Method and device for testing breakthrough pressure of low-permeability rock based on schlieren imaging

Abstract

The application discloses a low-permeability rock breakthrough pressure testing method based on a schlieren imaging system, which comprises the following steps: placing a test rock sample into a rock sample holder confining pressure cavity; connecting a rock sample holder injection end with a fluid injection module, connecting an outlet end with a schlieren cavity, and connecting the confining pressure cavity with a confining pressure injection module; vacuumizing a testing device pipeline, injecting degassed pure water medium into the schlieren cavity, and then performing light path debugging until a clear schlieren image is collected; loading confining pressure stress on the test rock sample, injecting water until the test rock sample is saturated, and then injecting carbon dioxide; observing the schlieren imaging system, and recording a schlieren image of carbon dioxide breakthrough by a high-precision camera when the carbon dioxide breaks through the saturated rock sample; at this moment, the breakthrough moment of the carbon dioxide is obtained, and a pressure difference between an upstream and a downstream at this moment is recorded, which is the breakthrough pressure of the carbon dioxide of the measured core sample. The application can visually observe the breakthrough of trace carbon dioxide in a rock sample, so that the breakthrough pressure of low-permeability rock can be accurately detected.

Description

Technical Field

[0001] This invention relates to the field of low-permeability rock testing technology, and in particular to a method and apparatus for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging. Background Technology

[0002] With the massive emission of greenhouse gases into the atmosphere, global climate change is becoming increasingly severe, and greenhouse gas emission reduction has become a key concern for countries worldwide. Carbon dioxide capture and storage (CCS) technology extracts carbon dioxide from industrial waste gases, converts it into supercritical carbon dioxide using industrial technology, and injects it into deep geological formations for long-term geological storage. Simultaneously, deep gas storage facilities, oil and gas reservoir sealing, and compressed air energy storage increasingly rely on the airtightness of deep rock formations to safely store important media such as oil, gas, hydrogen, and helium. In these technological fields, low-permeability rocks such as gypsum, mudstone, and shale are typically used as caprocks to seal gases, and the breakthrough pressure test of low-permeability caprocks is a key indicator for evaluating their airtightness.

[0003] Breakthrough pressure refers to the minimum displacement pressure required for a non-wetting phase (such as carbon dioxide) to displace a wetting phase (such as water) within the pores of a rock, forming a dominant seepage channel that penetrates the entire rock. Its magnitude is closely related to the capillary pressure at the minimum throat of the dominant seepage channel. Currently, breakthrough pressure is often tested using the rock displacement method. However, existing testing equipment lacks high-precision sensors capable of monitoring whether gas has broken through; typically, the moment of gas breakthrough is determined solely by the pressure curve. This method easily leads to significant deviations between the experimentally measured breakthrough pressure and the actual value. Low-permeability caprocks are typically composed of nanopores, and the process of gas displacing the internal liquid is very slow. Even if gas has broken through the rock, the amount of liquid discharged is extremely small, making it difficult for pressure sensors to detect pressure changes within the channel and determine the accurate moment of gas breakthrough in the core. This may lead to overlooking carbon dioxide breakthrough while continuing to increase the carbon dioxide injection pressure at the injection end, resulting in significant errors in the test results.

[0004] Therefore, it is necessary to design a high-precision testing device to improve the accuracy of testing breakthrough pressure in low-permeability rocks. Summary of the Invention

[0005] The main objective of this invention is to address the problems of insufficient sensitivity and delay in breakthrough pressure monitoring in existing breakthrough pressure testing devices and methods. This invention provides a breakthrough pressure testing method and device for low-permeability rocks based on schlieren imaging, which can visually observe breakthrough pressure in rock samples with trace concentrations (<10ppm) of carbon dioxide. This overcomes the problems of insufficient sensitivity and delay in determining the breakthrough time in low-permeability caprock testing, thus providing an efficient and accurate testing method and device for calculating the breakthrough pressure of low-permeability rocks.

[0006] The technical solution adopted in this invention is:

[0007] A method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging includes the following steps:

[0008] Process the test rock sample and place it into the confining pressure chamber of the rock sample holder;

[0009] Connect the injection end of the rock sample holder to the upstream injection pipeline and the outlet end to the inlet of the schlieren imaging system. Connect the confining pressure chamber of the rock sample holder to the confining pressure injection module. Connect the upstream injection pipeline to the fluid injection module for injecting water or carbon dioxide into the test rock sample. Install an upstream pressure sensor on the upstream injection pipeline. Connect the outlet of the schlieren cavity to the downstream outflow pipeline. Install a downstream pressure sensor on the downstream outflow pipeline.

[0010] The test device pipeline is evacuated through the downstream outflow pipeline; degassed pure water medium is injected into the schlieren cavity through the degassed pure water inlet on the schlieren cavity; the optical path of the schlieren imaging system is adjusted so that the observation light can acquire a clear schlieren image by the high-precision camera.

[0011] The entire measuring device is placed in a constant temperature control system. Confining pressure medium is injected into the confining pressure chamber through the confining pressure injection module to apply confining pressure stress to the test rock sample. Then the confining pressure injection module is closed, and water is injected into the rock sample holder through the water injection system of the fluid injection module until the test rock sample is completely saturated. Liquid or supercritical carbon dioxide is injected into the rock sample holder through the gas injection system of the fluid injection module.

