Dark field imaging-based nonlinear percolation experiment method and device for gas to break through cover layer

Abstract

The invention relates to a non-linear seepage experiment method for gas breaking through a cover layer based on dark field imaging. The method comprises the following steps: placing a microfluidic rock chip on an objective table of a dark field scattering microscope; the injection end of the microfluidic rock chip is connected with a fluid injection module, and the outlet end of the microfluidic rock chip is connected with a downstream outflow pipeline; injecting the formation water solution into the micro-fluidic rock chip until the micro-fluidic rock chip is filled with the formation water solution; opening a gas injection system, and adjusting the dark field scattering microscope until a clear image of a micro-fluidic rock chip pore structure and carbon dioxide displacement formation water is obtained; injecting carbon dioxide with different pressures into the micro-fluidic rock chip, and observing nonlinear seepage migration images of carbon dioxide and formation water in the micro-fluidic rock chip; when it is observed that carbon dioxide migrates to the outlet end of the rock chip, the breakthrough moment is obtained, and the upstream and downstream pressure difference value of the microfluidic rock chip is the breakthrough pressure. According to the method, the nonlinear seepage of the carbon dioxide displacement formation water in the nanopores can be accurately observed, and the breakthrough pressure can be determined.

Description

Technical Field

[0001] The present invention belongs to the technical field of low-permeability rock tightness evaluation, and in particular relates to a dark-field imaging-based nonlinear seepage experimental method and device for gas breakthrough caprock. Background Art

[0002] Geological energy storage technology is rapidly developing, encompassing diverse areas such as natural gas storage, carbon dioxide storage, and underground hydrogen storage. To prevent the leakage of gases like carbon dioxide, hydrogen, and natural gas from low-permeability geological formations, evaluating the sealing properties of the low-permeability rocks at the storage site is crucial. Consequently, current research is increasingly focusing on the nonlinear seepage characteristics of gases like carbon dioxide, hydrogen, and natural gas in low-permeability rocks.

[0003] Core flooding experiments are currently commonly used to study the fluid migration characteristics of low-permeability rocks. However, because existing flooding experimental equipment rarely allows for visual observation of gas migration in low-permeability rocks, the fluid flow in low-permeability rocks is often assumed to be linear, and the fluid flow characteristics are determined by pressure curves. However, due to the high formation stresses and sedimentary environment at depth, the pore size of deep rocks is nanometer-scale, resulting in an extremely slow, non-Darcy and nonlinear process of gas displacement of liquid within low-permeability rocks. Even if gas breaks through the rock, the minimal amount of liquid discharged makes it difficult for pressure sensors to detect small pressure changes, making it difficult to accurately determine the moment of gas breakthrough. This can easily lead to ignoring the discharged liquid and continuously increasing the pressure at the gas entry point, resulting in large errors in test results.

[0004] Therefore, in order to overcome the shortcomings of existing experimental measurement methods and devices in monitoring the nonlinear seepage of low permeability rocks, it is necessary to design a high-precision visual test device for measuring the nonlinear seepage of low permeability rocks.

[0005] Chinese patent CN 112858628 A discloses a microscopic visualization experimental device for simulating fluid displacement under high-temperature and high-pressure conditions. It uses a visualized micro- and nanoscale pore-throat model, or glass-etched model, to simulate the pore-throat characteristics of actual rocks. Through a reservoir temperature-pressure coordinated control system and a displacement reaction system, the flow behavior of fluids in micro- and nanochannels under high-temperature and high-pressure conditions is simulated. Combined with a data acquisition and processing system, the microscopic fluid migration characteristics within the porous medium are observed, enabling the quantification of residual oil and water saturation in the microscopic pore structure during the microscopic model displacement experiment. However, the pore structure of glass-etched models is typically relatively regular, making it difficult to accurately simulate the complex and heterogeneous pore structure of low-permeability rocks, particularly the irregular pore connectivity. Low-permeability rocks exhibit nonlinear seepage characteristics. Especially under high-temperature and high-pressure conditions, changes in fluid properties such as viscosity and surface tension, as well as complex processes such as adsorption and chemical reactions, are difficult to fully reproduce using glass-etched models alone. Digital photography systems, based on optical imaging technology and relying on external light sources, struggle to provide sufficient illumination intensity at the micro- and nanoscale, making it difficult to clearly capture microscopic pore structure details and fluid movement. Further improvements are needed to address the above issues. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to address the deficiencies in the above-mentioned existing technologies and provide a method and device for nonlinear seepage experiment of gas breaking through the caprock based on dark field imaging. This method applies the dark field scattering imaging platform to the nonlinear seepage test of low permeability rocks, and can accurately observe the nonlinear seepage of carbon dioxide displacing formation water in nanopores (4 to 200 nm), overcoming the problem of difficulty in observing two-phase seepage in low permeability rock testing.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is:

[0008] A dark field imaging-based nonlinear seepage experimental method for gas breakthrough caprock includes the following steps:

[0009] The rock cores drilled on site are cut into standard slices, polished and cleaned, and then dried until the quality is stable. The processed microfluidic rock chip is placed in a high-pressure resistant microfluidic fixture, and the microfluidic fixture is placed on the stage of a dark-field scattering microscope.

