Supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack propagation research device and method

Abstract

The invention discloses a supercritical carbon dioxide and liquid nitrogen collaborative fracturing crack propagation research device and method.The device comprises a temperature and pressure simulation mechanism, a simulation shaft, a supercritical carbon dioxide circulation mechanism, a liquid nitrogen cold impact mechanism and a monitoring mechanism, and the temperature and pressure simulation mechanism comprises a heat preservation box, a temperature regulation and control assembly and a triaxial pressure assembly; the simulation shaft plug is arranged in the drill hole; the monitoring mechanism is arranged on the periphery of the rock block and used for monitoring the crack expansion condition in the rock block. The device has the beneficial effects that the temperature and pressure simulation mechanism can accurately simulate temperature and pressure conditions in a stratum; and the supercritical carbon dioxide circulating mechanism realizes circulating injection of supercritical carbon dioxide. The supercritical carbon dioxide circulation mechanism and the liquid nitrogen cold impact mechanism work cooperatively. The monitoring mechanism can accurately monitor the expansion condition of the cracks in the rock in real time, detailed data and information are provided for researchers, and the researchers can deeply analyze the crack expansion mechanism and influence factors according to the monitoring result.

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 have significant advantages such as stability, cleanliness, environmental protection and high utilization coefficient. Promoting the development of related industries is a key measure for my country to achieve green and low-carbon energy transformation and ensure energy security. However, deep shale reservoirs and hot dry rock geothermal resource reservoirs have extremely low porosity and permeability. If resource exploitation is to be achieved, the reservoirs must be transformed.

[0003] This type of reservoir has typical "four high" characteristics, namely high temperature, high hardness, high stress and high density. This makes it difficult for the hydraulic fracturing reservoir transformation technology used by the traditional oil and gas industry to produce oil, gas or thermal energy resources with commercial development value. my country's research in this field started relatively late, and there is a lack of mature experience at home and abroad to learn from, resulting in a relatively slow progress in basic scientific research and industrial development.

[0004] Traditional hydraulic fracturing technology faces many challenges. First, the rock is strong, which causes high fracture pressure and easily induces earthquakes; second, it is difficult to form an artificial fracture network, the fracture morphology is single, and the extension direction is difficult to control, which leads to a low recovery rate; third, it consumes a lot of water and easily pollutes the environment.

[0005] In response to the above difficulties, scholars have proposed the concept of "flexible reservoir creation" based on hydraulic fracturing in recent years. Specifically, the pumping method of fracturing can be changed, such as shear fracturing, cyclic / fatigue fracturing, etc.; or the fracturing medium can be replaced, such as using low-temperature liquid nitrogen (LN2), supercritical carbon dioxide (SCCO2), etc., to reduce the fracture pressure, increase the scale of the fracture network and improve its complexity, improve the communication success rate between injection and production wells, and reduce the risk of earthquakes.

[0006] It can be seen that in-depth research on the fracture expansion characteristics of shale or hot dry rock under the synergistic action of SCCO2 and LN2 in the in-situ environment of the reservoir is of vital importance for using SCCO2 / LN2 as a fracturing medium to optimize hydraulic fracturing production technology. In the existing technology, there is a lack of effective research methods and devices 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, so as to reproduce the expansion evolution process of artificial fractures, so as to study the fracture expansion mechanism in rocks under the synergistic fracturing of SCCO2 circulation and LN2 cold shock. 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 supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack extension, so as to solve the technical problems in the prior art of lacking effective research means and devices 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.

[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 extension research device, comprising:

[0010] A temperature and pressure simulation mechanism, the temperature and pressure simulation mechanism comprising an insulation box, a temperature control component and a triaxial pressure component, the insulation box is used to place rock blocks, the rock blocks are provided with drill holes, the temperature control component is used to control the temperature in the insulation box, and the triaxial pressure component is used to apply triaxial pressure to the rock blocks in 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 connected to both the first channel and the second channel, and being used 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 for introducing 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 insulated 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, 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, 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 abutted against the corresponding loading plate.

[0018] In some embodiments, 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 abutted against the corresponding pressing 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 open. 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 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 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 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, and the first cone cylinder is in contact with the first cone sleeve. The contact surface of a tapered sleeve is tapered, 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 tapered, 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 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.

[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, and a second injection valve is provided on the second injection pipe.

