a molecular weight of 26.92 million, a concentration of 0.2%, and a viscosity of 262.362 mPa·s provided by the embodiment of the present invention;

[0039] Figure 10 It is the scanning electron microscope energy spectrum diagram provided by the embodiment of the present invention;

[0040] Figure 11 It is the electron microscope scan of the original rock sample provided by the embodiment of the present invention;

[0041] Figure 12 It is the electron microscope scan image of the polymer with a molecular weight of 12.75 million, a concentration of 0.50%, and a viscosity of 604.879 mPa·s provided by the embodiment of the present invention;

[0042] Figure 13 It is the electron microscope scan image of the polymer with a molecular weight of 25.55 million, a concentration of 0.4%, and a viscosity of 884.281 mPa·s provided by the embodiment of the present invention;

[0043] Figure 14 It is the electron microscope scanning image of the polymer provided by the embodiment of the present invention, with a molecular weight of 26.92 million, a concentration of 0.2%, and a viscosity of 262.362 mPa·s;

[0044] Figure 15 It is the flow chart of the method for analyzing the retention characteristics of the polymer fracturing fluid in the shale reservoir provided by the embodiment of the present invention;

[0045] Figure 16 It is the structural diagram of the system for analyzing the retention characteristics of the polymer fracturing fluid in the shale reservoir provided by the embodiment of the present invention;

[0046] Figure 17 It is the relationship diagram between the molecular weight and the retention index (RI) of the polymer with a concentration of 0.1% provided by the embodiment of the present invention;

[0047] Figure 18 It is the relationship diagram between the molecular weight and the viscosity of the polymer solution of the polymer with a concentration of 0.1% provided by the embodiment of the present invention;

[0048] Figure 19 It is the relationship diagram of the polymer molecular weight - concentration - retention index provided by the embodiment of the present invention. Detailed implementation manners

[0049] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0050] The method for analyzing the retention characteristics of the polymer fracturing fluid in the shale reservoir described in the present invention mainly simulates the displacement process of the fracturing fluid on the pore structure of the shale in the actual reservoir by injecting polymer solutions with different molecular weights and concentrations into the shale core at a set flow rate under room temperature and a certain confining pressure. Since the polymer has a certain viscosity and molecular chain length, it will have physical and chemical interactions (such as adsorption, blockage, etc.) with the rock pores during the flow process, thereby causing a certain degree of retention inside the core. By monitoring the injection pressure, confining pressure and liquid output flow rate during the experiment, the displacement law and flow characteristics of the polymer in the core can be obtained.

[0051] The molecular weight and concentration of the polymer have important effects on the fluid flow resistance and molecular chain entanglement degree. Polymer solutions with high molecular weight or high concentration have greater viscosity during flow and are more likely to be retained or blocked in the core pores. By conducting displacement experiments on polymer solutions with different combinations of molecular weights and concentrations, the retention mechanism and degree of the polymer in the shale pore structure can be systematically studied, thereby providing data support for the on-site fracturing fluid design. Such a multi-variable experimental design can compare and analyze the effects of different polymer characteristics on the retention distribution, retention amount and pore structure changes.

[0052] Micro-CT (X-ray three-dimensional reconstruction technology) can perform non-destructive internal structure imaging of shale cores at sub-micron resolution. By CT scanning the cores before and after displacement, the changes in pore structure, fractures, and mineral distribution can be directly observed. If the polymer remains in the pores, it will be manifested as a decrease in local pore volume or a significant density difference in the CT reconstruction map. Combining image segmentation and three-dimensional visualization analysis techniques, the distribution range and occupied volume of the polymer in shale pores can be quantitatively evaluated, providing an intuitive basis for revealing the retention mechanism.