[0010] Connecting the injection end of the microfluidic rock chip to the upstream injection pipeline and the outlet end to the downstream outflow pipeline; connecting the upstream injection pipeline to the fluid injection module for injecting formation water or carbon dioxide into the microfluidic rock chip, and setting an upstream pressure sensor on the upstream injection pipeline; connecting the downstream outflow pipeline to the fluid recovery module, and setting a downstream pressure sensor on the downstream outflow pipeline;

[0011] The entire experimental setup was placed in a constant temperature control system. The formation water solution was injected into the microfluidic rock chip through the water injection system of the fluid injection module and observed using a dark-field scattering microscope until the microfluidic rock chip was completely filled with the formation water solution. The water injection system was then closed, the gas injection system of the fluid injection module was opened, and the laser and high-precision camera of the dark-field scattering microscope were turned on. The image resolution was adjusted until a clear image of the pore structure of the microfluidic rock chip and the displacement of formation water by carbon dioxide could be obtained.

[0012] A step-by-step pressurization method was used to inject carbon dioxide at different pressures into the microfluidic rock chip, and the pressures upstream and downstream of the microfluidic rock chip were recorded in real time. The nonlinear seepage migration of carbon dioxide and formation water in the microfluidic rock chip was observed using a dark-field scattering microscope. When the dark-field scattering microscope observed carbon dioxide migrating to the outlet of the rock chip, this moment was the breakthrough moment of carbon dioxide. The pressure difference between the upstream and downstream of the microfluidic rock chip at this time was recorded, which was the breakthrough pressure of carbon dioxide in the tested core sample.

[0013] In the above solution, the length of the microfluidic rock chip is 10 to 12 cm, the width is 5 to 6 cm, and the thickness is 1 to 2 cm, and an inlet end and an outlet end are respectively provided at both ends of the microfluidic rock chip.

[0014] In the above scheme, the microfluidic fixture includes an upper visualization panel and a lower visualization panel respectively placed on the upper surface and lower surface of the microfluidic rock chip; the surfaces of the two visualization panels in contact with the microfluidic rock chip are provided with sealing rings, and the two visualization panels are fixedly connected by fasteners to prevent fluid leakage.

[0015] In the above scheme, the inlet end of the microfluidic rock chip is equipped with an inlet end head, and several inlet pipes connected to the upstream injection pipe are provided on the outside of the inlet end head, and an inlet flow groove is provided on the inside of the inlet end head. The inlet flow groove and the microfluidic rock chip are designed as an embedded structure, so that the inlet end of the microfluidic rock chip can be inserted into the inlet flow groove, so that the injected carbon dioxide or formation water passes through the inlet pipe and the inlet flow groove in sequence and finally enters the microfluidic rock chip; the outlet end of the microfluidic rock chip is equipped with an outlet end head, and several outlet pipes connected to the downstream outflow pipe are provided on the outside of the outlet end head, and an outlet flow groove is provided on the inside of the outlet end head. The outlet flow groove and the microfluidic rock chip are designed as an embedded structure, so that the outlet end of the microfluidic rock chip can be inserted into the outlet flow groove, so that the formation water or carbon dioxide displaced by the microfluidic rock chip can flow out.

[0016] In the above scheme, the dark field scattering microscope includes a light source, a focusing lens, a stage, a laser, a collimating beam expander, an annular focusing lens, an annular reflector, an objective lens, a prism, an eyepiece and a high-precision camera; the light source is placed under the stage to provide external light source illumination for the microfluidic rock chip, the focusing lens is located between the light source and the stage, and the focusing lens adjusts the focal length and angle of the lens so that the light from the light source illuminates the sample at a specific angle; the laser emits laser light, which is focused by the collimating beam expander and the annular focusing lens in turn to form a strong light beam, and the annular reflector refracts the strong light beam onto the microfluidic rock chip on the stage, the microfluidic rock chip scatters when encountering light, and the scattered light enters the objective lens to form a bright diffraction image of the rock chip, which is refracted by the prism to the eyepiece and the high-precision camera respectively, the rock chip diffraction image is directly observed through the eyepiece, and the image formed by the scattered light is recorded in real time by the high-precision camera, and the nonlinear seepage process of the two-phase flow is recorded in real time.

[0017] In the above solution, the resolution range of the dark field scattering microscope is 4 to 200 nm.

[0018] In the above scheme, the gas injection system includes a carbon dioxide gas cylinder, a carbon dioxide injection pump and a water bath heating box. The carbon dioxide gas cylinder and the carbon dioxide injection pump are connected by a pipeline, and a one-way valve (F1) is provided on the pipeline; the water bath heating box is connected to the carbon dioxide injection pump through a heating medium outlet pipe and a heating medium inlet pipe respectively. The heating medium in the water bath heating box enters the carbon dioxide injection pump through the heating medium outlet pipe to heat the carbon dioxide. After heat exchange, the medium returns to the water bath heating box through the heating medium inlet pipe for circulating heating. By adjusting the temperature of the water bath heating box and the pressure of the carbon dioxide injection pump, the carbon dioxide is converted from a gaseous state to a liquid state or a supercritical state. A three-way valve (F3) is provided on the outlet pipeline of the carbon dioxide injection pump to inject carbon dioxide into the upstream injection pipeline;

[0019] The water injection system includes a formation water injection pump and a liquid container, and the water injection system is connected to the liquid container through a three-way valve I (F2) and a pipeline. Before the experiment, formation water solution is injected into the liquid container, and the water suction direction of the three-way valve I (F2) is opened, so that the formation water injection pump draws in formation water. After the experiment starts, the water suction direction of the three-way valve I (F2) is closed, and the water injection direction is opened. The formation water injection pump injects formation water into the upstream injection pipeline by adjusting the injection pressure or injection rate.

