Shale supercritical CO2 fracturing experiment method and system

Abstract

The invention discloses a supercritical CO2 fracturing experiment method and system for shale. The method comprises the following steps: acquiring a natural crack distribution pattern of a shale fracturing test piece; testing the permeability of the shale fracturing test piece; then air tightness inspection before fracturing is carried out; loading set axial pressure and confining pressure, and stabilizing the pressure; injecting supercritical CO2 into the shale fracturing test piece at a constant temperature to carry out a fracturing experiment; testing the permeability of the fractured shale fracturing test piece; analyzing the shale fracturing permeability increasing effect according to the permeability data before and after fracturing; and obtaining the supercritical CO2 fracturing crack propagation form according to the fracturing crack distribution form and the natural crack distribution form. According to the method disclosed by the invention, experimental studies on a crack propagation mechanism and fracturing permeation enhancement in a shale supercritical CO2 fracturing process are carried out by virtue of an acoustic emission monitoring technology and an industrial CT technology, the feasibility of realizing shale gas reservoir volume fracturing transformation by virtue of supercritical CO2 fracturing is proved, and a reference is provided for exploring a shale gas reservoir anhydrous fracturing technology.

Description

Technical Field

[0001] The present invention belongs to the field of shale oil exploitation, and more specifically, relates to a supercritical CO2 fracturing experimental method and system for shale. Background Art

[0002] In view of the low porosity, low permeability and high clay content of my country's shale reservoirs, waterless fracturing technology has emerged, mainly including liquid CO2 fracturing, liquid N2 fracturing, liquid propane fracturing, supercritical CO2 fracturing, etc. In recent years, inspired by CO2 enhanced oil recovery and CO2 geological storage, combined with the characteristics of supercritical CO2, supercritical CO2 fracturing has become a new hotspot for global shale gas development. Although hydraulic fracturing can achieve large-scale commercial exploitation of shale gas, the problems it brings about, such as clay hydration expansion, water resource waste and environmental pollution, should not be underestimated.

[0003] As a new type of waterless fracturing method, supercritical CO2 fracturing is applied to shale gas development and can effectively solve the problem of water shortage in my country's shale gas-rich areas. At the same time, compared with traditional hydraulic fracturing technology, it can effectively avoid clay hydration and expansion in shale gas reservoirs, reduce reservoir damage, and increase shale gas production. Reservoir fracturing transformation with supercritical CO2 fluid can produce more tiny cracks in the reservoir, which is conducive to the exploitation of shale oil and gas. Since shale has a strong adsorption capacity for CO2, it can improve the recovery rate of shale oil and gas while realizing the geological storage of CO2. The distribution of fracture morphology and hydraulic characteristics have a decisive influence on the seepage and production law of shale gas in the reservoir. Supercritical CO2 fluid has the characteristics of low viscosity and strong diffusivity. In addition, the original discontinuity in the shale reservoir will make the fracture morphology and fracture formation mechanism more complicated.

[0004] The invention patent with application number 202211183137.6 discloses an integrated method of supercritical CO2 fracturing-in-situ leaching mining and CO2 storage of sandstone-type uranium mines, but this method can be applied to sandstone but not to shale. Therefore, how to obtain the crack propagation mechanism and fracturing and permeability enhancement of shale reservoirs during fracturing using supercritical CO2, provide a reference for exploring the waterless fracturing technology of shale gas reservoirs, and verify the feasibility of supercritical CO2 fracturing to achieve volume fracturing transformation of shale gas reservoirs is the problem to be solved by the present invention.

[0005] The information disclosed in the background technology section of the present invention is only intended to deepen the understanding of the general background technology of the present invention, and should not be regarded as acknowledging or suggesting in any form that the information constitutes the prior art already known to those skilled in the art. Summary of the invention

[0006] The purpose of the present invention is to propose a supercritical CO2 fracturing experimental method and system for shale, to realize the experimental study of the crack propagation mechanism and fracturing and permeability enhancement in the process of supercritical CO2 fracturing of shale, to provide a reference for exploring the waterless fracturing technology of shale gas reservoirs, and to confirm the feasibility of supercritical CO2 fracturing to achieve volume fracturing transformation of shale gas reservoirs.

[0007] To achieve the above objectives, the present invention proposes a supercritical CO2 fracturing experimental method and system for shale.

