A downhole supercritical carbon dioxide fracturing coal rock device and construction method

Abstract

The application provides a downhole supercritical carbon dioxide fracturing coal rock device and a construction method, which comprises a coal seam, a fracturing borehole, a hole blocking section, a hole sealing section, a fracturing section, a monitoring hole, a fracturing system, a temperature control system, a grouting system, a liquid carbon dioxide pressurizing system, a proppant mixing system, an extraction system and a data collector; the fracturing borehole is arranged on the coal seam and comprises the hole blocking section, the hole sealing section and the fracturing section; a plurality of monitoring holes are arranged on the coal seam and are spaced apart in the circumferential direction of the fracturing borehole; and the fracturing system is connected with the temperature control system. The proppant mixing system can improve the performance of the liquid carbon dioxide carrying proppant mixture, so that the proppant mixture effectively enters the fracturing fracture and maintains good flow conductivity. The temperature control system overcomes the difficulty in preparation of downhole supercritical carbon dioxide and can realize injection of carbon dioxide in a supercritical state into the fracturing coal seam.

Description

Technical Field

[0001] This application belongs to the field of coalbed methane extraction technology, specifically relating to an underground supercritical carbon dioxide fracturing coal and rock device and its construction method. Background Technology

[0002] Coalbed methane, also known as coalbed methane, is mainly composed of methane (CH4). It is an important clean energy source that can replace traditional energy sources such as coal, natural gas, and oil, reducing dependence on imported energy and improving the reliability and stability of energy supply. Underground gas drainage is an important way to improve the coalbed methane drainage rate and a necessary link to effectively manage deep outburst-prone coal seams and reduce their outburst risk. It is also a pressing problem in coalbed methane disaster management that needs to be solved.

[0003] Fracturing is a primary means of reservoir stimulation and production enhancement. Traditional coalbed methane extraction methods mainly employ hydraulic fracturing technology, but this suffers from problems such as high water consumption, difficulty in fracturing fluid recovery, and difficulty in controlling fracturing effects. Furthermore, hydraulic fracturing has drawbacks such as short extraction time and low extraction efficiency. Therefore, selecting effective permeability enhancement technologies to improve coal seam permeability is urgently needed. Supercritical carbon dioxide fracturing is a novel waterless fracturing technology that is environmentally friendly and can significantly improve fracturing efficiency. Supercritical carbon dioxide possesses unique physical properties, such as low viscosity and easy diffusion, allowing it to penetrate deep into the micropores of the reservoir. During fracturing, it achieves a unified high-pressure fluid flow inside and outside the rock, thereby increasing the scope and complexity of the stimulation. Existing research shows that compared to hydraulic fracturing and liquid carbon dioxide fracturing, supercritical carbon dioxide fracturing has lower fracturing pressure, higher wall roughness, and is more likely to form multiple branch fractures. Currently, supercritical carbon dioxide fracturing field tests are mainly focused on surface fracturing operations. For example, oil fields have successively carried out surface fracturing tests in shale reservoirs using large fracturing units that combine carbon dioxide booster pumps, sand mixing trucks, and fracturing trucks, and these tests have been successful.

[0004] However, due to the complex geological conditions and limited operating space underground, the requirements for the adaptability of fracturing technology and equipment, as well as its extraction processes, are high. Existing supercritical carbon dioxide surface fracturing equipment and construction techniques are not suitable for the underground environment, and there is a lack of underground supercritical carbon dioxide fracturing equipment and supporting processes. Therefore, there are still technical bottlenecks in carrying out supercritical carbon dioxide fracturing of coal seams underground, and similar underground on-site fracturing operations are rarely conducted. Furthermore, the temperature and pressure control of carbon dioxide is a key factor affecting the effectiveness of supercritical carbon dioxide fracturing. In surface fracturing operations, due to the deep burial of the target reservoir (generally greater than 1000 meters), the long wellbore path and large geothermal gradient between the surface and the target reservoir allow for sufficient heat exchange between carbon dioxide and the wellbore and formation, ensuring that the carbon dioxide reaches the target reservoir in a supercritical state. However, the underground pipeline length is limited, and how to achieve a supercritical state when injecting carbon dioxide into the target reservoir is a significant challenge in implementing underground supercritical carbon dioxide fracturing technology. Finally, in the process of fracturing and production enhancement, the evaluation of fracturing effect is an important step, and the gas extraction rate is a key metric for evaluation. Key parameters for fracturing effectiveness are often overlooked, but the underground environment (underground geology, hydrogeological conditions, coal and rock properties, etc.) is usually very complex and variable. These factors lead to significant spatial and temporal differences in the permeability enhancement effect after underground carbon dioxide fracturing. Moreover, underground operations are inherently complex and dangerous tasks. The limitations of the underground environment, the reliability of equipment, and technical issues such as data acquisition and transmission make obtaining relevant data and conducting real-time monitoring challenging. Ultimately, this hinders the precise and quantitative evaluation of the permeability enhancement effect after underground supercritical carbon dioxide fracturing, preventing the widespread adoption of supercritical carbon dioxide fracturing coal seams and enhanced extraction technology underground. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art or related art.

[0006] To address the aforementioned issues, this application provides a downhole supercritical carbon dioxide fracturing coal and rock device, comprising: a coal seam, a fracturing borehole, a plugging section, a sealing section, a fracturing section, a monitoring borehole, a fracturing system, a temperature control system, a grouting system, a liquid carbon dioxide pressurization system, a proppant mixing system, an extraction system, and a data acquisition device.

[0007] The coal seam is provided with fracturing boreholes, each fracturing borehole including a plugging section, a sealing section, and a fracturing section; a number of monitoring holes are provided on the coal seam and spaced apart around the fracturing boreholes in the circumferential direction.

[0008] The fracturing system is connected to the temperature control system, and the fracturing system is used to inject the fracturing fluid discharged from the temperature control system into the fracturing section of the coal seam.

[0009] The grouting system is connected to the fracturing borehole of the coal seam and is used to seal the plugging section and the sealing section;

[0010] The liquid carbon dioxide pressurization system is connected to the temperature control system and is used to provide fracturing fluid to the fracturing system. The proppant mixing system is located between the liquid carbon dioxide pressurization system and the temperature control system and is used to provide proppant mixture to the fracturing system.

[0011] The extraction system is connected to several monitoring holes for monitoring and extracting gas from the fracturing coal seam.

[0012] The data acquisition device is electrically connected to the liquid carbon dioxide pressurization system, temperature control system, fracturing system, grouting system and extraction system. The data acquisition device completes the fracturing of the coal seam and the acquisition and monitoring of data by controlling the operation of each electrically connected system.

