Ultra-low temperature and high pressure seepage and deformation test device and method based on in-situ CT scanning

Abstract

The present invention belongs to the field of rock physics testing equipment, and in particular relates to an ultra-low temperature, high-pressure seepage and deformation testing device and method based on in-situ CT scanning. The sample device includes a core clamping system, a seepage injection system, a confining pressure system, a backpressure system, a pressure acquisition system, and a temperature control system. The core clamping system includes a titanium alloy double-layer core clamp and an internal heat-shrinkable sleeve for wrapping the core. The core clamping system is fixed to the rotating stage of the CT scanning system by a clamp; the seepage injection system includes a gas injection pump; the backpressure system includes a backpressure control pump; the confining pressure system includes a nitrogen confining pressure pump; the pressure acquisition system includes a computer and a Keller pressure differential sensor; and the temperature control system includes a Dewar flask. The method includes a seepage test method and a deformation test method. The present invention can conduct transient permeability tests and ultra-low temperature pore water freeze heave-rock matrix cold shrinkage nonlinear coupled deformation tests, providing a theoretical basis for the construction of underground liquefied natural gas storage.

Description

Technical Field

[0001] The present invention belongs to the field of rock physics test devices, and in particular relates to an ultra-low temperature and high pressure seepage and deformation test device and method based on in-situ CT scanning. Background Art

[0002] Natural gas is a widely used clean energy source. Currently, the primary storage method for liquefied natural gas (LNG) is surface tanks, which pose challenges such as large floor space requirements, small reserves, high evaporation rates, and poor safety. Abandoned mines offer abundant underground space resources. Repurposing abandoned mines and tunnels into underground LNG storage facilities can effectively address the challenge of large-scale natural gas storage. Underground LNG storage facilities offer the advantages of large reserves, space savings, and high safety.

[0003] Converting abandoned shafts and tunnels into liquefied natural gas (LNG) storage facilities involves an ultra-low temperature storage environment of -162°C. Studying the evolution of water frost heave, rock contraction deformation, and permeability in these ultra-low temperature environments is crucial for the safety and stability of the reservoir. Conventional low-temperature triaxial rock mechanics testing machines, such as the utility model CN210923346U, can simulate temperatures above -30°C, but are unable to replicate the ultra-low temperature (-162°C) environment of LNG. Furthermore, studying the evolution of water frost heave, rock contraction deformation, and permeability in rock under ultra-low temperature conditions requires direct observation of changes in the rock's internal microstructure.

[0004] In recent years, in-situ CT imaging technology has been widely used in the field of geotechnical engineering as a visual non-destructive detection method. Invention CN114965076A proposes a device for measuring the microscopic deformation of the sediment skeleton during the seepage process of low-temperature hydrate mining based on CT imaging. However, it does not involve ultra-low temperature (-162°C) injection and production environments. The main problem is that it is difficult to achieve ultra-low temperature and high pressure in CT while taking into account the CT imaging quality.

[0005] Therefore, to address the above problems, an ultra-low temperature and high-pressure titanium alloy clamp with inner layer loading and outer layer temperature control was developed to meet the ultra-low temperature (-162℃) and high pressure (30MPa) experimental conditions, achieve precise temperature control (error less than ±0.05℃), and is suitable for in-situ CT scanning. It is of great significance to the study of rock deformation and permeability evolution under ultra-low temperature. Summary of the Invention

[0006] The present invention provides an ultra-low temperature and high pressure seepage and deformation testing device based on in-situ CT scanning, comprising a core clamping system, a seepage injection system, a confining pressure system, a back pressure system, a pressure acquisition system, and a temperature control system;

[0007] The core clamping system includes a titanium alloy double-layer core clamp and a heat shrinkable sleeve for wrapping the core inside, the titanium alloy double-layer core clamp includes an internal cavity and an annular cavity surrounding the internal cavity; two through holes are provided on the top surface of the heat shrinkable sleeve, and one through hole is provided on the bottom surface, and the heat shrinkable sleeve is provided in the internal cavity of the titanium alloy double-layer core clamp; three through holes are provided on the top of the titanium alloy double-layer core clamp, two of which correspond to the two through holes on the top of the heat shrinkable sleeve, and one is provided on the annular cavity; three through holes are provided on the bottom of the titanium alloy double-layer core clamp, one of which corresponds to the through hole at the bottom of the heat shrinkable sleeve, one is provided on the annular cavity, and the other is provided in the internal cavity and located outside the heat shrinkable sleeve; the core clamping system is fixed to the rotating stage of the CT scanning system by a clamp;