[0008] According to a first aspect of the present invention, a supercritical CO2 fracturing experimental method for shale is proposed, comprising:

[0009] Obtain the natural crack distribution morphology of shale fracturing specimens;

[0010] Conducting a permeability test on the shale fracturing specimen in the experimental chamber to obtain permeability data before fracturing;

[0011] After the permeability test is completed, the test chamber is inspected for air tightness before fracturing;

[0012] Load the set axial pressure and confining pressure, and maintain constant pressure during the fracturing experiment;

[0013] Injecting supercritical CO2 into the shale fracturing specimen to perform the fracturing experiment, and maintaining a constant temperature during the fracturing experiment to obtain real-time fracturing data;

[0014] Performing a permeability test on the shale fracturing specimen that has completed the fracturing experiment to obtain permeability data after fracturing;

[0015] Analyze the shale fracturing permeability enhancement effect according to the pre-fracturing permeability data and the post-fracturing permeability data;

[0016] Obtaining the distribution morphology of the shale fracturing specimen that has completed the fracturing experiment, and comparing it with the natural crack distribution morphology to obtain the supercritical CO2 fracturing crack extension morphology;

[0017] The real-time fracturing data is output.

[0018] Optionally, before obtaining the natural crack distribution morphology of the shale fracturing specimen, the method further includes making a shale fracturing specimen.

[0019] Optionally, the method of making a shale fracturing test piece specifically includes:

[0020] Drilling a cylinder of a first set specification in a direction perpendicular to the bedding plane of the outcropped shale;

[0021] Drilling a cylindrical hole of a second set specification at the bottom of the cylinder along the axial direction;

[0022] Filling industrial sodium chloride to a set depth of the cylindrical hole;

[0023] Inserting a high-strength steel pipe of a third set specification to the set depth of the cylindrical hole;

[0024] The gap between the high-strength steel pipe and the cylindrical hole is sealed by a high-strength adhesive, and the high-strength steel pipe is fixed.

[0025] Optionally, before the fracturing experiment, the natural crack distribution morphology of the shale fracturing specimen is obtained by performing CT scanning on the shale fracturing specimen, and photographing it with a high-definition digital camera;

[0026] After the fracturing experiment is completed, the shale fracturing specimen is CT scanned to obtain the distribution morphology of the fracturing cracks of the shale fracturing specimen, and photographed with a high-definition digital camera.

[0027] Optionally, the axial pressure is set to 16 MPa and the confining pressure is set to 12 MPa.

[0028] Optionally, the constant temperature is 35°C.

[0029] Optionally, the set axial pressure and the confining pressure are loaded by a triaxial servo hydraulic testing machine, and a constant pressure is maintained during the fracturing experiment.

[0030] Optionally, the real-time fracturing data includes:

[0031] Acoustic emission information of crack initiation and propagation process inside the shale fracturing specimen;

[0032] The pressure change of the shale fracturing specimen during the fracturing process;

[0033] The deformation data of the shale fracturing specimen during the fracturing process.

[0034] Optionally, helium is used to perform the permeability test.

[0035] According to a second aspect of the present invention, a supercritical CO2 fracturing experimental system for shale is provided, which is used to perform the supercritical CO2 fracturing experimental method for shale described in any one of the first aspects, comprising:

[0036] Industrial CT, used for scanning the shale fracturing specimen before the fracturing experiment and the shale fracturing specimen after the fracturing experiment, to obtain the natural crack distribution morphology and the fracturing crack distribution morphology of the shale fracturing specimen;

[0037] Gas supply subsystem, used to provide gas for permeability test, gas for air tightness check and CO2;

[0038] A CO2 pressurization control subsystem, used for pressurizing the CO2 to turn it into supercritical CO2;

[0039] A three-axis servo loading subsystem, used for loading the shale fracturing specimen with a set axial pressure and confining pressure, and maintaining a constant pressure during the fracturing experiment;

[0040] A temperature control subsystem for maintaining a constant temperature during the fracturing experiment;

[0041] An acoustic emission monitoring subsystem, used for collecting acoustic emission information of crack initiation and propagation process inside the shale fracturing specimen during the fracturing experiment;

[0042] The data acquisition subsystem is used to collect real-time fracturing data of the shale fracturing specimen during the fracturing experiment and output the real-time fracturing data.