[0013] Optionally, the liquid carbon dioxide pressurization system includes: a first high-pressure nitrogen pressurization device, a first connecting pipe, a storage tank, a first branch, a first pressure reducing valve, a first pressure gauge, a second pressure gauge, a first plunger pump, a second connecting pipe, and a first pressure sensor;

[0014] The first high-pressure nitrogen booster is connected to the storage tank via a first connecting pipe for pressurizing the material in the storage tank. The first branch is located on the first connecting pipe, and a shut-off valve, a first pressure reducing valve, and a second pressure gauge are sequentially installed on the first connecting pipe. The first branch is also equipped with a shut-off valve and a first pressure gauge. The storage tank is connected to the first plunger pump via a second connecting pipe, and a first pressure sensor and a shut-off valve are sequentially installed on the second connecting pipe. The first pressure sensor is electrically connected to the data acquisition unit.

[0015] Optionally, the storage tank includes a tank body, a rubber sleeve, and a sealing flange;

[0016] The rubber sleeve is disposed inside the tank, and the rubber sleeve has a space for storing liquid carbon dioxide. Sealing flanges are disposed on both sides of the tank and are respectively connected to the first connecting pipe and the second connecting pipe.

[0017] Optionally, the temperature control system includes: a third connecting pipe, a fourth connecting pipe, a fifth connecting pipe, a sixth connecting pipe, a heating box, a first temperature sensor, a second pressure sensor, and a third pressure sensor;

[0018] The output end of the first plunger pump is connected to the heating box through a third connecting pipe, a fourth connecting pipe, a fifth connecting pipe, and a sixth connecting pipe. A first temperature sensor, a second pressure sensor, and a shut-off valve are sequentially installed on the third connecting pipe. A shut-off valve is installed on the fifth connecting pipe. A shut-off valve and a third pressure sensor are sequentially installed on the sixth connecting pipe. The third pressure sensor is electrically connected to the data acquisition unit.

[0019] Optionally, the proppant mixing system includes: a seventh connecting pipe, a mixing tank, a proppant injection pump, an eighth connecting pipe, a ninth connecting pipe, and a first flow sensor;

[0020] One end of the seventh connecting pipe is connected to the fourth connecting pipe, and the other end of the seventh connecting pipe is connected to the feed inlet of the mixing tank. The proppant injection pump is connected to the feed inlet of the mixing tank through the eighth connecting pipe. Both the seventh and eighth connecting pipes are equipped with shut-off valves. The discharge port of the mixing tank is connected to the sixth connecting pipe through the ninth connecting pipe. The ninth connecting pipe is equipped with a first flow sensor, which is electrically connected to the data acquisition unit.

[0021] Optionally, the grouting system includes: a second high-pressure nitrogen booster, a tenth connecting pipe, a sealing grouting tank, a sealing grouting pipe, an eleventh connecting pipe, a plugging grouting pipe, a twelfth connecting pipe, a plugging grouting tank, a sealing return grouting pipe, a thirteenth connecting pipe, a slurry recovery tank, a capsule sealing device, a high-pressure nitrogen cylinder, an injection hose, a third pressure gauge, and a second pressure reducing valve;

[0022] The second high-pressure nitrogen booster is connected to the sealing grouting tank via the tenth connecting pipe. The first end of the sealing grouting pipe is located within the sealing section, and the second end of the sealing grouting pipe is connected to the sealing grouting tank via the eleventh connecting pipe. A shut-off valve is installed on the eleventh connecting pipe. The first end of the plugging grouting pipe is located within the plugging section, and the second end of the plugging grouting pipe is connected to the plugging grouting tank via the twelfth connecting pipe. A shut-off valve is installed on the twelfth connecting pipe. The first end of the sealing return grouting pipe is located at the top of the sealing section, and the second end of the sealing return grouting pipe is connected to the slurry recovery tank via the thirteenth connecting pipe. A shut-off valve is installed on the thirteenth connecting pipe. The capsule sealing device is located between the sealing section and the fracturing section of the coal seam. The high-pressure nitrogen cylinder is connected to the capsule sealing device via an injection hose. A third pressure gauge, a second pressure reducing valve, and a shut-off valve are sequentially installed on the injection hose.

[0023] Optionally, the fracturing system includes: a fracturing pipe, a fourteenth connecting pipe, a second flow sensor, a second temperature sensor, a fourth pressure sensor, a fifteenth connecting pipe, and a third temperature sensor;

[0024] The first end of the fracturing pipe is located within the fracturing section opened in the coal seam. The second end of the fracturing pipe is connected to the fifth connecting pipe via the fourteenth connecting pipe. The fourteenth connecting pipe is sequentially equipped with a shut-off valve, a second flow sensor, a second temperature sensor, and a fourth pressure sensor. The fifteenth connecting pipe is located between the output end of the heating box and the fourteenth connecting pipe. The fifteenth connecting pipe is sequentially equipped with a third temperature sensor and a shut-off valve. The second temperature sensor, the second flow sensor, the third temperature sensor, and the second pressure sensor are all electrically connected to the data acquisition unit.

[0025] Optionally, the sampling system includes: a third flow sensor and a concentration sensor;

[0026] Each of the monitoring holes is equipped with a third flow sensor and a concentration sensor. The third flow sensor is used to monitor the gas flow rate in the monitoring hole, and the concentration sensor is used to monitor the gas concentration in the monitoring hole. The third flow sensor and the concentration sensor are electrically connected to the data acquisition unit.

[0027] Optionally, it also includes a hydraulic booster system, which includes: a water supply tank, a second plunger pump, a sixteenth connecting pipe, a liquid level sensor, a seventeenth connecting pipe, and a fifth pressure sensor;

[0028] The water supply tank is connected to the second plunger pump via a sixteenth connecting pipe. A liquid level sensor and a shut-off valve are sequentially installed on the sixteenth connecting pipe. The output end of the second plunger pump is connected to the fourth connecting pipe via a seventeenth connecting pipe. A fifth pressure sensor and a shut-off valve are installed on the seventeenth connecting pipe. The liquid level sensor and the fifth pressure sensor are electrically connected to the data acquisition unit.

[0029] This application also provides a construction method for a downhole supercritical carbon dioxide fracturing coal and rock device, which uses the downhole supercritical carbon dioxide fracturing coal and rock device described in any of the above-mentioned methods, and includes the following steps:

[0030] S1. Coal seam parameter testing: Obtain coal samples from the target coal seam, test the coal seam gas pressure, gas content, coal firmness coefficient, initial venting velocity, coal failure type, and test the physical and mechanical properties of the target reservoir and the roof and floor of the coal seam.

[0031] S2. Arrangement and construction of fracturing boreholes and monitoring boreholes: The fracturing boreholes penetrate the rock strata and reach a coal seam depth of at least 5m; then, with the fracturing borehole as the center of a rectangle, four monitoring boreholes are arranged along the two diagonals of the rectangle, and a reference borehole is constructed far away from the fracturing borehole, which is not affected by the fracturing area; and the original gas concentration and extraction flow rate of the eight monitoring boreholes and one reference borehole are monitored and collected in real time.

[0032] S3. Fracturing borehole grouting and sealing construction: Open the high-pressure nitrogen cylinder and inject nitrogen into the capsule sealing device through the injection hose, so that it expands and squeezes the inner wall of the fracturing borehole. The plugging grouting pipe fills the plugging section with plugging grout. After it solidifies, open the second high-pressure nitrogen pressurization device to inject the sealing grout from the sealing grouting tank into the sealing section through the sealing grouting pipe. When the sealing grout flows into the grout recovery tank, the sealing is completed.

