A true triaxial high-temperature rock thermal shock hydraulic fracturing test system and a use method thereof

Abstract

The application discloses a true triaxial high-temperature rock thermal shock hydraulic fracturing test system and a use method, and relates to the technical field of rock mechanics test.The system comprises a supporting base, upper pressure heads, front, rear, left and right pressure heads, silicon-carbon rods, hydraulic pipelines, water pumps, water pressure sensors, automatic water valves, acoustic emission devices, rock samples, temperature measuring thermocouples, water delivery plugs and liquid nitrogen tanks.The silicon-carbon rods are integrated in the pressure heads to realize in-situ heating, and the working surfaces of the pressure heads are provided with spray holes which are communicated with the liquid nitrogen tanks to realize rapid cooling;the front and rear pressure heads are provided with through hydraulic channels which are connected with the central through holes of the rock samples through the water delivery plugs to form a liquid circulation loop;heat insulation plates are installed on the back surfaces of the left and right pressure heads and acoustic emission probes are fixed on the heat insulation plates.The application can realize real-time high-temperature, ultralow-temperature, thermal shock, temperature gradient, cold and hot cycle and thermal shock coupling hydraulic fracturing and other tests under the true triaxial stress state, and provides a test platform for rock mechanics research under complex temperature environments such as deep geothermal exploitation and polar engineering.

Description

Technical Field

[0001] This invention relates to the field of rock mechanics testing technology, and in particular to a true triaxial high-temperature rock thermal shock hydraulic fracturing test system and its usage method. Background Technology

[0002] Geothermal energy, as a abundant, clean, and low-carbon renewable energy source, has received widespread attention globally in recent years. Geothermal resources are primarily found deep underground in high-temperature rock formations. A common development method involves heat exchange through water injection: a low-temperature fluid is injected into the high-temperature rock layer, and after heating, the fluid is pumped back to the surface through production wells for power generation or direct heating. During this process, a dramatic heat exchange occurs between the high-temperature rock mass and the injected fluid. The sudden temperature change within the rock causes non-uniform thermal expansion and contraction, leading to the initiation and propagation of microcracks—a phenomenon known as "thermal shock." Thermal shock significantly affects the rock's strength, deformability, and permeability, thus impacting the efficiency and long-term safety of geothermal extraction.

[0003] Meanwhile, my country has a vast territory with widespread distribution of unique geological environments such as plateaus and permafrost. In the construction of highways, railways, and tunnels in regions like the Qinghai-Tibet Plateau, rocks are subjected to prolonged periods of ultra-low temperatures or repeated freeze-thaw cycles, resulting in significant differences in their mechanical behavior compared to those under normal temperature conditions. The brittle fracture and crack propagation of frozen rocks pose threats to engineering safety, necessitating systematic experimental research on low-temperature rock mechanics.

[0004] True triaxial testing is an important method for studying the mechanical properties of rocks under multi-directional stress. It can effectively reveal the influence of intermediate principal stress on rock strength, deformation, and failure modes, and is a key tool for studying the mechanisms of deep rock engineering and geological hazards. Introducing temperature factors into true triaxial testing to achieve mechanical loading under complex temperature paths such as high temperature, ultra-low temperature, thermal shock, and thermal cycling is of great significance for simulating actual working conditions such as geothermal energy extraction and high-altitude polar engineering construction.

[0005] Currently, some rock mechanics testing systems have attempted to integrate temperature control functions, such as achieving high temperatures through heating rods, hot air circulation, or infrared heating, and achieving low temperatures through liquid nitrogen or compressor cooling. However, existing systems still have shortcomings in simulating drastic in-situ temperature changes, rapid thermal shock processes, temperature gradient control within the sample, and multi-field coupled hydraulic fracturing of the thermal-fluid-solid system, making it difficult to fully meet the needs of rock mechanics testing under complex temperature conditions. Summary of the Invention The purpose of this invention is to provide a true triaxial high-temperature rock thermal shock hydraulic fracturing test system and its usage method, which can accurately simulate complex engineering conditions such as geothermal extraction, plateau and polar frozen rocks, deep rock mass temperature gradients and thermal cycles, accurately obtain the mechanical properties and crack evolution laws of rocks under multi-field coupling, and provide scientific experimental basis and technical support for geothermal development, plateau engineering and deep rock mass engineering.