[0008] The seepage injection system includes a gas phase injection pump, the outlet of which is connected to the second two-way valve, the first three-way connector, the bottom hole of the core holder, and the bottom hole of the heat shrink sleeve in sequence through pipelines;

[0009] The back pressure system includes a back pressure control pump, the outlet of which is connected to the back pressure valve, the sixth two-way valve, the second three-way connector, the small gas tank, the fifth two-way valve, the through hole on the top surface of the core holder, and the through hole on the top surface of the heat shrink tubing in sequence through pipelines;

[0010] The confining pressure system includes a nitrogen confining pressure pump, the inlet of the nitrogen confining pressure pump is connected to the nitrogen bottle through a pipeline, and the outlet of the nitrogen confining pressure pump is connected to the first two-way valve and the through hole arranged in the internal cavity and outside the heat shrinkable sleeve on the bottom surface of the core holder through a pipeline in sequence;

[0011] The pressure acquisition system includes a computer and a Keller differential pressure sensor connected thereto, wherein both ends of the pipeline where the Keller differential pressure sensor is located are respectively connected to a first three-way connector and a second three-way connector;

[0012] The temperature control system includes a Dewar flask, which is connected in sequence to a pressure and flow controller, an open nitrogen tank, an airflow temperature controller, a third two-way valve, and a through-hole arranged in an annular cavity at the bottom of a titanium alloy double-layer core holder through a stainless steel nitrogen pipeline wrapped in an insulation tube. The Dewar flask also includes a PID controller, which is respectively connected to the pressure and flow controller, the airflow temperature controller, and an ultra-low temperature resistant thermocouple, and the ultra-low temperature resistant thermocouple is wrapped in an ultra-low temperature resistant heat shrink tubing.

[0013] Preferably, the back pressure valve is also connected to the recovery tank through a pipeline.

[0014] Preferably, a nitrogen coil is provided in the open nitrogen tank.

[0015] Preferably, the temperature control system further comprises a recovery tank, which is sequentially connected to the fourth two-way valve and the through hole provided in the annular cavity at the top of the titanium alloy double-layer core holder through pipelines.

[0016] Preferably, the CT scanning system is a YXLON FF35 high-resolution micron CT scanning system.

[0017] Based on the ultra-low temperature and high pressure seepage and deformation test device described above, the present invention further proposes an ultra-low temperature and high pressure seepage and deformation test method based on in-situ CT scanning, including a seepage test method and a deformation test method. The seepage test method includes the following steps:

[0018] S1: Prepare several cores, clean them with methanol, dry them in a drying oven, measure and record the diameter and length of the cores; then evacuate the cores and completely saturate them with water;

[0019] S2: Place the saturated cores in a drying oven for natural evaporation. A pre-set saturation degree is obtained through quality control methods, so that different cores reach different saturations. Vaseline is then applied to the core surface. Cores with different saturations are then taken in sequence for subsequent testing.

[0020] S3: Place the core into a heat shrink tubing and heat shrink it to secure it. Wrap the entire core with the heat shrink tubing. Machine two through-holes on the top surface and one through-hole on the bottom surface of the heat shrink tubing. Set the heat shrink tubing inside the core holder. Assemble an ultra-low temperature, high-pressure seepage and deformation test device based on in-situ CT scanning.