[0033] S4. Fracturing Operation: Activate the first high-pressure nitrogen booster to maintain the liquid carbon dioxide pressure in the tank at 2.0-2.8 MPa; the first plunger pump boosts the liquid carbon dioxide input from the storage tank, transmits it to the heating box, heats it to the set temperature, and then transports the liquid carbon dioxide to the supercritical state into the fracturing pipeline. As the supercritical carbon dioxide is continuously injected into the target coal seam, the fluid pressure in the fracturing pipeline slowly rises. After reaching the fracturing pressure and causing the coal seam to form fractures, pumping continues. The liquid pressure is maintained at high pressure or shows several peak values ​​until the pumping volume reaches the design requirement and then stops.

[0034] After the target coal seam fractures are formed, proppant injection begins. Low-temperature liquid carbon dioxide, pressurized by the first plunger pump, is injected into the mixing tank. Thickener is injected into the mixing tank and stirred to form a proppant mixture. After being heated to the set temperature in the heating box, it is injected into the fracturing pipe for construction.

[0035] After the proppant mixture is injected, water is injected. The second plunger pump can supply pressurized water into the fracturing pipe through the seventeenth, fourth, fifth and fourteenth connecting pipes. The liquid water can carry the proppant mixture in the fracturing pipe and the fractures near the fracturing hole to a farther distance, and can also clean the fracturing pipe.

[0036] S5. Gas extraction monitoring: After coal seam fracturing, the gas concentration and flow rate of multiple monitoring holes arranged around the fracturing borehole are monitored and collected in real time through the third flow sensor and concentration sensor.

[0037] Beneficial effects

[0038] The embodiments of this invention provide an underground supercritical carbon dioxide fracturing coal and rock device and construction method. This device includes a fracturing system, a temperature control system, a grouting system, a liquid carbon dioxide pressurization system, a proppant mixing system, an extraction system, and a data acquisition device. The components can be quickly assembled and flexibly transported, adapting to the limited space underground in coal mines. The proppant mixing system improves the performance of liquid carbon dioxide in carrying the proppant mixture, enabling the proppant mixture to effectively enter the fracturing fractures and maintain good conductivity. The temperature control system overcomes the difficulties in preparing supercritical carbon dioxide underground, enabling the injection of carbon dioxide into the fracturing coal seam in a supercritical state. The grouting system effectively prevents the leakage of low-viscosity supercritical carbon dioxide into the borehole, effectively sealing the fracturing borehole. The extraction system enables multi-source data monitoring and acquisition of supercritical carbon dioxide fracturing of the coal seam, overcoming the difficulty of quantitatively evaluating the fracturing effect before and after fracturing in underground environments. Furthermore, the organic combination of these systems allows for the implementation of different fracturing methods, making it more suitable for coal seam modification with complex geological conditions. Attached Figure Description

[0039] Figure 1 This is a structural diagram of one embodiment of the present invention;

[0040] Figure 2 This is a structural diagram of a storage tank according to an embodiment of the present invention;

[0041] Figure 3 This is a coal seam borehole distribution diagram according to an embodiment of the present invention;

[0042] Figure 4 This is a flowchart of another embodiment of the present invention.

[0043] The reference numerals in the attached figures are as follows:

[0044] 1. Coal seam; 11. Fracturing borehole; 111. Pouring section; 112. Sealing section; 113. Fracturing section; 12. Monitoring borehole; 2. Fracturing system; 201. Fracturing pipe; 202. Fourteenth connecting pipe; 203. Second flow sensor; 204. Second temperature sensor; 205. Fourth pressure sensor; 206. Fifteenth connecting pipe; 207. Third temperature sensor; 3. Temperature control system; 301. Third connecting pipe; 302. Fourth connecting pipe; 303. Fifth connecting pipe; 304. Sixth connecting pipe; 305. 1. Heating box; 306. First temperature sensor; 307. Second pressure sensor; 308. Third pressure sensor; 4. Grouting system; 401. Second high-pressure nitrogen booster; 402. Tenth connecting pipe; 403. Sealing grouting tank; 404. Sealing grouting pipe; 405. Eleventh connecting pipe; 406. Plug grouting pipe; 407. Twelfth connecting pipe; 408. Plug grouting tank; 409. Sealing return grouting pipe; 410. Thirteenth connecting pipe; 411. Grout recovery tank; 412. Capsule sealer; 413. High pressure 414. Nitrogen cylinder; 415. Injection hose; 416. Third pressure gauge; 417. Second pressure reducing valve; 5. Liquid carbon dioxide pressurization system; 501. First high-pressure nitrogen pressurization device; 502. First connecting pipe; 503. Storage tank; 5031. Tank body; 5032. Rubber sleeve; 5033. Sealing flange; 504. First branch; 505. First pressure reducing valve; 506. First pressure gauge; 507. Second pressure gauge; 508. First plunger pump; 509. Second connecting pipe; 510. First pressure sensor; 6. Propionate Mixing system; 601, seventh connecting pipe; 602, mixing tank; 603, proppant injection pump; 604, eighth connecting pipe; 605, ninth connecting pipe; 606, first flow sensor; 7, extraction system; 701, third flow sensor; 702, concentration sensor; 8, data acquisition unit; 9, hydraulic booster system; 901, water supply tank; 902, second plunger pump; 903, sixteenth connecting pipe; 904, level sensor; 905, seventeenth connecting pipe; 906, fifth pressure sensor; 13, reference borehole. Detailed Implementation

[0045] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the present invention.

[0046] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0047] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0048] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0049] See also Figure 1-4As shown, the first embodiment of this application provides a downhole supercritical carbon dioxide fracturing coal and rock device, including: a coal seam 1, a fracturing borehole 11, a plugging section 111, a sealing section 112, a fracturing section 113, monitoring holes 12, a fracturing system 2, a temperature control system 3, a grouting system 4, a liquid carbon dioxide pressurization system 5, a proppant mixing system 6, an extraction system 7, and a data acquisition device 8; the coal seam 1 has a fracturing borehole 11, which includes a plugging section 111, a sealing section 112, and a fracturing section 113; a plurality of monitoring holes 12 are spaced apart circumferentially on the coal seam 1 and located at the fracturing borehole 11; the fracturing system 2 is connected to the temperature control system 3, and the fracturing system 2 is used to inject the fracturing fluid discharged from the temperature control system 3 into the fracturing section 113 of the coal seam 1; the grouting system 4 and the... The fracturing boreholes 11 of the coal seam 1 are connected to the plugging section 111 and the sealing section 112 for plugging the boreholes. The liquid carbon dioxide pressurization system 5 is connected to the temperature control system 3 for providing fracturing fluid to the fracturing system 2. The proppant mixing system 6 is located between the liquid carbon dioxide pressurization system 5 and the temperature control system 3 for providing proppant mixture to the fracturing system 2. The extraction system 7 is connected to several monitoring boreholes 12 for extracting and monitoring the gas in the fracturing coal seam 1. The data acquisition device 8 is electrically connected to the liquid carbon dioxide pressurization system 5, the temperature control system 3, the fracturing system 2, the grouting system 4, and the extraction system 7. The data acquisition device 8 completes the fracturing and data acquisition and monitoring of the coal seam 1 by controlling the operation of each electrical connection system.

