A triaxial loading core holder for a nuclear magnetic environment

Abstract

The utility model relates to a kind of triaxial loading core clamping devices for nuclear magnetic environment, its whole adopts non-magnetic material, it includes: core bearing unit, including cylinder, cylinder is provided with confining pressure liquid inlet and confining pressure liquid outlet, cylinder left and right ends are separately provided with left end cap and right end cap, cylinder is provided with for the left plug and right plug of compacting fixed core inside;Left end axial loading module, it includes the bellows with cavity inside and bellows liquid injection pipeline, bellows is arranged between left end cap and left plug;Right end force measuring module, it is arranged between right end cap and right plug, for real-time monitoring the stress condition of core in axial loading process.The utility model can load axial stress and confining pressure simultaneously, uses bellows to load axial force and embedded force measuring module, with good high temperature and high pressure adaptability and high-precision mechanical control performance, can be compatible with nuclear magnetic resonance measurement requirement, without interfering nuclear magnetic signal.

Description

Technical Field

[0001] This utility model relates to the field of rock physics experiments and nuclear magnetic resonance (NMR) measurement technology, and in particular to a triaxial loading core clamping device for use in a nuclear magnetic environment. Background Technology

[0002] Indoor seepage experiments are a core method for studying the transport mechanisms of multiphase fluids in porous media, and have significant application value in unconventional oil and gas development, carbon dioxide sequestration, and underground energy storage. Nuclear magnetic resonance (NMR) technology, as a non-invasive, high-resolution tool for characterizing pore structure and fluid distribution, has been widely introduced into seepage experimental systems in recent years to achieve real-time monitoring of dynamic processes inside rock cores.

[0003] However, existing core clamping devices under NMR conditions still suffer from several key technical limitations, severely impacting the accuracy and adaptability of experiments. On one hand, traditional NMR core clamps typically only provide confining pressure loading, failing to simultaneously apply axial stress. This causes experimental conditions to deviate from the actual triaxial stress state underground, limiting the realistic simulation of seepage-stress coupling behavior. The lack of axial pressure also results in loose contact between the core and the upper and lower plugs, creating micrometer- to millimeter-level voids. This not only traps fluids but also introduces errors in relaxation time (T2) measurement, potentially reaching 15–20% in dense rock samples, significantly affecting the accuracy of quantitative analysis of pore structure and saturation. On the other hand, existing loading systems mostly employ metal hydraulic structures, lacking NMR compatibility. Metal components, especially ferromagnetic materials, can interfere with the uniformity of the main NMR magnetic field, causing magnetic field drift, severely weakening NMR signal quality, and even leading to data distortion. In addition, existing triaxial loading devices are bulky and loosely structured, making them incompatible with low-field NMR probes with an inner diameter of only 50–60 mm. This results in limited real-time dynamic measurements and poor integration and operability of the experimental system. Utility Model Content

[0004] This invention aims to solve the problems of insufficient triaxial stress loading capacity and poor NMR compatibility in existing NMR experimental devices, and provides a compact, non-magnetic NMR-compatible core clamping device that can achieve triaxial stress loading.

[0005] To solve the above problems, the technical solution adopted by this utility model is as follows:

[0006] A triaxial loading core clamping device for use in nuclear magnetic resonance environments, the key feature of which is that the entire clamping device is made of non-magnetic material, comprising:

[0007] The core support unit includes a cylindrical body with a confining pressure loading cavity inside. The body is provided with a confining pressure fluid inlet and a confining pressure fluid outlet communicating with the confining pressure loading cavity. The left and right ends of the body are respectively provided with a left end cap and a right end cap to seal the confining pressure loading cavity. The body is provided with a left plug and a right plug for pressing and fixing the core.

[0008] The left-end axial loading module includes a bellows with an internal cavity and a bellows injection pipeline, wherein the bellows is disposed between the left end cap and the left plug;

[0009] The right-end force measuring module, located between the right end cap and the right end plug, is used to monitor the stress on the core during axial loading in real time.

[0010] In one embodiment of this utility model, the corrugated pipe is made of carbon fiber reinforced polyetheretherketone material.

[0011] In one embodiment of this utility model, the corrugated pipe adopts a U-shaped corrugated pipe structure.

[0012] In one embodiment of this utility model, a cylindrical corrugated pipe protector is provided on the outside of the corrugated pipe.

[0013] In one embodiment of this utility model, the bellows protector is made of silicon nitride material.

[0014] In one embodiment of this utility model, a heat-shrinkable sleeve is wrapped between the left plug and the right plug.