[0043] The beneficial effects of the present invention are as follows: the present invention obtains the natural crack distribution morphology of the shale fracturing specimen before fracturing through industrial CT technology, obtains the permeability data of the shale fracturing specimen before fracturing, injects supercritical CO2 into the shale fracturing specimen under a leakage-free, constant temperature, constant axial pressure and confining pressure environment to perform a fracturing experiment, obtains real-time fracturing data, obtains the permeability data and the distribution morphology of the fracturing cracks of the shale fracturing specimen after fracturing, and analyzes the permeability increase of shale fracturing according to the permeability data before fracturing and the permeability data after fracturing. The effect is obtained by obtaining the crack extension morphology of supercritical CO2 fracturing according to the distribution morphology of natural cracks and the distribution morphology of fracturing cracks. The feasibility of realizing volume fracturing transformation of shale gas reservoirs through supercritical CO2 fracturing is confirmed according to the permeability enhancement effect of shale fracturing, the crack extension morphology of supercritical CO2 fracturing and real-time fracturing data. The present invention uses acoustic emission monitoring technology and industrial CT technology to carry out experimental research on the crack extension mechanism and fracturing permeability enhancement in the process of shale supercritical CO2 fracturing, providing a reference for exploring waterless fracturing technology for shale gas reservoirs. The shale permeability after fracturing is increased by 3 to 4 orders of magnitude, indicating that the permeability enhancement effect of supercritical CO2 fracturing is obvious; supercritical fluid fracturing can form complex network cracks, which confirms the feasibility of realizing volume fracturing transformation of shale gas reservoirs through supercritical CO2 fracturing.

[0044] The system of the present invention has other characteristics and advantages, which will be apparent from the drawings incorporated herein and the following detailed description, or will be described in detail in the drawings incorporated herein and the following detailed description, which together serve to explain the specific principles of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention in conjunction with the accompanying drawings, in which like reference numerals generally represent like components.

[0046] Figure 1 A flow chart showing the steps of a shale supercritical CO2 fracturing experimental method according to the present invention.

[0047] Figure 2 A schematic diagram of a shale fracturing specimen according to Example 1 of the present invention is shown.

[0048] Figure 3 A schematic diagram of an experimental system for a supercritical CO2 fracturing experimental method for shale according to Example 1 of the present invention is shown.

[0049] Figure 4 A schematic diagram showing the variation of the permeability of a shale fracturing specimen with volume stress before fracturing according to Example 1 of the present invention is shown.

[0050] Figure 5 A schematic diagram showing the variation of the permeability of a shale fracturing specimen with volume stress after fracturing according to Example 1 of the present invention is shown.

[0051] Figure 6 A schematic diagram comparing the natural crack distribution morphology and the hydraulic crack distribution morphology of a shale hydraulic fracturing specimen according to Example 1 of the present invention is shown.

[0052] Description of reference numerals:

[0053] 1. Helium cylinder, 2. Carbon dioxide cylinder, 3. Control valve, 4. Control valve, 6. High-precision plunger pump, 5. Pressure reducing valve, 7. Pressure sensor, 8. Control valve, 9. Triaxial stress loading chamber, 10. Shale fracturing specimen, 11. Acoustic emission receiving probe, 12. Constant temperature oil bath, 13. Acoustic emission amplifier, 14. Acoustic emission device, 15. Pressure sensor, 16. Control valve, 17. Flow meter, 18. Control valve, 19. Vacuum pump, 20. Computer. DETAILED DESCRIPTION

[0054] The present invention will be described in more detail below with reference to the accompanying drawings. Although preferred embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0055] like Figure 1 As shown, a supercritical CO2 fracturing experimental method for shale according to the present invention comprises:

[0056] Obtain the natural crack distribution morphology of shale fracturing specimens;

[0057] Conduct permeability tests on shale fracturing specimens in the experimental chamber to obtain permeability data before fracturing;

[0058] After the permeability test is completed, the air tightness of the experimental chamber is checked before fracturing;

[0059] Load the set axial pressure and confining pressure, and maintain constant pressure during the fracturing experiment;

[0060] Inject supercritical CO2 into shale fracturing specimens to conduct fracturing experiments, and maintain a constant temperature during the fracturing experiment to obtain real-time fracturing data;

[0061] Conduct permeability test on shale fracturing specimens that have completed fracturing experiments to obtain permeability data after fracturing;

[0062] Analyze the shale fracturing permeability enhancement effect based on the pre-fracturing permeability data and post-fracturing permeability data;

[0063] Obtain the distribution morphology of the shale fracturing specimens that have completed the fracturing experiment, and compare it with the natural crack distribution morphology to obtain the supercritical CO2 fracturing crack extension morphology;

[0064] Output real-time fracturing data.