[0050] Specifically, the system includes a fracturing system 2, a temperature control system 3, a grouting system 4, a liquid carbon dioxide pressurization system 5, a proppant mixing system 6, an extraction system 7, and a data acquisition device 8. To adapt to the underground coal mine environment, each component in the above systems is equipped with an independent skid, which is fixed on a mine rail transport vehicle. It can be transported to the target fracturing location by the underground roadway rail, which is flexible and convenient, avoiding manual handling. Moreover, the components can be quickly assembled and flexibly transported, which can adapt to the limited space in the underground coal mine. A fracturing borehole 11 is opened in the coal seam 1. The fracturing borehole 11 includes a plugging section 111, a sealing section 112, and a fracturing section 113. The grouting system 4 is used to seal the plugging section 111 and the sealing section 112 of the fracturing borehole 11, so that the sealing section 112 will not leak during the fracturing operation, thereby improving the sealing effect. The liquid carbon dioxide pressurization system 5 pressurizes low-temperature, low-pressure carbon dioxide and discharges it at a set rate into the temperature control system 3, bringing it to a supercritical state. This supercritical carbon dioxide is then fed into the fracturing section 113 of coal seam 1 via the fracturing system 2, thus fracturing the coal seam 1. After the fractures in coal seam 1 are formed, the liquid carbon dioxide pressurization system 5 feeds the pressurized liquid carbon dioxide into the proppant mixing system 6. The material discharged from the proppant mixing system 6 is heated by the temperature control system 3 and then fed into the fracturing system 2, injecting a proppant mixture into the fracturing area of ​​coal seam 1. This reduces the probability of fracture closure under geostress, improves fracture conductivity, and ultimately increases production. After fracturing coal seam 1, the gas flow rate and concentration in the monitoring holes 12 opened on the fracturing coal seam 1 can be monitored and collected through the extraction system 7, overcoming the difficulty of quantitatively evaluating the fracturing effect before and after fracturing in the downhole environment; at the same time, the organic combination of various systems can realize the implementation of different fracturing methods, which is more applicable to the modification of coal seam 1 with complex geological conditions. The data acquisition device 8 collects and stores data, which facilitates the quantitative evaluation of the downhole supercritical carbon dioxide fracturing of coal seam 1 and its fracturing effect.

[0051] The liquid carbon dioxide pressurization system 5 includes: a first high-pressure nitrogen pressurization device 501, a first connecting pipe 502, a storage tank 503, a first branch 504, a first pressure reducing valve 505, a first pressure gauge 506, a second pressure gauge 507, a first plunger pump 508, a second connecting pipe 509, and a first pressure sensor 510. The first high-pressure nitrogen pressurization device 501 is connected to the storage tank 503 through the first connecting pipe 502 and is used to pressurize the material in the storage tank 503. The first branch 504 is arranged on the first connecting pipe 502. A shut-off valve, a first pressure reducing valve 505, and a second pressure gauge 507 are arranged sequentially on the first connecting pipe 502. A shut-off valve and a first pressure gauge 506 are arranged on the first branch 504. The storage tank 503 is connected to the first plunger pump 508 through the second connecting pipe 509. A first pressure sensor 510 and a shut-off valve are arranged sequentially on the second connecting pipe 509. The first pressure sensor 510 is electrically connected to the data acquisition device 8.

[0052] Specifically, the first high-pressure nitrogen booster 501 is connected to the storage tank 503 via the first connecting pipe 502, providing a continuous gas supply to the storage tank 503. The maximum output nitrogen pressure is 6 MPa. A shut-off valve, a first pressure reducing valve 505, and a second pressure gauge 507 are installed on the first connecting pipe 502, respectively controlling the connection between the first high-pressure nitrogen booster 501 and the outside environment, precisely controlling the output pressure of the first high-pressure nitrogen booster 501, and displaying the output pressure of the first pressure reducing valve 505. A first branch 504 is installed on the first connecting pipe 502, with a shut-off valve and a first pressure gauge 506 installed on the first branch 504, respectively displaying the gas pressure inside the first connecting pipe 502 and venting the gas inside the first connecting pipe 502. The storage tank 503 is connected to the first plunger pump 508 via the second connecting pipe 509. The first plunger pump 508 is a cryogenic hydraulic plunger pump, used to boost the liquid carbon dioxide output from the storage tank 503 and output it according to a set displacement. A first pressure sensor 510 and a shut-off valve are installed on the second connecting pipe 509. The first pressure sensor 510 is used to monitor the pressure of liquid carbon dioxide output from the storage tank 503. The shut-off valve is used to control the connection between the storage tank 503 and the first plunger pump 508. When the first pressure sensor 510 detects that the pressure is lower than 2.0 MPa, the storage tank 503 is replaced. The first pressure sensor 510 is connected to the data acquisition unit 8. The data acquisition unit 8 is used to receive and store the data from the first pressure sensor 510 to realize the control of the liquid carbon dioxide pressurization system 5.

[0053] The first plunger pump 508 has a rated output pressure, output flow rate, and inlet pressure of 40MPa, 100ml / min, and 2.0~2.8MPa, respectively. Its key components include a 45KW mining explosion-proof variable frequency motor, a plunger made of 45 steel and sprayed with nickel-based tungsten carbide, and a pump head made of 20Cr13 stainless steel forgings.

[0054] The storage tank 503 includes a tank body 5031, a rubber sleeve 5032, and a sealing flange 5033;

[0055] The rubber sleeve 5032 is disposed inside the tank body 5031, and the rubber sleeve 5032 is provided with a space for storing liquid carbon dioxide. The tank body 5031 is provided with sealing flanges 5033 on both sides, which are respectively connected to the first connecting pipe 502 and the second connecting pipe 509.

[0056] Specifically, the storage tank 503 includes a tank body 5031, a rubber sleeve 5032, and a sealing flange 5033. A skid is installed at the bottom of the storage tank 503. The rubber sleeve 5032 is a double-layer hollow structure installed inside the tank body 5031. Liquid carbon dioxide is stored inside the rubber sleeve 5032. When high-pressure nitrogen gas is injected into the first connecting pipe 502, the rubber sleeve 5032 will expand inward and compress the internal space.

[0057] Among them, the rubber sleeve 5032 is made of neoprene rubber, which is resistant to low temperature and chemical corrosion; the tank body 5031 is made of stainless steel, with an inner radius, length, and wall thickness of 1m, 2m, and 5cm respectively, and a maximum pressure resistance of 10MPa. It is also resistant to low temperature and chemical corrosion. The pressure and temperature of the liquid carbon dioxide inside the rubber sleeve 5032 are 2.0~2.8MPa and -25℃~-20℃ respectively.