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

[0024] S1. Place the rock block to be studied in an incubator with a pre-drilled hole in the rock block. Use the temperature control component to precisely control the temperature in the incubator to achieve the temperature conditions required to simulate the actual formation environment. At the same time, use the triaxial pressure component to apply triaxial pressure to the rock block in the incubator to simulate the actual ground stress state of the rock block in the formation.

[0025] S2. Install the simulated wellbore plug in the borehole of the rock block, ensuring that the first channel and the second channel of the simulated wellbore are connected to the borehole, so as to provide a channel for the subsequent injection and recovery of supercritical carbon dioxide and liquid nitrogen;

[0026] S3, the supercritical carbon dioxide circulation mechanism starts to work, injects supercritical carbon dioxide into the borehole of the rock block through the first channel, and 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, and the supercritical carbon dioxide circulation mechanism processes the recovered supercritical carbon dioxide to restore it to a state suitable for re-injection, and then the supercritical carbon dioxide circulation mechanism injects the treated supercritical carbon dioxide from the first channel into the borehole of the rock block again, and the cycle is repeated. During multiple cycles, the supercritical carbon dioxide continues to act on the rock block, and continuously promotes the development and expansion of cracks inside the rock block;

[0027] S4. After the supercritical carbon dioxide circulates for a preset time, the supercritical carbon dioxide in the borehole is released, and then the liquid nitrogen cold shock mechanism starts to work, and liquid nitrogen is quickly introduced into the borehole through the first channel. When the liquid nitrogen contacts the rock block, the temperature of the surface and the inside of the rock block will drop sharply. Due to the uneven thermal expansion and contraction of various parts of the rock block, huge temperature stress will be generated inside the rock block. This temperature stress will further promote the expansion of cracks inside the rock block. In particular, on the basis that the supercritical carbon dioxide has already exerted a certain effect on the rock block, the liquid nitrogen cold shock may cause the cracks to expand faster and more complexly.

[0028] 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 crack expansion 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 prior art, the beneficial effects of the supercritical carbon dioxide and liquid nitrogen synergistic 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 continuously and repeatedly act on the rock block. The supercritical carbon dioxide circulation mechanism and the liquid nitrogen cold shock mechanism work together, and the cyclic injection of supercritical carbon dioxide transforms the internal structure of the rock block in advance to increase 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, and researchers can deeply analyze the mechanism and influencing factors of crack expansion based on the monitoring results. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1It is a schematic diagram of the structure of a supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack extension research device 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] Description of the accompanying drawings: 1-temperature and pressure simulation mechanism, 11-insulation box, 111-box, 112-cover plate, 12-temperature control component, 121-heater, 122-temperature detection component, 13-triaxial pressure component, 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- The first tapered cylinder, 24-the first outer sleeve, 25-the second tapered cylinder, 26-the second tapered sleeve, 27-the second outer sleeve, 28-the locking nut, 3-the supercritical carbon dioxide circulation mechanism, 31-the low-temperature water bath, 32-the carbon dioxide storage tank, 33-the carbon dioxide pumping device, 34-the first injection pipe, 341-the first injection valve, 35-the heating element, 36-the circulation pipe, 361-the one-way valve, 362-the circulation valve, 37-the discharge pipe, 371-the discharge valve, 4-the liquid nitrogen cold shock mechanism, 41-the liquid nitrogen storage tank, 42-the liquid nitrogen pumping device, 43-the second injection pipe, 431-the second injection valve, 5-the monitoring mechanism, 51-the acoustic emission probe, 52-the acoustic emission monitor, 6-the rock block, 61-the drilling hole. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with 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 prior art of lacking effective research means and devices 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 extension research device and method, which can reproduce the expansion evolution process of artificial fractures, so as to study the fracture extension mechanism in rocks under the action of SCCO2 circulation and LN2 cold shock synergistic fracturing.

[0039] See also Figure 1-Figure 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 , and 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 at 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 5, the temperature control component 12 includes a plurality of heaters 121 and a temperature detection component 122, and the heaters 121 and the temperature detection component 122 are both arranged in the heat preservation box 11. Through the cooperation of the heater 121 and the temperature detection component 122, the temperature in the heat preservation box 11 can be accurately controlled to achieve the temperature conditions required to simulate the actual formation environment. In this way, the temperature state of the rock block 6 in the real formation can be highly restored, so that the experimental process is closer to the actual situation, and the reliability and accuracy of the research results are improved. For example, for the study of deep high-temperature formations, the temperature in the heat preservation box 11 can be adjusted to the corresponding high temperature value to accurately study the crack extension characteristics of the rock block 6 at this temperature.