[0020] In the above scheme, the fluid recovery module includes a gas-water separator, a gas flow meter and an outlet one-way valve (F4), the inlet of the gas-water separator is connected to the outlet of the microfluidic rock chip, the outlet of the gas-water separator is connected to the inlet of the gas flow meter, and the outlet of the gas flow meter is connected to the outlet one-way valve (F4).

[0021] In the above scheme, the upstream pressure sensor and the downstream pressure sensor are respectively connected to the data collector signal, the data collector is connected to the computer control terminal signal, and the computer control terminal obtains the breakthrough pressure by calculating the pressure difference between the upstream pressure sensor and the downstream pressure sensor at the breakthrough moment; the high-precision camera is connected to the computer control terminal signal.

[0022] Accordingly, the present invention also proposes a dark field imaging-based nonlinear seepage experimental device for gas breakthrough caprock, which is used to implement the above experimental method, including a dark field scattering imaging module, a fluid injection module, a fluid recovery module and a data acquisition module;

[0023] The dark field scattering imaging module includes a dark field scattering microscope and a microfluidic rock chip; the microfluidic rock chip is placed on the stage of the dark field scattering microscope, the injection end of the microfluidic rock chip is connected to the upstream injection pipeline, and the outlet end is connected to the downstream outflow pipeline; the upstream injection pipeline is connected to the fluid injection module for injecting formation water or carbon dioxide into the microfluidic rock chip, and an upstream pressure sensor is provided on the upstream injection pipeline; the downstream outflow pipeline is connected to the fluid recovery module, and a downstream pressure sensor is provided on the downstream outflow pipeline; the dark field scattering microscope includes a light source, a focusing lens, a stage, a laser, a collimating beam expander, an annular focusing lens, an annular reflector, an objective lens, a prism, an eyepiece and a high-precision camera; the light source is placed below the stage for the microfluidic The rock chip is provided with illumination by an external light source, and the focusing lens is located between the light source and the stage. The focusing lens adjusts the focal length and angle of the lens so that the light from the light source illuminates the sample at a specific angle. The laser emits laser light, which is focused by the collimating beam expander and the annular focusing lens in sequence to form a strong light beam, and the annular reflector refracts the strong light beam onto the microfluidic rock chip on the stage. The microfluidic rock chip scatters light when encountering light, and the scattered light enters the objective lens, forming a bright diffraction image of the rock chip. The diffraction image of the rock chip is refracted by a prism to an eyepiece and a high-precision camera respectively. The two-phase flow migration in the nanopores of the low-permeability rock is directly observed through the eyepiece, and the image formed by the scattered light is recorded in real time by the high-precision camera, thereby recording the nonlinear seepage process of the two-phase flow in real time.

[0024] The fluid injection module includes an air injection system and a water injection system, wherein the air injection system is used to inject carbon dioxide into the microfluidic rock chip, and the water injection system is used to inject formation water into the microfluidic rock chip;

[0025] The data acquisition module includes a data acquisition device and a computer control terminal. The upstream pressure sensor and the downstream pressure sensor are respectively connected to the data acquisition device by signal, and the data acquisition device is connected to the computer control terminal by signal; the high-precision camera is connected to the computer control terminal by signal.

[0026] The beneficial effects of the present invention are:

[0027] 1. The present invention innovatively applies dark-field scattering microscopy to nonlinear seepage testing of low-permeability rocks. Dark-field scattering microscopy utilizes the Tyndall effect. Through the focusing system, the central light beam is blocked by the annular shading plate. Only the light scattered by the edge of the object is observed in the eyepiece, and the background is black. The laser emitted by the laser is focused by the collimating beam expander and the annular focusing mirror to form a strong light beam, which is then refracted by the annular reflector to the microfluidic rock chip. The rock chip is scattered by light, and the scattered light enters the objective lens, forming a clear diffraction image, and then the two-phase flow migration in the nanopores (4-200nm) of low-permeability rocks is observed. The resolution is 50 times higher than that of ordinary microscopes. The high-precision camera records the scattered light image in real time and monitors the nonlinear seepage process of the two-phase flow in real time.

[0028] 2. The measuring device of the present invention is composed of a dark-field scattering imaging module, a fluid injection module, a data acquisition module and a fluid recovery module. It has a simple structure, good stability, easy-to-understand principles, efficient and convenient operation, and will be more convenient in actual application.

[0029] 3. The present invention is applicable to the evaluation of the tightness of low-permeability rocks in the fields of carbon dioxide geological storage, natural gas storage and underground hydrogen storage, and has the advantages of strong adaptability, high sensitivity, accurate measurement, safety and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0031] Figure 1 Schematic diagram of the overall structure of the experimental device for nonlinear seepage in low-permeability rocks based on dark-field scattering imaging of the present invention;

[0032] Figure 2 yes Figure 1 Schematic diagram of the structure of the microfluidic rock chip and fixture of the dark field scattering imaging module shown;

[0033] Figure 3 yes Figure 1 Another perspective structural diagram of the microfluidic rock chip and fixture of the dark field scattering imaging module shown;

[0034] Figure 4 1 is a test principle diagram for measuring breakthrough pressure and breakthrough position using the distributed pressurization method in an embodiment of the present invention, and is a graph showing the relationship between upstream and downstream pressure difference and carbon dioxide migration distance observed by dark field scattering and test time.