[0058] The temperature control system 3 includes: a third connecting pipe 301, a fourth connecting pipe 302, a fifth connecting pipe 303, a sixth connecting pipe 304, a heating box 305, a first temperature sensor 306, a second pressure sensor 307, and a third pressure sensor 308; the output end of the first plunger pump 508 is connected to the heating box 305 through the third connecting pipe 301, the fourth connecting pipe 302, the fifth connecting pipe 303, and the sixth connecting pipe 304; the third connecting pipe 301 is sequentially provided with the first temperature sensor 306, the second pressure sensor 307, and a shut-off valve; the fifth connecting pipe 303 is provided with a shut-off valve; and the sixth connecting pipe 304 is sequentially provided with a shut-off valve and the third pressure sensor 308; the third pressure sensor 308 is electrically connected to the data acquisition unit 8.

[0059] Specifically, the output end of the first plunger pump 508 is connected to the heating box 305 through the third connecting pipe 301, the fourth connecting pipe 302, the fifth connecting pipe 303, and the sixth connecting pipe 304 for heating the low-temperature carbon dioxide. The heating box 305 uses electromagnetic heating to heat the built-in serpentine tube, with a temperature range from room temperature to 100°C. A first temperature sensor 306, a second pressure sensor 307, and a shut-off valve are installed on the third connecting pipe 301, which can monitor the temperature and pressure of the high-pressure liquid carbon dioxide output by the first plunger pump 508. The data acquisition unit 8 can receive the data from the third pressure sensor 308 and control the temperature control system 3.

[0060] The proppant mixing system 6 includes: a seventh connecting pipe 601, a mixing tank 602, a proppant injection pump 603, an eighth connecting pipe 604, a ninth connecting pipe 605, and a first flow sensor 606; one end of the seventh connecting pipe 601 is connected to the fourth connecting pipe 302, and the other end of the seventh connecting pipe 601 is connected to the inlet of the mixing tank 602; the proppant injection pump 603 is connected to the inlet of the mixing tank 602 through the eighth connecting pipe 604; both the seventh connecting pipe 601 and the eighth connecting pipe 604 are equipped with shut-off valves; the outlet of the mixing tank 602 is connected to the sixth connecting pipe 304 through the ninth connecting pipe 605; the ninth connecting pipe 605 is equipped with a first flow sensor 606, and the first flow sensor 606 is electrically connected to the data acquisition unit 8.

[0061] Specifically, the proppant mixing system 6 is used to mix the liquid carbon dioxide discharged from the first plunger pump 508 with the thickener and proppant to form a proppant mixture. The output end of the first plunger pump 508 forms a passage with the mixing tank 602 through the third connecting pipe 301, the fourth connecting pipe 302 and the seventh connecting pipe 601, so that the liquid carbon dioxide can be discharged into the mixing tank 602. The proppant injection pump 603 is connected to the mixing tank 602 through the eighth connecting pipe 604, so that the thickener and proppant can be fed into the mixing tank 602 to achieve full mixing of liquid carbon dioxide with thickener and proppant. The proppant injection pump 603 contains a mixture of siloxane thickener and proppant, and can control the injection speed to ensure that the proppant and thickener are mixed in a predetermined ratio, which is used to thicken the liquid carbon dioxide injected into the mixing tank 602. The amount of proppant and thickener added is 2% or less of the volume of liquid carbon dioxide. The ratio of thickener to proppant is not less than 1:1, and can be flexibly set according to the sand addition requirements and site conditions. The discharge port of the mixing tank 602 is connected to the sixth connecting pipe 304 through the ninth connecting pipe 605, which is used to feed the stirred proppant mixture into the heating box 305 to heat the proppant mixture to reach the set temperature, which is at least greater than the critical temperature of carbon dioxide (31.26℃). A first flow sensor 606 is installed on the ninth connecting pipe 605. The first flow sensor 606 is used to detect the flow rate of the proppant mixture discharged from the mixing tank 602. The data acquisition unit 8 can receive the data from the first flow sensor 606 and control the proppant mixing system 6.

[0062] Among them, the proppant type is quartz sand or ceramsite, with a particle size of 80 / 100 mesh, 40 / 70 mesh or 20 / 40 mesh, and the 40 / 70 mesh proppant is the best choice.

[0063] The mixing tank 602 uses an explosion-proof variable frequency motor for its mixing pump, which is equipped with a rotating shaft to provide power to the rotor of the mixing tank 602. A fixed bracket is installed at the bottom of the mixing pump to prevent vibration. The base of the mixing pump is equipped with a skid. The tank body 5031 has an inner diameter of 1m and a height of 1.5m. It is equipped with a rotor and a mixing structure inside. The rotor and the mixing structure are driven by an internal gear to achieve full mixing of the proppant and liquid carbon dioxide.

[0064] The grouting system 4 includes: a second high-pressure nitrogen booster 401, a tenth connecting pipe 402, a sealing grouting tank 403, a sealing grouting pipe 404, an eleventh connecting pipe 405, a plugging grouting pipe 406, a twelfth connecting pipe 407, a plugging grouting tank 408, a sealing return grouting pipe 409, a thirteenth connecting pipe 410, a slurry recovery tank 411, a capsule sealing device 412, a high-pressure nitrogen cylinder 413, an injection hose 414, a third pressure gauge 415, and a second pressure reducing valve 416. The second high-pressure nitrogen booster 401 is connected to the sealing grouting tank 403 via the tenth connecting pipe 402. The first end of the sealing grouting pipe 404 is located within the sealing section 112, and the second end of the sealing grouting pipe 404 is connected to the sealing grouting tank 403 via the eleventh connecting pipe 405. The eleventh connecting pipe 405 is equipped with... The plugging grouting pipe 406 has a first end located inside the plugging section 111, and a second end connected to the plugging grouting tank 408 via the twelfth connecting pipe 407. A stop valve is installed on the twelfth connecting pipe 407. The sealing return slurry pipe 409 has a first end located at the top of the sealing section 112, and a second end connected to the slurry recovery tank 411 via the thirteenth connecting pipe 410. A stop valve is installed on the thirteenth connecting pipe 410. The capsule sealing device 412 is located between the sealing section 112 and the fracturing section 113 of the coal seam 1. The high-pressure nitrogen cylinder 413 is connected to the capsule sealing device 412 via an injection hose 414. A third pressure gauge 415, a second pressure reducing valve 416, and a stop valve are sequentially installed on the injection hose 414.