[0065] Specifically, the present invention uses industrial CT to perform tomography scanning on intact shale fracturing specimens before supercritical CO2 fracturing to obtain the natural crack distribution morphology of the specimens, and uses high-definition camera equipment, such as a high-definition digital camera, to take pictures to obtain pictures of the natural crack distribution morphology; then the shale fracturing specimens are placed in an experimental cabin to perform a shale permeability test on the shale fracturing specimens before supercritical CO2 fracturing, such as using He gas for a seepage experiment, placing the shale fracturing specimens in a triaxial stress loading chamber, and using different volume stresses and gas pressures in a constant temperature environment to perform shale permeability tests on the shale fracturing specimens before supercritical CO2 fracturing, to provide comparative data for subsequent fracturing permeability effect analysis, for example, at a temperature of 35 ℃, and the permeability tests with gas pressures of 1MPa and 1.8MPa were carried out at volume stresses of 0.07, 0.09, 0.16, 0.23, and 0.39MPa, respectively; after the shale permeability test was completed, the air tightness check of the experimental chamber was carried out before fracturing to ensure the authenticity and reliability of the experimental data, and to prevent leakage caused by poor air tightness during the fracturing process, thereby affecting the experimental data. For example, the air tightness of the experimental chamber was checked by 1.0MPa air pressure gas to ensure that the fracturing experiment was carried out only when the experimental chamber was leak-free; after the air tightness check was qualified, the set axial pressure and confining pressure were loaded into the experimental chamber, and the pressure was stabilized to maintain constant axial pressure and confining pressure during the experiment; after the pressure loading was completed, supercritical CO2 was injected into the shale fracturing specimen for Fracturing experiment, and keep a constant temperature during the experiment, and obtain the fracturing data of the whole experiment in real time; for example, collect the sound of shale fracturing specimens during fracturing, the pressure change of shale fracturing specimens during fracturing, and the deformation of shale fracturing specimens; after the experiment is completed, the permeability test of the shale fracturing specimens that have completed the fracturing experiment is carried out to obtain the permeability data after fracturing, for example, the permeability test of gas pressure 1MPa and 1.8MPa is carried out at a temperature of 35°C and a volume stress of 9, 15, 21, 27, and 33MPa, respectively. The permeability data after fracturing is compared with the permeability data before fracturing, and the permeability enhancement effect of shale fracturing is analyzed. The permeability of shale original rock is extremely low, and the permeability of shale is improved after fracturing. It is 3 to 4 orders of magnitude higher, indicating that the supercritical CO2 fracturing has an obvious permeability enhancement effect. Then the shale fracturing specimens that have completed the experiment are taken out, and the fracturing crack distribution morphology of the shale fracturing specimens that have completed the fracturing experiment is obtained, and compared with the natural crack distribution morphology to obtain the supercritical CO2 fracturing crack extension morphology. For example, the last scanning position of the shale fracturing specimen is tomographically scanned by industrial CT, and the shale fracturing specimen is photographed by a high-definition digital camera. The CT results before and after the specimen is fracturing are compared, and the surface of the shale fracturing specimen is directly observed with the naked eye to describe the supercritical fluid fracturing crack extension morphology. Supercritical fluid fracturing can form complex network cracks, which confirms the feasibility of supercritical CO2 fracturing to achieve volume fracturing transformation of shale gas reservoirs.

[0066] In one example, before obtaining the natural crack distribution morphology of the shale fracturing specimen, the method further includes making the shale fracturing specimen.