[0006] To achieve the above objectives, the present invention provides a true triaxial high-temperature rock thermal shock hydraulic fracturing test system, comprising a true triaxial loading cavity formed by a support base, an upper pressure head, a front pressure head, a rear pressure head, a left pressure head, and a right pressure head, wherein a rock cubic sample is placed inside the loading cavity; Each of the front, rear, left, and right pressure heads is equipped with a silicon carbide rod for in-situ heating of the sample. Each pressure head has a spray hole on its surface in contact with the sample, connected to a liquid nitrogen tank for rapid cooling. The front and rear pressure heads have a through-hole hydraulic channel, sealed to a through-hole in the center of the sample via a water plug, and connected to a hydraulic pipeline. A water pressure sensor and an automatic water valve are connected in series in the hydraulic pipeline, and a water pump, water tank, and wastewater treatment tank are also connected. A heat insulation plate is installed on the back of the left and right pressure heads, and an acoustic emission probe of an acoustic emission device is fixed on the heat insulation plate. The acoustic emission device also includes an acoustic emission instrument and an acoustic emission signal amplifier, and the acoustic emission instrument is connected to a computer signal. The temperature-measuring thermocouple is embedded inside the sample, and its wiring is led out through through-holes at the four corners of each pressure head.

[0007] Preferably, the hydraulic channel of the front pressure head is connected to a hydraulic pipeline, a water pump, and a water tank at the end opposite to the sample, and the hydraulic channel of the rear pressure head is connected to a hydraulic pipeline at the end opposite to the sample, with the end of the hydraulic pipeline connected to a wastewater treatment tank. The water pump can pressurize the solution in the water tank into the hydraulic pipeline. The water pump is connected to different water pipes in the water tank, and the different water pipes are connected to different solutions, such as water, calcium chloride solutions at different temperatures, etc. The water pump, automatic water valve, water pressure sensor, and other equipment are all connected to a computer. The operator uses the computer to adjust the size of the automatic water valve based on the data fed back by the water pressure sensor, thereby coordinating with the water pump to adjust the water pressure in the hydraulic pipeline.

[0008] Preferably, the support base is a cuboid structure, and the working surfaces of the upper, front, rear, left, and right pressure heads that contact the sample are all square. The support base is used to place the sample, support its weight, and bear the load applied by the pressure heads. Different sized bases can be replaced according to the sample size, and similarly, different pressure heads can be replaced according to the specific size of the sample. A cylindrical block is provided on the back of each of the upper, front, rear, left, and right pressure heads. The axis of the cylindrical block is aligned with the axis of the pressure head body. The end of the cylindrical block away from the pressure head is connected to the force application system, which applies force through an external hydraulic cylinder or electricity to facilitate the installation and movement of the pressure head. During the installation of the upper pressure head, its bottom surface must be aligned with the upper surface of the support base to ensure uniform force distribution. When installing the front and rear indenters, the upper edges of the front and rear indenters should be on the same plane as the lower surface of the upper indenter, and the lower edges should be on the same plane as the upper surface of the base. The front and rear indenters should also be aligned to ensure that the indenters are in perfect contact with the surface of the cubic sample and that the sample is subjected to uniform force. Before installing the left and right indenters onto the force application system, the heat insulation plate needs to be installed on the rear surface of the indenter. The surfaces of the heat insulation plate that contact the back of the left and right indenters also need to be aligned to ensure that the heat insulation plate is in close contact with the indenter.

[0009] Preferably, the upper surfaces of the front and rear pressure heads are provided with multiple heating holes, the silicon carbide rod is tightly embedded in the heating holes, and the silicon carbide rod is connected to an external heating power supply; the inner walls of the hydraulic channels of the front and rear pressure heads are provided with internal threads for sealing connection with the external threads of the water supply plug.

[0010] Preferably, the heat insulation plates of the left and right pressure heads are provided with probe mounting holes, the size of which is adapted to the cylindrical acoustic emission probe for inserting and positioning the acoustic emission probe; the acoustic emission probe is pressed and fixed to the surface of the heat insulation plate by an adsorption cap with a built-in spring, and the adsorption cap is fixed to the heat insulation plate by magnetic attraction or adhesive to ensure that the probe is installed firmly. The acoustic emission device consists of an acoustic emission probe, an acoustic emission signal amplifier, and an acoustic emission instrument. The acoustic emission instrument is connected to a computer. The acoustic emission probe is used to collect acoustic emission signals generated by rock fracture. The acoustic emission signal amplifier is placed between the acoustic emission probe and the acoustic emission instrument to amplify, impedance match, and filter weak signals to improve the signal-to-noise ratio. The acoustic emission instrument is used to record and store the processed signals. The computer is used to analyze waveform data and output parameters such as ring count, cumulative energy, and number of acoustic emission events. The acoustic emission device can realize crack location determination and propagation process monitoring, providing data support for the analysis of rock damage patterns under thermal shock.