[0012] Carbon dioxide at different pressures was injected into the rock sample holder using a stepwise pressurization method. During the experiment, the schlieren imaging system and each pressure sensor were observed in real time until the carbon dioxide broke through the saturation of the rock sample. The high-precision camera recorded the schlieren image of the carbon dioxide breakthrough formed after the carbon dioxide entered the schlieren cavity and changed the flow field distribution characteristics. This moment is the breakthrough moment of carbon dioxide. The pressure difference between the upstream and downstream of the measuring device at this time is recorded, which is the breakthrough pressure of carbon dioxide in the core sample being tested.

[0013] In the above scheme, a water separator is installed on the pipeline between the outlet of the rock sample holder and the inlet of the stencil cavity to absorb the water that breaks through and discharges the saturated test rock sample.

[0014] In the above scheme, the schlieren imaging system includes an LED light source, a slit, a plane-converting mirror (I), a spherical mirror (I), a schlieren cavity, a spherical mirror (II), a plane-converting mirror (II), a knife edge, and a high-precision camera. The plane-converting mirror (I) and the plane-converting mirror (II), as well as the spherical mirror (I) and the spherical mirror (II), are arranged symmetrically about the schlieren cavity. By adjusting the intensity of the LED light source, the emitted observation light passes sequentially through the slit, the plane-converting mirror (I), the spherical mirror (I), the schlieren cavity, the spherical mirror (II), the plane-converting mirror (II), and the knife edge before entering the high-precision camera.

[0015] In the above scheme, the schlieren cavity is a cylindrical cavity structure made of pressure- and temperature-resistant material, capable of withstanding a maximum pressure of 50 MPa and a maximum temperature of 100 °C; the side of the schlieren cavity is symmetrically provided with a schlieren observation window I and a schlieren observation window II along the propagation path of the observation light; the schlieren cavity is also provided with a conical gas inlet, a round hole gas outlet, and a degassed pure water injection outlet.

[0016] In the above scheme, the conical gas inlet is a cone-shaped integrated nozzle structure with a diameter that first decreases and then increases, and the diameter of the outlet end is larger than the diameter of the inlet end; a waterproof and breathable microporous membrane is provided inside the conical gas inlet.

[0017] In the above scheme, the circular gas outlet is a small circular nozzle structure with a radius of 0.2 to 0.3 mm; a waterproof and breathable microporous membrane is provided inside the circular gas outlet.

[0018] In the above scheme, the gas injection system includes a carbon dioxide cylinder, a carbon dioxide injection pump, and a water bath heating circulation tank. The carbon dioxide cylinder is connected to the inlet of the carbon dioxide injection pump, and a first control valve (F1) is installed on the connecting pipeline between the two. The outlet of the carbon dioxide injection pump is connected to a three-way control valve (F3). The water bath heating circulation tank is connected to the storage chamber of the carbon dioxide injection pump through a heating medium inlet pipeline and a heating medium outlet pipeline. The heating medium in the water bath heating circulation tank enters the storage chamber of the carbon dioxide injection pump through the heating medium inlet pipeline to heat the carbon dioxide. After heat exchange, the medium returns to the water bath heating circulation tank through the heating medium outlet pipeline for circulating heating. By adjusting the temperature of the water bath heating circulation tank 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.

[0019] The water injection system includes a fluid medium device and a high-precision liquid injection pump. The fluid medium device is connected to the inlet of the high-precision liquid injection pump, and a second control valve (F2) is provided on the connecting pipeline between the two. The outlet of the high-precision liquid injection pump is connected to a three-way control valve (F3).

[0020] The three-way control valve (F3) is connected to the rock sample holder via the upstream injection line.

[0021] In the above scheme, the confining pressure injection module includes a confining pressure medium device, a confining pressure injection pump, and a confining pressure sensor; the confining pressure medium device is connected to the inlet of the confining pressure injection pump, and a third control valve (F4) is provided on the connecting pipeline between the two; the outlet of the confining pressure injection pump is connected to the confining pressure chamber through a confining pressure injection pipeline, and a confining pressure sensor and a fourth control valve (F5) are provided on the confining pressure injection pipeline; the confining pressure medium device is filled with deionized water or oil, and the confining pressure injection pump is controlled to inject deionized water or oil at different pressures into the confining pressure chamber 20 to provide different circumferential pressures for the test rock sample.

[0022] In the above scheme, the rock sample holder includes a confining pressure cavity, a heating ring, and a holder cover arranged sequentially from the inside to the outside. The test rock sample is installed in the confining pressure cavity, which is filled with a confining pressure medium to apply a uniform circumferential load to the test rock sample. The heating ring heats the confining pressure medium through a heat-conducting material, so that the confining pressure cavity reaches the target temperature.