[0067] In one example, making a shale fracturing test piece specifically includes:

[0068] Drilling a cylinder of a first set specification in a direction perpendicular to the bedding plane of the outcropped shale;

[0069] Drilling a cylindrical hole of a second set specification along the axial direction at the bottom of the cylinder;

[0070] Fill the cylindrical hole with industrial sodium chloride to a set depth;

[0071] inserting a high-strength steel pipe of a third set specification to a set depth of the cylindrical hole;

[0072] The gap between the high-strength steel pipe and the cylindrical hole is sealed by a high-strength adhesive, and the high-strength steel pipe is fixed.

[0073] Specifically, a cylinder of a first set specification is drilled in a direction perpendicular to the bedding plane of the outcropped shale, that is, a section of cylindrical shale is drilled downward perpendicular to the bedding plane of the outcropped shale, and after measurement, cutting, grinding and other processing steps, a cylinder of a first set specification is obtained, and then a cylindrical hole of a second set specification is drilled in the cylinder along the axial direction at the upper bottom or lower bottom of the cylinder, and industrial sodium chloride is filled to a set depth of the cylindrical hole. The industrial sodium chloride filling area serves as an open hole section, and initial cracks will be formed in the section when simulating fracturing through the open hole section, and then a high-strength steel pipe of a third set specification is inserted to the set depth of the cylindrical hole, that is, industrial sodium chloride is filled to the set depth of the cylindrical hole, and a vertical wellbore is simulated through this steel pipe, and the high-strength steel pipe is inserted into the test piece, and finally the pore between the high-strength steel pipe and the cylindrical hole is sealed by a high-strength adhesive, and the high-strength steel pipe is fixed.

[0074] For example, a cylindrical specimen with a diameter of 100 mm and a length of 200 mm is drilled perpendicular to the bedding plane of the outcropped shale, a circular hole with a diameter of 14 mm and a depth of 130 mm is drilled in the center of the specimen, the hole depth of 80-130 mm is filled with industrial Nacl (sodium chloride), a high-strength steel pipe with an outer diameter of 8 mm, an inner diameter of 6 mm and a length of 80 mm is inserted into the specimen, and a high-strength adhesive is used to seal the 3 mm annular gap between the casing and the prefabricated wellbore.

[0075] In one example, before the fracturing experiment, the natural crack distribution morphology of the shale fracturing specimen is obtained by CT scanning the shale fracturing specimen, and photographed with a high-definition digital camera;

[0076] After the fracturing experiment is completed, the shale fracturing specimen is CT scanned to obtain the fracturing crack distribution morphology of the shale fracturing specimen, and a high-definition digital camera is used to photograph it.

[0077] Specifically, before the fracturing experiment, the natural crack distribution morphology of the shale fracturing specimen is obtained by performing CT scanning on the shale fracturing specimen, and photographed with a high-definition digital camera; after the fracturing experiment is completed, the fracturing crack distribution morphology of the shale fracturing specimen is obtained by performing CT scanning on the same scanning position of the shale fracturing specimen before the fracturing experiment, and photographed with a high-definition digital camera.

[0078] In one example, the set axial pressure is 16 MPa and the set confining pressure is 12 MPa.

[0079] In one example, the constant temperature is 35°C.

[0080] In one example, a set axial pressure and confining pressure are loaded by a triaxial servo hydraulic testing machine, and a constant pressure is maintained during the fracturing experiment.

[0081] Specifically, a shale fracturing specimen is placed in a three-axis servo hydraulic testing machine for testing. A set axial pressure and a set confining pressure are applied to the shale fracturing specimen by the three-axis servo hydraulic testing machine, and constant axial pressure and confining pressure are maintained during the fracturing experiment.

[0082] In one example, real-time fracturing data includes:

[0083] Acoustic emission information of crack initiation and propagation process inside shale fracturing specimens;

[0084] Pressure changes of shale fracturing specimens during fracturing;

[0085] Deformation data of shale fracturing specimens during the fracturing process.

[0086] Specifically, during the experiment, real-time data such as acoustic emission information of the initiation and propagation process of cracks inside the shale fracturing specimen, pressure changes during the fracturing process of the shale fracturing specimen, and deformation data of the shale fracturing specimen during the fracturing process are collected. For example, acoustic emission information of the initiation and propagation process of cracks inside the shale fracturing specimen is collected through an acoustic emission monitoring system, and pressure changes of the shale fracturing specimen during the fracturing process are collected by using pressure sensors at the upper and lower ends of the shale fracturing specimen to determine the initiation pressure, and deformation data of the shale fracturing specimen during the fracturing process is obtained through a three-axis servo hydraulic testing machine.