[0011] Preferably, the silicon carbide rod has a cylindrical structure that matches the size of the heating holes in each pressure head; the liquid nitrogen tank independently controls the spray path of each pressure head through a multi-channel valve. The liquid nitrogen tank stores liquid nitrogen inside, and its upper part has an interface, pressure gauge, and valve, which can be used for rapid cooling of the sample. When installing the liquid nitrogen pipeline, first connect the threaded holes on the upper surface of the upper, lower, left, and right pressure heads to the pipeline for conveying liquid nitrogen. Then connect the pipelines at the top of each pressure head to the multi-channel valves. Different pressure heads correspond to different valves and different liquid nitrogen tanks. Next, connect the pipeline to the upper interface of the liquid nitrogen tank, and connect this pipeline to the corresponding multi-channel valve.

[0012] Preferably, the rock cube specimen includes a temperature test specimen and a mechanical test specimen, both of which have a through hole in the center. The temperature test specimen also has temperature measuring holes on multiple surfaces for embedding thermocouples. The thermocouples are connected to external sensors and a computer to test the internal temperature changes during the heating process.

[0013] Preferably, the water plug is an integral structure, with its lower part being an externally threaded cylinder that matches the internal threads of the hydraulic channels of the front and rear pressure heads, and its upper part being a cylinder that is sealed and inserted into the central through hole of the sample.

[0014] A method for using a true triaxial high-temperature rock thermal shock hydraulic fracturing test system, characterized by the following steps: S1. System Assembly: Place the support base on the test bench, and install the upper pressure head, front pressure head, rear pressure head, left pressure head, and right pressure head in sequence, ensuring that the working surface of each pressure head is in close contact with the sample surface; insert the silicon carbide rod into the heating hole of each pressure head and connect it to an external heating power supply; connect the liquid nitrogen inlet of each pressure head to the liquid nitrogen tank through a connecting pipe and a multi-channel valve; connect the outer end of the hydraulic channel of the front and rear pressure heads to the hydraulic pipeline, and connect the hydraulic pipeline of the front pressure head to the water pump and water tank in sequence, and connect the hydraulic pipeline of the rear pressure head to the wastewater treatment tank; connect the water pressure sensor and the automatic water valve in series in the hydraulic pipeline, and connect the water pressure sensor and the automatic water valve to the computer signal; install the acoustic emission probe on the heat insulation plate on the back of the left and right pressure heads and connect it to the acoustic emission instrument; embed the temperature measuring thermocouple into the temperature measuring hole of the temperature measuring test sample, and lead the connecting wire of the temperature measuring thermocouple out from the through hole at the four corners of each pressure head to the acquisition device; S2. Temperature Measurement Calibration: Place the temperature measurement test sample in the loading chamber, apply a pre-contact force to each pressure head so that it makes slight contact with the sample surface; start the silicon carbide rod to heat or open the liquid nitrogen valve to cool down, monitor the temperature change at different positions inside the sample through the temperature measuring thermocouple, record the heating or cooling time required for the sample center through hole to reach the target temperature, and parameters such as the opening degree of the liquid nitrogen valve. S3. Mechanical test preparation: Remove the temperature test specimen, place the mechanical test specimen in the cavity, align the central through hole of the specimen with the water supply plugs on the front and rear pressure heads and tighten them to seal; apply pre-contact force to each pressure head; heat or cool the specimen according to the parameters calibrated in S2 to bring it to the target temperature. S4. Loading and Data Acquisition: According to the test requirements, the preset triaxial principal stress is applied to the sample through the external force application system; while maintaining the temperature conditions, conventional compression, thermal shock hydraulic fracturing or thermal cycling tests are carried out; during the test, the stress of each pressure head, the water pressure fed back by the water pressure sensor, the temperature data of the thermocouple, and the ringing count, cumulative energy, and event count parameters of the acoustic emission device are collected simultaneously. S5. Post-processing: After the test, turn off the heating and cooling system, relieve the stress, take out the sample to observe the failure mode, and organize and analyze the collected data.