[0053] Scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS) can observe the morphological characteristics of the rock pore surface at the microscale and identify the types and contents of elements contained in the sample by measuring the characteristic X-ray energy. When the polymer solution adheres or blocks at the pores or throats, polymer films or viscous residues can be observed in the SEM images, and the corresponding characteristic elements (such as C, O, N, etc.) can be detected in the EDS elemental analysis, thereby judging the retention position and morphology of the polymer on the rock surface and inside the pores. Combining these microscale evidences with the macroscale distribution information of micro-CT can comprehensively reveal the polymer retention mechanism.

[0054] During the displacement experiment, a syringe pump is used to inject the polymer solution at a speed of 0.2 mL / min, and a confining pressure of 10 MPa is applied through a core holder. During the experiment, the injection pressure, flow rate, and confining pressure changes are recorded in real time to monitor the displacement process of the polymer solution inside the core. When liquid starts to flow out at the outlet end, it indicates that the polymer has penetrated and passed through some pore channels. Subsequently, if the pressure increases significantly or the liquid output decreases, it may indicate local blockage or increased retention inside the core. After completing a set of experiments, the polymer solution or the core is replaced, and the above steps are repeated, and micro-CT and SEM / EDS tests are performed on the core before and after displacement. By comparing with the pressure curve and flow rate curve during the displacement process, the dynamic evolution process of polymer retention can be judged.

[0055] After obtaining the results of displacement experiments, micro-CT scans, and SEM / EDS, through image processing and data analysis methods, the retention distribution, retention amount of the polymer in the shale reservoir, and the degree of change in pore structure can be accurately evaluated. The retention mechanisms and laws of polymers with different molecular weights and concentrations in the pores are also revealed, providing a basis for optimizing the field fracturing fluid formula and construction parameters, and avoiding or reducing the productivity loss caused by polymer retention. This method has important guiding value in the fracturing fluid design and effectiveness evaluation of unconventional reservoirs such as shale gas and tight oil.

[0056] 1. Polymer solution displacement experiment

[0057] The experimental materials mainly include: polymer powders with different molecular weights. The basic information of the polymers is shown in Table 1. The permeability and porosity of the shale samples used are 0.1897×10 -3 μm 2 and 2.653%. The experimental equipment and instruments mainly include: Teledyne SSI ReaXus horizontal flow pump, core holder, piston intermediate container, hand-cranked confining pressure pump, six-way valve, pressure gauge, etc. Figure 1 As shown. When conducting the polymer displacement experiment, first, adjust the temperature of the experimental environment to room temperature to ensure that the experimental conditions are natural and stable. Next, connect the pipeline and set the fluid injection rate to 0.2mL / min. The injection rate is adjusted according to the specific requirements of the experiment and the characteristics of the core to ensure that the polymer solution can effectively displace the fluid in the core. Then, place the core in the core holder, set the initial confining pressure to 10MPa, and adjust the confining pressure according to the change in the displacement pressure difference. During the experiment, closely monitor the changes in the injection pressure and confining pressure, and observe the liquid discharge at the outlet of the core holder. When the outlet starts to discharge liquid, record the pressure, flow rate and time at this moment. Subsequently, clean the core holder and related equipment and replace the polymer solution in the intermediate container, conduct the next set of displacement experiments, and repeat the above steps.

[0058] Table 1 Basic information of polymer samples

[0059] Experiment Serial Number Polymer Molecular Weight / 10,000 Mass Concentration of Polymer Solution / % Viscosity of Polymer Solution / mPa·s 1 1275 0.5 604.879 2 1754 0.1 33.579 3 2555 0.4 884.281 4 2692 0.2 262.362

[0060] 2. Micrometer CT scan before and after polymer solution flooding

[0061] The CT scan was performed using a Versa XRM-500 micrometer CT scanner with a resolution of 1 μm. Figure 2 The shale samples were micron CT scanned before and after displacement, with a test temperature of 20°C and an operating voltage of 150KV. First, the original dry core was CT scanned to obtain the three-dimensional pore space distribution of the initial core; then, core displacement experiments with different polymer solutions were carried out, and the cores after polymer solution displacement were CT scanned to compare the core pore distribution before and after polymer solution displacement, thereby obtaining the retention and migration of polymers inside the rock.

