Low-permeability rock breakthrough pressure testing method and device based on schlieren imaging

Abstract

The invention discloses a low-permeability rock breakthrough pressure testing method based on schlieren imaging. The method comprises the following steps: putting a test rock sample into a confining pressure cavity of a rock sample holder; the injection end of the rock sample holder is connected with the fluid injection module, the outlet end of the rock sample holder is connected with the schlieren cavity, and the confining pressure cavity is connected with the confining pressure injection module; vacuumizing a pipeline of the testing device, injecting a degassed purified water medium into the schlieren cavity, and then carrying out optical path debugging until a clear schlieren image is acquired; loading confining pressure stress on the test rock sample, injecting water until the test rock sample is saturated, injecting carbon dioxide, observing a schlieren imaging system, recording a schlieren image of carbon dioxide breakthrough by a high-precision camera when the carbon dioxide breaks through the saturated rock sample, recording the breakthrough moment of the carbon dioxide, and recording the upstream and downstream pressure difference value at the moment; the breakthrough pressure is the breakthrough pressure of the carbon dioxide of the tested rock core sample. According to the invention, a trace concentration of carbon dioxide can be visually observed to break through the rock sample, so that the breakthrough pressure of the low-permeability rock can be accurately detected.

Description

Technical Field

[0001] The present invention relates to the technical field of low-permeability rock testing, and in particular to a method and device for testing the breakthrough pressure of low-permeability rock based on schlieren imaging. Background Art

[0002] As large quantities of greenhouse gases are emitted into the atmosphere, global climate change is becoming increasingly serious, and greenhouse gas emission reduction has become one of the key issues of concern to countries around the world. Carbon dioxide capture and geological storage technology extracts carbon dioxide from industrial waste gas, uses industrial technology to convert carbon dioxide into supercritical carbon dioxide, and injects it into deep formations to achieve long-term geological storage. At the same time, fields such as deep gas storage, oil and gas reservoir storage, and compressed air energy storage are increasingly relying on the tightness of deep rock formations to safely store important media such as oil, gas, hydrogen, and helium. In the above-mentioned technical fields, low-permeability rocks such as gypsum, mudstone, and shale are usually used as caprocks to seal gases, and the breakthrough pressure test of low-permeability caprocks is a key indicator for evaluating the tightness of caprocks.

[0003] Breakthrough pressure refers to the minimum displacement pressure at which a non-wetting phase (such as CO2) displaces a wetting phase (such as water) in rock pores, forming a dominant seepage channel through the intact rock. Its magnitude is closely related to the capillary pressure at the smallest pore throat of the dominant seepage channel. Currently, breakthrough pressure is often measured using the rock displacement method. However, existing testing equipment lacks high-precision sensors capable of detecting gas breakthrough. Consequently, the moment of gas breakthrough is typically determined based on pressure curves. This method can lead to significant deviations between experimentally measured breakthrough pressures and actual values. Low-permeability caprocks are often composed of nanopores, and gas displacement of liquid within the rock is very slow. Even if gas breakthrough occurs, the minimal amount of liquid expelled makes it difficult for pressure sensors to detect pressure changes within the pipeline, making it difficult to determine the precise moment of gas breakthrough. This can lead to overlooking CO2 breakthrough while continuing to increase the injection pressure at the injection point, resulting in significant errors in test results.

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

[0005] The main purpose of the present invention is to address the problems of insufficient sensitivity and delay in breakthrough pressure monitoring in existing breakthrough pressure testing devices and methods, and to provide a low-permeability rock breakthrough pressure testing method and device based on schlieren imaging. The method can visualize the breakthrough rock samples of trace concentrations (<10ppm) of carbon dioxide, overcome the problems of insufficient sensitivity and delay in determining the breakthrough moment in low-permeability cap rock testing, and thus provide an efficient and accurate testing method and device for calculating the breakthrough pressure of low-permeability rocks.