[0015] Preferably, the thermal shock hydraulic fracturing test mode specifically includes: heating the mechanical test specimen to a preset high temperature and holding it at that temperature according to S2 and S2 to make the temperature field of the specimen uniform; applying triaxial true triaxial stress to a preset value through an external force application system; starting the water pump, opening the automatic water valve of the front pressure head hydraulic pipeline, and injecting low-temperature coolant into the central through hole of the specimen to make the specimen undergo thermal shock from the inside; monitoring the water pressure in real time through a water pressure sensor, and using a computer to adjust the opening of the automatic water valve and the water pump outlet pressure to gradually increase the water pressure at a preset rate; continuously injecting water and pressurizing until the specimen undergoes hydraulic fracturing failure, and recording the rupture pressure and acoustic emission event location information; and discharging the liquid flowing out of the rear pressure head hydraulic pipeline into a wastewater treatment tank during the water injection process.

[0016] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects: First, the present invention integrates silicon carbide rods inside each pressure head to achieve in-situ heating under true triaxial stress. At the same time, spray holes connected to liquid nitrogen tanks are opened on the contact surface between each pressure head and the sample, and water injection cooling is carried out in conjunction with the central through hole of the sample. This can independently or collaboratively realize the construction of multiple thermal shock modes and internal temperature gradients of the sample, and truly simulate the stress state of rocks under complex temperature paths such as geothermal mining.

[0017] Secondly, the present invention forms a liquid flow loop through the sample center through hole by sealing the hydraulic channel between the front and rear pressure heads with the water supply plug. Combined with the computer closed-loop control of the water pressure sensor and the automatic water valve, hydraulic fracturing tests can be carried out under high temperature or thermal shock conditions, so as to realize the study of rock fracture mechanism under the multi-field coupling of heat-fluid-solid.

[0018] Third, the present invention installs the acoustic emission probe on the heat insulation plate on the back of the left and right pressure heads to avoid damage to the probe by extreme temperatures, and realizes real-time positioning and monitoring of crack initiation and propagation during thermal shock and fracturing; at the same time, the design of distinguishing between temperature test specimens and mechanical test specimens ensures the accuracy of temperature calibration and the repeatability of the test.

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the overall system structure according to Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the rear pressure head structure according to Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the temperature measurement test specimen structure according to Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the mechanical test specimen structure according to Embodiment 1 of the present invention; Figure 5 This is a schematic diagram of the water-transporting plug structure according to Embodiment 1 of the present invention; Figure 6 This is a schematic diagram of the right pressure head structure according to Embodiment 1 of the present invention.

[0022] Figure Labels 1. Support base; 2. Upper pressure head; 3. Front pressure head; 4. Rear pressure head; 5. Left pressure head; 6. Right pressure head; 7. Silicon carbide rod; 8. Hydraulic pipeline; 81. Water tank; 82. Wastewater treatment tank; 9. Water pump; 10. Water pressure sensor; 11. Automatic water valve; 12. Acoustic emission device; 121. Acoustic emission probe; 122. Acoustic emission instrument; 13. Temperature measurement test specimen; 14. Mechanical test specimen; 15. Temperature measurement thermocouple; 16. Water supply plug; 17. Liquid nitrogen tank; 171. Connecting pipe; 172. Multi-channel valve. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] Example 1: Real-time high-temperature true triaxial test This embodiment is used to test the triaxial mechanical properties of rocks under high temperature conditions.

[0026] First, a granite cube specimen with a side length of 100 mm is prepared as mechanical test specimen 14. A cylindrical hole with a diameter of 5 mm and a penetration length of 100 mm is opened at the center of the surface of the specimen that contacts the front indenter 3 and the rear indenter 4.

[0027] Installation System: Fix the support base 1, which is 100mm long and wide and 50mm thick, to the test bench. Install the upper pressure head 2. The cuboid of the upper pressure head 2 that contacts the sample is 100mm long and wide and 50mm thick, and its upper cylinder has a diameter of 50mm and a thickness of 50mm. Install the front pressure head 3 and the rear pressure head 4, ensuring that their upper and lower edges are flush with the lower surface of the upper pressure head 2 and the upper surface of the base. The cuboids of the front pressure head 3 and the rear pressure head 4 that contact the sample are 100mm long and wide and 50mm thick, and their upper cylinders have a diameter of 50mm and a thickness of 50mm. Install the left pressure head 5 and the right pressure head 6, again ensuring they are aligned and fitted. First, install a heat insulation plate with a side length of 100mm and a thickness of 10mm on the back of the left pressure head 5 and the right pressure head 6. The heat insulation plate has a circular hole with a diameter of 50mm in the center for the pressure head cylinder to pass through.