[0023] Accordingly, this invention also proposes a low-permeability rock breakthrough pressure testing device based on schlieren imaging. This testing device can be used to implement the above-mentioned testing method, including a fluid injection module, a confining pressure injection module, a rock sample holder module, and a high-precision schlieren imaging module. The fluid injection module includes an air injection system and a water injection system. The air injection system is used to inject carbon dioxide into the test rock sample, and the water injection system is used to inject water into the test rock sample. The rock sample holder module includes a rock sample holder and a test rock sample. The rock sample holder includes a confining pressure chamber, a heating ring, and a holder cover arranged sequentially from the inside to the outside. The test rock sample is installed in the confining pressure chamber, which is filled with a confining pressure medium to apply a uniform circumferential load to the test rock sample. The heating ring heats the confining pressure medium through a heat-conducting material, so that the confining pressure chamber reaches the target temperature. The injection end of the rock sample holder is connected to the fluid injection module through an upstream injection pipeline, and the outlet end is connected to the schlieren imaging module. The inlet of the schlieren imaging module is connected to the confining pressure chamber of the rock sample holder, which is connected to the confining pressure injection module. An upstream pressure sensor is installed on the upstream injection pipeline. The confining pressure injection module is used to inject deionized water or oil into the confining pressure chamber to provide confining pressure for the test rock sample. The high-precision schlieren imaging module includes a water remover, a schlieren imaging system, a downstream pressure sensor, and a vacuum pump. The schlieren imaging system includes an LED light source, a slit, a plane mirror (I), a spherical mirror (I), a schlieren observation window (I), a schlieren cavity, a schlieren observation window (II), a spherical mirror (II), a plane mirror (II), a knife edge, and a high-precision camera. The schlieren cavity is equipped with a conical gas inlet, a circular gas outlet, and a degassed pure water injection outlet. The water remover is installed in the pipeline between the rock sample holder outlet and the conical gas inlet at the schlieren cavity inlet end, and is used to absorb and drain water saturated with the test rock sample. The circular gas outlet at the schlieren cavity outlet end is connected to the downstream outflow pipeline, and a downstream pressure sensor is installed on the downstream outflow pipeline.

[0024] The beneficial effects of this invention are:

[0025] 1. Schlieren imaging is an optical method for visualizing density changes in transparent media, widely used in fluid mechanics, aerodynamics, and thermodynamics. This invention innovatively applies a high-precision schlieren imaging system to breakthrough pressure testing of low-permeability rocks. By capturing the light deflection caused by the refractive index gradient change in the fluid medium during breakthrough with trace concentrations (<10 ppm) of carbon dioxide, a schlieren image of the carbon dioxide breakthrough is ultimately displayed on the image plane of a high-speed camera, thereby accurately detecting the initial moment of carbon dioxide breakthrough in the rock.

[0026] 2. The measuring device of the present invention consists of a fluid injection module, a confining pressure loading module, a rock sample holder module, and a high-precision schlieren imaging module. The entire testing device has a simple structure, a simple and easy-to-understand testing principle, is highly efficient and convenient to operate, and has good stability, and can effectively test breakthrough pressure.

[0027] 3. This invention is applicable to the evaluation of low-permeability caprock breakthrough pressure and caprock tightness in fields such as carbon dioxide geological storage, deep gas storage, oil and gas reservoir storage, and compressed air energy storage. It has the advantages of strong adaptability, high sensitivity, accurate measurement, and safety and reliability. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the overall structure of the low-permeability rock breakthrough pressure testing device based on schlieren imaging of the present invention.

[0030] Figure 2 yes Figure 1 A schematic diagram of the high-precision schlieren imaging test principle for carbon dioxide breakthrough using the measurement device shown.

[0031] Figure 3 yes Figure 1 A schematic diagram of the schlieren cavity of the measuring device shown;

[0032] Figure 4 yes Figure 3 A cross-sectional view of the conical gas inlet of the schlieren cavity shown;

[0033] Figure 5 yes Figure 3 A cross-sectional view of the circular gas outlet of the schlieren cavity shown.

[0034] Figure 6 This is a test principle diagram of the present invention, which uses the step-by-step pressurization method to measure the breakthrough pressure and breakthrough time. It is a curve of the upstream and downstream pressure difference and the intensity of schlieren imaging light changing with the test time.

[0035] Figure 7 It is a diffraction schlieren image formed by the change in light intensity caused by carbon dioxide breakthrough in the schlieren cavity in the embodiment of the method of the present invention.

[0036] In the diagram: A. Fluid injection module; B. Confining pressure injection module; C. Rock sample holder module; D. High-precision schlieren imaging module;

[0037] 1. Carbon dioxide cylinder; 2. Carbon dioxide injection pump; 3. Water bath heating circulation chamber; 4. Heating medium inlet pipeline; 5. Heating medium outlet pipeline; 6. Carbon dioxide injection pump storage chamber; 7. Fluid medium appliance; 9. Liquid injection pump; 10. Injection pipeline of the gas injection system; 11. Injection pipeline of the liquid injection system; 12. Confining pressure medium appliance; 13. Confining pressure injection pump; 14. Confining pressure injection pipeline; 15. Confining pressure sensor; 16. Upstream pressure sensor; 17. Rock sample holder; 18. Heating ring; 19. Test rock sample; 20. Confining pressure chamber; 21. Dehydrator; 22. Computer data acquisition system; 23. Data transmission line; 24. LED light source; 25. Slit; 26. I. Plane convection mirror; 27. I. Spherical mirror; 28. I. Schlieren observation window; 29. ​​Schlieren cavity; 30. II. Schlieren observation window; 31. Conical gas inlet; 32. Circular gas outlet; 33. Degassed pure water; 34. II. Spherical mirror; 35. II. Plane convection mirror; 36. Knife edge; 37. High-precision camera; 38. Downstream pressure sensor; 39. Observation light beam; 40. Degassed pure water inlet / outlet; 41. Waterproof and breathable microporous membrane; 42. Vacuum pump;

[0038] F1, First control valve; F2, Second control valve; F3, Three-way control valve; F4, Third control valve; F5, Fourth control valve; F6, Fifth control valve. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0040] It should be noted that the illustrations provided in the embodiments of the present invention are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the number, shape and size of the components in actual implementation. In actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0041] In this invention, it should also be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. Furthermore, the terms "first" and "second" are used only for descriptive and distinguishing purposes and should not be construed as indicating or implying relative importance.