[0006] The technical solution adopted in the present invention is:

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

[0008] processing a test rock sample and placing it into a confining pressure chamber of a rock sample holder;

[0009] The injection end of the rock sample holder is connected to the upstream injection pipeline, the outlet end is connected to the Schlieren cavity inlet of the Schlieren imaging system, and the confining pressure cavity 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 set on the upstream injection pipeline; the Schlieren cavity outlet is connected to the downstream outflow pipeline, and a downstream pressure sensor is set on the downstream outflow pipeline;

[0010] The test device pipeline is vacuumed through the downstream outflow pipeline; degassed pure water medium is injected into the Schlieren cavity through the degassed pure water injection outlet on the Schlieren cavity; the optical path of the Schlieren imaging system is debugged so that the observation light can capture a clear Schlieren image on the high-precision camera;

[0011] The entire measuring device is placed in a constant temperature control system, and the 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] A step-by-step pressurization method is used to inject carbon dioxide at different pressures into the rock sample holder. During the experiment, the schlieren imaging system and various pressure sensors are observed in real time until the carbon dioxide breaks through the saturated rock sample. The high-precision camera will record the schlieren image of the carbon dioxide breakthrough formed after the carbon dioxide enters the schlieren cavity and changes 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 measured core sample.

[0013] In the above scheme, a water remover is provided on the pipeline between the outlet of the rock sample holder and the inlet of the schlieren cavity, so as 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 turning mirror I, a spherical reflector, a Schlieren cavity, a spherical reflector II, a plane turning mirror II, a knife edge and a high-precision camera; the plane turning mirror I and the plane turning mirror II, the spherical reflector I and the spherical reflector II are respectively arranged in a centrally symmetrical manner about the Schlieren cavity; by adjusting the light source intensity of the focusing LED light source, the observation light is emitted and passes through the slit, the plane turning mirror I, the spherical reflector I, the Schlieren cavity, the spherical reflector II, the plane turning mirror II and the knife edge in sequence and then enters the high-precision camera.

[0015] In the above scheme, the Schlieren cavity is a cylindrical cavity structure made of pressure-resistant and temperature-resistant materials, which can withstand a maximum pressure of 50MPa 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 on the propagation path of the observation light; the Schlieren cavity is also provided with a cone gas inlet, a circular hole gas outlet and a degassed pure water injection outlet.

[0016] In the above solution, the cone gas inlet is a cone-shaped integrated nozzle structure whose diameter 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 in the cone gas inlet.

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

[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 circulation box. The carbon dioxide gas cylinder is connected to the inlet of the carbon dioxide injection pump. A first control valve (F1) is provided on the connecting pipeline between the two. The outlet of the carbon dioxide injection pump is connected to the three-way control valve (F3). The water bath heating circulation box is connected to the carbon dioxide injection pump storage chamber through a heating medium inlet pipeline and a heating medium outlet pipeline respectively. The heating medium in the water bath heating circulation box enters the carbon dioxide injection pump storage chamber through the heating medium inlet pipeline to heat the carbon dioxide. After heat exchange, the medium returns to the water bath heating circulation box through the heating medium outlet pipeline for circulation heating. By adjusting the temperature of the water bath heating circulation 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.

[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. 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 through the upstream injection pipeline.

[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 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 the confining pressure injection pipeline, and a confining pressure 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 deionized water or oil of different pressures is injected into the confining pressure chamber 20 by controlling the confining pressure injection pump to provide different annular pressures for the test rock sample.

[0022] In the above scheme, the rock sample holder includes a confining pressure chamber, a heating ring and a holder cover arranged in sequence 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, and a uniform circumferential load is applied to the test rock sample; the heating ring heats the confining pressure medium through a heat-conducting material so that the target temperature is reached inside the confining pressure chamber.