[0028] A silicon carbide rod 7, with a total length of 80 mm and a diameter of 6 mm, is embedded in heating holes with a diameter of 6 mm and a depth of 90 mm on the upper surface of the front, rear, left, and right pressure heads 6. The silicon carbide rod 7 is connected to an external heating power supply. A 10 mm diameter through-hydraulic channel, formed along the central axis of the front pressure head 3 and the rear pressure head 4, is connected to a hydraulic pipeline 8. The pipeline of the front pressure head 3 is sequentially connected to a water pump 9 and a water tank 81, while the pipeline of the rear pressure head 4 is connected to a wastewater treatment tank 82. A water pressure sensor 10 and an automatic water valve 11 are connected in series in the hydraulic pipeline 8, both of which are connected to a computer signal. An acoustic emission probe 121 is installed in four holes with a radius of 2.5 mm and a depth of 2 mm in the heat insulation plates of the left pressure head 5 and the right pressure head 6, and secured using a suction cap with an internal spring. The acoustic emission probe 121 is connected to an acoustic emission instrument 122, which is connected to a computer.

[0029] The mechanical test specimen 14 is placed in the loading chamber, with the central through-hole of the specimen aligned with the water plugs 16 on the front and rear pressure heads 3 and tightly sealed. The lower part of the water plug 16 is a 10mm diameter, 20mm high externally threaded cylinder, which is sealed to the 20mm long thread on the inner wall of the hydraulic channel of the pressure head. The upper part is a 5mm outer diameter, 4mm inner diameter, and 5mm high cylinder, which is inserted into the central through-hole of the specimen. A pre-contact force is applied to each pressure head. Based on the pre-test results of the temperature test specimen 13, the silicon carbide rod 7 is heated to 300℃ and held for 2 hours. After the holding period, a preset triaxial principal stress (e.g., σ1=20MPa, σ2=10MPa, σ3=5MPa) is applied to the specimen through an external force application system. The temperature is kept constant, and the specimen is loaded at a displacement rate of 0.5mm / min until it fails. Stress, strain, temperature, and acoustic emission data are collected simultaneously during the test.

[0030] Example 2: Thermal Shock True Triaxial Hydraulic Fracturing Test This embodiment is used to simulate the working condition of high-temperature rock mass encountering cold water and undergoing hydraulic fracturing during geothermal extraction.

[0031] The system assembly is the same as in Example 1. First, the mechanical test specimen 14 is heated to 300℃ and held at that temperature for 2 hours to ensure a uniform temperature field. Triaxial true triaxial stress is applied to preset values ​​(σ1=15MPa, σ2=8MPa, σ3=8MPa) through an external force application system. The water pump 9 is started, and the automatic water valve 11 of the hydraulic line 8 of the front pressure head 3 is opened, injecting a -20℃ calcium chloride solution into the central through-hole of the specimen, causing the specimen to undergo thermal shock from the inside. The water pressure is monitored in real time by the water pressure sensor 10, and the opening of the automatic water valve 11 and the outlet pressure of the water pump 9 are adjusted by a computer to gradually increase the water pressure at a rate of 0.5MPa / min. Water injection and pressurization continue until the specimen undergoes hydraulic fracturing failure, and the fracturing pressure and acoustic emission event location information are recorded. During the water injection process, the liquid flowing out from the hydraulic line 8 of the rear pressure head 4 is discharged into the wastewater treatment tank 82.

[0032] Alternatively, liquid nitrogen spraying can be activated simultaneously after heating: open valve 17 of liquid nitrogen tank, and through connection 171, control the spray holes of the front, rear, left, and right pressure heads 6 to spray liquid nitrogen onto the sample surface via multi-channel valve 172. Each spray hole has a diameter of 2mm and a depth of 10mm. On the upper surface, eight centers are arranged with a center every 10mm, four centers on each side inward, for a total of eight centers. On the sample contact surface, nineteen centers are arranged with a center every 5mm from each side, four rows of holes on each side, forming a sprinkler-like uniform spray to achieve combined internal and external cooling thermal shock before hydraulic fracturing.

[0033] Example 3: Ultra-low temperature true triaxial test This embodiment is used to test the mechanical properties of rocks in ultra-low temperature environments in polar or high-altitude permafrost regions.