[0062] Figure 3 This is the CT scan of the original rock sample. First, the edge effect is removed from the original grayscale image of the core, where black represents pores and gray represents matrix; then, the pores are redefined, where blue represents pores and gray represents matrix; finally, the pore cluster rendering is further rendered, where bright colors represent pores, and it can be seen that there are many pores in the original core.

[0063] Figure 4It is the core CT scan image of Experiment 1 (polymer molecular weight is 127.5 million, concentration is 0.50%, viscosity is 604.879 mPa·s). When the polymer molecular weight is small and the concentration is large, the polymer solution can smoothly enter the core, and its distribution along the core is relatively uniform, with an insignificant retention effect. The pore space size in the CT scan image after injection is relatively close to that before injection.

[0064] Figure 5 It is the core CT scan image of Experiment 2 (polymer molecular weight is 17.54 million, concentration is 0.1%, viscosity is 33.579 mPa·s). When the polymer molecular weight is large and the concentration is low, with an increased displacement pressure, the polymer solution can enter the core relatively smoothly and form a certain degree of retention in the core.

[0065] Figures 6 to 9 It is the core CT scan image of Experiment 3 and Experiment 4 (polymer molecular weight is 25.55 million, concentration is 0.4%, viscosity is 884.281 mPa·s and polymer molecular weight is 26.92 million, concentration is 0.2%, viscosity is 262.362 mPa·s). When the polymer molecular weight is large, it is difficult for the polymer solution to smoothly enter the core, and obvious retention occurs in the core. The polymer aggregates at the injection end, resulting in a significant reduction in the pore space at the injection end. There are significant differences between the injection end and the outlet section in the CT scan image, and the pore space at the injection end is significantly larger than that at the outlet end, indicating that a large amount of polymer aggregates and retains at the injection end.

[0066] 3. SEM scan results before and after polymer flooding

[0067] The energy spectrometer determines the composition of a sample by analyzing the energy of the characteristic X-rays released by the sample. Since there is a certain functional relationship between the energy of the characteristic X-rays and the atomic number of the atoms in the sample, by measuring the energy of a certain characteristic X-ray, the corresponding element in the sample can be identified. By analyzing the rock sample with a scanning electron microscope energy spectrometer, the elemental composition of different substances in the rock sample can be obtained, as Figure 10 shown. At the position of spectrum point 21, the contents of Si (silicon) element and Al (aluminum) element are relatively high ( Figure 10 (a)), and it can be judged that this may be a siliceous-aluminous mineral here. While at the position of spectrum point 20, the main elements are C (carbon) and O (oxygen), which can be judged as polymer ( Figure 10 (b)). Thus, the state of the polymer under the scanning electron microscope can be determined, which is in a dark irregular shape, attached to the surface or in the pore spaces, so as to observe the subsequent experimental phenomena.

[0068] Figure 11 It is the SEM image of the original rock sample. During the SEM scan of the original core, the voltage is set at 1 KV - 10 KV. By adjusting the different voltages of the scanning electron microscope, the rock structure in the core, as well as obvious pores and fractures, can be clearly observed.

[0069] Figure 12 It is the SEM image of Experiment 1 (polymer molecular weight is 12.75 million, concentration is 0.50%, viscosity is 604.879 mPa·s). When the polymer molecular weight is small and the concentration is large, both the cracks and pores near the injection end and the outlet end contain polymers. The SEM images are not very different, and the polymer retention effect is not obvious. The presence of polymers can be easily found at both the injection end and the outlet end.

[0070] Figure 13 and Figure 14 They are the SEM images of Experiment 3 and Experiment 4 (polymer molecular weight is 25.55 million, concentration is 0.4%, viscosity is 884.281 mPa·s and polymer molecular weight is 26.92 million, concentration is 0.2%, viscosity is 262.362 mPa·s). When the polymer molecular weight is large, it is difficult for the polymer solution to enter the core, and obvious retention occurs in the core. It can be seen from the SEM that more polymers accumulate at the injection end and are easy to find, while at the outlet end, a small amount of polymers can be found only by magnifying the electron microscope magnification.