[0021] S4: Open the first two-way valve and start the confining pressure pump to gradually increase the confining pressure to the first pressure;

[0022] S5: Open the second two-way valve, the first three-way connector, the fifth two-way valve, the second three-way connector, and the sixth two-way valve; turn on the gas injection pump to maintain the upstream injection second pressure, turn on the back pressure control pump to pressurize to the second pressure, and wait for the upstream and downstream pressure differential measured by the Keller differential pressure sensor to be zero; increase the gas injection pump pressure to the third pressure, and gradually increase the back pressure control pump pressure to the third pressure until the pressure differential measured by the Keller differential pressure sensor gradually decreases to zero, record the pressure differential decay, and calculate the permeability according to the following formula

[0023]

[0024] Where k is the permeability, α is the slope of the pressure drop curve, μ g is the gas viscosity, L is the core length, c g is the gas compressibility, V down is the volume of the small gas tank, A is the cross-sectional area of ​​the core;

[0025] S6: Close the first three-way connector, the second two-way valve, the fifth two-way valve, the second three-way connector, the sixth two-way valve, and the gas injection pump;

[0026] Turn on the temperature control system and the valve of the Dewar flask. The nitrogen passes through the pressure and flow controller and then through the copper tube in the open liquid nitrogen tank to cool to a first temperature. Then, it is introduced into the air flow temperature controller through the pipeline to raise the temperature to the desired temperature. The nitrogen is introduced into the through hole at the bottom of the annular cavity of the low-temperature resistant core holder through the third two-way valve. The core is slowly cooled to multiple preset low temperatures in sequence through the annular cavity, and the temperature is maintained at each temperature level for a certain period of time. Refer to step S5 to calculate the permeability at different temperatures;

[0027] The deformation test method comprises the following steps:

[0028] B1: Prepare several cores, clean them with methanol, dry them in a drying oven, measure and record the diameter and length of the cores; then evacuate the cores and completely saturate them with water;

[0029] B2: Place the saturated cores in a drying oven for natural evaporation. Use quality control methods to achieve a pre-set saturation, so that different cores reach different saturations. Apply vaseline to the core surface. Sequentially select cores with different saturations from the lowest to the highest saturation for subsequent testing.

[0030] B3: Place the core into a heat shrink tubing and heat shrink it to secure it. Wrap the entire core in the heat shrink tubing and place the heat shrink tubing inside the core holder. Assemble the ultra-low temperature, high pressure seepage and deformation test apparatus based on in-situ CT scanning.

[0031] B4: The simulated formation pressure is the fourth pressure, and the pore pressure is the fifth pressure, that is, the effective stress is the fourth pressure minus the fifth pressure; open the first two-way valve, start the nitrogen confining pressure pump and gradually increase the confining pressure to the effective stress;

[0032] B5: Scan the core at room temperature using CT;

[0033] B6: Turn on the temperature control system and slowly cool the core to multiple preset low-temperature levels through the temperature control system. The temperature is maintained at each temperature level for a certain period of time, and the core is scanned by CT.

[0034] B7: Data processing: AVIZO image processing software is used to pre-process high-precision CT images of cores with different water saturations and obtained under different temperature conditions, mainly including image cropping, correction and image noise reduction.

[0035] Preferably, in step B8, the CT image of the core at room temperature is used as a reference, and the digital volume technology is used to first grid the image before deformation and mark each subdomain, then the coordinates between the freezing process image and the reference image are tracked, and the position changes between the rock images at different freezing temperatures and the reference image are monitored to obtain the deformation of the subdomain; all subdomains are calculated and analyzed to obtain the global deformation information inside the core at different freezing temperatures, and the volume frost heave or contraction coefficient at the corresponding freezing temperature is further calculated to analyze the nonlinear change stage and mode of freezing deformation; based on the watershed segmentation method of mathematical morphology topology theory, the rock particles and pores in the CT images at different temperatures are segmented, the pore water frost heave strain and the rock matrix contraction strain are studied in different zones, and the nonlinear competition and coupling law of pore water frost heave-rock matrix contraction at ultra-low temperatures is analyzed.

[0036] The beneficial technical effects of the present invention are as follows: the test device of the present invention can carry out transient permeability tests and ultra-low temperature pore water frost heave-rock matrix cold contraction nonlinear coupled deformation tests, providing a theoretical basis for the construction of underground liquefied natural gas storage.