[0046] In one embodiment, see Figure 2 , Figure 5 and Figure 6 The triaxial pressure assembly 13 includes 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 the two mutually perpendicular lateral directions of the rock block 6, and 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 respectively apply a first preset pressure to the two mutually perpendicular lateral directions of the rock block 6, and the lower pressure member 132 applies a second preset pressure to the top surface of the rock block 6, so as to simulate the real ground stress state of the rock block 6 in the formation. This triaxial pressure application method more accurately restores the stress condition of the rock block 6 underground, making the experimental results more valuable for reference. At the same time, by adjusting the size of the lateral pressure and the vertical pressure, the expansion of the cracks in the rock block 6 under different stress conditions can be studied, providing theoretical support for optimizing the fracturing process parameters and improving the fracturing effect. For example, in high-stress formations, understanding the crack expansion law of the rock block 6 under a specific stress combination is helpful to formulate a more effective fracturing plan.

[0047] In one embodiment, see Figure 2 and Figure 5 The 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 to approach 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 wall of the heat preservation 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 is in contact with 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 driving member 1322. The pressing plate 1321 is attached to the top surface of the rock block 6. The pressing driving member 1322 is used to drive the pressing plate 1321 to move downward to press the rock block 6. The pressing driving 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 is in contact with 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 incubator 11 includes a box body 111 and a cover plate 112. The upper end of the box body 111 is provided with an opening. 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 pressing power rod 13221 is fixed to the cover plate 112. The design of the box body 111 and the cover plate 112 of the incubator 11 can effectively maintain the internal temperature and pressure stability and reduce the interference of external environmental factors on the experiment. The cover plate 112 is detachably fixedly connected to the box body 111, which is convenient for putting in and taking out the rock block 6. At the same time, it also ensures the sealing of the incubator 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 cone sleeve 22, a first tapered cylinder 23, a first outer sleeve 24, a second cone cylinder 25, a second cone 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. A plug 213 is formed at the lower end of the center rod 21. The first cone sleeve 22 is slidably sleeved on the center rod 21. One end of the first cone sleeve 22 abuts against the plug 213. The first cone cylinder 23 is sleeved on the center rod 21 and contacts with the first cone sleeve 22. The first cone 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 physical model 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 function, 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, place the simulated wellbore 2 into the borehole 61 (blind hole) of the rock block 6, and the simulated wellbore 2 will be close to the wellbore wall. When the simulated wellbore 2 is placed to a certain depth, rotate the locking nut 28, and the locking nut 28 presses 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 downward. In this process, the second tapered cylinder 25 and the first tapered cylinder 23 are deformed and further pressed against the wellbore wall. Next, use a small hammer to hammer the simulated wellbore 2 to a predetermined depth, and continue to twist the locking nut 28 during this period to strengthen the sealing effect, and then form the first seal.

[0053] After the simulated wellbore 2 is placed, the rock block 6 in the heat preservation box 11 is heated by the temperature control component 12 of the temperature and pressure simulation mechanism 1. Since the thermal expansion coefficient of the second tapered cylinder 25 and the first tapered cylinder 23 (made of copper material) is relatively large, when the target temperature is reached, the second tapered cylinder 25 and the first tapered cylinder 23 will expand, thereby becoming more closely attached to the rock, and at this time, a second seal is formed.

[0054] Subsequently, the supercritical carbon dioxide circulation mechanism 3 is started to introduce supercritical carbon dioxide (SCCO2) into the borehole 61 through the first channel 211, or the liquid nitrogen cold shock mechanism 4 is started to introduce liquid nitrogen (LN2) into the borehole 61 through the first channel 211 for fracturing operation. During the fluid injection process, the fluid pressure will support the first cone sleeve 22 and move it upward. However, due to the limiting effect of the first tapered cylinder 23, the entire first tapered cylinder 23 will fit more closely to the well wall, and the last seal will be formed at this time. At this point, the rock block 6 sample is ready, and the next step of supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research test can be carried out.

[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 connected to the borehole 61 is also formed in the center rod 21; 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 connected to 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 arranged on the first injection pipe 34, one end of the circulation pipe 36 is connected to the second channel 212, 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 arranged 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 arranged on the discharge pipe 37.

