Device and method for studying crack expansion by synergistic fracturing of supercritical carbon dioxide and liquid nitrogen

Abstract

The present invention discloses a device and method for studying the expansion of cracks by synergistic fracturing of supercritical carbon dioxide and liquid nitrogen. The device includes a temperature and pressure simulation mechanism, a simulated wellbore, a supercritical carbon dioxide circulation mechanism, a liquid nitrogen cold shock mechanism, and a monitoring mechanism. The temperature and pressure simulation mechanism includes an insulation box, a temperature control component, and a triaxial pressure component; the simulated wellbore plug is provided in a borehole; the monitoring mechanism is arranged on the periphery of the rock block and is used to monitor the expansion of cracks in the rock block. The beneficial effects of the present invention are as follows: the temperature and pressure simulation mechanism can accurately simulate the temperature and pressure conditions in the formation; the supercritical carbon dioxide circulation mechanism realizes the cyclic injection of supercritical carbon dioxide. The supercritical carbon dioxide circulation mechanism and the liquid nitrogen cold shock mechanism work together. The monitoring mechanism can monitor the expansion of cracks in the rock block in real time and accurately, providing researchers with detailed data and information. Researchers can deeply analyze the mechanism and influencing factors of crack expansion based on the monitoring results.

Description

Technical Field

[0001] The present invention relates to the technical field of reservoir transformation methods, and in particular to a device and method for studying supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion. Background Art

[0002] Deep shale gas and deep geothermal energy offer significant advantages, including stability, cleanliness, environmental friendliness, and high utilization rates. However, both deep shale reservoirs and hot dry rock geothermal resource reservoirs suffer from extremely low porosity and permeability, necessitating reservoir modification for resource extraction.

[0003] These reservoirs typically exhibit "four highs": high temperature, high hardness, high stress, and high density. This makes it difficult for the hydraulic fracturing reservoir stimulation technology used in the traditional oil and gas industry to extract commercially viable oil, gas, or thermal energy resources. my country's research in this field began relatively late, and there is a lack of mature experience both domestically and internationally to draw upon, resulting in slow progress in both basic scientific research and industrial development.

[0004] Traditional hydraulic fracturing technology faces numerous challenges. First, the rock is very strong, resulting in high fracture pressures. Second, the formation of artificial fracture networks is difficult, resulting in a single fracture pattern and difficulty controlling its extension direction, which in turn leads to low recovery rates. Third, it consumes a lot of water.

[0005] To address these challenges, researchers have recently proposed the concept of "flexible reservoir creation," building on hydraulic fracturing. Specifically, by changing the pumping method, such as shear fracturing or cyclic / fatigue fracturing, or by replacing the fracturing medium, such as cryogenic liquid nitrogen (LN2) or supercritical carbon dioxide (SCCO2), these approaches can reduce fracture pressure, increase the size and complexity of the fracture network, and improve the success rate of communication between injection and production wells.

[0006] Therefore, in-depth research on the fracture propagation characteristics of shale or hot dry rock under the synergistic action of SCCO2 and LN2 in the in-situ reservoir environment is of vital importance for optimizing hydraulic fracturing production enhancement technology by using SCCO2 / LN2 as a fracturing medium. In the existing technology, there is a lack of effective research methods and equipment for supercritical carbon dioxide circulation and liquid nitrogen cold shock synergistic fracturing of deep shale and hot dry rock reservoirs, and basic scientific research and industrial development are slow. Therefore, there is an urgent need for a wellbore device and method that can be used for SCCO2 circulation and LN2 cold shock synergistic fracturing to reproduce the expansion and evolution process of artificial fractures, so as to facilitate the study of the fracture propagation mechanism in rocks under the synergistic action of SCCO2 circulation and LN2 cold shock synergistic fracturing. Summary of the Invention

[0007] The purpose of the present invention is to overcome the above-mentioned technical deficiencies and propose a device and method for studying crack expansion by synergistic fracturing of supercritical carbon dioxide and liquid nitrogen, so as to solve the technical problems in the prior art of the lack of effective research means and devices for supercritical carbon dioxide circulation and liquid nitrogen cold shock synergistic fracturing of deep shale and hot dry rock reservoirs, and the slow development of basic scientific research and industry.

[0008] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0009] The present invention provides a supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device, comprising:

[0010] A temperature and pressure simulation mechanism, comprising an insulation box, a temperature control assembly, and a triaxial pressure assembly. The insulation box is used to place rock blocks with drill holes formed therein. The temperature control assembly is used to control the temperature within the insulation box. The triaxial pressure assembly is used to apply triaxial pressure to the rock blocks within the insulation box.

[0011] A simulated wellbore, wherein the simulated wellbore plug is disposed in the borehole, and the simulated wellbore has a first passage and a second passage communicating with the borehole;

[0012] a supercritical carbon dioxide circulation mechanism, the supercritical carbon dioxide circulation mechanism being in communication with both the first channel and the second channel, and being configured to introduce supercritical carbon dioxide into the borehole via the first channel and to recover the supercritical carbon dioxide introduced into the borehole via the second channel;

[0013] a liquid nitrogen cold shock mechanism, the liquid nitrogen cold shock mechanism being in communication with the first channel and being used to introduce liquid nitrogen into the borehole via the first channel; and

[0014] A monitoring mechanism is arranged at the periphery of the rock block and is used to monitor the expansion of cracks in the rock block.

[0015] In some embodiments, the temperature control component includes a plurality of heaters and temperature detection components, and the heaters and the temperature detection components are both disposed in the thermal insulation box.

[0016] In some embodiments, the triaxial pressure assembly includes two lateral pressure members and a lower pressure member, the two lateral pressure members are respectively used to apply a first preset pressure to two mutually perpendicular lateral directions of the rock block, and the lower pressure member is used to apply a second preset pressure to the top surface of the rock block.