[0015] In one embodiment of this utility model, both the left and right end caps are provided with ventilation rods, and the left plug, right plug, and corrugated pipe are all provided with through holes along the axis for the ventilation rods to pass through. The ventilation rods on both sides are embedded in the shaft holes of the rock core.

[0016] As one embodiment of this utility model, the right-end force measuring module adopts a center-opening piezoelectric sensor, which supports real-time stress detection of 0.1-80MPa.

[0017] The beneficial effects of adopting the above technical solution are as follows:

[0018] This invention provides a triaxial loading core clamping device for NMR environments, capable of simultaneously loading axial stress and confining pressure. It employs a bellows-like structure for axial force loading and an embedded force measurement module for real-time stress detection, exhibiting excellent high-temperature and high-pressure adaptability and high-precision mechanical control performance. The entire device is constructed from non-magnetic materials, ensuring compatibility with NMR measurement requirements and preventing interference with NMR signals. The use of a non-magnetic bellows structure for axial force loading replaces traditional hydraulic systems, avoiding magnetic field interference caused by metal components. It allows for the application of adjustable axial loads without interfering with NMR signals, ensuring high compatibility with low-field NMR systems. Furthermore, in conjunction with a confining pressure system, it constructs a triaxial stress environment closely resembling real geological conditions. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of this utility model.

[0020] Figure 2 This is a three-dimensional structural diagram of the corrugated pipe of this utility model.

[0021] Figure 3 This is a cross-sectional structural diagram of the corrugated pipe of this utility model.

[0022] Figure 4 This is a schematic diagram of the structure of the left end cover of this utility model, which has a slot provided on the inside.

[0023] The components include: 1. Vent rod, 2. Confining pressure liquid inlet, 3. Confining pressure liquid outlet, 4. Bellows, 5. Bellows liquid inlet, 6. Bellows protector, 7. Left plug, 8. Core, 9. Right plug, 10. Heat shrink tubing, 11. Confining pressure loading chamber, 12. Right end cap, 13. Sealing ring, 14. Left end cap, 15. Pressure sensor, 16. Wire, 17. Bellows liquid injection pipeline, 18. Cylinder body, 19. Slot. Detailed Implementation

[0024] To make the objectives, technical solutions and advantages of this utility model clearer, the utility model will be clearly and completely described below in conjunction with specific embodiments.

[0025] In this embodiment, the left end and the right end are based on Figure 1 The orientations shown are relative positions for reference and are not intended as absolute limitations on the technical solutions.

[0026] like Figures 1 to 3 The diagram shows a triaxial loading core clamping device for use in a nuclear magnetic resonance environment, which includes a core bearing unit, a left-end axial loading module, and a right-end force measuring module. The core bearing unit includes a cylindrical body 18, and a confining pressure loading cavity 11 is formed inside the cylindrical body 18. A confining pressure liquid inlet 2 and a confining pressure liquid outlet 3 communicating with the confining pressure loading cavity 11 are provided on the cylindrical body 18. The confining pressure liquid inlet 2 is located at the upper left position of the cylindrical body 18, and the confining pressure liquid outlet 3 is located at the lower right position of the cylindrical body 18.

[0027] The left end cap 14 and the right end cap 12 are respectively provided at the left and right ends of the cylinder 18 to seal the confining pressure loading cavity 11. The left end cap 14 and the right end cap 12 are connected to the two ends of the cylinder 18 by threads, and O-rings 13 are provided between the left end cap 14 and the right end cap 12 and the cylinder 18.

[0028] The cylinder 18 is equipped with a left plug 7 and a right plug 9 for clamping and fixing the core 8. In this embodiment, the inner diameter of the cylinder 18 is Φ60mm, used to accommodate and fix a standard Φ25mm core sample. The core 8 is located in the center of the confining pressure loading cavity 11, and its two ends are clamped and fixed by the left plug 7 and the right plug 9 respectively. The diameter of the left plug 7 and the right plug 9 is the same as that of the core 8. The left plug 7 and the right plug 9 are made of non-magnetic material and are in direct contact with the end face of the core 8.

[0029] The left-end axial loading module includes a corrugated pipe 4 with an internal cavity and a corrugated pipe injection line 17. The corrugated pipe 4 is located between the left end cap 14 and the left plug 7. The two sides of the corrugated pipe 4 are in contact with the left end cap 14 and the left plug 7, respectively.

[0030] The right-end force measuring module is located between the right end cap 12 and the right plug 9, and is used to monitor the stress on the core in real time during axial loading. The right-end force measuring module supports real-time data transmission based on the UART serial port protocol, and can synchronously upload the loaded stress to the computer, meeting the requirements for high-precision acquisition and automatic recording of stress data during the experiment.