[0065] Specifically, the grouting system 4 is used to seal the fracturing borehole 11 drilled in the coal seam 1 to ensure the sealing of the fracturing section 113. The fracturing borehole 11 has an inner diameter of 84 mm and includes a plugging section 111, a sealing section 112, and a fracturing section 113. The plugging section 111 is 2-3 m long and is used to seal the rock end of the fracturing borehole 11 and prevent the sealing grout from flowing out. The sealing section 112 is 3 / 4 the length of the total length of the fracturing borehole 11 and is used to seal the fracturing borehole 11 over a large area to prevent fracturing fluid leakage. The fracturing section 113 is at least 5 m long and is the main injection point for fracturing fluid into the fracturing coal seam 1. Location: The capsule sealer 412 is located at the junction of the sealing section 112 and the fracturing section 113. It has an inflation pressure of 4–8 MPa and a compressive strength of 40 MPa. It is used to prevent fracturing fluid leakage and the entry of sealing slurry into the fracturing section 113. An injection hose 414 is connected to the capsule sealer 412, with its other end connected to a high-pressure nitrogen cylinder 413. The high-pressure nitrogen cylinder 413 injects high-pressure nitrogen into the capsule sealer 412, causing it to expand and compress the inner wall of the fracturing borehole 11. A third pressure gauge 415 and a second pressure reducing valve 416 are installed on the injection hose 414 to display the high-pressure nitrogen level. The bottle 413 outputs gas pressure and precisely controls its output pressure; the first end of the sealing grouting pipe 404 is set inside the sealing section 112, used to fill the sealing section 112 with the sealing grout from the sealing grouting tank 403; its second end is connected to the sealing grouting tank 403 via the eleventh connecting pipe 405; the second high-pressure nitrogen booster device 401 is connected to the sealing grouting tank 403 via the tenth connecting pipe 402, and the second high-pressure nitrogen booster device 401 is used to adjust the pressure inside the sealing grouting tank 403 and improve the grouting capacity of the sealing grouting tank 403; the sealing grouting tank 403 has an inner diameter of 1m and a height of... The 1.5m tank-like structure is equipped with an internal stirring mechanism to ensure thorough mixing of the sealing grout. The sealing grout can be cement mortar, wooden wedges, yellow mud, polyurethane foam, etc. The first end of the plugging grouting pipe 406 is located inside the plugging section 111, used to fill the plugging section 111 with the sealing grout from the plugging grouting tank 408. The second end of the plugging grouting pipe 406 is connected to the plugging grouting tank 408 via a twelfth connecting pipe 407. The plugging grouting tank 408 contains sealing grout, which is a slow-reaction polyurethane A+B liquid, capable of sealing the plugging section 111. The top of the sealing return pipe 409 is installed at the top of the sealing section 112, and its bottom is connected to the grout recovery tank 411 via a thirteenth connecting pipe 410. The grout recovery tank 411 is a tank-like structure with an inner diameter of 1m and a height of 1m, used to collect excess sealing grout.

[0066] The fracturing system 2 includes: a fracturing pipe 201, a fourteenth connecting pipe 202, a second flow sensor 203, a second temperature sensor 204, a fourth pressure sensor 205, a fifteenth connecting pipe 206, and a third temperature sensor 207. The first end of the fracturing pipe 201 is located in a fracturing section 113 opened on the coal seam 1. The second end of the fracturing pipe 201 is connected to the fifth connecting pipe 303 through the fourteenth connecting pipe 202. A shut-off valve, a second flow sensor 203, a second temperature sensor 204, and a fourth pressure sensor 205 are sequentially installed on the fourteenth connecting pipe 202. The fifteenth connecting pipe 206 is located between the output end of the heating box 305 and the fourteenth connecting pipe 202. A third temperature sensor 207 and a shut-off valve are sequentially installed on the fifteenth connecting pipe 206. The second temperature sensor 204, the second flow sensor 203, the third temperature sensor 207, and the second pressure sensor 307 are all electrically connected to the data acquisition unit 8.

[0067] Specifically, the fracturing system 2 is used to inject carbon dioxide fracturing fluid into the fracturing coal seam 1. After the plugging section 111 and sealing section 112 of the fracturing borehole 11 are plugged, the first plunger pump 508 feeds pressurized liquid carbon dioxide into the heating box 305 through the third connecting pipe 301, the fourth connecting pipe 302, the fifth connecting pipe 303 and the sixth connecting pipe 304. After the heating box 305 heats it, the temperature reaches the set temperature (greater than 35°C), so that the liquid carbon dioxide reaches the supercritical state. Then, it is fed into the fracturing pipe 201 through the fifteenth connecting pipe 206 and the fourteenth connecting pipe 202. The top of the fracturing pipe 201 is installed in the fracturing section 113, thereby realizing the feeding of fracturing fluid into the fracturing section 113, ensuring that the carbon dioxide is injected into the coal seam 1 in a supercritical state, and realizing the fracturing operation of the coal seam. A third temperature sensor 207 and a shut-off valve are installed on the fifteenth connecting pipe 206. A shut-off valve, a second flow sensor 203, a second temperature sensor 204, and a fourth pressure sensor 205 are installed on the fourteenth connecting pipe 202 to monitor the flow rate, temperature, and pressure of the fracturing fluid entering the fracturing pipe 201. As supercritical carbon dioxide is continuously injected into the target coal seam 1, the fluid pressure in the fracturing pipe 201 slowly increases. When the pumping pressure reaches the fracturing pressure and a fracture is formed, pumping continues, and the liquid pressure remains high or experiences several peaks until the pumping rate reaches the design requirement. During fracturing, the first pressure sensor 510 and the fourth pressure sensor 205 on the first connecting pipe 502 output pressure. If the pressure of the first pressure sensor 510 is lower than 2 MPa, the storage tank 503 is replaced. If the pressure of the fourth pressure sensor 205 exceeds the pressure threshold of 40 MPa, the fracturing operation is stopped immediately, and the pipe blockage is checked. During the fracturing process, the gas concentration at the fracturing site is measured in real time using a DGC gas content rapid analyzer. If the concentration exceeds the limit, the fracturing operation is stopped and ventilation is carried out until the gas concentration is reduced to a safe value before the fracturing operation is restarted.

[0068] The extraction system 7 includes: a third flow sensor 701 and a concentration sensor 702;

[0069] Each of the monitoring holes 12 is provided with a third flow sensor 701 and a concentration sensor 702. The third flow sensor 701 is used to monitor the gas flow rate in the monitoring hole 12, and the concentration sensor 702 is used to monitor the gas concentration in the monitoring hole 12. The third flow sensor 701 and the concentration sensor 702 are electrically connected to the data acquisition unit 8.

[0070] Specifically, after fracturing, eight monitoring holes 12 arranged around the fracturing borehole 11 are monitored. The third flow sensor 701 and the concentration sensor 702 independently monitor and collect the gas concentration and extraction flow rate in the eight monitoring holes 12 for 30 days. The data are compared with the extraction data of the reference borehole 13 in the nearby unfractured area. By analyzing the extraction situation of the monitoring holes 12 at different locations, the relationship between fracturing parameters and extraction effect is quantitatively evaluated, providing data support for subsequent optimization of fracturing.