[0017] In some embodiments, the lateral pressure member includes two loading plates and two lateral driving members, the two loading plates are respectively attached to the opposite side walls of the rock block, and the two lateral driving members are respectively used to drive the two loading plates closer to each other to clamp the rock block. The lateral driving member includes a lateral power rod and a lateral pressure sensor, one end of the lateral power rod is fixed to the inner wall of the insulation box, and the other end of the lateral power rod is connected to the fixed end of the lateral pressure sensor, and the detection end of the lateral pressure sensor is in contact with the corresponding loading plate.

[0018] In some embodiments, the downward pressure member includes a pressure plate and a downward pressure driving member, the pressure plate is attached to the top surface of the rock block, the downward pressure driving member is used to drive the pressure plate downward to press the rock block, the downward pressure driving member includes a downward pressure power rod and a vertical pressure sensor, one end of the downward pressure power rod is fixed to the top surface of the insulation box, the other end of the downward pressure power rod is connected to the fixed end of the vertical pressure sensor, and the detection end of the vertical pressure sensor is in contact with the corresponding pressure plate.

[0019] In some embodiments, the thermal insulation box includes a box body and a cover plate. The upper end of the box body is opened. The cover plate is used to cover the upper end opening of the box body and is detachably fixed to the box body. One end of the downward power rod is fixed to the cover plate.

[0020] In some embodiments, the simulated wellbore includes a center rod, a first cone sleeve, a first tapered cylinder, a first outer sleeve, a second cone sleeve, a second outer sleeve and a locking nut. The first channel and the second channel are formed in the center rod, an external thread is formed on the outer wall of the center rod, a plug is formed at the lower end of the center rod, the first cone sleeve is slidably mounted on the center rod, one end of the first cone sleeve is in contact with the plug, the first cone cylinder is sleeved on the center rod and in contact with the first cone sleeve, the first cone cylinder is in contact with the first cone sleeve, and the second cone The contact surface of a tapered sleeve is conical, the first outer sleeve is sleeved on the center rod, and its lower end is fixedly connected to the first tapered cylinder, the second tapered cylinder is sleeved on the center rod, and the lower end of the second tapered cylinder is fixedly connected to the first outer sleeve, the second tapered sleeve is slidably sleeved on the center rod, the contact surface between the second tapered cylinder and the second tapered sleeve is conical, the second outer sleeve is sleeved on the center rod, and its lower end is fixedly connected to the second tapered sleeve, the locking nut is threadedly sleeved on the center rod, and its lower end abuts against the second outer sleeve.

[0021] In some embodiments, a third channel connected to the borehole is further formed in the center rod; the supercritical carbon dioxide circulation mechanism includes a low-temperature water bath, a carbon dioxide storage tank, a carbon dioxide pumping device, a first injection pipe, a heating element, a first injection valve, a circulation pipe, a one-way valve, a circulation valve, a discharge pipe and a discharge valve, the carbon dioxide storage tank is arranged in the low-temperature water bath, the inlet of the carbon dioxide pumping device is connected to the carbon dioxide storage tank, the outlet of the carbon dioxide pumping device is connected to one end of the first injection pipe, the other end of the first injection pipe is connected to the first channel, the heating element is used to heat the carbon dioxide in the first injection pipe, the first injection valve is arranged on the first injection pipe, one end of the circulation pipe is connected to the second channel, the other end of the circulation pipe is connected to the carbon dioxide storage tank, the one-way valve and the circulation valve are both arranged on the circulation pipe, one end of the discharge pipe is connected to the third channel, and the discharge valve is arranged on the discharge pipe.

[0022] In some embodiments, the liquid nitrogen cold shock mechanism includes a liquid nitrogen storage tank, a liquid nitrogen pumping device and a second injection pipe. The liquid nitrogen storage tank is used to store liquid nitrogen. The inlet of the liquid nitrogen pumping device is connected to the liquid nitrogen storage tank. The outlet of the liquid nitrogen pumping device is connected to one end of the second injection pipe. The other end of the second injection pipe is connected to the first channel. A second injection valve is provided on the second injection pipe.

[0023] The present invention also provides a method for studying crack propagation by synergistic fracturing of supercritical carbon dioxide and liquid nitrogen, which is applicable to the above-mentioned apparatus for studying crack propagation by synergistic fracturing of supercritical carbon dioxide and liquid nitrogen, and includes the following method:

[0024] S1. Place the rock block to be studied in an insulated box with pre-drilled holes in it. The temperature inside the box is precisely controlled by a temperature control component to achieve the temperature conditions required to simulate the actual formation environment. Simultaneously, a triaxial pressure component is used to apply triaxial pressure to the rock block in the insulated box to simulate the actual ground stress state of the rock block in the formation.

[0025] S2. Installing the simulated wellbore plug in the drilled hole of the rock block, ensuring that the first channel and the second channel of the simulated wellbore are connected to the drilled hole, providing a channel for subsequent injection and recovery of supercritical carbon dioxide and liquid nitrogen;

[0026] S3. The supercritical carbon dioxide circulation mechanism starts to work, injecting supercritical carbon dioxide into the borehole of the rock block through the first channel. After a certain amount of supercritical carbon dioxide is injected, the supercritical carbon dioxide in the borehole is discharged from the second channel and re-enters the supercritical carbon dioxide circulation mechanism. The supercritical carbon dioxide circulation mechanism processes the recovered supercritical carbon dioxide to restore it to a state suitable for re-injection. Thereafter, the supercritical carbon dioxide circulation mechanism again injects the treated supercritical carbon dioxide into the borehole of the rock block through the first channel. This cycle is repeated. During multiple cycles, the supercritical carbon dioxide continues to act on the rock block, continuously promoting the development and expansion of cracks within the rock block.