[0035] In the figure: A, fluid injection module; B, dark field scattering imaging module; C, data acquisition module; D, fluid recovery module.

[0036] 1. Water bath; 2. CO2 cylinder; 3. CO2 injection pump; 4. Formation water injection pump; 5. Liquid container; 6. Upstream injection pipeline; 7. Upstream pressure sensor; 8. Downstream pressure sensor; 9. Downstream outflow pipeline; 10. Light source; 11. Condenser lens; 12. Stage; 13. Microfluidic rock chip; 14. Objective lens; 15. Annular reflector; 16. Collimating beam expander; 17. Annular focusing lens; 18. Prism; 19. Laser Optical device; 20. Eyepiece; 21. High-precision camera; 22. Computer control terminal; 23. Data acquisition unit; 24. Data transmission line; 25. Gas-water separator; 26. Gas flow meter; 27. Water injection pipeline; 28. Gas injection pipeline; 29. ​​Inlet end; 30. Outlet end; 31. Discharge pipeline; 32. Inlet flow channel; 33. Outlet flow channel; 34. Upper visual panel; 35. Lower visual panel; 36. Fasteners; 37. Sealing ring.

[0037] F1, one-way valve; F2, I three-way valve; F3, II three-way valve; F4, outlet one-way valve; F5, gas injection inlet one-way valve. DETAILED DESCRIPTION

[0038] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, specific embodiments of the present invention are now described in detail with reference to the accompanying drawings.

[0039] The present invention provides a dark field imaging-based nonlinear seepage experimental method for gas breakthrough cap layer, comprising the following steps:

[0040] The rock core drilled on site is cut into standard slices, polished and cleaned, and then dried until the quality is stable; the processed microfluidic rock chip 13 is placed in a high-pressure resistant microfluidic fixture, and the microfluidic fixture is placed on the stage of a dark-field scattering microscope.

[0041] The injection end of the microfluidic rock chip 13 is connected to the upstream injection pipeline 6, and the outlet end is connected to the downstream outflow pipeline 9; the upstream injection pipeline 6 is connected to the fluid injection module A to inject formation water or carbon dioxide into the microfluidic rock chip 13, and an upstream pressure sensor 7 is set on the upstream injection pipeline 6; the downstream outflow pipeline 9 is connected to the fluid recovery module D, and a downstream pressure sensor 8 is set on the downstream outflow pipeline 9.

[0042] The entire experimental device was placed in a constant temperature control system, and the formation water solution was injected into the microfluidic rock chip 13 through the water injection system of the fluid injection module A. The solution was observed through a dark-field scattering microscope until the microfluidic rock chip 13 was completely saturated with the formation water solution. The water injection system was then turned off, and the laser 19 and high-precision camera 21 of the dark-field scattering microscope were turned on. The image resolution was adjusted until a clear image of the pore structure of the microfluidic rock chip 13 and the displacement of formation water by carbon dioxide could be obtained.

[0043] The gas injection system of the fluid injection module A is turned on, and carbon dioxide at different pressures is injected into the microfluidic rock chip 13 using a step-by-step pressurization method. The pressures upstream and downstream of the microfluidic rock chip 13 are recorded in real time, and the nonlinear seepage migration image of carbon dioxide and formation water in the microfluidic rock chip 13 is observed using a dark-field scattering microscope. When the dark-field scattering microscope observes the migration of carbon dioxide to the outlet of the rock chip, this moment is the breakthrough moment of carbon dioxide. The pressure difference between the upstream and downstream of the microfluidic rock chip 13 at this time is recorded, which is the breakthrough pressure of carbon dioxide in the core sample being tested.

[0044] Figure 1 The invention discloses an experimental device for the nonlinear seepage experiment method of gas breaking through the cap layer based on dark field imaging, which comprises a dark field scattering imaging module B, a fluid injection module A, a fluid recovery module D and a data acquisition module C.

[0045] The dark field scattering imaging module B includes a dark field scattering microscope and a microfluidic rock chip 13. Figure 2-3As shown, the microfluidic rock chip 13 is cut from a real rock sample using wire cutting into a rectangular rock chip with a length of 10-12 cm, a width of 5-6 cm, and a thickness of 1-2 cm. An inlet and an outlet are provided at each end of the microfluidic rock chip 13. The processed microfluidic rock chip 13 is then mounted in a high-pressure-resistant visualization microfluidic fixture. The microfluidic fixture includes an upper visualization panel 34 and a lower visualization panel 35, which are placed on the upper and lower surfaces of the microfluidic rock chip 13, respectively. Sealing rings 37 are installed on the surfaces of the two visualization panels that contact the microfluidic rock chip 13, and the two visualization panels are fixed together by fasteners 36 to prevent fluid leakage. The inlet end of the microfluidic rock chip 13 is equipped with an inlet head 29. Multiple gas injection lines 28 and water injection lines 27 are located outside the inlet head 29. Each gas injection line 28 is equipped with a gas inlet check valve F5. Both the gas injection line 28 and the water injection line 27 are connected to an upstream injection line 6, which is connected to a fluid injection module A for injecting formation water or carbon dioxide into the microfluidic rock chip 13. The upstream injection line 6 is also equipped with an upstream pressure sensor 7. An inlet flow channel 32 is located inside the inlet head 29. The inlet flow channel 32 and the microfluidic rock chip 13 are designed as a fast-embedded structure, allowing the inlet end of the microfluidic rock chip 13 to be precisely inserted into the inlet flow channel. This allows the injected carbon dioxide or formation water to pass through the inlet line and inlet flow channel in sequence, ultimately entering the microfluidic rock chip 13. The outlet end of the microfluidic rock chip 13 is equipped with an outlet head 30. Four outlets are located on the outside of the head, each connected to a discharge line 31. These lines are connected to a downstream outflow line 9, which is connected to a fluid recovery module D. Downstream outflow line 9 is also equipped with a downstream pressure sensor 8. An outlet flow channel 33 is located inside the head 30. This flow channel 33 and the microfluidic rock chip 13 are designed as a fast-fitting structure, allowing the outlet end of the microfluidic rock chip 13 to be precisely inserted into the flow channel, allowing the formation water or carbon dioxide displaced by the microfluidic rock chip 13 to flow out. During the water saturation experiment, the gas injection inlet check valve F5 is first closed, and formation water is injected into the microfluidic rock chip 13 via the formation water injection pump 4, completely saturating the microfluidic rock chip 13 with water. Then, the gas injection inlet check valve F5 is opened, and carbon dioxide is injected into the microfluidic rock chip 13 via the carbon dioxide injection pump 3.