[0023] Correspondingly, the present invention also proposes a low permeability rock breakthrough pressure testing device based on schlieren imaging, which 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 in sequence from the inside to the outside, the test rock sample is installed in the confining pressure chamber, the confining pressure chamber is filled with confining pressure medium, and a uniform annular load is applied 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 of the high-precision schlieren imaging module. The inlet of the Schlieren cavity is connected, and the confining pressure cavity of the rock sample holder is connected to the confining pressure injection module; an upstream pressure sensor is provided on the upstream injection pipeline; the confining pressure injection module is used to inject deionized water or oil into the confining pressure cavity to provide the confining pressure of the test rock sample; the high-precision Schlieren imaging module includes a dehydrator, 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 turning mirror I, a spherical reflector, a Schlieren observation window I, a Schlieren cavity, a Schlieren observation window II, a spherical reflector, a plane turning mirror II, a knife edge and a high-precision camera; the Schlieren cavity is provided with a cone gas inlet, a circular hole gas outlet and a degassed pure water injection outlet; the dehydrator is arranged in the pipeline between the outlet of the rock sample holder and the cone gas inlet at the inlet end of the Schlieren cavity, for absorbing water that breaks through and discharges the saturated test rock sample; the circular hole gas outlet at the outlet end of the Schlieren cavity is connected to the downstream outflow pipeline, and a downstream pressure sensor is provided on the downstream outflow pipeline.

[0024] The beneficial effects produced by the present invention are:

[0025] Schlieren imaging is an optical method used to visualize density changes in transparent media and is widely used in fields such as fluid mechanics, aerodynamics, and thermodynamics. This invention innovatively applies a high-precision schlieren imaging system to low-permeability rock breakthrough pressure testing. By capturing the light deflection caused by the refractive index gradient change of the fluid medium when trace concentrations of carbon dioxide (<10 ppm) break through, the system ultimately presents a schlieren image of the CO2 breakthrough on the high-speed camera image plane, thereby accurately detecting the initial moment of CO2 breakthrough.

[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, an easy-to-understand testing principle, efficient and convenient operation, and good stability, and can effectively test breakthrough pressure.

[0027] 3. The present invention is applicable to the evaluation of breakthrough pressure and tightness of low permeability caprocks 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, safety and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 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 Schematic diagram of the test principle of high-precision Schlieren imaging of carbon dioxide breakthrough of the measuring device shown;

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

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

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

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

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

[0036] In the figure: 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 box; 4. Heating medium inlet pipeline; 5. Heating medium outlet pipeline; 6. Carbon dioxide injection pump storage chamber; 7. Fluid medium device; 9. Liquid injection pump; 10. Injection pipeline of gas injection system; 11. Injection pipeline of liquid injection system; 12. Confining pressure medium device; 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. Water eliminator. 22. Computer data acquisition system; 23. Data transmission line; 24. LED light source; 25. Slit; 26. Plane turning mirror I; 27. Spherical reflector I; 28. Schlieren observation window I; 29. Schlieren cavity; 30. Schlieren observation window II; 31. Conical gas inlet; 32. Circular gas outlet; 33. Degassed purified water; 34. Spherical reflector II; 35. Plane turning mirror II; 36. Knife edge; 37. High-precision camera; 38. Downstream pressure sensor; 39. Observation light; 40. Degassed purified water injection and discharge port; 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 DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present 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 only used to explain the present invention and are not intended to limit the present invention.

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

[0041] In the present invention, it should also be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer" and the like are used to indicate positions or locations based on those shown in the accompanying drawings. These terms are intended solely to facilitate the description of the present application and to simplify the description. They are not intended to indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limiting the present application. Furthermore, the terms "first" and "second" are used solely for descriptive and distinguishing purposes and should not be construed as indicating or implying relative importance.