[0087] In one example, permeability testing is performed using helium.

[0088] Specifically, helium is used to carry out permeability tests on shale fracturing specimens before and after fracturing. The permeability tests are carried out with the same gas pressure and different volume stresses. Then the gas pressure is changed and the above steps are repeated to carry out permeability tests. The permeability law of the shale fracturing specimens before and after fracturing is analyzed based on the permeability test results before and after fracturing. During the test, a constant temperature is maintained.

[0089] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments, but is not intended to be limiting of the present invention. It should be noted that the embodiments and features in the embodiments of the present invention may be combined with each other without conflict.

[0090] Example 1

[0091] This embodiment provides a supercritical CO2 fracturing experimental method for shale, comprising:

[0092] To make shale fracturing specimens, a cylindrical specimen with a diameter of 100 mm and a length of 200 mm was drilled perpendicular to the bedding plane of the outcropped shale. A circular hole with a diameter of 14 mm and a depth of 130 mm was drilled in the center of the specimen. The hole depth of 80-130 mm was filled with industrial NaCl to leave a 50 mm open hole section at the bottom of the high-strength steel pipe. Initial cracks will be formed in this section during simulated fracturing. A high-strength steel pipe with an outer diameter of 8 mm, an inner diameter of 6 mm, and a length of 80 mm was inserted into the specimen to simulate a vertical wellbore. A high-strength adhesive was used to seal the 3 mm annular pore between the casing and the prefabricated wellbore. After the shale fracturing specimen was made, the shale fracturing specimen was as follows: Figure 2 As shown in FIG, the shale fracturing specimen is subjected to industrial CT tomography to obtain its natural crack distribution morphology, and is photographed with a high-definition digital camera; Figure 3As shown, a shale fracturing specimen 10 is placed in a triaxial stress loading chamber 9, two acoustic emission receiving probes 11 are symmetrically placed on the left and right end surfaces of a triaxial pressure rod simulating the vertical ground stress direction, and a coupling agent is used to bond the probe to the pressure rod so as to effectively monitor the acoustic emission information of the crack initiation and expansion process inside the specimen. Pressure sensors are used at the upper and lower ends of the shale fracturing specimen to collect pressure changes during the fracturing process so as to determine the initiation pressure; a shale permeability test is performed on the shale fracturing specimen 10 before the supercritical CO2 fracturing experiment to provide comparative data for the subsequent analysis of the fracturing permeability effect; a seepage experiment is performed using helium, and the temperature of the constant temperature oil bath 12 is maintained at 35°C, so that the temperature of the triaxial stress loading chamber 9 is also maintained at 35°C, that is, the test environment temperature is maintained at 35°C, and then the valve 3 is opened to transport the helium in the helium bottle 1 to the high-precision plunger pump 6, and the volume stress of the helium is adjusted by the high-precision plunger pump 6, and at the same time, the pressure sensor 7 is used to adjust the volume stress of the helium. The gas pressure of helium is monitored and adjusted through valve 3. When the volume stress is 0.07, 0.09, 0.16, 0.23, and 0.39 MPa, the permeability tests with gas pressures of 1 MPa and 1.8 MPa are carried out respectively. That is, after the volume stress is adjusted to 0.07 MPa and the gas pressure is adjusted to 1 MPa, valve 8 is opened, and helium with a volume stress of 0.07 MPa and a gas pressure of 1 MPa is input into the triaxial stress loading chamber 9 for permeability test. After the test is