[0071] It also includes a hydraulic booster system 9, which comprises: a water supply tank 901, a second plunger pump 902, a sixteenth connecting pipe 903, a level sensor 904, a seventeenth connecting pipe 905, and a fifth pressure sensor 906; the water supply tank 901 is connected to the second plunger pump 902 through the sixteenth connecting pipe 903, and the sixteenth connecting pipe 903 is sequentially equipped with a level sensor 904 and a shut-off valve; the output end of the second plunger pump 902 is connected to the fourth connecting pipe 302 through the seventeenth connecting pipe 905, and the seventeenth connecting pipe 905 is equipped with a fifth pressure sensor 906 and a shut-off valve; the level sensor 904 and the fifth pressure sensor 906 are electrically connected to the data acquisition unit 8.

[0072] Specifically, after the proppant mixture is injected, the second plunger pump 902 can supply pressurized water into the fracturing pipe 201 through the seventeenth connecting pipe 905, the fourth connecting pipe 302, the fifth connecting pipe 303 and the fourteenth connecting pipe 202, so that the proppant mixture in the fracturing pipe 201 and the fractures near the fracturing hole can be carried to a farther distance by the water, and the fracturing pipe 201 can be cleaned. The injection volume is 10-15 cubic meters.

[0073] A liquid level sensor 904 and a shut-off valve are installed on the sixteenth connecting pipe 903. The liquid level sensor 904 can monitor the liquid level in the water supply tank 901. When the liquid level is lower than 3 / 4, the water supply tank 901 should be replaced.

[0074] The second plunger pump 902 pressurizes the liquid in the water supply tank 901 and can also be used as a fracturing fluid to perform separate fracturing of the coal seam 11. The process is as follows: the second plunger pump 902 pressurizes the liquid in the water supply tank 901, which is then fed into the fracturing pipe 201 through the seventeenth connecting pipe 905, the fourth connecting pipe 302, the fifth connecting pipe 303, and the fourteenth connecting pipe 202. The liquid water is then discharged into the fracturing section 113 to achieve fracturing of the coal seam 1. After fracturing is completed, the second plunger pump 902 pressurizes the liquid in the water supply tank 901 again. The pressurized liquid enters the mixing tank 602 through the seventeenth connecting pipe 905, the fourth connecting pipe 302, and the seventh connecting pipe 601, where it is mixed with the thickener and proppant to form a proppant mixture. The proppant mixture is then directly fed into the fracturing pipe 201 through the ninth connecting pipe 605, the sixth connecting pipe 304, and the fourteenth connecting pipe 202 to support the fractured cracks.

[0075] See also Figure 4 As shown, the second embodiment of this application provides a construction method for a downhole supercritical carbon dioxide fracturing coal and rock device, using any one of the downhole supercritical carbon dioxide fracturing coal and rock devices described above, including the following steps:

[0076] S1, Coal Seam 1 Parameter Test: Obtain coal sample of target coal seam 1, test the gas pressure, gas content, coal firmness coefficient, initial venting velocity, and coal failure type of coal seam 1, and test the physical and mechanical properties of the target reservoir and the roof and floor of coal seam 1.

[0077] Specifically, a reservoir was selected, and coal samples from coal seam 1 were obtained using a kilometer-wide directional large-diameter drilling rig. The gas pressure, gas content, coal firmness coefficient, initial venting velocity ΔP, and coal failure type of coal seam 1 were tested. The physical and mechanical properties of the target reservoir and the roof and floor of coal seam 1 were also tested. Based on the actual site conditions, fracturing borehole 11 was drilled.

[0078] In-situ stress tests were conducted to obtain the maximum and minimum horizontal in-situ stresses of coal seam 1. The fracturing pressure range was then calculated to determine subsequent fracturing parameters, with the upper limit of the fracturing pressure being:

[0079] P b =σ H -3σ h +σ t -P0

[0080] Where, σ H and σ h These are the maximum and minimum horizontal ground stresses (MPa), σ t P0 is the tensile strength of the coal seam (MPa), and P0 is the initial gas pressure of the coal seam (MPa).

[0081] The lower limit of rupture pressure is:

[0082]

[0083] Where α is the Biot coefficient of the coal sample (dimensionless), and υ is the Poisson's ratio of the coal sample (dimensionless).

[0084] It should be noted that when performing fracturing on coal seam 1, both cross-seam fracturing and in-seam fracturing techniques can be used. Cross-seam fracturing refers to drilling holes in the roof or floor of coal seam 1 and connecting the coal seam through segmented fracturing. In-seam fracturing refers to drilling holes along the strike or dip direction of coal seam 1. The fracturing technique should be flexibly selected according to the occurrence conditions of coal seam 1. The following uses cross-seam fracturing as an example. It should be noted that this device is not limited to a single fracturing technique.

[0085] Using the aforementioned fracturing pressure range as a reference, the first plunger pump 508 of the liquid carbon dioxide pressurization system 5 is selected and its parameters are determined. The determination of the rated pressure of the first plunger pump 508, in addition to meeting the fracturing pressure of the target coal seam 1, should also comprehensively consider the pressure loss along the carbon dioxide delivery pipeline, and can be calculated using the Darcy-Weisbach formula.

[0086]

[0087] Where ΔP is the pressure loss along the pipe (Pa), f is the pipe friction coefficient (dimensionless), L is the pipe length (m), D is the pipe diameter (m), is the liquid density (kg / m^3), and is the liquid velocity (m / s).

[0088] Since the compressibility of water is typically around 8.5 × 10⁻⁶ -10 The compressibility of carbon dioxide is around 1 Pa, while the compressibility of carbon dioxide is several orders of magnitude greater than that of water, and it varies greatly with temperature and pressure. Therefore, carbon dioxide has higher compressibility than water, meaning that its volume changes more easily under the same pressure. Thus, the volume of carbon dioxide required for fracturing coal seam 1 is much larger than the volume of water injected. Based on site conditions and experience from other construction cases, the rated flow rate of the first plunger pump 508 is initially set at 40–60 L / min.

[0089] The fracturing system 2, temperature control system 3, grouting system 4, liquid carbon dioxide pressurization system 5, proppant mixing system 6, extraction system 7, and data acquisition device 8 are independently fixed on a mine rail transport vehicle and transported to the underground fracturing site. The transport vehicles are rigidly connected and installed. The equipment operation is checked and the maximum pressure limit test is carried out on the construction pipeline to prevent leakage at the pipe and valve interfaces.

[0090] S2. Arrangement and construction of fracturing borehole 11 and monitoring borehole 12: Fracturing borehole 11 penetrates the rock strata and enters coal seam 1 to a depth of at least 5m; then, with fracturing borehole 11 as the center of a rectangle, four monitoring boreholes 12 are arranged along the two diagonals of the rectangle respectively, and a reference borehole 13 is constructed away from fracturing borehole 11, which is not affected by the fracturing area; and the original gas concentration and extraction flow rate of the eight monitoring boreholes 12 and one reference borehole 13 are monitored and collected in real time.