[0042] The present 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 is processed into a cylinder, and the end faces and side faces are cut flat and smooth; the rock sample and the metal end pad are sealed into a whole with a heat shrink tube, and then placed in the confining pressure chamber of the rock sample holder;

[0044] The injection end of the rock sample holder is connected to the upstream injection pipeline, the outlet end is connected to the Schlieren cavity inlet of the Schlieren imaging system, and the confining pressure cavity 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 set on the upstream injection pipeline; the Schlieren cavity outlet is connected to the downstream outflow pipeline, and a downstream pressure sensor is set on the downstream outflow pipeline;

[0045] The test device pipeline is vacuumed through the downstream outflow pipeline; degassed pure water medium is injected into the Schlieren cavity through the degassed pure water injection and discharge port on the Schlieren cavity (to ensure that the Schlieren cavity is filled with degassed pure water to avoid the presence of dissolved gas that affects the Schlieren imaging); the optical path of the Schlieren imaging system is debugged so that the observation light can capture a clear Schlieren image on the high-precision camera;

[0046] The entire measuring device is placed in a constant temperature control system, and the 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] A step-by-step pressurization method is used to inject carbon dioxide at different pressures into the rock sample holder. During the experiment, the schlieren imaging system and various pressure sensors are observed in real time until the carbon dioxide breaks through the saturated rock sample. The high-precision camera will record the schlieren image of the carbon dioxide breakthrough formed after the carbon dioxide enters the schlieren cavity and changes 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 measured core sample.

[0048] Figure 1 The invention discloses a measuring device for the low permeability rock breakthrough pressure test method based on schlieren imaging, comprising 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] Fluid injection module A comprises 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 and includes a carbon dioxide cylinder 1, a carbon dioxide injection pump 2, and a water bath heating circulation box 3. The carbon dioxide cylinder 1 is connected to the inlet of the carbon dioxide injection pump 2, and the connecting pipeline between the two is equipped with a first control valve F1. The outlet of the carbon dioxide injection pump 1 is connected to a three-way control valve F3 via the gas injection system's injection pipeline 10. The water bath heating circulation box 3 is connected to the carbon dioxide injection pump storage chamber 6 via a heating medium inlet pipeline 4 and a heating medium outlet pipeline 5, respectively. The heating medium in the water bath heating circulation box 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 box 3 through the heating medium outlet pipeline 5 for circulation and heating. By adjusting the temperature of the water bath heating circulation box 3 and the pressure of the carbon dioxide injection pump 2, the carbon dioxide can be converted from a gaseous state to a liquid or supercritical state and injected into the rock sample holder 17.

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

[0051] Confining pressure injection module B is used to inject deionized water or oil into the confining pressure chamber 20 to provide confining pressure for the test rock sample 19. It 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 in the connecting pipeline between the two. The outlet of the confining pressure injection pump 13 is connected to the confining pressure chamber 20 of the rock sample holder module C via 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 is filled with deionized water or oil. By controlling the confining pressure injection pump 13, deionized water or oil of different pressures is injected into the confining pressure chamber 20 of the rock sample holder module C, providing different annular pressures for the test rock sample 19.

[0052] The rock sample holder module C, used to load and test a test rock sample 19, comprises a rock sample holder 17 and the test rock sample 19. The rock sample holder 17 comprises, arranged from the inside out, a confining pressure chamber 20, a heating ring 18, and a holder housing. The test rock sample 19 is installed within the confining pressure chamber 20, which is filled with a confining pressure medium, applying a uniform circumferential load to the test rock sample 19. The heating ring 18 heats the confining pressure medium via a heat-conducting material, bringing the confining pressure chamber to the target temperature. The holder housing 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 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, designed to accurately monitor the initial moment of CO2 breakthrough through low-permeability rock, comprises a dewatering device 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 plane deflecting mirror I 26, a spherical reflector I 27, a temperature- and pressure-resistant Schlieren cavity 29, a spherical reflector II 34, a plane deflecting mirror II 35, a knife edge 36, and a high-precision camera 37. The Schlieren cavity 29 is equipped with a cone-shaped gas inlet 31, a circular gas outlet 32, and a degassed pure water inlet and outlet 40. The dewatering device 21, located in the outlet pipe of the rock sample holder 17, absorbs water that breaks through and discharges the saturated test rock sample 19. The outlet of the dewatering device 21 is connected to the cone-shaped gas inlet 31 at the inlet of the Schlieren cavity 29. The circular gas outlet 32 at the outlet of the Schlieren cavity 29 is connected to the downstream pressure sensor 38 and to the fifth control valve F6 and vacuum pump 42 at the downstream outlet.