[0037] The present invention adopts an improved transient method - constant pressure pulse decay method to measure permeability, which reduces experimental errors and does not require the installation of gas tanks upstream; the low-temperature nitrogen coil contact refrigeration method is adopted to strengthen the control of nitrogen flow rate and cooling medium distribution, and the nitrogen temperature and flow rate are controlled according to the core temperature to form a PID control system, which can accurately regulate the temperature and cooling rate, and the temperature fluctuation is less than 0.05 degrees Celsius. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings constituting the present invention are used to provide a further understanding of the present invention, and the exemplary embodiments of the present invention and their descriptions are used to explain the present invention, but do not constitute improper limitations on the present invention.

[0039] Figure 1 This is an overall schematic diagram of the ultra-low temperature and high pressure seepage and deformation test device based on in-situ CT scanning of the present invention;

[0040] Explanation of the accompanying symbols: 1. Ultra-low temperature resistant thermocouple; 2. Ultra-low temperature resistant heat shrink tubing; 3. Titanium alloy double-layer core clamp; 4. First two-way valve; 5. Second two-way valve; 6. First three-way connector; 7. Third two-way valve; 8. Fourth two-way valve; 9. Fifth two-way valve; 10. Small gas tank; 11. Second three-way connector; 12. Sixth two-way valve; 13. Back pressure valve; 14. Recovery tank; 15. Back pressure control pump; 16. Nitrogen confining pressure pump; 17. Gas phase injection pump; 18. Air flow temperature controller; 19. Open nitrogen tank; 20. Dewar flask; 21. Pressure and flow controller; 22. PID controller; 23. Recovery tank; 24. Computer; 25. Keller pressure differential sensor. DETAILED DESCRIPTION

[0041] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0042] Example 1

[0043] like Figure 1 As shown, the ultra-low temperature and high pressure seepage and deformation testing device based on in-situ CT scanning of the present invention includes a core clamping system, a seepage injection system, a confining pressure system, a back pressure system, a pressure acquisition system and a temperature control system;

[0044] The core clamping system includes a titanium alloy double-layer core clamp 3 and a heat shrinkable sleeve 2 for wrapping the core inside, the titanium alloy double-layer core clamp 3 includes an internal cavity and an annular cavity surrounding the internal cavity; two through holes are set on the top surface of the heat shrinkable sleeve 2, and one through hole is set on the bottom surface, and the heat shrinkable sleeve 2 is set in the internal cavity of the titanium alloy double-layer core clamp 3; three through holes are set on the top of the titanium alloy double-layer core clamp 3, two of which correspond to the two through holes on the top of the heat shrinkable sleeve 2, and one is set on the annular cavity; three through holes are set on the bottom of the titanium alloy double-layer core clamp 3, one of which corresponds to the through hole at the bottom of the heat shrinkable sleeve 2, one is set on the annular cavity, and the other is set in the internal cavity and located outside the heat shrinkable sleeve 2; the core clamping system is fixed to the rotating stage of the CT scanning system by a clamp;

[0045] The seepage injection system includes a gas injection pump 17, the inlet of the gas injection pump 17 is connected to the gas cylinder through a pipeline, and the outlet of the gas injection pump 17 is connected to the second two-way valve 5, the first three-way connector 6, the bottom hole of the core holder 3, and the bottom hole of the heat shrink sleeve 2 through a pipeline in sequence;

[0046] The back pressure system includes a back pressure control pump 15, the outlet of which is connected to the back pressure valve 13, the sixth two-way valve 12, the second three-way connector 11, the small gas tank 10, the fifth two-way valve 9, the through hole on the top surface of the core holder 3, and the through hole on the top surface of the heat shrink tubing 2 through a pipeline in sequence; the back pressure valve 13 is also connected to the recovery tank 14 through a pipeline;

[0047] The confining pressure system includes a nitrogen confining pressure pump 16, the inlet of the nitrogen confining pressure pump 16 is connected to the nitrogen bottle through a pipeline, and the outlet of the nitrogen confining pressure pump 16 is connected to the first two-way valve 4 and the through hole on the bottom surface of the core holder 3 arranged in the internal cavity and located outside the heat shrinkable sleeve 2 through a pipeline in sequence;