[0042] This invention provides a method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging, comprising the following steps:

[0043] The test rock sample was processed into a cylinder, with the two end faces and sides cut flat and smooth; the rock sample and the metal end pad were sealed together as a whole by heat shrink tubing and placed in the confining pressure cavity of the rock sample holder;

[0044] Connect the injection end of the rock sample holder to the upstream injection pipeline and the outlet end to the inlet of the schlieren imaging system. Connect the confining pressure chamber of the rock sample holder to the confining pressure injection module. Connect the upstream injection pipeline to the fluid injection module for injecting water or carbon dioxide into the test rock sample. Install an upstream pressure sensor on the upstream injection pipeline. Connect the outlet of the schlieren cavity to the downstream outflow pipeline. Install a downstream pressure sensor on the downstream outflow pipeline.

[0045] The test device pipeline is evacuated through the downstream outflow pipeline; degassed pure water medium is injected into the schlieren cavity through the degassed pure water inlet and outlet on the schlieren cavity (in order to ensure that the schlieren cavity contains degassed pure water and avoid the presence of dissolved gas, which would affect schlieren imaging); the optical path of the schlieren imaging system is adjusted so that the observation light can acquire a clear schlieren image by the high-precision camera.

[0046] The entire measuring device is placed in a constant temperature control system. Confining pressure medium is injected into the confining pressure chamber through the confining pressure injection module to apply confining pressure stress to the test rock sample. Then the confining pressure injection module is closed, and water is injected into the rock sample holder through the water injection system of the fluid injection module until the test rock sample is completely saturated. Liquid or supercritical carbon dioxide is injected into the rock sample holder through the gas injection system of the fluid injection module.

[0047] Carbon dioxide at different pressures was injected into the rock sample holder using a stepwise pressurization method. During the experiment, the schlieren imaging system and each pressure sensor were observed in real time until the carbon dioxide broke through the saturation of the rock sample. The high-precision camera recorded the schlieren image of the carbon dioxide breakthrough formed after the carbon dioxide entered the schlieren cavity and changed the flow field distribution characteristics. This moment is the breakthrough moment of carbon dioxide. The pressure difference between the upstream and downstream of the measuring device at this time is recorded, which is the breakthrough pressure of carbon dioxide in the core sample being tested.

[0048] Figure 1 The measuring device used in the low-permeability rock breakthrough pressure test method based on schlieren imaging of this invention includes a fluid injection module A, a confining pressure injection module B, a rock sample holder module C, and a high-precision schlieren imaging module D.

[0049] The fluid injection module A includes a gas injection system and a liquid injection system. The gas injection system is used to inject carbon dioxide into the test rock sample 19. It includes a carbon dioxide cylinder 1, a carbon dioxide injection pump 2, and a water bath heating circulation tank 3. The carbon dioxide cylinder 1 is connected to the inlet of the carbon dioxide injection pump 2, and a first control valve F1 is installed on the connecting pipeline between the two. The outlet of the carbon dioxide injection pump 1 is connected to a three-way control valve F3 through the injection pipeline 10 of the gas injection system. The water bath heating circulation tank 3 is connected to the carbon dioxide injection pump storage chamber 6 through the heating medium inlet pipeline 4 and the heating medium outlet pipeline 5. The heating medium in the water bath heating circulation tank 3 enters the carbon dioxide injection pump storage chamber 6 through the heating medium inlet pipeline 4 to heat the carbon dioxide. After heat exchange, the medium returns to the water bath heating circulation tank 3 through the heating medium outlet pipeline 5 for circulation heating. By adjusting the temperature of the water bath heating circulation tank 3 and the pressure of the carbon dioxide injection pump 2, the carbon dioxide can be converted from a gaseous state to a liquid state or a supercritical state and injected into the rock sample holder 17.

[0050] The injection system is used to supply injection liquid (deionized water) to the test rock sample 19. It includes a fluid medium device 7 and a high-precision injection pump 9. The fluid medium device 7 is connected to the inlet of the high-precision injection pump 9, and a second control valve F2 is installed on the connecting pipeline between the two. The outlet of the high-precision injection pump 9 is connected to a three-way control valve F3 through the injection pipeline 11 of the injection system. The three-way control valve F3 is connected to the rock sample holder module C through the upstream injection pipeline.

[0051] The confining pressure injection module B, used to inject deionized water or oil into the confining pressure chamber 20 to provide confining pressure for the test rock sample 19, includes a confining pressure medium device 12, a confining pressure injection pump 13, and a confining pressure sensor 15. The confining pressure medium device 12 is connected to the inlet of the confining pressure injection pump 13, and a third control valve F4 is installed on the connecting pipeline. The outlet of the confining pressure injection pump 13 is connected to the confining pressure chamber 20 of the rock sample holder module C through a confining pressure injection pipeline 14, which is equipped with a confining pressure sensor 15 and a fourth control valve F5. The confining pressure medium device 12 contains deionized water or oil. By controlling the confining pressure injection pump 13, deionized water or oil at different pressures is injected into the confining pressure chamber 20 of the rock sample holder module C, providing different circumferential pressures for the test rock sample 19.