[0071] As Figure 15 shown, the method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs provided by the embodiments of the present invention includes:

[0072] S1, performing displacement experiments using polymer solutions with different molecular weights and concentrations;

[0073] S2, observing the retention of polymers in the core before and after displacement through micro-CT and scanning electron microscope (SEM) techniques;

[0074] S3, analyzing the influence of polymer solutions with different molecular weights and concentrations on the retention characteristics in shale reservoirs.

[0075] As Figure 16 shown, the system for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs provided by the embodiments of the present invention includes:

[0076] A displacement experiment module for performing displacement experiments using polymer solutions with different molecular weights and concentrations;

[0077] An SEM module for observing the retention of polymers in the core before and after displacement through micro-CT and scanning electron microscope (SEM) techniques;

[0078] A retention characteristic analysis module for analyzing the influence of polymer solutions with different molecular weights and concentrations on the retention characteristics in shale reservoirs.

[0079] An application embodiment of the present invention provides a computer device, which includes a memory and a processor. The memory stores a computer program. When the computer program is executed by the processor, the processor executes the steps of the method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs.

[0080] An application embodiment of the present invention provides a computer-readable storage medium storing a computer program. When the computer program is executed by the processor, the processor executes the steps of the method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs.

[0081] An application embodiment of the present invention provides an information data processing terminal, which includes a system for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs.

[0082] The present invention conducts displacement experiments using polymer solutions with different molecular weights and concentrations to simulate the migration and retention behavior of fracturing fluid in shale reservoirs. By controlling the fluid injection rate (0.2 mL / min) and confining pressure (10 MPa), the seepage characteristics of polymer fluids under real reservoir conditions are simulated. During the displacement process, the polymer solution enters the shale pores and replaces the original fluid. Part of the polymer is trapped within the pore structure due to adsorption, retention, or shear degradation, affecting the subsequent reservoir permeability and pore connectivity.

[0083] High-resolution three-dimensional scanning of the rock sample before and after displacement is carried out using micro-CT (XRM-500) to reconstruct the changes in the pore structure. The CT scan before displacement can be used to obtain the initial pore distribution of the core, while the CT scan after displacement is used to identify the polymer retention area and pore plugging situation. By comparing the CT images before and after, the retention characteristics of polymer solutions with different molecular weights and concentrations in the pores are analyzed, and their impact on reservoir permeability is quantified.

[0084] Scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) is used to observe the polymer retention situation on the core surface and inside the pores. SEM provides images at the microscale, revealing how polymers form adsorption layers or gel deposits on the pore walls, thereby reducing pore connectivity. EDS energy spectrum analysis can further determine the chemical composition of the retained substances, confirm the deposition characteristics of polymers in different pore environments, and combined with CT scan data, comprehensively evaluate the impact of different polymer solutions on reservoir fluid migration.

[0085] By integrating micro-CT, SEM, and EDS energy spectrum data, comparing the retention characteristics under different experimental conditions, and exploring the effects of molecular weight, concentration, and displacement rate on polymer retention. High molecular weight polymers are generally more likely to be retained in nanoscale pores, while low molecular weight polymers have stronger seepage ability and lower retention rates. The experimental results can provide a scientific basis for optimizing the fracturing fluid formula and reducing fracturing fluid damage, thereby improving the exploitation efficiency of shale reservoirs.

[0086] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any modification, equivalent replacement, improvement, etc. made by any person skilled in the art within the technical scope disclosed by the present invention and within the spirit and principle of the present invention shall be covered by the protection scope of the present invention.