[0048] The pressure acquisition system includes a computer 24 and a Keller pressure differential sensor 25 connected thereto. The two ends of the pipeline where the Keller pressure differential sensor 25 is located are respectively connected to the first three-way connector 6 and the second three-way connector 11; the Keller pressure differential sensor 25 is used to monitor the pressure difference between the upstream and downstream of the core;

[0049] The temperature control system includes a Dewar flask 20, which is connected to the pressure and flow control device 21, the open nitrogen tank 19, the airflow temperature controller 18, the third two-way valve 7, and the through-hole set in the annular cavity at the bottom of the titanium alloy double-layer core clamp 3 through a stainless steel nitrogen pipeline wrapped in an insulation tube. It also includes a PID controller 22, which is respectively connected to the pressure and flow control device 21, the airflow temperature controller 18, and the ultra-low temperature resistant thermocouple 1, and the ultra-low temperature resistant thermocouple 1 is wrapped in an ultra-low temperature resistant heat shrink tubing 2; it also includes a recovery tank 14, and the recovery tank 23 is connected to the fourth two-way valve 8 and the through-hole set in the annular cavity at the top of the titanium alloy double-layer core clamp 3 through a pipeline.

[0050] A nitrogen coil is provided in the open nitrogen tank 19 .

[0051] Among them, the CT scanning system uses the YXLON FF35 high-resolution micron CT scanning system.

[0052] Example 2

[0053] Based on the ultra-low temperature and high pressure seepage and deformation test device of Example 1, the present invention further proposes an ultra-low temperature and high pressure seepage and deformation test method based on in-situ CT scanning, including a seepage test method and a deformation test method. The seepage test method includes the following steps:

[0054] S1: Prepare several cores, wash them with methanol for 5 hours, and dry them in a drying oven at 105°C (until the mass change does not exceed 0.1%). Measure and record the diameter and length of the cores; in this example, the core diameter is 10 mm and the length is 20 mm. Then, evacuate the cores and completely saturate them with water using a 20% mass fraction KI solution.

[0055] S2: Place the saturated cores in a drying oven at 50°C for natural evaporation. A pre-set saturation (e.g., 20%, 40%, 60%, 80%, 100%) is obtained through quality control methods. Different cores are saturated to different degrees, and the core surfaces are coated with vaseline. Cores with different saturations are then taken in sequence for subsequent testing.

[0056] S3: Place the core into the heat shrink tubing 2 and heat shrink it to fix it. The heat shrink tubing 2 wraps the entire core. Two through holes are machined on the top surface of the heat shrink tubing 2 and one through hole is machined on the bottom surface. The heat shrink tubing 2 is set in the inner cavity of the core holder 3. Assemble the ultra-low temperature and high pressure seepage and deformation test device based on in-situ CT scanning ( Figure 1 );

[0057] S4: Open the first two-way valve 4 and start the confining pressure pump 16 to gradually increase the confining pressure to 4 MPa;

[0058] S5: Open the second two-way valve 5, the first three-way connector 6, the fifth two-way valve 9, the second three-way connector 11 and the sixth two-way valve 12; turn on the gas phase injection pump 17 to maintain the upstream injection pressure at 0.4 MPa, turn on the back pressure control pump 15 to increase the pressure to 0.4 MPa, and wait for the upstream and downstream pressure differential measured by the Keller differential pressure sensor to be zero; close the sixth two-way valve 12, increase the gas phase injection pump pressure by 0.4 MPa, until the pressure differential measured by the Keller differential pressure sensor 25 gradually decreases to zero, record the pressure differential decay, and the permeability can be calculated according to the following formula

[0059]

[0060] Where k is the permeability, α is the slope of the pressure decay curve (lnΔp~t, Δp is the upstream and downstream pressure difference, t is the test time), μ g is the gas viscosity, L is the core length, c g is the gas compressibility, V down is the volume of the small gas tank, A is the cross-sectional area of ​​the core;

[0061] S6: Close the first three-way connector 6, the second two-way valve 5, the fifth two-way valve 9, the second three-way connector 11, the sixth two-way valve 12, and the gas injection pump 17;