[0052] The rock sample holder module C is used to load and test the test rock sample 19. It includes a rock sample holder 17 and the test rock sample 19. The rock sample holder 17 includes a confining pressure chamber 20, a heating ring 18, and a holder cover arranged sequentially from the inside out. The test rock sample 19 is installed inside the confining pressure chamber 20, which is filled with a confining pressure medium to apply a uniform circumferential load to the test rock sample 19. The heating ring 18 heats the confining pressure medium through a heat-conducting material, allowing the confining pressure chamber to reach the target temperature. The holder cover is made of high-temperature and high-pressure resistant 316 stainless steel. The injection end of the rock sample holder 17 is connected to the fluid injection module A via an upstream injection pipeline, and the outlet end is connected to the inlet of the schlieren imaging chamber 29 of the high-precision schlieren imaging module D. The confining pressure chamber 20 of the rock sample holder 17 is connected to the confining pressure injection module B. An upstream pressure sensor 16 is installed on the upstream injection pipeline to detect the pressure in the upstream injection pipeline.

[0053] The high-precision schlieren imaging module D is used to accurately monitor the initial moment of carbon dioxide breakthrough in low-permeability rocks. It includes a water separator 21, a schlieren imaging system, a downstream pressure sensor 38, and a vacuum pump 42. The schlieren imaging system includes an LED light source 24, a slit 25, a planar deflector 26, a spherical reflector 27, a temperature- and pressure-resistant schlieren cavity 29, a spherical reflector 34, a planar deflector 35, a knife edge 36, and a high-precision camera 37. The schlieren cavity 29 has a conical gas inlet 31, a circular gas outlet 32, and a degassed pure water injection / discharge port 40. The water separator 21 is located in the outlet pipe of the rock sample holder 17, absorbing water that has broken through and saturated the test rock sample 19. The outlet of the water separator 21 is connected to the conical gas inlet 31 at the inlet end of the schlieren cavity 29, and the circular gas outlet 32 ​​at the outlet end of the schlieren cavity 29 is connected to the downstream pressure sensor 38, and also connected to the fifth control valve F6 and the vacuum pump 42 at the downstream outlet end.

[0054] The real-time data from the upstream pressure sensor 16, the confining pressure sensor 15, the downstream pressure sensor 38, and the high-precision camera 37 are transmitted back to the computer data acquisition system 22 via data transmission line 23. Before the experiment, after all the devices are connected, the test device is evacuated to a vacuum state by opening the fifth control valve F6 and using the vacuum pump 42. Then, degassed pure water 33 is injected into the cavity through the inlet / outlet port 40.

[0055] Schlieren imaging systems utilize the principle that the refractive index gradient of diffracted light in the measured flow field is proportional to the airflow density. The testing principle is as follows: Figure 2 As shown, by adjusting the intensity of the LED light source 24, observation light 39 is emitted and sequentially passes through slit 25, plane contra-reflector 26, spherical reflector 27, temperature and pressure resistant schlieren cavity 29, spherical reflector 34, plane contra-reflector 35, and knife edge 36. After reflection and refraction, the observation light 39 enters the high-precision camera 37. When carbon dioxide breaks through the saturated test rock sample 19, low-concentration (<20ppm) carbon dioxide enters the schlieren cavity 29, causing a change in the density of the flow field medium, which in turn causes a change in the refractive index of the flow field medium. As a result, a change in light intensity caused by carbon dioxide breakthrough appears in the cavity, thus producing a schlieren image of carbon dioxide breakthrough. The high-precision camera 37 records the diffracted light schlieren image and transmits it to the computer data acquisition system 22, thereby accurately determining the moment of carbon dioxide breakthrough.

[0056] like Figure 3 As shown, the schlieren chamber 29 has a cylindrical structure made of 316 stainless steel, which is resistant to high pressure and high temperature, and can withstand a maximum pressure of 50 MPa and a maximum temperature of 100℃. The axial length of the schlieren chamber 29 is 0.2–0.5 m, and the diameter is 0.25–0.5 m. Symmetrically arranged on the side of the schlieren chamber 29 along the propagation path of the observation light 39 are two schlieren observation windows: I (28) and II (30). Both observation windows are made of cylindrical glass with a thickness of 20–25 mm, embedded in cylindrical caps of 316 stainless steel, and sealed with screws. The schlieren chamber 29 is surrounded by a conical gas inlet 31, a circular gas outlet 32, and a degassed pure water inlet 40.

[0057] like Figure 4As shown, the conical gas inlet 31 adopts a conical integrated nozzle structure. The left side of the nozzle is a conical nozzle with a radius of 0.5 mm, and the right side is a reverse conical nozzle with a radius of 1.0 mm. After breakthrough, carbon dioxide can effectively enter the conical gas inlet 31 and, through the amplification effect of the reverse conical nozzle, form a beam within the schlieren cavity 29, thereby enhancing the clarity of the schlieren imaging. The conical gas inlet 31 is equipped with a waterproof and breathable microporous membrane 41, which can be made of polytetrafluoroethylene (PTFE) polymer material. During the carbon dioxide breakthrough test of the saturated rock sample 19, water is blocked from entering the schlieren cavity 29 by the waterproof and breathable microporous membrane 41, while carbon dioxide can pass through the membrane and enter the schlieren cavity 29. Simultaneously, it prevents the isobaric degassed water within the schlieren cavity 29 from entering the pipeline through the conical gas inlet 31.