[0031] As a further improvement of this utility model, both the left end cap 14 and the right end cap 12 are provided with venting rods 1. The venting rods 1 on both sides pass through the left end cap 14 and the right end cap 12 respectively, and are sealed to the left end cap 14 and the right end cap 12. The left plug 7, the right plug 9, and the bellows 4 are all provided with through holes along the axis for the venting rods 1 to pass through. The venting rods 1 on both sides are embedded in the shaft hole of the core 8. The right end force measuring module adopts a center-opening type piezoelectric sensor 15 (e.g., Kistler 9341B), which supports real-time stress detection of 0.1-80MPa, facilitating the passage of the venting rods 1.

[0032] The ventilation rod 1 and the core 8 form a complete injection-production pathway, achieving full connectivity of the gas channel. This supports fluid displacement, adsorption, and NMR testing, allowing for realistic simulation of core seepage behavior under triaxial stress conditions without interfering with NMR signals. It is particularly suitable for dynamic seepage experiments requiring multi-field coupling. This device is widely applicable to experimental research in complex geological conditions such as unconventional oil and gas reservoir development and CO2 geological storage. Furthermore, its compact structure allows it to be embedded in an NMR probe with an inner diameter of 50–60 mm, maintaining the continuity of the central gas channel. With no metal components introducing electromagnetic interference, it is widely applicable to dynamic testing scenarios such as multiphase seepage, gas adsorption, and diffusion under NMR conditions.

[0033] In this embodiment, the clamping device is made entirely of non-magnetic materials. For example, the cylinder 18 and the bellows 4 are made of carbon fiber reinforced polyether ether ketone (PEEK-CF). The bellows 4 adopts a U-shaped bellows structure, possessing excellent non-magnetic properties, thermal stability, and high yield strength (≥160MPa). This ensures structural safety and reusability under high-pressure loading conditions while maintaining compatibility with the nuclear magnetic resonance environment. The bellows injection line 17 communicates with the internal chamber of the bellows 4 through the bellows inlet 5. The bellows inlet 5 is equipped with a high-temperature and high-pressure valve. During the experiment, the bellows 4 contains a staged pressurized liquid to provide the required axial pressure. To stabilize the axial pressure, the valve is closed after reaching the preset pressure, thus stabilizing the pressure inside the bellows 4 and preventing external pressure fluctuations from causing errors in the experiment. The valve is opened again when the axial pressure needs to be changed. Perfluoropolyether liquid (viscosity approximately 50cSt, no nuclear magnetic response) is injected into the bellows 4 through the bellows injection line 17 as the driving medium, with a hydraulic loading transmission efficiency ≥98%. The corrugated pipe 4 has a vent hole (radius 2.5mm) in the center axis that connects to the vent pipe of core 8. The fluid resistance coefficient is ≤0.05, which supports multiphase displacement experiments at a rate of 10mL / min or higher.

[0034] The bellows 4 has the following two implementation methods:

[0035] (1) Single-story structure scheme:

[0036] A typical U-shaped corrugated design is adopted, with structural parameters as follows: wave pitch 2mm, wave height 2.5mm, wave number 3, inner diameter 3.6mm, wave thickness 2mm, wall thickness 0.7mm, and outer diameter 10mm. During axial loading, the U-shaped corrugated joint absorbs external forces through elastic micro-bending, exhibiting good compliance and nonlinear compression characteristics, making it suitable for medium loading pressure and conventional dynamic experimental scenarios. Considering the periodicity and local load-bearing characteristics of the U-shaped corrugated joint, the thin-shell circumferential stress theory can be used to mechanically verify the arc segment of each corrugated joint. Setting the corrugated joint bending radius to 1mm, the loading pressure to 70MPa, and the safety factor to 1.5, combined with the material yield strength of 160MPa, the minimum wall thickness is calculated to be ≥0.656mm. In this embodiment, a value of 0.7mm is used to ensure that each corrugated joint does not yield or locally become unstable under independent stress, guaranteeing the overall uniform strength distribution and loading stability of the loaded structure.