[0062] Turn on the temperature control system. At this time, open the Dewar flask valve 20. The nitrogen passes through the pressure and flow controller and then cools down to about -180°C through the copper tube in the open liquid nitrogen tank 19. Then, it passes through the pipeline into the air flow temperature controller 18 to raise the temperature to the required temperature. The nitrogen passes through the third two-way valve 7 into the through hole at the bottom of the annular cavity of the low-temperature resistant core holder 3, passes through the annular cavity, and then exits the annular cavity through the fourth two-way valve 8. The core is slowly cooled to six preset temperature levels of 0°C, -20°C, -40°C, -80°C, -120°C, and -160°C in sequence. The temperature is maintained at each temperature level for 30 minutes. Refer to step S5 to calculate the permeability at different temperatures.

[0063] The deformation test method comprises the following steps:

[0064] B1: Prepare several cores, wash them with methanol for 5 hours, and dry them in a drying oven at 105°C (until the mass change does not exceed 0.1%). Measure and record the diameter and length of the cores; in this example, the core diameter is 10 mm and the length is 20 mm. The cores are then evacuated and completely saturated with water using a 20% mass fraction of KI solution.

[0065] B2: The saturated cores were placed in a drying oven at 50°C for natural evaporation. Pre-set saturations were achieved through quality control methods, with different cores reaching different saturations (0%, 20%, 40%, 60%, 80%, and 100%). A core with a saturation of 0% was considered completely dry. Vaseline was applied to the core surface. Cores with different saturations were then sampled from the lowest to the highest saturations for subsequent testing.

[0066] B3: Place the core into the heat shrink tubing 2 and heat shrink it to fix it. Wrap the entire core with the heat shrink tubing 2. Set the heat shrink tubing 1 in the inner cavity of the core holder 2. Assemble the ultra-low temperature and high pressure seepage and deformation test device based on in-situ CT scanning ( Figure 1 );

[0067] B4: Simulate a formation pressure of 20 MPa and a pore pressure of 5 MPa, i.e., an effective stress of approximately 15 MPa; open the first two-way valve 3 and start the nitrogen confining pressure pump 14 to gradually increase the confining pressure to 15 MPa;

[0068] B5: Scanning of the core at room temperature using a YXLON FF35 high-resolution micron CT scanner.

[0069] B6: Turn on the temperature control system and slowly cool the core to six preset temperature levels: 0°C, -20°C, -40°C, -80°C, -120°C, and -160°C. Maintain the temperature at each level for at least 10 minutes. Scan the core using a YXLON FF35 high-resolution micron CT scanner.

[0070] CT scans were performed at the same location each time, and each scan lasted 40 minutes;

[0071] B7: Data processing: AVIZO image processing software is used to pre-process high-precision CT images of cores with different water saturations and obtained under different temperature conditions, mainly including image cropping, correction and image noise reduction;

[0072] Using CT images of rock cores at room temperature as a benchmark, the pre-deformation image is first meshed and each subdomain is labeled using Digital Volume Correlation (DVC). The coordinates between the freezing process image and the benchmark image are then tracked, and the positional changes between the rock images at different freezing temperatures and the benchmark image are monitored to obtain the deformation of the subdomains. All subdomains are then analyzed to obtain global deformation information within the rock core at different freezing temperatures. The volumetric freeze-height or contraction coefficients at the corresponding freezing temperatures are further calculated, and the stages and patterns of nonlinear changes in freezing deformation are analyzed.

[0073] Based on the watershed segmentation method of mathematical morphology topology theory, the rock particles and pores in CT images at different temperatures are segmented, and the pore water frost heave strain and rock matrix cooling shrinkage strain are studied in different regions. The nonlinear competition and coupling laws between pore water frost heave and rock matrix cooling shrinkage at ultra-low temperatures are analyzed.

[0074] Of course, the above description is only a preferred embodiment of the present invention, and the present invention is not limited to the above-mentioned embodiments. It should be noted that all equivalent substitutions and obvious deformation forms made by any technician familiar with this field under the guidance of this specification fall within the substantive scope of this specification and should be protected by the present invention.