completed, valve 8 is closed, and the gas pressure is adjusted to 0.09 MPa, valve 8 is opened, and helium with a volume stress of 0.09 MPa and a gas pressure of 1.8 MPa is input into the triaxial stress loading chamber 9 for permeability test. In this way, permeability tests with gas pressures of 1 MPa and 1.8 MPa are carried out respectively at volume pressures of 0.09, 0.16, 0.23, and 0.39. The test results are shown in FIG. Figure 3 As shown in Figure 2, the shale permeability before fracturing is between (0.068 and 0.393)×10 － 18 m 2After the test is completed, gas with a gas pressure of 1.0Mpa is introduced into the triaxial stress loading chamber 9 for airtightness inspection to ensure the authenticity and reliability of the experimental data. The fracturing experiment is carried out only when the system is leak-proof. After the inspection is qualified, the axial pressure and confining pressure strip loading are completed through the triaxial stress loading chamber 9, and the axial pressure is kept constant at 16MPa and the confining pressure is kept constant at 12Mpa; the high-precision plunger pump 6 is started to pressurize the CO2 medium output by the carbon dioxide bottle 2 into supercritical CO2 and then injected into the pre-supported wellbore of the shale fracturing specimen 10 in the triaxial stress loading chamber 9. The constant flow operation mode is adopted, and the injection rate is set to 30mL / min to carry out the fracturing experiment. In order to ensure that CO2 reaches the supercritical state (critical pressure 7.38MPa, temperature 31.4℃), the temperature is kept constant at 35℃ during the experiment, and the temperature of the shale fracturing specimen 10 is kept the same as the oil bath temperature of the constant temperature oil bath box 12. The pump pressure-time variation law, acoustic emission signal, deformation and other fracturing data of the shale fracturing specimen 10 during the fracturing process are collected in real time and synchronously through the acoustic emission amplifier 13, the acoustic emission device 14, the pressure sensors arranged at the upper and lower ends of the shale fracturing specimen 10 and the computer 20. After the fracturing test is completed, the high-precision plunger pump 6 and the acoustic emission device 14 are stopped, the axial pressure and the confining pressure of the triaxial stress loading chamber 9 are stably unloaded to 0, and the post-fracturing permeability test of helium gas with a gas pressure of 1MPa and 1.8MPa is carried out at a temperature of 35°C and a volume stress of 9, 15, 21, 27, and 33MPa, respectively. The permeability enhancement effect of shale fracturing is analyzed according to the pre-fracturing permeability test results and the post-fracturing permeability test results; Figure 4 As shown in Figure 2, the shale permeability before fracturing is between (0.068 and 0.393)×10 －18 m 2 ,like Figure 5 As shown in Figure 2, the shale permeability after fracturing is between (0.732 and 1.520)×10 －15 m 2 After fracturing, the shale permeability increased by 3 to 4 orders of magnitude, indicating that supercritical CO2 fracturing has a significant effect on permeability enhancement; the shale fracturing specimens after fracturing were disassembled, their surfaces were directly observed with the naked eye, photographed with a high-definition digital camera, and industrial CT tomography was performed on the shale fracturing specimens after fracturing at the same position as before fracturing. The CT results before and after the specimens were compared to describe the crack extension morphology of supercritical CO2 fracturing, and the plane extension morphology and extension law were obtained, such as Figure 6 As shown, the left figure shows the distribution of natural cracks, and the right figure shows the distribution of hydraulic cracks. Supercritical fluid fracturing can form complex network cracks, which confirms the feasibility of supercritical CO2 fracturing to achieve volume fracturing transformation of shale gas reservoirs.