[0054] 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 devices were connected, the fifth control valve F6 was opened, and the test apparatus was evacuated to a vacuum state using a vacuum pump 42. Degassed purified water 33 was then injected into the chamber through the injection and discharge port 40.

[0055] The Schlieren imaging system uses the principle that the refractive index gradient of diffracted light in the measured flow field is proportional to the airflow density of the flow field to select and measure. The test principle is as follows: Figure 2 As shown, by adjusting the intensity of the focused LED light source 24, observation light 39 is emitted, which passes through the slit 25, plane deflecting mirror I 26, spherical reflector I 27, the temperature- and pressure-resistant Schlieren cavity 29, spherical reflector II 34, plane deflecting mirror II 35, and knife edge 36. After reflection and refraction, observation light 39 enters the high-precision camera 37. When CO2 breaks through the saturated test rock sample 19, low-concentration CO2 (<20 ppm) enters the Schlieren cavity 29, causing a change in the density of the flow field medium and, in turn, a change in the refractive index of the flow field medium. This causes a change in light intensity within the cavity due to CO2 breakthrough, generating a Schlieren image of the CO2 breakthrough. The high-precision camera 37 records the diffracted light Schlieren image and transmits it to the computer data acquisition system 22, allowing the precise determination of the CO2 breakthrough moment.

[0056] like Figure 3 As shown, the Schlieren cavity 29 is a cylindrical structure constructed from high-pressure and high-temperature-resistant 316 stainless steel, capable of withstanding a maximum pressure of 50 MPa and a maximum temperature of 100°C. The cavity 29 has an axial length of 0.2 to 0.5 m and a diameter of 0.25 to 0.5 m. Schlieren observation windows I and II are symmetrically positioned on the sides of the cavity 29, along the propagation path of the observation light 39. These windows are constructed from cylindrical glass 20 to 25 mm thick, embedded in the 316 stainless steel cylindrical screw cap and sealed with a screw. The cavity 29 is surrounded by a conical gas inlet 31, a circular gas outlet 32, and a degassed purified water inlet and outlet 40.

[0057] like Figure 4As shown, the cone gas inlet 31 adopts a conical integrated nozzle structure. The left side of the nozzle is a cone nozzle with a radius of 0.5mm, and the right side is a reverse cone nozzle with a radius of 1.0mm. After the breakthrough, carbon dioxide can effectively enter the cone gas inlet 31 and form a beam in the Schlieren cavity 29 through the amplification effect of the reverse cone nozzle, thereby enhancing the clarity of the Schlieren imaging. A waterproof and breathable microporous membrane 41 is provided in the cone gas inlet 31, and the material can be selected from polytetrafluoroethylene polymer material. In the process of carbon dioxide breaking through the saturated test rock sample 19, water is blocked by the waterproof and breathable microporous membrane 41 from entering the Schlieren cavity 29, while carbon dioxide can pass through the waterproof and breathable microporous membrane 41 and enter the Schlieren cavity 29. At the same time, it prevents the isoenclosed degassed water in the Schlieren cavity 29 from entering the pipeline through the cone gas inlet 31.