[0046] See also Figure 1The dark-field scattering microscope includes a light source 10, a condenser lens 11, a stage 12, a laser 19, a collimating beam expander 16, an annular focusing lens 17, an annular reflector 15, an objective lens 14, a prism 18, an eyepiece 20, and a high-precision camera 21. The light source 10 is placed below the stage 12, providing external light source illumination for the microfluidic rock chip 13. The condenser lens 11 is located between the light source 10 and the stage 12. By adjusting the focal length and angle of the lens, the condenser lens 11 allows the light from the light source 10 to illuminate the sample at a specific angle. The function of the light source 10 and the condenser lens 11 is to adjust specific lighting conditions to enhance the contrast of the microfluidic rock chip 13, making the fine structures and fluid flow on the microfluidic rock chip more clearly visible. The direct light generated by the light source 10 is blocked by the light shield of the annular reflector 15. The eyepiece 20 only observes the light scattered from the edge of the object, and the background is black. Laser 19 emits laser light, which is then focused by collimating beam expander 16 and annular focusing lens 17 to form a strong beam. The annular reflector 15 refracts the strong beam onto microfluidic rock chip 13 on stage 12. The microfluidic rock chip 13 scatters light upon encountering it, and the scattered light enters objective lens 14, forming a bright diffraction image of the rock chip. This diffraction image of the rock chip is then refracted by prism 18 to eyepiece 20 and high-precision camera 21, respectively. Through eyepiece 20, two-phase flow migration within the nanopores (4-200nm) of low-permeability rocks is directly observed, with a resolution 50 times higher than that of a conventional microscope. The high-precision camera 21 records the image formed by the scattered light in real time, recording the nonlinear seepage process of the two-phase flow in real time.

[0047] Continue to see Figure 1The fluid injection module A is used to provide a two-phase flow for the microfluidic rock chip 13, including a gas injection system and a water injection system. The gas injection system is used to inject carbon dioxide into the microfluidic rock chip 13, and the water injection system is used to inject formation water into the microfluidic rock chip 13. The gas injection system includes a carbon dioxide cylinder 2, a carbon dioxide injection pump 3, and a water bath heating tank 1. The carbon dioxide cylinder 2 and the carbon dioxide injection pump 3 are connected by a pipeline equipped with a one-way valve F1. The water bath heating tank 1 is connected to the carbon dioxide injection pump 3 via a heating medium outlet pipe and a heating medium inlet pipe, respectively. The heating medium in the water bath heating tank 1 enters the carbon dioxide injection pump 3 through the heating medium outlet pipe to heat the carbon dioxide. After heat exchange, the medium returns to the water bath heating tank 1 through the heating medium inlet pipe for circulation heating. By adjusting the temperature of the water bath heating tank 1 and the pressure of the carbon dioxide injection pump 3, the carbon dioxide is converted from a gaseous state to a liquid or supercritical state, simulating the phase changes of carbon dioxide in a real formation environment. A three-way valve II F3 is installed on the outlet pipe of the carbon dioxide injection pump 3 to inject carbon dioxide into the upstream injection pipe 6. The water injection system includes a formation water injection pump 4 and a liquid container 5, connected to the liquid container 5 via a three-way valve I F2 and a pipeline. Before the experiment, liquid container 5 was filled with formation water (including a mineralized aqueous solution), and the suction direction of three-way valve I F2 was opened, allowing formation water injection pump 4 to draw in formation water. After the experiment began, the suction direction of three-way valve I F2 was closed, and the injection direction was opened. Formation water injection pump 4 injected formation water into upstream injection pipeline 6 by adjusting the injection pressure or injection rate. Three-way valve II F3 of the gas injection system was connected to three-way valve I F2 of the water injection system.

[0048] Data acquisition module C includes a data collector 23 and a computer control terminal 22. The upstream pressure sensor 7 and the downstream pressure sensor 8 are connected to the data collector 23 via data transmission lines 24, which in turn are connected to the computer control terminal 22. The computer control terminal 22 calculates the breakthrough pressure by calculating the pressure difference between the upstream and downstream pressure sensors 7 and 8 at the breakthrough moment. A high-precision camera 21 is also connected to the computer control terminal 22 via data transmission lines 24.