[0037] (2) Multi-layer structure scheme:

[0038] To enhance loading strength and fatigue life, this embodiment employs a double-layer nested U-shaped corrugated pipe structure. The inner and outer corrugated pipes are made of carbon fiber reinforced PEEK-CF material, bonded together by hot pressing or slip-fitting to form a compact composite loading body with coordinated axial response. Each layer's structural parameters are identical to a single layer, with a single-layer wall thickness of 0.35 mm and a total thickness of 0.7 mm, achieving layered pressure bearing and coordinated deformation. This multi-layered structure disperses load stress, significantly improving the structure's compressive strength, fatigue resistance, and long-term loading stability, making it particularly suitable for high-frequency dynamic loading or impact loading scenarios. It also possesses structural redundancy, ensuring that even if a single layer fails, the overall load-bearing capacity remains unaffected, enhancing experimental safety and stability.

[0039] In summary, the left-end loading module offers a "single-layer / multi-layer selectable corrugated pipe structure," which, while meeting the requirements of different loading intensities and experimental frequencies, also boasts high adaptability, high safety, and NMR compatibility, making it an ideal axial loading solution for high-pressure dynamic seepage experiments.

[0040] To further ensure the axial stability and structural safety of the bellows 4 during high-pressure loading, a cylindrical bellows protector 6 is fitted around the outside of the bellows 4. The bellows protector 6 is made of silicon nitride (Si3N4) material. This bellows protector 6 possesses high modulus, high hardness, low coefficient of thermal expansion, and electromagnetic transparency. Its dimensional parameters are: inner diameter 25.2 mm, outer diameter 35.2 mm, wall thickness 5.0 mm, and its total length covers the corrugated section of the bellows 4 while allowing for an appropriate compression stroke. Figure 4As shown, the inner side of the left end cap 14 is provided with a slot 19 for fixing the bellows 4. The bellows 4 is embedded in the slot 19, so that the bellows 4 is fixed without relying on adhesives or other external materials. When the bellows 4 fails, it can be flexibly replaced. During loading, the radial expansion and lateral buckling of the corrugations are restricted, which significantly improves the cyclic loading life and stress transmission stability.

[0041] To improve interface stability and sealing, a heat-shrinkable sleeve 10 is wrapped between the left plug 7 and the right plug 9. The core 8 is wrapped inside the heat-shrinkable sleeve 10, and the core-plug connection is also wrapped with the heat-shrinkable sleeve 10, fitting the outer surface of the core-plug assembly structure to form a primary sealing and fixing structure. The entire core-plug assembly is installed in the center of the confining pressure loading chamber 11, and the two ends are connected by threads through the left end cap and the right end cap and sealed with O-rings to ensure that a completely sealed confining pressure loading space is formed, effectively isolating the confining pressure medium from the ventilation path, ensuring that no leakage or cross-contamination occurs under high pressure, and guaranteeing the purity and functional integrity under the nuclear magnetic resonance environment.

[0042] In this embodiment, the two ends of the core 8 are directly pressed together by non-magnetic plugs, and the heat shrink tubing 10 and O-ring sealing structure are used to effectively eliminate interface voids and retained volume, which can significantly reduce the measurement error of relaxation time (T2) and improve the accuracy and stability of pore structure identification.

[0043] This utility model adopts an embedded piezoelectric force measurement system to realize real-time load feedback: the right end integrates a high-frequency response piezoelectric sensor module, which supports real-time acquisition and digital transmission of load throughout the process, and can meet the force-seepage synergistic response testing requirements in multi-field coupling experiments such as fracturing simulation and cyclic loading.

[0044] The piezoelectric sensor 15 has a thickness of no more than 2 mm and a central aperture of 2–3 mm, allowing gas or liquid to pass freely during displacement, and enabling synchronous load measurement without affecting the axial ventilation channel. The piezoelectric sensor 15 is clamped between the core plug and the external rigid limiting end cap on both sides. As the axial load advances, the minute deformation it experiences is converted into a continuous charge signal.

[0045] The connection method of the force measurement signal acquisition system of the bearing device provided by this utility model is as follows: Lead the BNC output port of the piezoelectric sensor 15 (Kistler 9341B) out from the side of the right end cap 12 through a low-noise coaxial cable 16, and connect it to the Charge Input (channel 1) of the 5080A charge amplifier. Then, power the amplifier through a 24V DC power adapter, ensuring correct polarity. After powering on, the LED indicator on its front panel should be constantly lit, indicating that the system is working normally. To achieve real-time computer reception and recording, connect the RS-232 interface on the rear panel of the 5080A charge amplifier to an RS-232 to USB converter, then insert it into the USB interface of the host computer, install the corresponding driver (such as the FTDI driver), and complete the communication initialization. The analog output channel (Analog Output) of the 5080A charge amplifier can be connected to an oscilloscope or data acquisition card through a BNC cable to achieve the observation and storage of continuous mechanical response curves. This module enables real-time load monitoring within the range of 0.1–80 MPa, providing stable and traceable stress data support for dynamic loading-seepage experiments under NMR conditions, and is suitable for high-precision rock mechanics response analysis scenarios.