[0058] like Figure 5 As shown, the circular gas outlet 32 utilizes a small circular nozzle structure, with a radius of 0.2-0.3 mm on both the left and right sides of the circular nozzle. A waterproof, breathable microporous membrane 41, made of a polymer material such as polytetrafluoroethylene, is located within 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 have been made. The LED light source 24 is a color LED with a central wavelength of 532nm and a power of 250W. The intensity of the test light 39 is adjusted by adjusting the focusing lens of the LED light source 24. The slit 25 is a circular hole with a diameter of less than 2-3mm and sharp edges without burrs. The effective aperture of the spherical reflector 27 I and the spherical reflector 34 II is φ100-150mm, with a focal length of 2.0-2.5m. The edge thickness of the spherical reflector is 50-60mm, and the surface accuracy is 1 / 4 wavelength. The effective aperture of the plane deflection mirror 26 I and the plane deflection mirror 35 is φ200-250mm, with a surface accuracy of 1 / 8 wavelength. The Schlieren observation window I 28 and the Schlieren observation window II 30 are made of high-strength, non-refractive and non-reflective glass with an effective aperture of φ150mm, a K9 material, and a thickness of 20-25mm. The opening width of the knife edge 36 ranges from 0-5mm, with a stepping accuracy of 0.01mm. The above-mentioned high-precision Schlieren imaging devices are all installed on the installation and adjustment structure, with a center height of 1.1m and a lifting and lowering adjustment accuracy of ±100mm.

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

[0061] Step 1: Place the processed cylindrical test rock sample 19 in a drying oven and dry it until it is completely dry (the drying temperature is set at 60° C.), and then measure its dry density, height, diameter, volume and other parameters.

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

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

[0064] Step 4: Place the entire measurement device in a constant temperature environment and adjust the positions of the various devices in the Schlieren imaging system so that the observation light 39 can capture a clear diffraction image on the high-precision camera 37. Adjust the light intensity of the focusing LED light source 24, the slit 25 width, and the position of the knife edge 36 to make the Schlieren imaging reasonable. During debugging, ensure the following key points: (1) The slit position must be at the focal point of the light source; (2) The distance between the slit and reflector I should be 1 times the focusing distance of the light source; (3) The knife edge position must be at the focal position between reflectors II. With a good optical path debugging effect, the high-precision camera can clearly observe the Schlieren image.

[0065] Step 5: Open the third and fourth control valves F4 and F5, and inject the confining pressure medium into the confining pressure chamber 20 via the confining pressure injection pump 13, loading the test rock sample 19 to the target confining pressure and maintaining the target confining pressure. Next, open the three-way control valve F3, and inject deionized water into the test rock sample 19 via the liquid injection pump 9 until it is saturated. Subsequently, adjust the three-way control valve F3, and use the water bath heating circulation box 3 to convert carbon dioxide into liquid or supercritical carbon dioxide. This carbon dioxide is then injected into the test rock sample 19 via the carbon dioxide injection pump 9.

[0066] Step 6: Using a step-by-step pressurization method, gradually increase the CO2 injection pressure using a high-precision CO2 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 CO2 breaks through the saturated test rock sample 19, the high-precision camera 37 captures the changes in the density of the flow field medium within the cavity and the resulting changes in the light intensity of the medium after CO2 enters the schlieren cavity 29. The schlieren image of the CO2 breakthrough is recorded, and this moment is determined as the CO2 breakthrough moment. The pressure difference between the upstream and downstream pressures of the measuring device at this moment is recorded as the CO2 breakthrough pressure of the tested core.