[0034] First, prepare the temperature measurement test specimen 13: On the five surfaces of the cube specimen with a side length of 100mm, make a hole with a diameter of 2mm and a depth of 50mm every 21mm along the diagonal of each face, starting from the midpoint and going upwards or downwards (the holes are made in opposite directions along the diagonals on opposite faces). Make four holes on each face, for a total of 20 temperature measurement holes, and embed the temperature measurement thermocouple 15.

[0035] Place the temperature test specimen 13 into the loading chamber and apply a pre-contact force to each pressure head. Open the valve of the liquid nitrogen tank 17 and supply liquid nitrogen to the spray hole through the liquid nitrogen inlet at the top of each pressure head. The liquid nitrogen inlet is located 10 mm from the edge of the upper surface where it contacts the specimen. Starting from 5 mm from the left and right short sides of the upper surface, take a center every 10 mm. Take four centers inward from each side, and make an opening with a diameter of 2 mm and a depth of 96 mm downward. Make a threaded hole with a depth of 4 mm and a diameter of 4 mm at the opening on the upper surface for connecting the liquid nitrogen pipeline. Measure the surface temperature of the loading plate with a temperature gun and simultaneously observe the temperature fed back by the temperature measuring thermocouple 15. When the two temperatures are consistent and reach -50℃, record the time taken and the opening degree of the liquid nitrogen valve.

[0036] Replace the mechanical test specimen 14 (with only the center through hole open), perform cryogenic treatment with the same parameters, and after the specimen temperature stabilizes at -50℃, apply triaxial true triaxial stress to perform a compression test, and record the stress-strain curve and failure mode.

[0037] Example 4: True Triaxial Test of Thermal Cycling This embodiment is used to simulate the mechanical behavior of rocks under repeated freeze-thaw cycles or alternating hot and cold environments.

[0038] A temperature-measuring test specimen 13 was placed in the loading chamber after embedding a thermocouple 15, and a pre-contact force was applied to each pressure head. A thermal cycling program was set: first, the silicon carbide rod 7 was started to heat to 200℃ and held for 1 hour; then the heating power was turned off, the valve of the liquid nitrogen tank 17 was opened, and liquid nitrogen was sprayed onto the specimen surface through the spray holes of each pressure head for 30 seconds, rapidly cooling the specimen surface to -50℃; then it was heated again to 200℃. This cycle was repeated 5 times. The temperature changes at different locations inside the specimen were monitored using the thermocouple 15, and the time required to reach the target temperature and the opening degree of the liquid nitrogen valve were recorded for each cycle.

[0039] After temperature calibration, the specimen was replaced with mechanical test specimen 14 (with only the center through-hole). The specimen underwent the same five cycles of thermal cycling according to the calibration parameters. Immediately after the final cooling (or heating) cycle, a true triaxial stress was applied for compression testing, and the stress-strain curve and failure mode were recorded. Throughout the test, the acoustic emission device 12 monitored the process, and the acoustic emission probe 121 collected parameters such as ring count, cumulative energy, and number of acoustic emission events through the mounting hole (2.5 mm radius, 2 mm depth) on the heat insulation plate.

[0040] Example 5: True Triaxial Test with Temperature Gradient This embodiment is used to simulate the stress state of rocks with temperature differences inside (such as around water injection wells in geothermal extraction).

[0041] The system was assembled according to the method in Example 1. The temperature measurement test sample 13 was placed in the cavity, and thermocouples 15 were embedded in the temperature measurement holes at each depth. The test conditions were set: the silicon carbide rods 7 in the left pressure head 5 and the front pressure head 3 were heated to 250°C, while the right pressure head 6 and the rear pressure head 4 were not heated. At the same time, liquid nitrogen was introduced into the spray holes of the right pressure head 6 and the rear pressure head 4 through the liquid nitrogen tank 17 for cooling, so that a temperature gradient was formed inside the sample from one side to the other. The temperature gradient was maintained stable for 2 hours by independent temperature control of each pressure head. The readings of the thermocouples 15 at different positions inside the sample were monitored, and the temperature distribution and the time to reach steady state were recorded. The wiring of the thermocouples 15 passed through holes with a diameter of 2 mm and a thickness of 1 mm made at the four corners of each pressure head and was connected to the external data acquisition equipment.

[0042] Replace the mechanical test specimen 14 and perform temperature gradient treatment with the same parameters. After the internal temperature gradient of the specimen stabilizes, apply triaxial true triaxial stress to perform a compression test and record the failure mode, crack propagation direction and acoustic emission signal.