[0027] S4. After the supercritical carbon dioxide circulates for a preset time, the supercritical carbon dioxide in the borehole is released. Then, the liquid nitrogen cold shock mechanism begins to operate, rapidly introducing liquid nitrogen into the borehole through the first channel. When the liquid nitrogen contacts the rock, the temperature of the rock surface and interior drops sharply. Due to the uneven thermal expansion and contraction of different parts of the rock, huge temperature stress is generated within the rock. This temperature stress further promotes the expansion of cracks within the rock. Especially since the supercritical carbon dioxide has already had a certain effect on the rock, the liquid nitrogen cold shock may cause the cracks to expand more rapidly and complexly.

[0028] S5. During the entire supercritical carbon dioxide circulation and liquid nitrogen cold shock process, the monitoring mechanism continues to work. The monitoring mechanism is arranged on the periphery of the rock block to monitor the expansion of cracks in the rock block in real time. Acoustic emission monitoring can capture the elastic wave signals generated when the cracks inside the rock block expand. By analyzing these signals, the location, time and expansion speed of the crack expansion can be determined.

[0029] Compared with the existing technology, the beneficial effects of the supercritical carbon dioxide and liquid nitrogen collaborative fracturing crack expansion research device and method provided by the present invention are: the temperature and pressure simulation mechanism can accurately simulate the temperature and pressure conditions in the formation, so that the experimental process is highly close to the actual formation conditions, greatly improving the reliability and accuracy of the research results; the supercritical carbon dioxide circulation mechanism realizes the cyclic injection of supercritical carbon dioxide, allowing supercritical carbon dioxide to act on the rock block continuously and multiple times. The supercritical carbon dioxide circulation mechanism and the liquid nitrogen cold shock mechanism work together, and the cyclic injection of supercritical carbon dioxide pre-modifies the internal structure of the rock block and increases the porosity and permeability of the rock block. The monitoring mechanism can monitor the expansion of cracks in the rock block in real time and accurately, providing researchers with detailed data and information. Researchers can deeply analyze the mechanism and influencing factors of crack expansion based on the monitoring results. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1This is a schematic structural diagram of a device for studying supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack extension provided by one embodiment of the present invention;

[0031] Figure 2 yes Figure 1 Schematic diagram of the structure of the temperature and pressure simulation mechanism;

[0032] Figure 3 yes Figure 2 A partial enlarged view of the middle area A;

[0033] Figure 4 yes Figure 2 A partial enlarged view of the middle area B;

[0034] Figure 5 yes Figure 2 A partial enlarged view of the middle area C;

[0035] Figure 6 yes Figure 2 A partial enlarged view of the middle area D;

[0036] Explanation of the reference numerals: 1-temperature and pressure simulation mechanism, 11-insulation box, 111-box, 112-cover plate, 12-temperature control assembly, 121-heater, 122-temperature detection component, 13-triaxial pressure assembly, 131-lateral pressure component, 1311-loading plate, 1312-lateral driving component, 13121-lateral power rod, 13122-lateral pressure sensor, 132-downward pressure component, 1321-pressure plate, 1322-downward pressure driving component, 13221-downward pressure power rod, 13222-vertical pressure sensor, 2-simulated wellbore, 21-center rod, 211-first channel, 212-second channel, 213-plug, 214-third channel, 22-first cone sleeve, 23- First tapered cylinder, 24-first outer sleeve, 25-second tapered cylinder, 26-second tapered sleeve, 27-second outer sleeve, 28-locking nut, 3-supercritical carbon dioxide circulation mechanism, 31-low-temperature water bath, 32-carbon dioxide storage tank, 33-carbon dioxide pumping device, 34-first injection pipe, 341-first injection valve, 35-heating element, 36-circulation pipe, 361-one-way valve, 362-circulation valve, 37-discharge pipe, 371-discharge valve, 4-liquid nitrogen cold shock mechanism, 41-liquid nitrogen storage tank, 42-liquid nitrogen pumping device, 43-second injection pipe, 431-second injection valve, 5-monitoring mechanism, 51-acoustic emission probe, 52-acoustic emission monitor, 6-rock block, 61-drilling hole. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0038] In order to solve the technical problems in the existing technology of lacking effective research methods and equipment in supercritical carbon dioxide circulation and liquid nitrogen cold shock synergistic fracturing for deep shale and hot dry rock reservoirs, and slow basic scientific research and industrial development, the present invention provides a supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device and method, which can reproduce the expansion evolution process of artificial fractures, so as to facilitate the study of the fracture expansion mechanism in rocks under the action of SCCO2 circulation and LN2 cold shock synergistic fracturing.

[0039] See also Figures 1-6 , Figure 1 This is a structural schematic diagram of a supercritical carbon dioxide and liquid nitrogen collaborative fracturing crack extension research device in one embodiment of the present invention. The supercritical carbon dioxide and liquid nitrogen collaborative fracturing crack extension research device includes a temperature and pressure simulation mechanism 1, a simulated wellbore 2, a supercritical carbon dioxide circulation mechanism 3, a liquid nitrogen cold shock mechanism 4 and a monitoring mechanism 5.

[0040] The temperature and pressure simulation mechanism 1 includes an insulation box 11, a temperature control component 12 and a triaxial pressure component 13. The insulation box 11 is used to place a rock block 6, and a drill hole 61 is opened on the rock block 6. The temperature control component 12 is used to control the temperature in the insulation box 11, and the triaxial pressure component 13 is used to apply triaxial pressure to the rock block 6 in the insulation box 11.

[0041] The simulated wellbore 2 is plugged in the borehole 61 . The simulated wellbore 2 has a first channel 211 and a second channel 212 communicating with the borehole 61 .