[0067] Step-by-step pressurization method Figure 6As shown, the carbon dioxide injection pressure of the test rock sample 19 is increased step by step, and the upstream and downstream pressure differences and the changes in the light intensity of the schlieren imaging system are monitored. First, the carbon dioxide pressure is injected into the rock sample holder 17 step by step through the carbon dioxide injection pump 2. The first stage is injected with a relatively low pressure P1 (0-2 MPa), the second stage pressure P2 is ΔP1 greater than P1, the third stage pressure P3 is ΔP2 greater than P2, and so on. The pressure P at the i+1 stage is ΔP2 greater than P2. i+1 P i Large ΔP i Each increment is within the range of 0.01-1 MPa, and the CO2 pressure is always less than the confining pressure. When CO2 breaks through test rock sample 19, it enters Schlieren cavity 29, causing changes in the fluid medium and the refractive index of the flow field medium within the cavity. This, in turn, results in changes in light intensity within the cavity caused by the CO2 breakthrough. A high-precision camera 37 captures the moment when the CO2 breakthrough triggers the change in light intensity within the Schlieren cavity 29 and transmits this information to a computer data acquisition system 22, which records the pressure difference between the upstream and downstream sides of the measuring device at that moment, representing the CO2 breakthrough pressure of the test rock sample 19. Figure 7 The diffraction schlieren image formed by the light intensity change caused by the breakthrough of carbon dioxide in the schlieren cavity 29 is shown. The horizontal axis represents the length of the schlieren image, reflecting the response distance of the change in light intensity of the fluid medium caused by the breakthrough of carbon dioxide; the vertical axis represents the height of the carbon dioxide schlieren image, and the zero point corresponds to the position of the cone gas inlet 31, indicating the height of the change in light intensity of the fluid medium caused by the breakthrough of carbon dioxide. Since the density of carbon dioxide is different from the density of the fluid medium (degassed pure water) in the schlieren cavity 29, carbon dioxide enters the schlieren cavity from the cone gas inlet 31 after the breakthrough, causing the change in light intensity of the fluid medium, forming Figure 7 The Schlieren image of the carbon dioxide breakthrough moment is shown. Figure 7 In the Schlieren image, it is possible to accurately determine Figure 6 The change in light intensity is correlated with the breakthrough moment of low-concentration carbon dioxide, thereby accurately deriving the breakthrough pressure of low-permeability rocks.

[0068] Step 7: After the experiment, the CO2 injection pump 9 and three-way control valve F3 are closed, and the fifth control valve F6 is opened to release the CO2 and pressure to zero. Subsequently, the confining pressure injection pump 13 is adjusted to unload the confining pressure to zero. After the pressure is released, the test rock sample 19 is removed from the rock sample holder 17, and the breakthrough pressure measurement experiment is complete.

[0069] Correspondingly, 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, including 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-mentioned method and will not be repeated here.

[0070] It should be pointed out that, according to the needs of implementation, the various steps / components described in this application can be split into more steps / components, or two or more steps / components or partial operations of steps / components can be combined into new steps / components to achieve the purpose of the present invention.

[0071] The size of the serial numbers of the steps in the above embodiments does not mean 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 the present application.

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

Claims

1. A method for testing the breakthrough pressure of low permeability rocks based on schlieren imaging, characterized in that: The following steps are involved: processing a test rock sample and placing it into a confining pressure chamber of a rock sample holder; The injection end of the rock sample holder is connected to the upstream injection pipeline, the outlet end is connected to the Schlieren cavity inlet of the Schlieren imaging system, and the confining pressure cavity 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 set on the upstream injection pipeline; the Schlieren cavity outlet is connected to the downstream outflow pipeline, and a downstream pressure sensor is set on the downstream outflow pipeline; Evacuate the test device piping through the downstream outflow line; Degassed pure water medium is injected into the Schlieren cavity through the degassed pure water injection port on the Schlieren cavity; the optical path of the Schlieren imaging system is debugged so that the observation light can capture a clear Schlieren image with a high-precision camera; The entire measuring device is placed in a constant temperature control system, and the 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. A step-by-step pressurization method is used to inject carbon dioxide at different pressures into the rock sample holder. During the experiment, the schlieren imaging system and various pressure sensors are observed in real time until the carbon dioxide breaks through the saturated rock sample. The high-precision camera will record the schlieren image of the carbon dioxide breakthrough formed after the carbon dioxide enters the schlieren cavity and changes 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 measured core sample.

2. The low permeability rock breakthrough pressure testing method based on schlieren imaging according to claim 1, characterized in that: A water remover is provided on the pipeline between the outlet of the rock sample holder and the inlet of the schlieren cavity to absorb the water that breaks through and discharges the saturated test rock sample.