[0043] Example 6: True triaxial thermal shock test (internal water injection) This embodiment is used to simulate the effect of water injection heat exchange process on rock mechanical properties during geothermal extraction.

[0044] The system was assembled according to the method in Example 1. The mechanical test specimen 14 was heated to 300°C and held at that temperature for 2 hours. After the holding period, room temperature water (approximately 20°C) was immediately injected into the central through-hole of the specimen through the hydraulic line 8 of the front pressure head 3. The flow rate of the water pump 9 was controlled at 100 ml / min, which rapidly cooled the specimen from the inside, generating a thermal shock effect. During the water injection process, the pressure change was monitored by the water pressure sensor 10, and the water injection continued for 3 minutes. After the water injection was completed, a triaxial true triaxial stress was immediately applied for a compression test. The mechanical parameters were compared with those of the specimen without thermal shock to analyze the effect of thermal shock on the rock strength and deformation characteristics. During the test, the acoustic emission device 12 monitored the initiation and propagation of microcracks inside the specimen.

[0045] The remaining technical features in the above embodiments can be flexibly selected by those skilled in the art to meet different specific practical needs according to actual circumstances. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims. In the above description, numerous specific details have been set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that these specific details are not necessary to implement the present invention. In other instances, to avoid obscuring the present invention, well-known techniques, such as specific construction details, operating conditions, and other technical conditions, have not been specifically described.

[0046] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A true triaxial high-temperature rock thermal shock hydraulic fracturing test system, characterized in that: It includes a true triaxial loading cavity formed by a support base, an upper pressure head, a front pressure head, a rear pressure head, a left pressure head, and a right pressure head, and the loading cavity contains a rock cube sample. Each of the front, rear, left, and right pressure heads is equipped with a silicon carbide rod for in-situ heating of the sample. Each pressure head has a spray hole on its surface in contact with the sample, connected to a liquid nitrogen tank for rapid cooling. The front and rear pressure heads have a through-hole hydraulic channel, sealed to a through-hole in the center of the sample via a water plug, and connected to a hydraulic pipeline. A water pressure sensor and an automatic water valve are connected in series in the hydraulic pipeline, and a water pump, water tank, and wastewater treatment tank are also connected. A heat insulation plate is installed on the back of the left and right pressure heads, and an acoustic emission probe of an acoustic emission device is fixed on the heat insulation plate. The acoustic emission device also includes an acoustic emission instrument and an acoustic emission signal amplifier, and the acoustic emission instrument is connected to a computer signal. The temperature-measuring thermocouple is embedded inside the sample, and its wiring is led out through through-holes at the four corners of each pressure head.

2. The true triaxial high-temperature rock thermal shock hydraulic fracturing test system according to claim 1, characterized in that: The hydraulic channel of the front pressure head is connected to a hydraulic pipeline, a water pump, and a water tank at the end opposite to the sample. The hydraulic channel of the rear pressure head is connected to a hydraulic pipeline at the end opposite to the sample, and the end of the hydraulic pipeline is connected to a wastewater treatment tank.

3. The true triaxial high-temperature rock thermal shock hydraulic fracturing test system according to claim 1, characterized in that: The support base is a cuboid structure, and the working surfaces of the upper pressure head, front pressure head, rear pressure head, left pressure head, and right pressure head that contact the sample are all square.

4. The true triaxial high-temperature rock thermal shock hydraulic fracturing test system according to claim 1, characterized in that: The upper surfaces of the front and rear pressure heads are provided with multiple heating holes, and the silicon carbide rod is tightly embedded in the heating holes. The silicon carbide rod is connected to an external heating power supply. The inner walls of the hydraulic channels of the front and rear pressure heads are provided with internal threads for sealing connection with the external threads of the water supply plug.

5. The true triaxial high-temperature rock thermal shock hydraulic fracturing test system according to claim 1, characterized in that: The heat insulation plates of the left and right pressure heads are provided with probe mounting holes. The acoustic emission probe is fixed in the mounting holes by an adsorption cap with a built-in spring. The adsorption cap is fixed to the heat insulation plate by magnetic attraction or adhesive.

6. The true triaxial high-temperature rock thermal shock hydraulic fracturing test system according to claim 1, characterized in that: The silicon carbide rod has a cylindrical structure that matches the size of the heating holes in each pressure head; the liquid nitrogen tank independently controls the spray holes of each pressure head through a multi-channel valve.