[0058] like Figure 5 As shown, the circular gas outlet 32 ​​adopts a small circular nozzle structure, with the left and right radii of the nozzle being 0.2–0.3 mm. A waterproof and breathable microporous membrane 41, made of polytetrafluoroethylene (PTFE) polymer material, is installed inside the circular gas inlet. Carbon dioxide in the schlieren cavity 29 flows out of the cavity through the circular gas outlet 32.

[0059] Further optimizations were made. LED light source 24 is a colored LED with a center wavelength of 532nm and a power of 250W. The intensity of the test light 39 is adjusted by regulating the focusing lens of LED light source 24. Slit 25 is a circular hole with a diameter less than 2-3mm and sharp, burr-free edges. The effective aperture of spherical mirrors Ⅰ27 and Ⅱ34 is φ100-150mm, with a focal length of 2.0-2.5m, an edge thickness of 50-60mm, and a surface accuracy of 1 / 4 wavelength. The effective aperture of planar convoluted mirrors Ⅰ26 and Ⅱ35 is φ200-250mm, with a surface accuracy of 1 / 8 wavelength. Schlieren observation windows Ⅰ28 and Ⅱ30 are made of high-strength, non-refractive, and non-reflective glass with an effective aperture of φ150mm, material K9, and a thickness of 20-25mm. The opening width of the knife edge 36 ranges from 0-5mm, with a step accuracy of 0.01mm. The aforementioned high-precision schlieren imaging devices are all mounted on the mounting structure, with a center height of 1.1m and a lifting adjustment accuracy of ±100mm.

[0060] The following are the specific implementation steps of an embodiment of the breakthrough pressure testing method for low-permeability rocks based on schlieren imaging of the present invention:

[0061] Step 1: Place the processed cylindrical test rock sample 19 into a drying oven and dry it until completely dry (drying temperature set to 60℃). Then measure its dry density, height, diameter and volume.

[0062] Step 2: Use corrosion-resistant heat shrink tubing, silicone, or rubber to seal the test rock sample 19 and the metal pads at both ends into a sealed whole.

[0063] Step 3: Place the sealed test rock sample 19 into the rock sample holder 17, connect all the stainless steel pipes, and check the working status of the pressure sensor and schlieren imaging system to ensure they are properly connected to the computer data acquisition system 22. After evacuating the entire pipeline system using the vacuum pump 42, inject degassed pure water 33 into the schlieren chamber through the degassed pure water inlet / outlet 40.

[0064] Step 4: Place the entire measuring device in a constant temperature environment. Adjust the positions of each component of the schlieren imaging system so that the observation light 39 can acquire a clear diffraction image by the high-precision camera 37. Adjust the light intensity of the LED light source 24, the slit width of the slit 25, and the position of the blade edge 36 to ensure reasonable schlieren imaging. During debugging, ensure the following key points: (1) The slit position must be located at the focal point of the light source; (2) The distance between the slit and the reflector I should be 1 times the focusing distance of the light source; (3) The blade edge position must be located at the focal point between the reflectors II. With the optical path properly adjusted, the high-precision camera can clearly observe the schlieren image.

[0065] Step 5: Open the third control valve F4 and the fourth control valve F5 to inject the confining pressure medium into the confining pressure chamber 20 through the confining pressure injection pump 13, loading the test rock sample 19 to the target confining pressure and maintaining the target confining pressure unchanged. Then, open the three-way control valve F3 to inject deionized water into the test rock sample 19 through the liquid injection pump 9 until saturation. Subsequently, adjust the three-way control valve F3 to convert carbon dioxide into liquid or supercritical carbon dioxide using the water bath heating circulation tank 3, and inject carbon dioxide into the test rock sample 19 through the carbon dioxide injection pump 9.

[0066] Step 6: Employing a stepwise pressurization method, the carbon dioxide injection pressure is gradually increased using a high-precision carbon dioxide injection pump 9. During the experiment, data from the confining pressure sensor 15, upstream pressure sensor 16, high-precision camera 37, and downstream pressure sensor 38 are recorded in real time. When carbon dioxide breaks through the saturated test rock sample 19, the high-precision camera 37 captures the change in medium density and light intensity caused by carbon dioxide entering the schlieren cavity 29, recording the schlieren image of the carbon dioxide breakthrough. This moment is determined as the breakthrough time of carbon dioxide. The pressure difference between the upstream and downstream of the measuring device at this time is recorded as the breakthrough pressure of the carbon dioxide in the tested rock core.