3. The low permeability rock breakthrough pressure testing method based on schlieren imaging according to claim 1, characterized in that: The Schlieren imaging system includes an LED light source, a slit, a plane turning mirror I, a spherical reflector I, a Schlieren cavity, a spherical reflector II, a plane turning mirror II, a knife edge and a high-precision camera; the plane turning mirror I and the plane turning mirror II, the spherical reflector I and the spherical reflector II are respectively arranged in a centrally symmetrical manner about the Schlieren cavity; by adjusting the light source intensity of the focusing LED light source, the observation light is emitted and passes through the slit, the plane turning mirror I, the spherical reflector I, the Schlieren cavity, the spherical reflector II, the plane turning mirror II and the knife edge in sequence before entering the high-precision camera.

4. The low permeability rock breakthrough pressure testing method based on schlieren imaging according to claim 3, characterized in that: The Schlieren cavity is a cylindrical cavity structure made of pressure-resistant and temperature-resistant materials, which can withstand a maximum pressure of 50MPa 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 on the propagation path of the observation light; the Schlieren cavity is also provided with a cone gas inlet, a circular hole gas outlet and a degassed pure water injection outlet.

5. The low permeability rock breakthrough pressure testing method based on schlieren imaging according to claim 4, characterized in that: The cone gas inlet is a cone-shaped integrated nozzle structure whose diameter 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 in the cone gas inlet.

6. The low permeability rock breakthrough pressure testing method based on schlieren imaging according to claim 4, characterized in that: The circular hole gas outlet is a small circular hole nozzle structure with a radius of 0.2 to 0.3 mm; a waterproof and breathable microporous membrane is arranged inside the circular hole gas outlet.

7. The low permeability rock breakthrough pressure testing method based on schlieren 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 circulation box. The carbon dioxide gas cylinder is connected to the inlet of the carbon dioxide injection pump. A first control valve (F1) is provided 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 box is connected to the carbon dioxide injection pump storage cavity through a heating medium inlet pipeline and a heating medium outlet pipeline respectively. The heating medium in the water bath heating circulation box enters the carbon dioxide injection pump storage cavity through the heating medium inlet pipeline to heat the carbon dioxide. After heat exchange, the medium returns to the water bath heating circulation box through the heating medium outlet pipeline to circulate and heat. By adjusting the temperature of the water bath heating circulation 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. 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. 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 through the upstream injection pipeline.

8. The low permeability rock breakthrough pressure testing method 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 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 the confining pressure injection pipeline, and the confining pressure injection pipeline is provided with a confining pressure pressure sensor and a fourth control valve (F5); the confining pressure medium device is filled with deionized water or oil, and deionized water or oil of different pressures is injected into the confining pressure chamber 20 by controlling the confining pressure injection pump to provide different annular pressures for the test rock sample.

9. The low permeability rock breakthrough pressure testing method 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, which are arranged in sequence 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, and a uniform circumferential load is applied 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.

10. A low permeability rock breakthrough pressure testing device based on schlieren imaging, characterized in that: The testing device implements the testing method according to any one of claims 1 to 9, comprising 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 a gas injection system and a water injection system, wherein the gas 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 in sequence 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 temperature in 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 chamber 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 provided 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 dewatering device, 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 turning mirror I, a spherical reflector, a Schlieren observation window I, a Schlieren cavity, a Schlieren observation window II, a spherical reflector II, a plane turning mirror II, a knife edge and a high-precision camera; the Schlieren cavity is provided with a cone gas inlet, a circular hole gas outlet and a degassed pure water injection outlet; the dewatering device is arranged in a pipeline between the outlet of the rock sample holder and the cone gas inlet at the inlet end of the Schlieren cavity, and is used to absorb water that breaks through and discharges the saturated test rock sample; the circular hole gas outlet at the outlet end of the Schlieren cavity is connected to the downstream outflow pipeline, and a downstream pressure sensor is provided on the downstream outflow pipeline.

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