7. The true triaxial high-temperature rock thermal shock hydraulic fracturing test system according to claim 1, characterized in that: The cubic stone specimens include temperature test specimens and mechanical test specimens, each with a through hole in its center. Multiple surfaces of the temperature test specimens also have temperature measurement holes for embedding thermocouples.

8. The true triaxial high-temperature rock thermal shock hydraulic fracturing test system according to claim 1, characterized in that: The water plug is an integral structure, with an externally threaded cylinder at the bottom that matches the internal threads of the hydraulic channels of the front and rear pressure heads, and a cylinder at the top that is sealed and inserted into the central through hole of the sample.

9. The method of using the true triaxial high-temperature rock thermal shock hydraulic fracturing test system as described in any one of claims 1-8, characterized in that, The steps are as follows: S1. System Assembly: Place the support base on the test bench, and install the upper pressure head, front pressure head, rear pressure head, left pressure head, and right pressure head in sequence, ensuring that the working surface of each pressure head is in close contact with the sample surface; insert the silicon carbide rod into the heating hole of each pressure head and connect it to an external heating power supply; connect the liquid nitrogen inlet of each pressure head to the liquid nitrogen tank through a connecting pipe and a multi-channel valve; connect the outer end of the hydraulic channel of the front and rear pressure heads to the hydraulic pipeline, and connect the hydraulic pipeline of the front pressure head to the water pump and water tank in sequence, and connect the hydraulic pipeline of the rear pressure head to the wastewater treatment tank; connect the water pressure sensor and the automatic water valve in series in the hydraulic pipeline, and connect the water pressure sensor and the automatic water valve to the computer signal; install the acoustic emission probe on the heat insulation plate on the back of the left and right pressure heads and connect it to the acoustic emission instrument; embed the temperature measuring thermocouple into the temperature measuring hole of the temperature measuring test sample, and lead the connecting wire of the temperature measuring thermocouple out from the through hole at the four corners of each pressure head to the acquisition device; S2. Temperature Measurement Calibration: Place the temperature measurement test sample in the loading chamber, apply a pre-contact force to each pressure head so that it makes slight contact with the sample surface; start the silicon carbide rod to heat or open the liquid nitrogen valve to cool down, monitor the temperature change at different positions inside the sample through the temperature measuring thermocouple, record the heating or cooling time required for the sample center through hole to reach the target temperature, and parameters such as the opening degree of the liquid nitrogen valve. S3. Mechanical test preparation: Remove the temperature test specimen, place the mechanical test specimen in the cavity, align the central through hole of the specimen with the water supply plugs on the front and rear pressure heads and tighten them to seal; apply pre-contact force to each pressure head; heat or cool the specimen according to the parameters calibrated in step two to bring it to the target temperature. S4. Loading and Data Acquisition: According to the test requirements, the preset triaxial principal stress is applied to the sample through the external force application system; while maintaining the temperature conditions, conventional compression, thermal shock hydraulic fracturing or thermal cycling tests are carried out; during the test, the stress of each pressure head, the water pressure fed back by the water pressure sensor, the temperature data of the thermocouple, and the ringing count, cumulative energy, and event count parameters of the acoustic emission device are collected simultaneously. S5. Post-processing: After the test, turn off the heating and cooling system, relieve the stress, take out the sample to observe the failure mode, and organize and analyze the collected data.

10. The method of using the true triaxial high-temperature rock thermal shock hydraulic fracturing test system according to claim 1, characterized in that: The thermal shock hydraulic fracturing test mode specifically includes: heating the mechanical test specimen to a preset high temperature according to S2 and S2 and holding it at that temperature to make the temperature field of the specimen uniform; applying triaxial true triaxial stress to a preset value through an external force application system; starting the water pump, opening the automatic water valve of the front pressure head hydraulic pipeline, and injecting low-temperature coolant into the central through hole of the specimen to make the specimen undergo thermal shock from the inside; monitoring the water pressure in real time through a water pressure sensor, and using a computer to adjust the opening of the automatic water valve and the water pump outlet pressure to gradually increase the water pressure at a preset rate; continuously injecting water and pressurizing until the specimen undergoes hydraulic fracturing failure, and recording the rupture pressure and acoustic emission event location information; during the water injection process, the liquid flowing out from the rear pressure head hydraulic pipeline is discharged into a wastewater treatment tank.

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