[0067] Stepwise pressurization method Figure 6As shown, the carbon dioxide injection pressure of the test rock sample 19 was gradually increased, and the upstream and downstream pressure differences and the light intensity changes of the schlieren imaging system were monitored. First, carbon dioxide pressure was injected into the rock sample holder 17 in stages via the carbon dioxide injection pump 2. The first stage injected a relatively small pressure P1 (0–2 MPa), the second stage pressure P2 was ΔP1 greater than P1, the third stage pressure P3 was ΔP2 greater than P2, and so on, with the (i+1)th stage pressure P... i+1 P i Large ΔP i Each pressure increase is within the range of 0.01-1 MPa, and the carbon dioxide pressure is always less than the confining pressure. When carbon dioxide breaks through the test rock sample 19, it enters the schlieren cavity 29, causing a change in the fluid medium and inducing a change in the refractive index of the medium within the schlieren cavity 29. Consequently, a change in light intensity occurs within the cavity due to the carbon dioxide breakthrough. A high-precision camera 37 captures the moment when the light intensity change caused by the carbon dioxide breakthrough in the schlieren cavity 29 is triggered and transmits the information to the computer data acquisition system 22, thereby recording the upstream and downstream pressure difference of the measuring device at this time, which is the carbon dioxide breakthrough pressure of the test rock sample 19. Figure 7 This diagram illustrates the diffraction schlieren image formed by the light intensity change induced by carbon dioxide breakthrough within the schlieren cavity 29. The horizontal axis represents the length of the schlieren image, reflecting the response distance of the light intensity change in the fluid medium caused by the carbon dioxide breakthrough; the vertical axis represents the height of the carbon dioxide schlieren image, with the zero point corresponding to the position of the conical gas inlet 31, indicating the height of the light intensity change in the fluid medium caused by the carbon dioxide breakthrough. Because the density of carbon dioxide differs from that of the fluid medium (degassed pure water) within the schlieren cavity 29, after the carbon dioxide breakthrough, it enters the schlieren cavity from the conical gas inlet 31, triggering a change in the light intensity of the fluid medium and forming... Figure 7 The image shown is a schlieren image of the carbon dioxide breakthrough moment. According to... Figure 7 Medium schlieren images can accurately determine Figure 6 By analyzing changes in light intensity and the breakthrough moment of low-concentration carbon dioxide, the breakthrough pressure of low-permeability rocks can be accurately determined.

[0068] Step 7: After the experiment, turn off the carbon dioxide injection pump 9 and the three-way control valve F3 in sequence, and open the fifth control valve F6 to release the carbon dioxide and pressure until it is zero. Then, by adjusting the confining pressure injection pump 13, unload the confining pressure to zero. After the pressure is released, take the test rock sample 19 out of the rock sample holder 17. The breakthrough pressure measurement experiment is now complete.

[0069] Accordingly, the present invention also proposes a low-permeability rock breakthrough pressure testing device based on schlieren imaging, which can realize the above-mentioned testing method. It includes a fluid injection module A, a confining pressure injection module B, a rock sample holder module C, and a high-precision schlieren imaging module D. The structural composition and connection relationship of each module have been described in detail in the above method and will not be repeated here.

[0070] It should be noted that, depending on the implementation needs, the various steps / components described in this application can be broken down into more steps / components, or two or more steps / components or parts of the operation of steps / components can be combined into new steps / components to achieve the purpose of this invention.

[0071] The order of the steps in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0072] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging, characterized in that, Includes the following steps: Process the test rock sample and place it into the confining pressure chamber of the rock sample holder; The rock sample holder's injection end is connected to the upstream injection pipeline, and its outlet end is connected to the conical gas inlet of the schlieren imaging system's schlieren cavity. The confining pressure chamber of the rock sample holder is connected to the confining pressure injection module. The upstream injection pipeline is connected to the fluid injection module for injecting water or carbon dioxide into the test rock sample, and an upstream pressure sensor is installed on the upstream injection pipeline. The circular gas outlet of the schlieren cavity is connected to the downstream outflow pipeline, and a downstream pressure sensor is installed on the downstream outflow pipeline. The schlieren cavity also has a degassed pure water injection / discharge port. The conical gas inlet is a cone-shaped integrated nozzle structure with a diameter that first decreases and then increases, and the diameter of the outlet end is larger than the diameter of the inlet end. A waterproof and breathable microporous membrane is installed inside the conical gas inlet. The circular gas outlet is a circular nozzle structure, and a waterproof and breathable microporous membrane is installed inside the circular gas outlet. The test device pipeline is evacuated through the downstream outflow pipeline; degassed pure water medium is injected into the schlieren cavity through the degassed pure water inlet on the schlieren cavity; the optical path of the schlieren imaging system is adjusted so that the observation light can acquire a clear schlieren image by the high-precision camera. The entire measuring device is placed in a constant temperature control system. Confining pressure medium is injected into the confining pressure chamber through the confining pressure injection module to apply confining pressure stress to the test rock sample. Then the confining pressure injection module is closed, and water is injected into the rock sample holder through the water injection system of the fluid injection module until the test rock sample is completely saturated. Liquid or supercritical carbon dioxide is injected into the rock sample holder through the gas injection system of the fluid injection module. Carbon dioxide at different pressures was injected into the rock sample holder using a stepwise pressurization method. During the experiment, the schlieren imaging system and each pressure sensor were observed in real time until the carbon dioxide broke through the saturation of the rock sample. The high-precision camera recorded the schlieren image of the carbon dioxide breakthrough formed after the carbon dioxide entered the schlieren cavity and changed the flow field distribution characteristics. This moment is the breakthrough moment of carbon dioxide. The pressure difference between the upstream and downstream of the measuring device at this time is recorded, which is the breakthrough pressure of carbon dioxide in the core sample being tested.

2. The method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging according to claim 1, characterized in that, A water separator is installed on the pipeline between the outlet of the rock sample holder and the inlet of the schlieren cavity to absorb the water that will be discharged from the saturated test rock sample.