Claims

1. A method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs, characterized in that Including: 1) Conduct displacement experiments using polymer solutions with different molecular weights and concentrations; 2) Observe the retention of polymers in the core before and after displacement through micro-CT and scanning electron microscopy techniques; 3) Analyze the effects of polymer solutions with different molecular weights and concentrations on the retention characteristics in shale reservoirs.

2. The method for analyzing the retention characteristics of the polymer fracturing fluid in the shale reservoir according to claim 1, wherein The specific steps of the displacement experiment are as follows: Select polymer powders with different molecular weights to prepare solutions, and control the permeability and porosity of the used shale core samples at 0.1897×10^-3μm^2 and 2.653% respectively; Use equipment such as a peristaltic pump, core holder, piston intermediate container, confining pressure pump, six-way valve, and pressure gauge to displace the core at a set fluid injection rate.

3. The method for analyzing the retention characteristics of the polymer fracturing fluid in the shale reservoir according to claim 1, wherein The specific process of the displacement experiment is as follows: Adjust the experimental environment temperature to room temperature, connect the pipelines and set the fluid injection rate at 0.2 mL / min; Place the core in the core holder, with an initial confining pressure of 10 MPa, and adjust the confining pressure according to the change of displacement pressure difference; Monitor the injection pressure, confining pressure, and liquid output during the experiment, and record the pressure, flow rate, and time when liquid starts to flow out at the outlet end; After completing one displacement experiment, clean the equipment and replace the polymer solution, and repeat the above steps.

4. The method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs according to claim 1, wherein, The specific steps of the micro-CT scanning are as follows: Use a Versa XRM-500 type micro-CT scanner with a resolution of 1μm to perform CT scans on the core before and after displacement, with a test temperature of 20°C and a working voltage of 150 KV; Compare the changes in pore distribution in the core before and after scanning to determine the retention and migration of polymers inside the rock.

5. The method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs according to claim 1, wherein The specific process of the scanning electron microscopy observation is as follows: Take core cross-section samples before and after the displacement experiment for scanning electron microscopy imaging; Combine an energy dispersive spectrometer (EDS) to analyze the characteristic X-rays released by the samples, identify the elemental composition of the samples, and then judge the adhesion and blockage degree of polymers on the rock surface.

6. The method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs according to claim 1, wherein The results of the scanning electron microscopy and energy spectrum analysis show that: Polymers form viscous films or local deposits at pores or throats; The energy dispersive spectrometer detects the distribution of characteristic elements related to polymers, indicating that polymers are retained on the rock surface and inside the pores.

7. The method for analyzing the retention characteristics of polymer fracturing fluid in shale reservoirs according to claim 1, wherein During the comprehensive analysis: Evaluate the retention amount and distribution of polymers in the pore structure of the shale reservoir by comparing the micro-CT scans and scanning electron microscopy images before and after displacement; Combine data such as the pressure difference and liquid output caused by polymers with different molecular weights and concentrations during the flow process to obtain the retention law and mechanism of polymer fracturing fluids in the shale reservoir, providing a reference basis for on-site fracturing fluid formulations and construction processes.

8. A shale reservoir polymer fracturing fluid retention characteristic analysis system for implementing the shale reservoir polymer fracturing fluid retention characteristic analysis method according to any one of claims 1 to 7, characterized in that Including: A displacement experiment module for conducting displacement experiments using polymer solutions with different molecular weights and concentrations; An electron microscopy scanning module for observing the retention of polymers in the core before and after displacement through micro-CT and scanning electron microscopy techniques; A retention characteristic analysis module for analyzing the effects of polymer solutions with different molecular weights and concentrations on the retention characteristics in shale reservoirs.

9. A computer-readable storage medium stores a computer program. When the computer program is executed by a processor, the processor is caused to execute the steps of the method for analyzing the retention characteristics of polymer fracturing fluids in a shale reservoir according to any one of claims 1 to 6.

10. An information data processing terminal, the information data processing terminal includes the shale reservoir polymer fracturing fluid retention characteristic analysis system as described in claim 7.

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