[0042] The supercritical carbon dioxide circulation mechanism 3 is used to communicate with both the first channel 211 and the second channel 212 , and is used to introduce supercritical carbon dioxide into the borehole 61 through the first channel 211 , and to recover the supercritical carbon dioxide introduced into the borehole 61 through the second channel 212 .

[0043] The liquid nitrogen cold shock mechanism 4 is in communication with the first channel 211 and is used to introduce liquid nitrogen into the borehole 61 via the first channel 211 .

[0044] The monitoring mechanism 5 is arranged on the periphery of the rock block 6 and is used to monitor the expansion of cracks in the rock block 6 .

[0045] In one embodiment, see Figure 2 and Figure 5The temperature control assembly 12 includes several heaters 121 and temperature sensors 122, both of which are located within the insulation box 11. The heaters 121 and temperature sensors 122 work together to precisely control the temperature within the insulation box 11, achieving the desired temperature conditions required to simulate actual formation environments. This allows for a close reproduction of the temperature state of the rock block 6 in the actual formation, making the experimental process more realistic and improving the reliability and accuracy of the research results. For example, for research in deep, high-temperature formations, the temperature within the insulation box 11 can be adjusted to a corresponding high temperature value to accurately study the fracture propagation characteristics of the rock block 6 at that temperature.

[0046] In one embodiment, see Figure 2 、 Figure 5 and Figure 6 The triaxial pressure assembly 13 comprises two lateral pressure members 131 and a lower pressure member 132. The two lateral pressure members 131 are respectively used to apply a first preset pressure to two mutually perpendicular lateral directions of the rock block 6, while the lower pressure member 132 is used to apply a second preset pressure to the top surface of the rock block 6. The two lateral pressure members 131 apply the first preset pressure to the two mutually perpendicular lateral directions of the rock block 6, while the lower pressure member 132 applies the second preset pressure to the top surface of the rock block 6. This simulates the actual in-situ stress state experienced by the rock block 6 in the formation. This triaxial pressure application method more accurately reproduces the stress conditions experienced by the rock block 6 underground, making the experimental results more valuable. Furthermore, by adjusting the magnitude of the lateral and vertical pressures, the propagation of cracks in the rock block 6 under different stress conditions can be studied, providing theoretical support for optimizing fracturing process parameters and improving fracturing effectiveness. For example, in high-stress formations, understanding the crack propagation patterns of the rock block 6 under specific stress combinations can help develop more effective fracturing strategies.

[0047] In one embodiment, see Figure 2 and Figure 5The lateral pressure member 131 includes two loading plates 1311 and two lateral driving members 1312. The two loading plates 1311 are respectively attached to the opposite side walls of the rock block 6. The two lateral driving members 1312 are respectively used to drive the two loading plates 1311 toward each other to clamp the rock block 6. The lateral driving member 1312 includes a lateral power rod 13121 and a lateral pressure sensor 13122. One end of the lateral power rod 13121 is fixed to the inner side wall of the thermal insulation box 11, and the other end of the lateral power rod 13121 is connected to the fixed end of the lateral pressure sensor 13122. The detection end of the lateral pressure sensor 13122 abuts the corresponding loading plate 1311. In this embodiment, the lateral power rod 13121 can be a hydraulic cylinder or a pneumatic cylinder.

[0048] In one embodiment, see Figure 2 and Figure 6 The pressing member 132 includes a pressing plate 1321 and a pressing drive member 1322. The pressing plate 1321 is attached to the top surface of the rock block 6. The pressing drive member 1322 is used to drive the pressing plate 1321 downward to compress the rock block 6. The pressing drive member 1322 includes a pressing power rod 13221 and a vertical pressure sensor 13222. One end of the pressing power rod 13221 is fixed to the top surface of the heat preservation box 11, and the other end of the pressing power rod 13221 is connected to the fixed end of the vertical pressure sensor 13222. The detection end of the vertical pressure sensor 13222 abuts against the corresponding pressing plate 1321. In this embodiment, the pressing power rod 13221 can be a hydraulic cylinder or a pneumatic cylinder.

[0049] In one embodiment, see Figure 2 and Figure 6 The insulated box 11 includes a box body 111 and a cover plate 112. The upper end of the box body 111 is open. The cover plate 112 is used to cover the upper end opening of the box body 111 and is detachably fixedly connected to the box body 111. One end of the downward pressure rod 13221 is fixed to the cover plate 112. The design of the box body 111 and cover plate 112 of the insulated box 11 effectively maintains stable internal temperature and pressure, reducing interference from external environmental factors on the experiment. The cover plate 112 is detachably fixedly connected to the box body 111, facilitating the placement and removal of the rock block 6. It also ensures the sealing of the insulated box 11, which is conducive to maintaining a stable experimental environment.

[0050] In one embodiment, see Figure 2 and Figure 3The simulated wellbore 2 includes a center rod 21, a first tapered sleeve 22, a first tapered cylinder 23, a first outer sleeve 24, a second tapered cylinder 25, a second tapered sleeve 26, a second outer sleeve 27 and a locking nut 28. The center rod 21 is formed with the first channel 211 and the second channel 212. The outer wall of the center rod 21 is formed with an external thread. The lower end of the center rod 21 is formed with a plug 213. The first tapered sleeve 22 is slidably mounted on the center rod 21. One end of the first tapered sleeve 22 abuts against the plug 213. The first tapered cylinder 23 is mounted on the center rod 21 and contacts the first tapered sleeve 22. The first tapered cylinder 23 abuts against the plug 213. The contact surface of the first tapered sleeve 22 is conical, the first outer sleeve 24 is sleeved on the center rod 21, and the lower end of the first tapered cylinder 23 is fixedly connected to the second tapered cylinder 25, the second tapered cylinder 25 is sleeved on the center rod 21, and the lower end of the second tapered cylinder 25 is fixedly connected to the first outer sleeve 24, the second tapered sleeve 26 is slidably sleeved on the center rod 21, and the contact surface between the second tapered cylinder 25 and the second tapered sleeve 26 is conical, the second outer sleeve 27 is sleeved on the center rod 21, and the lower end of the second tapered sleeve 26 is fixedly connected, the locking nut 28 is threadedly sleeved on the center rod 21, and the lower end of the locking nut abuts against the second outer sleeve 27.