[0049] The fluid recovery module D includes a gas-water separator 25, a gas flow meter 26 and an outlet one-way valve F4. The inlet of the gas-water separator 25 is connected to the outlet of the microfluidic rock chip 13, the outlet of the gas-water separator 25 is connected to the inlet of the gas flow meter 26, and the outlet of the gas flow meter 26 is connected to the outlet one-way valve F4.

[0050] Preferably, the upstream pressure sensor 7 and the downstream pressure sensor 8 have a measuring range of 0-30 MPa and an accuracy level of 1 kPa.

[0051] Preferably, the resolution range of the dark field scattering microscope is 4 to 200 nm.

[0052] Preferably, the two visualization panels can be made of transparent pressure-resistant glass or acrylic, and the fasteners 36 can be 316 stainless steel bolts. The inlet and outlet ports 29 and 30 can also be made of 316 stainless steel. The various pipelines in the experimental apparatus are also made of 316 stainless steel to meet the experimental conditions of high temperature and high pressure.

[0053] The following are specific implementation steps of an embodiment of the nonlinear seepage experimental method for gas breaking through the cap layer based on dark field imaging of the present invention:

[0054] (1) The rock core drilled on site is cut into standard slices by wire cutting or other methods, polished and cleaned with alcohol, and then placed in a drying oven until the quality is stable; the processed microfluidic rock chip 13 is placed in a high-pressure resistant microfluidic fixture, and the microfluidic fixture is placed on the stage 12 of a dark-field scattering microscope.

[0055] (2) The injection end of the microfluidic rock chip 13 is connected to the upstream injection pipeline 6, and the outlet end is connected to the downstream outflow pipeline 9; the upstream injection pipeline 6 is connected to the fluid injection module A for injecting formation water or carbon dioxide into the microfluidic rock chip 13, and an upstream pressure sensor 7 is set on the upstream injection pipeline 6; the downstream outflow pipeline 9 is connected to the fluid recovery module D, and a downstream pressure sensor 8 is set on the downstream outflow pipeline 9.

[0056] (3) The entire experimental device is placed in a constant temperature control system, and the formation water solution is injected into the microfluidic rock chip 13 through the formation water injection pump 4, and observed through a dark field scattering microscope until the microfluidic rock chip 13 is completely filled with the formation water solution; then the Harvard high-performance injection pump and the fluid injection direction of the II three-way valve F3 are closed, the gas injection direction of the II three-way valve F3 is opened, and carbon dioxide is injected into the microfluidic chip through the carbon dioxide injection pump 3; the laser 19 and high-precision camera 21 of the dark field scattering microscope are turned on, and the image resolution is adjusted through the eyepiece 20 and the objective lens 14, so that the computer control terminal 22 in the data acquisition module CC can obtain a clear image of the pore structure of the microfluidic rock chip 13 and the displacement of formation water by carbon dioxide.

[0057] (4) A step-by-step pressurization method is used to inject carbon dioxide at different pressures into the microfluidic rock chip 13. The pressure curves upstream and downstream of the microfluidic rock chip 13 are recorded in real time. The nonlinear seepage migration image of carbon dioxide and formation water in the microfluidic rock chip 13 is observed by dark field scattering microscopy. When the dark field scattering microscope observes the migration of carbon dioxide to the outlet end of the rock chip, this moment is the breakthrough moment of carbon dioxide. The pressure difference between the upstream and downstream of the microfluidic rock chip 13 at this time is recorded as the breakthrough pressure of carbon dioxide in the core sample under test. After carbon dioxide displaces the formation water in the microfluidic rock chip 13, the mixture of carbon dioxide and formation water enters the gas-water separator 25 through the downstream outlet pipeline. The gas-water separator 25 contains water with a certain salinity. When the mixture of carbon dioxide and formation water passes through this device, the water is retained in the gas-water separator 25, while the carbon dioxide passes through the insoluble liquid and is discharged. The measurement is performed by the gas flow meter 26. According to the scale of the gas-water separator 25, the volume of the discharged water can be recorded, and then the saturation change of the microfluidic rock chip 13 during the carbon dioxide displacement process can be calculated.

[0058] The experimental process of the present invention adopts a step-by-step pressurization method to gradually increase the carbon dioxide injection pressure upstream of the microfluidic rock chip 13 to increase the injection pressure of the microfluidic rock chip 13, and simultaneously monitor the upstream and downstream pressure differences and the gas displacement position of the dark field scattering imaging to obtain the carbon dioxide breakthrough moment and breakthrough pressure. First, open the one-way valve F5, and load the injection pressure to the inlet end 29 of the microfluidic rock chip 13 in stages through the carbon dioxide injection pump 3. The first stage injects a relatively small pressure P1 (0-10MPa), the second stage pressure P2 is ΔP1 greater than P1, the third stage pressure P3 is ΔP2 greater than P2, and so on. The nth stage pressure P n P n-1 Large ΔP n-1 n is a positive integer, and each level increment is within the range of 0.01-1 MPa. The upstream pressure sensor 7 and the downstream pressure sensor 8 record the curve changes of the upstream pressure and the downstream pressure, and the high-precision camera 21 observes the nonlinear seepage migration image of carbon dioxide and formation water in the microfluidic rock chip 13 in real time. When the dark field scattering microscope observes the migration of carbon dioxide to the rock chip outlet end 30, this is the breakthrough moment of carbon dioxide. The difference between the upstream and downstream pressures of the measuring device at this time is recorded as the breakthrough pressure of carbon dioxide, which is referred to as the reference pressure. Figure 4 .