Claims

1. An ultra-low temperature, high pressure seepage and deformation testing device based on in-situ CT scanning, comprising a core clamping system, a seepage injection system, a confining pressure system, a back pressure system, a pressure acquisition system, and a temperature control system; characterized by: The core clamping system includes a titanium alloy double-layer core clamp and a heat shrinkable sleeve for wrapping the core inside, the titanium alloy double-layer core clamp includes an internal cavity and an annular cavity surrounding the internal cavity; two through holes are provided on the top surface of the heat shrinkable sleeve, and one through hole is provided on the bottom surface, and the heat shrinkable sleeve is provided in the internal cavity of the titanium alloy double-layer core clamp; three through holes are provided on the top of the titanium alloy double-layer core clamp, two of which correspond to the two through holes on the top of the heat shrinkable sleeve, and one is provided on the annular cavity; three through holes are provided on the bottom of the titanium alloy double-layer core clamp, one of which corresponds to the through hole at the bottom of the heat shrinkable sleeve, one is provided on the annular cavity, and the other is provided in the internal cavity and located outside the heat shrinkable sleeve; the core clamping system is fixed to the rotating stage of the CT scanning system by a clamp; The seepage injection system includes a gas phase injection pump, the outlet of which is connected to the second two-way valve, the first three-way connector, the bottom hole of the core holder, and the bottom hole of the heat shrink sleeve in sequence through pipelines; The back pressure system includes a back pressure control pump, the outlet of which is connected to the back pressure valve, the sixth two-way valve, the second three-way connector, the small gas tank, the fifth two-way valve, the through hole on the top surface of the core holder, and the through hole on the top surface of the heat shrink tubing in sequence through pipelines; The confining pressure system includes a nitrogen confining pressure pump, the inlet of the nitrogen confining pressure pump is connected to the nitrogen bottle through a pipeline, and the outlet of the nitrogen confining pressure pump is connected to the first two-way valve and the through hole arranged in the internal cavity and outside the heat shrinkable sleeve on the bottom surface of the core holder through a pipeline in sequence; The pressure acquisition system includes a computer and a Keller pressure differential sensor connected thereto, wherein both ends of the pipeline where the Keller pressure differential sensor is located are connected to a first three-way connector and a second three-way connector respectively; The temperature control system includes a Dewar flask, which is connected in sequence to a pressure and flow controller, an open nitrogen tank, an airflow temperature controller, a third two-way valve, and a through-hole arranged in an annular cavity at the bottom of a titanium alloy double-layer core holder through a stainless steel nitrogen pipeline wrapped in an insulation tube. The Dewar flask also includes a PID controller, which is respectively connected to the pressure and flow controller, the airflow temperature controller, and an ultra-low temperature resistant thermocouple, and the ultra-low temperature resistant thermocouple is wrapped in an ultra-low temperature resistant heat shrink tubing.

2. The test device according to claim 1, characterized in that The back pressure valve is also connected to the recovery tank through a pipeline.

3. The test device according to claim 1, characterized in that A nitrogen coil is arranged in the open nitrogen tank.

4. The test device according to claim 1, characterized in that The temperature control system further comprises a recovery tank, which is sequentially connected to a fourth two-way valve and a through hole arranged in the annular cavity at the top of the titanium alloy double-layer core holder through pipelines.

5. The test device according to claim 1, characterized in that The CT scanning system uses the YXLON FF35 high-resolution micron CT scanning system.