[0057] In this embodiment, when in use, the first injection valve 341 and the circulation valve 362 are opened, and the discharge valve 371 is closed. The carbon dioxide pumping device 33 is started, and carbon dioxide is extracted from the carbon dioxide storage tank 32 and enters the first injection pipe 34 through the carbon dioxide pumping device 33. When the carbon dioxide flows through the first injection pipe 34, the heating element 35 heats it so that the carbon dioxide reaches 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, and the supercritical carbon dioxide diffuses and penetrates inside the rock block 6, which may promote the expansion of microcracks inside the rock block 6. When the supercritical carbon dioxide in the borehole 61 reaches a certain amount, the supercritical carbon dioxide 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 to prevent backflow. The supercritical carbon dioxide returns to the carbon dioxide storage tank 32 through the circulation pipe 36 to complete a cycle. The carbon dioxide pumping device 33 works continuously, so that the supercritical carbon dioxide is continuously circulated and injected into the borehole 61. During multiple cycles, the supercritical carbon dioxide continues to act on the rock block 6, promoting the development and expansion of the internal cracks of the rock block 6. When the supercritical carbon dioxide circulation experiment reaches the preset time or number of cycles, the carbon dioxide pumping device 33, the first injection valve 341 and the circulation valve 362 are closed to stop the circulation injection of supercritical carbon dioxide. 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 then is 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] Start the liquid nitrogen pumping device 42, the liquid nitrogen pumping device 42 starts working, and the liquid nitrogen is extracted from the liquid nitrogen storage tank 41. The extracted liquid nitrogen enters the second injection pipe 43 through the outlet of the liquid nitrogen pumping device 42. In the process of liquid nitrogen flowing through the second injection pipe 43, it is necessary to pay attention to the temperature change and pressure of the pipeline to prevent the abnormal increase of the pressure in the pipeline due to the gasification of liquid nitrogen. The liquid nitrogen flows along the second injection pipe 43 and is finally quickly injected into the borehole 61 of the rock block 6 through the first channel 211. The injection speed 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 best cold shock effect. During the liquid nitrogen cold shock process, the monitoring mechanism 5 continues to work. The acoustic emission probe 51 captures the elastic wave signal generated when the internal cracks of the rock block 6 expand in real time, and the acoustic emission monitor 52 receives and analyzes these signals to determine the position, time and expansion speed of the crack expansion.

[0061] In one embodiment, see Figure 1 , Figure 2 and Figure 5 The monitoring mechanism 5 includes a plurality of acoustic emission probes 51 and an acoustic emission monitor 52. The acoustic emission probes 51 are arranged on the periphery of the rock block 6, and the acoustic emission monitor 52 is electrically connected to each of the acoustic emission probes 51. In this embodiment, the core working principle of the monitoring mechanism 5 is based on acoustic emission technology. During the co-fracturing process of supercritical carbon dioxide and liquid nitrogen, elastic waves will be generated inside the rock block 6 due to crack expansion. The acoustic emission probes 51 can sense these elastic waves and convert them into electrical signals. The acoustic emission monitor 52 receives and processes these electrical signals, thereby obtaining relevant information about the crack expansion inside the rock block 6.

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

[0063] S1. Place the rock block 6 to be studied in an incubator 11. A drill hole 61 is pre-opened on the rock block 6. The temperature in the incubator 11 is precisely regulated by the temperature control component 12 to achieve the temperature condition required for simulating the actual formation environment. At the same time, a triaxial pressure component 13 is used to apply triaxial pressure to the rock block 6 in the incubator 11 to simulate the actual ground stress state of the rock block 6 in the formation.

[0064] S2, plug the simulated wellbore 2 into the borehole 61 of the rock block 6, ensure that the first channel 211 and the second channel 212 of the simulated wellbore 2 are connected to the borehole 61, and provide 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 to work, and injects the 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. After that, the supercritical carbon dioxide circulation mechanism 3 injects the processed supercritical carbon dioxide from the first channel 211 into the borehole 61 of the rock block 6 again, and the cycle is repeated. During multiple cycles, the supercritical carbon dioxide continues to act on the rock block 6, and continuously promotes 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, and then the liquid nitrogen cold shock mechanism 4 starts to work, and liquid nitrogen is quickly introduced into the borehole 61 through the first channel 211. When the liquid nitrogen contacts the rock block 6, the temperature of the surface and the inside of the rock block 6 will drop sharply. Due to the uneven thermal expansion and contraction of various parts of the rock block 6, huge temperature stress will be generated inside the rock block 6. This temperature stress will further promote the expansion of cracks inside the rock block 6. In particular, on the basis that 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 more complexly.