[0059] Correspondingly, the present invention also proposes a nonlinear seepage experimental device for gas breakthrough of the cap layer based on dark field imaging, which can be used to implement the above-mentioned experimental method, including a dark field scattering imaging module B, a fluid injection module A, a fluid recovery module D and a data acquisition module C. The structural composition and connection relationship of each module have been described in detail in the above-mentioned method and will not be repeated here.

[0060] Dark-field scattering imaging is commonly used in the study of biological cell structures and metal nanoparticles. The present invention extends its application to the study of nonlinear seepage in low-permeability rocks, providing an efficient detection and tracking system for the calculation of breakthrough pressure of two-phase flow, morphological changes of displacing fluids, and breakthrough positions of nonlinear seepage.

[0061] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0062] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are protected by the present invention.

Claims

1. A nonlinear seepage experimental method for gas breakthrough caprock based on dark field imaging, characterized in that: The following steps are involved: The rock cores drilled on site are cut into standard slices, polished and cleaned, and then dried until the quality is stable. The processed microfluidic rock chip is placed in a microfluidic fixture, and the microfluidic fixture is placed on the stage of a dark field scattering microscope. Connecting the injection end of the microfluidic rock chip to the upstream injection pipeline and the outlet end to the downstream outflow pipeline; connecting the upstream injection pipeline to the fluid injection module for injecting formation water or carbon dioxide into the microfluidic rock chip, and setting an upstream pressure sensor on the upstream injection pipeline; connecting the downstream outflow pipeline to the fluid recovery module, and setting a downstream pressure sensor on the downstream outflow pipeline; The entire experimental setup was placed in a constant temperature control system. The formation water solution was injected into the microfluidic rock chip through the water injection system of the fluid injection module and observed using a dark-field scattering microscope until the microfluidic rock chip was completely filled with the formation water solution. The water injection system was then closed, the gas injection system of the fluid injection module was opened, and the laser and high-precision camera of the dark-field scattering microscope were turned on. The image resolution was adjusted until a clear image of the pore structure of the microfluidic rock chip and the displacement of formation water by carbon dioxide could be obtained. A step-by-step pressurization method was used to inject carbon dioxide at different pressures into the microfluidic rock chip, and the pressures upstream and downstream of the microfluidic rock chip were recorded in real time. The nonlinear seepage migration of carbon dioxide and formation water in the microfluidic rock chip was observed using a dark-field scattering microscope. When the dark-field scattering microscope observed carbon dioxide migrating to the outlet of the rock chip, this moment was the breakthrough moment of carbon dioxide. The pressure difference between the upstream and downstream of the microfluidic rock chip at this time was recorded, which was the breakthrough pressure of carbon dioxide in the tested core sample.

2. The nonlinear seepage experimental method of gas breaking through the cap layer based on dark field imaging according to claim 1 is characterized in that: The microfluidic rock chip has a length of 10 to 12 cm, a width of 5 to 6 cm, and a thickness of 1 to 2 cm, and an inlet end and an outlet end are respectively arranged at both ends of the microfluidic rock chip.

3. The nonlinear seepage experimental method of gas breaking through the cap layer based on dark field imaging according to claim 1, characterized in that: The microfluidic fixture includes an upper visualization panel and a lower visualization panel respectively placed on the upper surface and lower surface of the microfluidic rock chip; the surfaces of the two visualization panels in contact with the microfluidic rock chip are provided with sealing rings, and the two visualization panels are fixedly connected by fasteners to prevent fluid leakage.

4. The method for the nonlinear seepage experiment of gas breaking through the cap layer based on dark field imaging according to claim 3, characterized in that: The inlet end of the microfluidic rock chip is equipped with an inlet end head, and a plurality of inlet pipes connected to the upstream injection pipe are provided on the outside of the inlet end head, and an inlet flow groove is provided on the inside of the inlet end head. The inlet flow groove and the microfluidic rock chip are designed as an embedded structure, so that the inlet end of the microfluidic rock chip can be inserted into the inlet flow groove, so that the injected carbon dioxide or formation water passes through the inlet pipe and the inlet flow groove in sequence and finally enters the microfluidic rock chip; the outlet end of the microfluidic rock chip is equipped with an outlet end head, and a plurality of outlet pipes connected to the downstream outflow pipe are provided on the outside of the outlet end head, and an outlet flow groove is provided on the inside of the outlet end head. The outlet flow groove and the microfluidic rock chip are designed as an embedded structure, so that the outlet end of the microfluidic rock chip can be inserted into the outlet flow groove, so that the formation water or carbon dioxide displaced by the microfluidic rock chip flows out.

5. The nonlinear seepage experimental method of gas breaking through the cap layer based on dark field imaging according to claim 1, characterized in that: The dark field scattering microscope includes a light source, a focusing lens, a stage, a laser, a collimating beam expander, an annular focusing lens, an annular reflector, an objective lens, a prism, an eyepiece and a high-precision camera; the light source is placed below the stage to provide external light source illumination for the microfluidic rock chip, the focusing lens is located between the light source and the stage, and the focusing lens adjusts the focal length and angle of the lens so that the light from the light source illuminates the sample at a specific angle; the laser emits laser light, which is focused by the collimating beam expander and the annular focusing lens in turn to form a strong light beam, and the annular reflector refracts the strong light beam onto the microfluidic rock chip on the stage, the microfluidic rock chip scatters when encountering light, and the scattered light enters the objective lens to form a bright diffraction image of the rock chip, which is refracted by the prism to the eyepiece and the high-precision camera respectively, the rock chip diffraction image is directly observed through the eyepiece, and the image formed by the scattered light is recorded in real time by the high-precision camera, thereby recording the nonlinear seepage process of the two-phase flow in real time.