[0051] At present, in the vast majority of fracturing simulation tests, the wellbore sealing process for the specimens adopts the method of epoxy resin glue or epoxy resin glue plus sealing ring. This sealing method has indeed achieved good results for normal temperature tests, but the sealing effect is greatly reduced for hydraulic fracturing, ScCO2 fracturing, and LN2 fracturing tests of high-temperature rock masses. The most important reason is that the properties of the sealant will be weakened in special fluid media and extreme environments. Even if a sealing ring is added to assist in sealing, the sealing ring will lose its effect, and the fracturing medium will easily leak along the well wall, resulting in test failure. In this embodiment, when it is necessary to use a simulated wellbore 2 to conduct a study on the expansion of supercritical carbon dioxide and liquid nitrogen synergistic fracturing cracks, follow the steps below:

[0052] First, the simulated wellbore 2 is lowered into the borehole 61 (blind hole) in the rock block 6. The simulated wellbore 2 will cling tightly to the wellbore wall. Once the simulated wellbore 2 reaches a certain depth, the locking nut 28 is rotated, pressing downward against the second outer sleeve 27, the second tapered sleeve 26, the second tapered cylinder 25, the first outer sleeve 24, the first tapered cylinder 23, and the first tapered sleeve 22. During this process, the second tapered cylinder 25 and the first tapered cylinder 23 deform, further pressing against the wellbore wall. Next, the simulated wellbore 2 is hammered to the desired depth using a small hammer. During this time, the locking nut 28 is continuously torqued to reinforce the seal, thus forming a primary seal.

[0053] After the simulated wellbore 2 is placed, the temperature control assembly 12 of the temperature-pressure simulation mechanism 1 heats the rock 6 within the insulation box 11. Because the second tapered cylinder 25 and the first tapered cylinder 23 (made of copper) have large thermal expansion coefficients, they expand when the target temperature is reached, tightening against the rock and forming a second seal.

[0054] Subsequently, the supercritical carbon dioxide circulation mechanism 3 is activated to introduce supercritical carbon dioxide (SCCO2) into the borehole 61 via the first channel 211, or the liquid nitrogen cold shock mechanism 4 is activated to introduce liquid nitrogen (LN2) into the borehole 61 via the first channel 211 to perform a fracturing operation. During the fluid injection process, the fluid pressure pushes against the first cone sleeve 22, causing it to move upward. However, the restraining action of the first tapered cylinder 23 causes the entire first tapered cylinder 23 to fit more closely against the wellbore wall, forming the final seal. At this point, the rock block 6 sample is ready for the next step of the supercritical carbon dioxide and liquid nitrogen synergistic fracturing experiment to study crack propagation.

[0055] In this embodiment, the first channel 211, the second channel 212 and the third channel 214 formed in the central rod 21 provide reliable channels for the injection, recovery and discharge of supercritical carbon dioxide and liquid nitrogen, ensuring the normal circulation of fluids during the experiment.

[0056] In one embodiment, see Figure 2 and Figure 3 , a third channel 214 is further formed in the center rod 21 and communicated with the bore 61; the supercritical carbon dioxide circulation mechanism 3 includes a low-temperature water bath 31, a carbon dioxide storage tank 32, a carbon dioxide pumping device 33, a first injection pipe 34, a heating element 35, a first injection valve 341, a circulation pipe 36, a one-way valve 361, a circulation valve 362, a discharge pipe 37 and a discharge valve 371, the carbon dioxide storage tank 32 is arranged in the low-temperature water bath 31, the inlet of the carbon dioxide pumping device 33 is communicated with the carbon dioxide storage tank 32, and the outlet of the carbon dioxide pumping device 33 is connected to the first injection pipe 3 4, the other end of the first injection pipe 34 is connected to the first channel 211, the heating element 35 is used to heat the carbon dioxide in the first injection pipe 34, the first injection valve 341 is provided on the first injection pipe 34, one end of the circulation pipe 36 is connected to the second channel 212, and the other end of the circulation pipe 36 is connected to the carbon dioxide storage tank 32, the one-way valve 361 and the circulation valve 362 are both provided on the circulation pipe 36, one end of the discharge pipe 37 is connected to the third channel 214, and the discharge valve 371 is provided on the discharge pipe 37.

[0057] In this embodiment, during use, the first injection valve 341 and circulation valve 362 are opened, and the discharge valve 371 is closed. The carbon dioxide pumping device 33 is activated, and carbon dioxide is pumped from the carbon dioxide storage tank 32 and enters the first injection pipe 34 through the carbon dioxide pumping device 33. As the carbon dioxide flows through the first injection pipe 34, the heating element 35 heats it, causing it to reach a supercritical state. The supercritical carbon dioxide enters the first channel 211 of the simulated wellbore 2 through the first injection pipe 34 and then flows into the borehole 61 of the rock block 6. As the supercritical carbon dioxide is continuously injected, the pressure in the borehole 61 gradually increases. The supercritical carbon dioxide diffuses and permeates within the rock block 6, potentially causing microcracks within the rock block 6 to expand. When the supercritical carbon dioxide in the borehole 61 reaches a certain level, it enters the circulation pipe 36 through the second channel 212. The one-way valve 361 on the circulation pipe 36 ensures that the supercritical carbon dioxide can only flow in one direction, preventing backflow. The supercritical carbon dioxide is returned to the carbon dioxide storage tank 32 through the circulation pipe 36, completing one cycle. The carbon dioxide pumping device 33 operates continuously, circulating the supercritical carbon dioxide into the borehole 61. During multiple cycles, the supercritical carbon dioxide continuously acts on the rock block 6, promoting the development and expansion of internal cracks in the rock block 6. When the supercritical carbon dioxide circulation experiment reaches a predetermined time or number of cycles, the carbon dioxide pumping device 33, the first injection valve 341, and the circulation valve 362 are closed, halting the supercritical carbon dioxide circulation. The discharge valve 371 is opened, and the carbon dioxide gas in the borehole 61 enters the discharge pipe 37 through the third channel 214 and is then discharged to the external environment.