3. The method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging according to claim 1, characterized in that, The schlieren imaging system includes an LED light source, a slit, a plane-converting mirror (I), a spherical mirror (I), a schlieren cavity, a spherical mirror (II), a plane-converting mirror (II), a knife edge, and a high-precision camera. The plane-converting mirror (I) and the plane-converting mirror (II), as well as the spherical mirror (I) and the spherical mirror (II), are arranged symmetrically about the schlieren cavity. By adjusting the intensity of the LED light source, the emitted observation light passes sequentially through the slit, the plane-converting mirror (I), the spherical mirror (I), the schlieren cavity, the spherical mirror (II), the plane-converting mirror (II), and the knife edge before entering the high-precision camera.

4. The method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging according to claim 3, characterized in that, The schlieren cavity is a cylindrical cavity structure made of pressure- and temperature-resistant material, capable of withstanding a maximum pressure of 50 MPa and a maximum temperature of 100°C; the side of the schlieren cavity is symmetrically provided with schlieren observation window I and schlieren observation window II along the propagation path of the observation light.

5. The method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging according to claim 1, characterized in that, The radius of the circular gas outlet is 0.2~0.3mm.

6. The method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging according to claim 1, characterized in that, The gas injection system includes a carbon dioxide cylinder, a carbon dioxide injection pump, and a water bath heating circulation tank. The carbon dioxide cylinder is connected to the inlet of the carbon dioxide injection pump, and a first control valve (F1) is installed on the connecting pipeline between the two. The outlet of the carbon dioxide injection pump is connected to a three-way control valve (F3). The water bath heating circulation tank is connected to the storage chamber of the carbon dioxide injection pump through a heating medium inlet pipeline and a heating medium outlet pipeline. The heating medium in the water bath heating circulation tank enters the storage chamber of the carbon dioxide injection pump through the heating medium inlet pipeline to heat the carbon dioxide. After heat exchange, the medium returns to the water bath heating circulation tank through the heating medium outlet pipeline for circulation heating. By adjusting the temperature of the water bath heating circulation tank 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. The water injection system includes a fluid medium device and a high-precision liquid injection pump. The fluid medium device is connected to the inlet of the high-precision liquid injection pump, and a second control valve (F2) is provided on the connecting pipeline between the two. The outlet of the high-precision liquid injection pump is connected to a three-way control valve (F3). The three-way control valve (F3) is connected to the rock sample holder via the upstream injection line.

7. The method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging according to claim 1, characterized in that, The confining pressure injection module includes a confining pressure medium device, a confining pressure injection pump, and a confining pressure sensor. The confining pressure medium device is connected to the inlet of the confining pressure injection pump, and a third control valve (F4) is installed on the connecting pipeline between the two. The outlet of the confining pressure injection pump is connected to the confining pressure chamber through a confining pressure injection pipeline, which is equipped with a confining pressure sensor and a fourth control valve (F5). The confining pressure medium device is filled with deionized water or oil. By controlling the confining pressure injection pump, deionized water or oil at different pressures is injected into the confining pressure chamber to provide different circumferential pressures for the test rock sample.

8. The method for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging according to claim 1, characterized in that, The rock sample holder includes a confining pressure chamber, a heating ring, and a holder cover arranged sequentially from the inside to the outside. The test rock sample is installed in the confining pressure chamber, which is filled with a confining pressure medium to apply a uniform circumferential load to the test rock sample. The heating ring heats the confining pressure medium through a heat-conducting material, so that the confining pressure chamber reaches the target temperature.

9. A device for testing the breakthrough pressure of low-permeability rocks based on schlieren imaging, characterized in that, The testing device implements the testing method according to any one of claims 1-8, and includes a fluid injection module, a confining pressure injection module, a rock sample holder module, and a high-precision schlieren imaging module; The fluid injection module includes an air injection system and a water injection system. The air injection system is used to inject carbon dioxide into the test rock sample, and the water injection system is used to inject water into the test rock sample. The rock sample holder module includes a rock sample holder and a test rock sample. The rock sample holder includes a confining pressure chamber, a heating ring, and a holder cover arranged sequentially from the inside to the outside. The test rock sample is installed in the confining pressure chamber, which is filled with a confining pressure medium to apply a uniform circumferential load to the test rock sample. The heating ring heats the confining pressure medium through a heat-conducting material, so that the confining pressure chamber reaches the target temperature. The injection end of the rock sample holder is connected to the fluid injection module through an upstream injection pipeline, and the outlet end is connected to the schlieren cavity inlet of the high-precision schlieren imaging module. The confining pressure chamber of the rock sample holder is connected to the confining pressure injection module. An upstream pressure sensor is installed on the upstream injection pipeline. The confining pressure injection module is used to inject deionized water or oil into the confining pressure chamber to provide confining pressure for the test rock sample; The high-precision schlieren imaging module includes a water separator, a schlieren imaging system, a downstream pressure sensor, and a vacuum pump. The schlieren imaging system includes an LED light source, a slit, a planar conical mirror, a spherical mirror, a schlieren observation window, a schlieren cavity, a schlieren observation window, a spherical mirror, a planar conical mirror, a knife edge, and a high-precision camera. The schlieren cavity is equipped with a conical gas inlet, a circular gas outlet, and a degassed pure water injection / discharge port. The water separator is installed in the pipeline between the rock sample holder outlet and the conical gas inlet at the schlieren cavity inlet end, used to absorb and drain water saturated in the test rock sample. The circular gas outlet at the schlieren cavity outlet end is connected to the downstream outflow pipeline, and a downstream pressure sensor is installed on the downstream outflow pipeline.

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