6. The dark field imaging-based nonlinear seepage experimental method for gas breakthrough cap layer according to claim 1, characterized in that: The resolution range of the dark field scattering microscope is 4 to 200 nm.

7. The method for the nonlinear seepage experiment of gas breaking through the cap layer based on dark field imaging according to claim 1, characterized in that: The gas injection system includes a carbon dioxide gas cylinder, a carbon dioxide injection pump, and a water bath heating box. The carbon dioxide gas cylinder and the carbon dioxide injection pump are connected by a pipeline, and a one-way valve (F1) is provided on the pipeline. The water bath heating box is connected to the carbon dioxide injection pump via a heating medium outlet pipe and a heating medium inlet pipe, respectively. The heating medium in the water bath heating box enters the carbon dioxide injection pump via the heating medium outlet pipe to heat the carbon dioxide. After heat exchange, the medium returns to the water bath heating box via the heating medium inlet pipe for circulating heating. By adjusting the temperature of the water bath heating box and the pressure of the carbon dioxide injection pump, the carbon dioxide is converted from a gaseous state to a liquid state or a supercritical state. A three-way valve (F3) is provided on the outlet pipe of the carbon dioxide injection pump to inject carbon dioxide into the upstream injection pipe. The water injection system includes a formation water injection pump and a liquid container, and the water injection system is connected to the liquid container through a three-way valve I (F2) and a pipeline. Before the experiment, formation water solution is injected into the liquid container, and the water suction direction of the three-way valve I (F2) is opened, so that the formation water injection pump draws in formation water. After the experiment starts, the water suction direction of the three-way valve I (F2) is closed, and the water injection direction is opened. The formation water injection pump injects formation water into the upstream injection pipeline by adjusting the injection pressure or injection rate.

8. The dark field imaging-based nonlinear seepage experimental method for gas breakthrough cap layer according to claim 1, characterized in that: The fluid recovery module includes a gas-water separator, a gas flow meter and an outlet one-way valve (F4), the inlet of the gas-water separator is connected to the outlet of the microfluidic rock chip, the outlet of the gas-water separator is connected to the inlet of the gas flow meter, and the outlet of the gas flow meter is connected to the outlet one-way valve (F4).

9. The nonlinear seepage experimental method of gas breaking through the cap layer based on dark field imaging according to claim 1, characterized in that: The upstream pressure sensor and the downstream pressure sensor are respectively connected to the data collector signal, the data collector is connected to the computer control terminal signal, and the computer control terminal obtains the breakthrough pressure by calculating the pressure difference between the upstream pressure sensor and the downstream pressure sensor at the breakthrough moment; the high-precision camera is connected to the computer control terminal signal.

10. A nonlinear seepage experimental device for gas breakthrough cap layer based on dark field imaging, characterized in that: Used to implement the experimental method according to any one of claims 1 to 9, comprising a dark field scattering imaging module, a fluid injection module, a fluid recovery module and a data acquisition module; The dark field scattering imaging module includes a dark field scattering microscope and a microfluidic rock chip; the microfluidic rock chip is placed on the stage of the dark field scattering microscope, the injection end of the microfluidic rock chip is connected to the upstream injection pipeline, and the outlet end is connected to the downstream outflow pipeline; The upstream injection pipeline is connected to the fluid injection module and is used to inject formation water or carbon dioxide into the microfluidic rock chip, and an upstream pressure sensor is provided on the upstream injection pipeline; the downstream outflow pipeline is connected to the fluid recovery module, and a downstream pressure sensor is provided on the downstream outflow pipeline; the dark field scattering microscope includes a light source, a focusing lens, a stage, a laser, a collimating beam expander, an annular focusing lens, an annular reflector, an objective lens, a prism, an eyepiece and a high-precision camera; the light source is placed under the stage to provide external light source illumination for the microfluidic rock chip, the focusing lens is located between the light source and the stage, and the focusing lens adjusts the lens focal length and angle by adjusting the lens focal length and angle , so that the light from the light source illuminates the sample at a specific angle; the laser emits laser light, which is focused by the collimating beam expander and the annular focusing lens in turn to form a strong light beam, and the annular reflector refracts the strong light beam onto the microfluidic rock chip on the stage, and the microfluidic rock chip scatters light when it encounters light, and the scattered light enters the objective lens, forming a bright diffraction image of the rock chip, which is refracted by the prism to the eyepiece and the high-precision camera respectively, and the two-phase flow migration in the nanopores of the low-permeability rock is directly observed through the eyepiece, and the image formed by the scattered light is recorded in real time by the high-precision camera, thereby recording the nonlinear seepage process of the two-phase flow in real time; The fluid injection module includes an air injection system and a water injection system, wherein the air injection system is used to inject carbon dioxide into the microfluidic rock chip, and the water injection system is used to inject formation water into the microfluidic rock chip; The data acquisition module includes a data acquisition device and a computer control terminal. The upstream pressure sensor and the downstream pressure sensor are respectively connected to the data acquisition device by signal, and the data acquisition device is connected to the computer control terminal by signal; the high-precision camera is connected to the computer control terminal by signal.

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