[0046] This invention features a closed confining pressure loading chamber inside the clamping device. This chamber surrounds the space between the core sample and its supporting structure. A confining pressure medium (fluorinated liquid) is injected through the confining pressure liquid inlet 2, forming a uniform and stable coaxial confining pressure field within the confining pressure loading chamber 11. The confining pressure liquid applies radial hydrostatic pressure to the outer wall of the core sample 8 to simulate the formation confining pressure state in an underground reservoir environment.

[0047] The loading direction of the confining pressure loading chamber 11 is orthogonally coupled with the axial stress applied by the left-end bellows 4. The two act together on the core 8, forming a three-dimensional triaxial stress loading environment. This device can effectively simulate the multi-directional stress state of rocks in real strata, improve the representativeness and accuracy of the experiment, and is particularly suitable for multiphase fluid seepage and stress response experiments in complex media such as tight coal, shale, and sandstone.

[0048] It is worth noting that in this device, the axial stress applied by the bellows 4 actually depends on the pressure difference between the pressure of the working medium inside it and the liquid pressure in the confining pressure loading chamber 11. By precisely controlling these two pressure sources respectively, the axial stress of the rock sample can be adjusted independently, ensuring that the required triaxial stress conditions can still be accurately applied in the nuclear magnetic resonance experimental environment.

[0049] The clamping device provided by this invention features a compact and modular design, facilitating maintenance and integration. Each module can be quickly disassembled and assembled, simplifying core sample replacement, maintenance and cleaning, and integration with NMR measurement systems. It exhibits strong adaptability to high temperatures and pressures, making it suitable for various extreme working conditions. The PEEK material corrugated pipe possesses excellent thermal stability, corrosion resistance, and mechanical support capabilities, demonstrating good reliability and repetitive loading life in high-pressure seepage experiments in oil and gas, geothermal, and CO2 storage applications.

[0050] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A triaxial loading core clamping device for use in nuclear magnetic resonance environments, characterized in that, The clamping device is made entirely of non-magnetic materials and includes: The core support unit includes a cylindrical body (18), inside which a confining pressure loading cavity (11) is formed. The body (18) is provided with a confining pressure fluid inlet (2) and a confining pressure fluid outlet (3) communicating with the confining pressure loading cavity (11). The left end cap (14) and the right end cap (12) are respectively provided at the left and right ends of the body (18) to seal the confining pressure loading cavity (11). The body (18) is provided with a left plug (7) and a right plug (9) for pressing and fixing the core (8). The left-end axial loading module includes a bellows (4) with an internal cavity and a bellows injection line (17), wherein the bellows (4) is disposed between the left end cap (14) and the left plug (7); The right end force measuring module is located between the right end cover (12) and the right end cap (9) and is used to monitor the stress on the core during axial loading in real time.

2. The triaxial loading core clamping device for nuclear magnetic resonance environment according to claim 1, characterized in that, The corrugated pipe (4) is made of carbon fiber reinforced polyether ether ketone material.

3. A triaxial loading core clamping device for nuclear magnetic resonance environment according to claim 2, characterized in that, The corrugated pipe (4) adopts a U-shaped corrugated pipe structure.

4. A triaxial loading core clamping device for nuclear magnetic resonance environment according to claim 1, characterized in that, The corrugated pipe (4) is fitted with a cylindrical corrugated pipe protector (6).

5. A triaxial loading core clamping device for nuclear magnetic resonance environment according to claim 4, characterized in that, The bellows protector (6) is made of silicon nitride material.

6. A triaxial loading core clamping device for nuclear magnetic resonance environment according to claim 1, characterized in that, A heat-shrinkable sleeve (10) is wrapped between the left plug (7) and the right plug (9).

7. A triaxial loading core clamping device for nuclear magnetic resonance environment according to claim 1, characterized in that, Both the left end cap (14) and the right end cap (12) are provided with ventilation rods (1). The left plug (7), the right plug (9), and the corrugated pipe (4) are all provided with through holes along the axis for the ventilation rods (1) to pass through. The ventilation rods (1) on both sides are embedded in the shaft hole of the core (8).

8. A triaxial loading core clamping device for nuclear magnetic resonance environment according to claim 7, characterized in that, The right-end force measuring module adopts a center-opening piezoelectric sensor (15) that supports real-time stress detection of 0.1-80MPa.

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