[0067] S5. During the entire supercritical carbon dioxide circulation and liquid nitrogen cold shock process, the monitoring mechanism 5 works continuously. 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 real 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, the expansion law of rock block 6 cracks in various complex geological environments can be deeply studied.

[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 give full play to 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 the actual fracturing process, 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 circulation injection of supercritical carbon dioxide pre-modifies the internal structure of the rock block 6 and increases the porosity and permeability of the rock block 6. The liquid nitrogen cold shock uses temperature stress to further promote crack expansion. The two cooperate with each other 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 monitor the expansion of cracks in the rock block 6 in real time and accurately, providing researchers with detailed data and information. Based on the monitoring results, researchers can deeply analyze 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 supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack expansion research experiment to improve the sealing effect of the device.

[0074] The specific implementation of the present invention described above does not constitute a limitation on the protection scope 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 protection scope 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, the temperature and pressure simulation mechanism comprising an insulation box, a temperature control component and a triaxial pressure component, the insulation box is used to place rock blocks, the rock blocks are provided with drill holes, the temperature control component is used to control the temperature in the insulation box, and the triaxial pressure component is used to apply triaxial pressure to the rock blocks in the insulation box; 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; a supercritical carbon dioxide circulation mechanism, the supercritical carbon dioxide circulation mechanism being connected to both the first channel and the second channel, and being used 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 for introducing 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 extension research device according to claim 1 is 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 heat preservation box.

3. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack extension research device according to claim 1 is 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 extension research device according to claim 3 is 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 to approach 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, 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 abutted against the corresponding loading plate.

5. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack extension research device according to claim 3 is 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 heat preservation 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 extension 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 extension research device according to claim 1, characterized in that: 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 abuts against the plug. The first cone cylinder is mounted on the center rod and contacts the first cone sleeve. The first cone cylinder and the first cone sleeve are connected to each other. The contact surface 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.

8. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack extension research device according to claim 7, 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.

9. The supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack extension 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.

10. A method for studying crack expansion by synergistic fracturing of supercritical carbon dioxide and liquid nitrogen, characterized in that: The invention is applicable to the supercritical carbon dioxide and liquid nitrogen synergistic fracturing crack extension research device as claimed in any one of claims 1 to 9, and includes the following method: S1. Place the rock block to be studied in an incubator with a pre-drilled hole in the rock block. Use the temperature control component to precisely control the temperature in the incubator to achieve the temperature conditions required to simulate the actual formation environment. At the same time, use the triaxial pressure component to apply triaxial pressure to the rock block in the incubator to simulate the actual ground stress state of the rock block in the formation. S2. Install the simulated wellbore plug in the borehole of the rock block, ensuring that the first channel and the second channel of the simulated wellbore are connected to the borehole, so as to provide a channel for the subsequent injection and recovery of supercritical carbon dioxide and liquid nitrogen; S3, the supercritical carbon dioxide circulation mechanism starts to work, injects supercritical carbon dioxide into the borehole of the rock block through the first channel, and 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, and the supercritical carbon dioxide circulation mechanism processes the recovered supercritical carbon dioxide to restore it to a state suitable for re-injection, and then the supercritical carbon dioxide circulation mechanism injects the treated supercritical carbon dioxide from the first channel into the borehole of the rock block again, and the cycle is repeated. During multiple cycles, the supercritical carbon dioxide continues to act on the rock block, and continuously promotes the development and expansion of cracks inside the rock block; S4. After the supercritical carbon dioxide circulates for a preset time, the supercritical carbon dioxide in the borehole is released, and then the liquid nitrogen cold impact mechanism starts to work, and liquid nitrogen is quickly introduced into the borehole through the first channel. When the liquid nitrogen contacts the rock block, the temperature of the surface and inside of the rock block will drop sharply. Due to the uneven thermal expansion and contraction of various parts of the rock block, huge temperature stress will be generated inside the rock block, and this temperature stress will further promote the expansion of cracks inside 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 crack expansion 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.

Images