[0058] In one embodiment, see Figure 1-Figure 3 The liquid nitrogen cold shock mechanism 4 includes a liquid nitrogen storage tank 41, a liquid nitrogen pumping device 42 and a second injection pipe 43. The liquid nitrogen storage tank 41 is used to store liquid nitrogen. The inlet of the liquid nitrogen pumping device 42 is connected to the liquid nitrogen storage tank 41, and the outlet of the liquid nitrogen pumping device 42 is connected to one end of the second injection pipe 43. The other end of the second injection pipe 43 is connected to the first channel 211. The second injection pipe 43 is provided with a second injection valve 431.

[0059] In this embodiment, when in use, the second injection valve 431 is slowly opened to put the second injection pipe 43 in a conducting state.

[0060] The liquid nitrogen pumping device 42 is activated and begins operation, pumping liquid nitrogen from the liquid nitrogen storage tank 41. The pumped liquid nitrogen enters the second injection pipe 43 through the outlet of the liquid nitrogen pumping device 42. As the liquid nitrogen flows through the second injection pipe 43, the temperature and pressure of the pipe must be carefully monitored to prevent abnormal pressure increases due to vaporization of the liquid nitrogen. The liquid nitrogen flows along the second injection pipe 43 and is ultimately rapidly injected into the borehole 61 of the rock block 6 through the first channel 211. The injection rate can be controlled by adjusting the power of the liquid nitrogen pumping device 42 or the opening of the second injection valve 431 to achieve the optimal cold shock effect. During the liquid nitrogen cold shock process, the monitoring device 5 continues to operate. The acoustic emission probe 51 captures the elastic wave signals generated by the expansion of cracks within the rock block 6 in real time. The acoustic emission monitor 52 receives and analyzes these signals to determine information such as the location, timing, and rate of crack expansion.

[0061] In one embodiment, see Figure 1 、 Figure 2 and Figure 5 The monitoring mechanism 5 comprises several acoustic emission probes 51 and acoustic emission monitors 52. The acoustic emission probes 51 are arranged around the periphery of the rock block 6, and the acoustic emission monitors 52 are electrically connected to each of the acoustic emission probes 51. In this embodiment, the core operating principle of the monitoring mechanism 5 is based on acoustic emission technology. During the co-fracturing process using supercritical carbon dioxide and liquid nitrogen, elastic waves are generated within the rock block 6 due to crack expansion. The acoustic emission probes 51 sense these elastic waves and convert them into electrical signals. The acoustic emission monitors 52 receive and process these electrical signals to obtain information about the crack expansion within the rock block 6.

[0062] The present invention also provides a method for studying crack propagation by synergistic fracturing of supercritical carbon dioxide and liquid nitrogen, which is applicable to the above-mentioned apparatus for studying crack propagation by synergistic fracturing of supercritical carbon dioxide and liquid nitrogen, and includes the following method:

[0063] S1. Place the rock block 6 to be studied in an insulated box 11 with a pre-drilled hole 61 in the rock block 6. The temperature in the insulated box 11 is precisely controlled by the temperature control assembly 12 to achieve the temperature conditions required to simulate the actual formation environment. Simultaneously, a triaxial pressure assembly 13 applies triaxial pressure to the rock block 6 in the insulated box 11 to simulate the actual in-situ stress state of the rock block 6 in the formation.

[0064] S2. Insert the simulated wellbore 2 into the borehole 61 of the rock block 6, ensuring that the first channel 211 and the second channel 212 of the simulated wellbore 2 are connected to the borehole 61, providing a channel for the subsequent injection and recovery of supercritical carbon dioxide and liquid nitrogen;

[0065] S3, the supercritical carbon dioxide circulation mechanism 3 starts working, injecting supercritical carbon dioxide into the borehole 61 of the rock block 6 through the first channel 211. After a certain amount of supercritical carbon dioxide is injected, the supercritical carbon dioxide in the borehole 61 is discharged from the second channel 212 and re-enters the supercritical carbon dioxide circulation mechanism 3. The supercritical carbon dioxide circulation mechanism 3 processes the recovered supercritical carbon dioxide (such as adjusting parameters such as temperature and pressure) to restore it to a state suitable for re-injection. Afterwards, the supercritical carbon dioxide circulation mechanism 3 again injects the processed supercritical carbon dioxide from the first channel 211 into the borehole 61 of the rock block 6. This cycle is repeated. During multiple cycles, the supercritical carbon dioxide continues to act on the rock block 6, continuously promoting the development and expansion of cracks inside the rock block 6.

[0066] S4. After the supercritical carbon dioxide circulates for a preset time, the supercritical carbon dioxide in the borehole 61 is released. Then, the liquid nitrogen cold shock mechanism 4 begins to operate, rapidly introducing liquid nitrogen into the borehole 61 through the first channel 211. When the liquid nitrogen contacts the rock block 6, the temperature on the surface and inside of the rock block 6 drops sharply. Due to the uneven thermal expansion and contraction of various parts of the rock block 6, huge temperature stress is generated inside the rock block 6. This temperature stress further promotes the expansion of cracks in the rock block 6. In particular, since the supercritical carbon dioxide has already exerted a certain effect on the rock block 6, the liquid nitrogen cold shock may cause the cracks to expand more rapidly and complexly.