6. An ultra-low temperature and high pressure seepage and deformation test method based on in-situ CT scanning, using the test apparatus according to any one of claims 1 to 5, characterized in that: Including seepage test method and deformation test method; The seepage test method comprises the following steps: S1: Prepare several cores, clean them with methanol, dry them in a drying oven, measure and record the diameter and length of the cores; then evacuate the cores and completely saturate them with water; S2: Place the saturated cores in a drying oven for natural evaporation. A pre-set saturation degree is obtained through quality control methods, so that different cores reach different saturations. Vaseline is then applied to the core surface. Cores with different saturations are then taken in sequence for subsequent testing. S3: Place the core into a heat shrink tubing and heat shrink it to secure it. Wrap the entire core with the heat shrink tubing. Machine two through-holes on the top surface and one through-hole on the bottom surface of the heat shrink tubing. Set the heat shrink tubing inside the core holder. Assemble an ultra-low temperature, high-pressure seepage and deformation test device based on in-situ CT scanning. S4: Open the first two-way valve and start the confining pressure pump to gradually increase the confining pressure to the first pressure; S5: Open the second two-way valve, the first three-way connector, the fifth two-way valve, the second three-way connector, and the sixth two-way valve; turn on the gas injection pump to maintain the upstream injection second pressure, turn on the back pressure control pump to pressurize to the second pressure, and wait for the upstream and downstream pressure differential measured by the Keller pressure differential sensor to be zero; close the sixth two-way valve, increase the gas injection pump pressure to the third pressure until the pressure differential measured by the Keller pressure differential sensor gradually decreases to zero, record the pressure differential decay, and calculate the permeability according to the following formula Where k is the permeability, α is the slope of the pressure drop curve, μ g is the gas viscosity, L is the core length, c g is the gas compressibility, V down is the volume of the small gas tank, A is the cross-sectional area of ​​the core; S6: Close the first three-way connector, the second two-way valve, the fifth two-way valve, the second three-way connector, the sixth two-way valve, and the gas injection pump; Turn on the temperature control system and the valve of the Dewar flask. The nitrogen passes through the pressure and flow controller and then through the copper tube in the open liquid nitrogen tank to cool to a first temperature. Then, it is introduced into the air flow temperature controller through the pipeline to raise the temperature to the desired temperature. The nitrogen is introduced into the through hole at the bottom of the annular cavity of the low-temperature resistant core holder through the third two-way valve. The core is slowly cooled to multiple preset low temperatures in sequence through the annular cavity, and the temperature is maintained at each temperature level for a certain period of time. Refer to step S5 to calculate the permeability at different temperatures; The deformation test method comprises the following steps: B1: Prepare several cores, clean them with methanol, dry them in a drying oven, measure and record the diameter and length of the cores; then evacuate the cores and completely saturate them with water; B2: Place the saturated cores in a drying oven for natural evaporation. Use quality control methods to achieve a pre-set saturation, so that different cores reach different saturations. Apply vaseline to the core surface. Sequentially select cores with different saturations from the lowest to the highest saturation for subsequent testing. B3: Place the core into a heat shrink tubing and heat shrink it to secure it. Wrap the entire core in the heat shrink tubing and place the heat shrink tubing inside the core holder. Assemble the ultra-low temperature, high pressure seepage and deformation test apparatus based on in-situ CT scanning. B4: The simulated formation pressure is the fourth pressure, and the pore pressure is the fifth pressure, that is, the effective stress is the fourth pressure minus the fifth pressure; open the first two-way valve, start the nitrogen confining pressure pump and gradually increase the confining pressure to the effective stress; B5: Scan the core at room temperature using CT; B6: Turn on the temperature control system and slowly cool the core to multiple preset low-temperature levels through the temperature control system. The temperature is maintained at each temperature level for a certain period of time, and the core is scanned by CT. B7: Data processing: AVIZO image processing software is used to pre-process high-precision CT images of cores with different water saturations and obtained under different temperature conditions, mainly including image cropping, correction and image noise reduction.

7. The test method according to claim 6, characterized in that In step B8, the core CT image at room temperature is used as a reference. The pre-deformation image is first meshed and each subdomain is marked using digital volume technology. Then, the coordinates between the freezing process image and the reference image are tracked, and the position changes between the rock images at different freezing temperatures and the reference image are monitored to obtain the deformation of the subdomain. All subdomains are calculated and analyzed to obtain global deformation information inside the core at different freezing temperatures. The volume frost heave or contraction coefficient at the corresponding freezing temperature is further calculated, and the nonlinear change stages and patterns of freezing deformation are analyzed. Based on the watershed segmentation method of mathematical morphology topology theory, the rock particles and pores in CT images at different temperatures are segmented, and the pore water frost heave strain and rock matrix contraction strain are studied in different regions. The nonlinear competition and coupling laws of pore water frost heave and rock matrix contraction at ultra-low temperatures are analyzed.

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