[0093] Example 2

[0094] This embodiment provides a supercritical CO2 fracturing experimental system for shale, which is used to perform the supercritical CO2 fracturing experimental method for shale described in Example 1, including:

[0095] Industrial CT is used to scan shale fracturing specimens before and after fracturing experiments to obtain the natural crack distribution morphology and fracturing crack distribution morphology of shale fracturing specimens;

[0096] High-definition digital camera, used to photograph the natural crack distribution morphology and hydraulic crack distribution morphology of shale fracturing specimens;

[0097] Gas supply subsystem, used to provide gas for permeability test, gas for air tightness check and CO2;

[0098] The CO2 pressurization control subsystem is used to pressurize CO2 to make it supercritical CO2;

[0099] The three-axis servo loading subsystem is used to load the shale fracturing specimen with the set axial pressure and confining pressure, and maintain constant pressure during the fracturing experiment;

[0100] Temperature control subsystem, used to maintain a constant temperature during the fracturing experiment;

[0101] Acoustic emission monitoring subsystem, used to collect acoustic emission information of crack initiation and expansion process inside shale fracturing specimens during fracturing experiments;

[0102] The data acquisition subsystem is used to collect real-time fracturing data of shale fracturing specimens during the fracturing experiment and output the real-time fracturing data.

[0103] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A supercritical CO2 fracturing experimental method for shale, characterized in that: include: Obtain the natural crack distribution morphology of shale fracturing specimens; Conducting a permeability test on the shale fracturing specimen in the experimental chamber to obtain permeability data before fracturing; After the permeability test is completed, the test chamber is inspected for air tightness before fracturing; Load the set axial pressure and confining pressure, and maintain constant pressure during the fracturing experiment; Injecting supercritical CO2 into the shale fracturing specimen to perform the fracturing experiment, and maintaining a constant temperature during the fracturing experiment to obtain real-time fracturing data; Performing a permeability test on the shale fracturing specimen that has completed the fracturing experiment to obtain permeability data after fracturing; Analyze the shale fracturing permeability enhancement effect according to the pre-fracturing permeability data and the post-fracturing permeability data; Obtaining the distribution morphology of the shale fracturing specimen that has completed the fracturing experiment, and comparing it with the natural crack distribution morphology to obtain the supercritical CO2 fracturing crack extension morphology; The real-time fracturing data is output.

2. The supercritical CO2 fracturing experimental method for shale according to claim 1, characterized in that: Before obtaining the natural crack distribution morphology of the shale fracturing specimen, the shale fracturing specimen is also prepared.

3. The supercritical CO2 fracturing experimental method for shale according to claim 2, characterized in that: The preparation of the shale fracturing test piece specifically includes: Drilling a cylinder of a first set specification in a direction perpendicular to the bedding plane of the outcropped shale; Drilling a cylindrical hole of a second set specification at the bottom of the cylinder along the axial direction; Filling industrial sodium chloride to a set depth of the cylindrical hole; Inserting a high-strength steel pipe of a third set specification to the set depth of the cylindrical hole; The gap between the high-strength steel pipe and the cylindrical hole is sealed by a high-strength adhesive, and the high-strength steel pipe is fixed.

4. The supercritical CO2 fracturing experimental method for shale according to claim 1, characterized in that: Before the fracturing experiment, the natural crack distribution morphology of the shale fracturing specimen is obtained by performing CT scanning on the shale fracturing specimen, and photographing it with a high-definition digital camera; After the fracturing experiment is completed, the shale fracturing specimen is CT scanned to obtain the distribution morphology of the fracturing cracks of the shale fracturing specimen, and photographed with a high-definition digital camera.

5. The supercritical CO2 fracturing experimental method for shale according to claim 1, characterized in that: The set axial pressure is 16 MPa and the set confining pressure is 12 MPa.

6. The supercritical CO2 fracturing experimental method for shale according to claim 1, characterized in that: The constant temperature was 35°C.

7. The supercritical CO2 fracturing experimental method for shale according to claim 1, characterized in that: The set axial pressure and the confining pressure are loaded by a triaxial servo hydraulic testing machine, and a constant pressure is maintained during the fracturing experiment.

8. The supercritical CO2 fracturing experimental method for shale according to claim 1, characterized in that: The real-time fracturing data includes: Acoustic emission information of crack initiation and propagation process inside the shale fracturing specimen; The pressure change of the shale fracturing specimen during the fracturing process; The deformation data of the shale fracturing specimen during the fracturing process.

9. The supercritical CO2 fracturing experimental method for shale according to claim 1, characterized in that: The permeability test was conducted using helium gas.

10. A supercritical CO2 fracturing experimental system for shale, used to perform the supercritical CO2 fracturing experimental method for shale according to any one of claims 1 to 9, characterized in that: include: Industrial CT, used for scanning the shale fracturing specimen before the fracturing experiment and the shale fracturing specimen after the fracturing experiment, to obtain the natural crack distribution morphology and the fracturing crack distribution morphology of the shale fracturing specimen; Gas supply subsystem, used to provide gas for permeability test, gas for air tightness check and CO2; A CO2 pressurization control subsystem, used for pressurizing the CO2 to turn it into supercritical CO2; A three-axis servo loading subsystem, used for loading the shale fracturing specimen with a set axial pressure and confining pressure, and maintaining a constant pressure during the fracturing experiment; A temperature control subsystem for maintaining a constant temperature during the fracturing experiment; An acoustic emission monitoring subsystem, used for collecting acoustic emission information of crack initiation and propagation process inside the shale fracturing specimen during the fracturing experiment; The data acquisition subsystem is used to collect real-time fracturing data of the shale fracturing specimen during the fracturing experiment and output the real-time fracturing data.

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