[0067] S5. During the entire supercritical carbon dioxide circulation and liquid nitrogen cold shock process, the monitoring mechanism 5 continues to work. The monitoring mechanism 5 is arranged on the periphery of the rock block 6 to monitor the crack expansion in the rock block 6 in real time. The acoustic emission monitoring can capture the elastic wave signals generated when the cracks inside the rock block 6 expand. By analyzing these signals, the position, time and expansion speed of the crack expansion can be determined.

[0068] The beneficial effects of the technical solution provided by the present invention include:

[0069] (1) Simulating the actual formation environment: The temperature and pressure simulation mechanism 1 can accurately simulate the temperature and pressure conditions in the formation, making the experimental process highly close to the actual formation conditions, greatly improving the reliability and accuracy of the research results. By simulating the temperature and pressure conditions of different formations, it is possible to conduct in-depth research on the expansion patterns of rock block 6 cracks in various complex geological environments.

[0070] (2) Supercritical carbon dioxide circulation: The supercritical carbon dioxide circulation mechanism 3 realizes the cyclic injection of supercritical carbon dioxide, allowing the supercritical carbon dioxide to continuously and repeatedly act on the rock block 6. This circulation process can more fully exert the characteristics of supercritical carbon dioxide, continuously promote the expansion and connection of micro-cracks inside the rock block 6, more realistically simulate the effect of fluid on rock during actual fracturing, and provide more accurate data for studying the fracture expansion mechanism.

[0071] (3) Synergistic effect of supercritical carbon dioxide and liquid nitrogen: The supercritical carbon dioxide circulation mechanism 3 and the liquid nitrogen cold shock mechanism 4 work together. The cyclic injection of supercritical carbon dioxide pre-modifies the internal structure of the rock block 6, increasing the porosity and permeability of the rock block 6. The liquid nitrogen cold shock utilizes temperature stress to further promote crack expansion. The two work together to more effectively simulate the generation and expansion of cracks in the actual fracturing process, providing a theoretical basis for improving the fracturing effect.

[0072] (4) Real-time monitoring of crack expansion: The monitoring mechanism 5 can accurately monitor the expansion of cracks in the rock block 6 in real time, providing researchers with detailed data and information. Based on the monitoring results, researchers can conduct in-depth analysis of the mechanism and influencing factors of crack expansion, thereby optimizing fracturing process parameters and improving fracturing efficiency and oil and gas recovery.

[0073] (5) The synergistic sealing effect of the second outer sleeve 27, the second cone sleeve 26, the second tapered cylinder 25, the first outer sleeve 24, the first tapered cylinder 23 and the first cone sleeve 22 can be applied to the research experiment of the synergistic fracturing crack expansion of supercritical carbon dioxide and liquid nitrogen, thereby improving the sealing effect of the device.

[0074] The specific embodiments of the present invention described above do not limit the scope of protection of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device, characterized in that: include: A temperature and pressure simulation mechanism, comprising an insulation box, a temperature control assembly, and a triaxial pressure assembly. The insulation box is used to place rock blocks with drill holes formed therein. The temperature control assembly is used to control the temperature within the insulation box. The triaxial pressure assembly is used to apply triaxial pressure to the rock blocks within the insulation box. A simulated wellbore, wherein the simulated wellbore plug is arranged in the drill hole, and the simulated wellbore has a first channel and a second channel connected to the drill hole, and the simulated wellbore includes a center rod, a first cone sleeve, a first tapered cylinder, a first outer sleeve, a second cone cylinder, a second cone sleeve, a second outer sleeve and a locking nut, the first channel and the second channel are formed in the center rod, an external thread is formed on the outer side wall of the center rod, a plug is formed at the lower end of the center rod, the first cone sleeve is slidably sleeved on the center rod, one end of the first cone sleeve is in contact with the plug, the first cone sleeve is sleeved on the center rod, and is in contact with the first cone A tapered sleeve is in contact, the contact surface between the first tapered cylinder and the first tapered sleeve is tapered, the first outer sleeve is sleeved on the center rod, and the lower end of the first tapered cylinder is fixedly connected to the first outer sleeve, the second tapered cylinder is sleeved on the center rod, and the lower end of the second tapered cylinder is fixedly connected to the first outer sleeve, the second tapered sleeve is slidably sleeved on the center rod, the contact surface between the second tapered cylinder and the second tapered sleeve is tapered, the second outer sleeve is sleeved on the center rod, and the lower end of the second tapered sleeve is fixedly connected to the second tapered sleeve, the locking nut is threadedly sleeved on the center rod, and the lower end of the locking nut abuts against the second outer sleeve; a supercritical carbon dioxide circulation mechanism, the supercritical carbon dioxide circulation mechanism being in communication with both the first channel and the second channel, and being configured to introduce supercritical carbon dioxide into the borehole via the first channel and to recover the supercritical carbon dioxide introduced into the borehole via the second channel; a liquid nitrogen cold shock mechanism, the liquid nitrogen cold shock mechanism being in communication with the first channel and being used to introduce liquid nitrogen into the borehole via the first channel; as well as, A monitoring mechanism is arranged at the periphery of the rock block and is used to monitor the expansion of cracks in the rock block.

2. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device according to claim 1, characterized in that: The temperature control component includes a plurality of heaters and temperature detection components, and the heaters and the temperature detection components are both arranged in the thermal insulation box.

3. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device according to claim 1, characterized in that: The triaxial pressure assembly includes two lateral pressure members and a lower pressure member. The two lateral pressure members are respectively used to apply a first preset pressure to two mutually perpendicular lateral directions of the rock block, and the lower pressure member is used to apply a second preset pressure to the top surface of the rock block.

4. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device according to claim 3, characterized in that: The lateral pressure member includes two loading plates and two lateral driving members. The two loading plates are respectively attached to the opposite side walls of the rock block. The two lateral driving members are respectively used to drive the two loading plates closer to each other to clamp the rock block. The lateral driving member includes a lateral power rod and a lateral pressure sensor. One end of the lateral power rod is fixed to the inner wall of the insulation box, and the other end of the lateral power rod is connected to the fixed end of the lateral pressure sensor. The detection end of the lateral pressure sensor is in contact with the corresponding loading plate.

5. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device according to claim 3, characterized in that: The downward pressing member includes a pressing plate and a downward pressing driving member, the pressing plate is attached to the top surface of the rock block, the downward pressing driving member is used to drive the pressing plate to move downward to press the rock block, the downward pressing driving member includes a downward pressing power rod and a vertical pressure sensor, one end of the downward pressing power rod is fixed to the top surface of the insulation box, the other end of the downward pressing power rod is connected to the fixed end of the vertical pressure sensor, and the detection end of the vertical pressure sensor is in contact with the corresponding pressing plate.

6. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device according to claim 5, characterized in that: The thermal insulation box includes a box body and a cover plate. The upper end of the box body is opened. The cover plate is used to cover the upper end opening of the box body and is detachably fixedly connected to the box body. One end of the downward pressing power rod is fixed to the cover plate.

7. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device according to claim 1, characterized in that: A third channel connected to the borehole is also formed in the center rod; the supercritical carbon dioxide circulation mechanism includes a low-temperature water bath, a carbon dioxide storage tank, a carbon dioxide pumping device, a first injection pipe, a heating element, a first injection valve, a circulation pipe, a one-way valve, a circulation valve, a discharge pipe and a discharge valve. The carbon dioxide storage tank is arranged in the low-temperature water bath, the inlet of the carbon dioxide pumping device is connected to the carbon dioxide storage tank, the outlet of the carbon dioxide pumping device is connected to one end of the first injection pipe, and the other end of the first injection pipe is connected to the first channel. The heating element is used to heat the carbon dioxide in the first injection pipe. The first injection valve is arranged on the first injection pipe, one end of the circulation pipe is connected to the second channel, and the other end of the circulation pipe is connected to the carbon dioxide storage tank. The one-way valve and the circulation valve are both arranged on the circulation pipe, one end of the discharge pipe is connected to the third channel, and the discharge valve is arranged on the discharge pipe.

8. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research device according to claim 1, characterized in that: The liquid nitrogen cold shock mechanism includes a liquid nitrogen storage tank, a liquid nitrogen pumping device and a second injection pipe. The liquid nitrogen storage tank is used to store liquid nitrogen. The inlet of the liquid nitrogen pumping device is connected to the liquid nitrogen storage tank. The outlet of the liquid nitrogen pumping device is connected to one end of the second injection pipe. The other end of the second injection pipe is connected to the first channel. The second injection pipe is provided with a second injection valve.

9. A method for studying crack expansion by synergistic fracturing with supercritical carbon dioxide and liquid nitrogen, characterized in that: The device for studying the crack extension of supercritical carbon dioxide and liquid nitrogen synergistic fracturing is applicable to any one of claims 1 to 8, and includes the following method: S1. Place the rock block to be studied in an insulated box with pre-drilled holes in it. The temperature inside the box is precisely controlled by a temperature control component to achieve the temperature conditions required to simulate the actual formation environment. Simultaneously, a triaxial pressure component is used to apply triaxial pressure to the rock block in the insulated box to simulate the actual ground stress state of the rock block in the formation. S2. Installing the simulated wellbore plug in the drilled hole of the rock block, ensuring that the first channel and the second channel of the simulated wellbore are connected to the drilled hole, providing a channel for subsequent injection and recovery of supercritical carbon dioxide and liquid nitrogen; S3. The supercritical carbon dioxide circulation mechanism starts to work, injecting supercritical carbon dioxide into the borehole of the rock block through the first channel. After the supercritical carbon dioxide is injected, the supercritical carbon dioxide in the borehole is discharged from the second channel and re-enters the supercritical carbon dioxide circulation mechanism. The supercritical carbon dioxide circulation mechanism processes the recovered supercritical carbon dioxide to restore it to a state suitable for re-injection. Thereafter, the supercritical carbon dioxide circulation mechanism injects the treated supercritical carbon dioxide into the borehole of the rock block through the first channel again. This cycle is repeated. During multiple cycles, the supercritical carbon dioxide continues to act on the rock block, continuously promoting the development and expansion of cracks within the rock block. S4. After the supercritical carbon dioxide circulates for a preset time, the supercritical carbon dioxide in the borehole is released. Then, the liquid nitrogen cold shock mechanism begins to operate, rapidly introducing liquid nitrogen into the borehole through the first channel. When the liquid nitrogen contacts the rock block, the temperature of the rock block's surface and interior drops sharply. Due to the uneven thermal expansion and contraction of different parts of the rock block, huge thermal stress is generated inside the rock block, which further promotes the expansion of internal cracks in the rock block. S5. During the entire supercritical carbon dioxide circulation and liquid nitrogen cold shock process, the monitoring mechanism works continuously. The monitoring mechanism is arranged on the periphery of the rock block to monitor the expansion of cracks in the rock block in real time. The acoustic emission monitoring captures the elastic wave signals generated when the cracks inside the rock block expand. By analyzing these signals, the location, time and expansion speed of the crack expansion are determined.

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