[0091] Specifically, a construction site is set up in the roadway below the target reservoir using a kilometer-long directional drilling rig. Directional drilling is carried out according to the drilling layout plan. The fracturing borehole 11 penetrates the rock strata and drills into the coal seam 1 to a depth of at least 5m. Then, with the fracturing borehole 11 as the center of the rectangle, four monitoring holes 12 are arranged along the two diagonals of the rectangle, with two adjacent monitoring holes 12 on the diagonal spaced 25m apart. A reference borehole 13 is drilled 100m away from the fracturing borehole 11. The reference borehole 13 is not affected by the fracturing area. The original gas concentration and extraction flow rate of the eight monitoring holes 12 and the reference borehole 13 are monitored and collected in real time.

[0092] S3. Grouting and sealing construction of fracturing borehole 11: Open the high-pressure nitrogen cylinder 413 and inject nitrogen into the capsule sealing device 412 through the injection hose 414, so that it expands and squeezes the inner wall of the fracturing borehole 11. The plugging grouting pipe 406 fills the plugging section 111 with plugging grout. After it solidifies, open the second high-pressure nitrogen pressurization device 401 so that the sealing grouting tank 403 injects the sealing grouting into the sealing section 112 through the sealing grouting pipe 404. The sealing is completed when the sealing grouting flows into the grout recovery tank 411.

[0093] Specifically, the fracturing pipe 201, the plugging grouting pipe 406, the sealing grouting pipe 404, the sealing return grouting pipe 409, the capsule sealing device 412, and the gas injection hose 414 are all placed inside the fracturing borehole 11. The high-pressure nitrogen cylinder 413 is opened, and the high-pressure nitrogen inside is injected into the capsule sealing device 412 through the gas injection hose 414, causing it to expand and squeeze the inner wall of the fracturing borehole 11. The injection pressure is not less than 5 MPa. After the capsule sealer 412 achieves sealing, the sealing grout in the sealing grouting tank 408 is injected into the sealing section 111 through the twelfth connecting pipe 407 and the sealing grouting pipe 406. The sealing grout is a slow-reaction polyurethane A+B liquid mixed with cotton yarn. After the sealing section 111 is filled, the shut-off valve on the twelfth connecting pipe 407 is closed. After it solidifies, the second high-pressure nitrogen booster device 401 is turned on. The second high-pressure nitrogen booster device 401 adjusts the pressure in the sealing grouting tank 403 through the tenth connecting pipe 402, so that the sealing grout inside is fed into the sealing section 112 through the eleventh connecting pipe 405 and the sealing grouting pipe 404. After the sealing section 112 is filled, the grout inside is allowed to solidify for 24 hours before the next operation is carried out. This can improve the sealing section 112 from leaking during fracturing operations and further ensure the sealing effect.

[0094] S4. Fracturing Operation. The first high-pressure nitrogen booster 401 is turned on to maintain the liquid carbon dioxide pressure in the tank at 2.0-2.8 MPa; the first plunger pump 508 pressurizes the liquid carbon dioxide input from the storage tank 503 and transmits it to the heating box 305 to heat it to the set temperature, so that the liquid carbon dioxide reaches the supercritical state and is transported to the fracturing pipe 201. As the supercritical carbon dioxide is continuously injected into the target coal seam 1, the fluid pressure in the fracturing pipe 201 slowly rises. After reaching the fracturing pressure and causing the coal seam to form fractures, the pumping continues. The liquid pressure is kept high or has several peak values ​​until the pumping volume reaches the design requirement and then stops.

[0095] After the coal seam 1 fracture is formed, the proppant mixture is injected. Low-temperature liquid carbon dioxide pressurized by the first plunger pump 508 is fed into the mixing tank 602. Thickener is injected into the mixing tank 602 for stirring to form the proppant mixture. After being heated to the set temperature by the heating box 305, it is fed into the fracturing pipe 201 for construction.

[0096] After the proppant mixture is injected, water injection begins. The second plunger pump 902 can supply pressurized water into the fracturing pipe 201 through the seventeenth connecting pipe 905, the fourth connecting pipe 302, the fifth connecting pipe 303, and the fourteenth connecting pipe 202. The liquid water allows the proppant mixture in the fracturing pipe 201 and the fractures near the fracturing hole to be carried to a farther distance, and also enables the fracturing pipe 201 to be cleaned.

[0097] Specifically, the fracturing system 2 is used to inject carbon dioxide fracturing fluid into the fracturing coal seam 1. After the plugging section 111 and sealing section 112 of the fracturing borehole 11 are plugged, the first plunger pump 508 feeds the pressurized hydraulic carbon dioxide into the heating box 305 through the third connecting pipe 301, the fourth connecting pipe 302, the fifth connecting pipe 303, and the sixth connecting pipe 304. After the heating box 305 heats it, the temperature reaches the set temperature (greater than 35°C), so that the liquid carbon dioxide reaches the supercritical state. Then, it is fed into the fracturing pipe 201 through the fifteenth connecting pipe 206 and the fourteenth connecting pipe 202. The top of the fracturing pipe 201 is installed in the fracturing section 113, thereby realizing the feeding of fracturing fluid into the fracturing section 113 and ensuring that the carbon dioxide is injected into the coal seam 1 in a supercritical state. A third temperature sensor 207 and a shut-off valve are installed on the fifteenth connecting pipe 206. A shut-off valve, a second flow sensor 203, a second temperature sensor 204, and a fourth pressure sensor 205 are installed on the fourteenth connecting pipe 202 to monitor the flow rate, temperature, and pressure of the fracturing fluid entering the fracturing pipe 201. As supercritical carbon dioxide is continuously injected into the target coal seam 1, the fluid pressure in the fracturing pipe 201 slowly increases. When the pumping pressure reaches the fracturing pressure and a fracture is formed, pumping continues, and the liquid pressure remains high or experiences several peaks until the pumping rate reaches the design requirement. During fracturing, the output pressure of the first pressure sensor 510 and the fourth pressure sensor 205 on the first connecting pipe 502 should be constantly monitored. If the pressure of the first pressure sensor 510 is lower than 2 MPa, the storage tank 503 should be replaced. If the pressure of the fourth pressure sensor 205 exceeds the pressure threshold of 40 MPa, the fracturing operation should be stopped immediately, and the pipe blockage should be checked. In addition, the gas concentration at the fracturing site should be measured in real time using a DGC gas content rapid analyzer during the fracturing process. If the concentration exceeds the limit, the fracturing operation should be stopped and ventilation should be carried out until the gas concentration drops to a safe value before the fracturing operation is restarted.

[0098] After the formation of fractures in the target coal seam 1, the proppant mixture is injected. The first plunger pump 508 supplies pressurized hydraulic carbon dioxide into the heating box 305 via the third connecting pipe 301, the fourth connecting pipe 302, and the seventh connecting pipe 601. Meanwhile, the proppant injection pump 603 supplies thickener and proppant into the mixing tank 602 via the eighth connecting pipe 604, ensuring thorough mixing of the liquid carbon dioxide with the thickener and proppant. The proppant injection pump 603 contains a mixture of siloxane-based thickener and proppant, allowing for controlled injection speed to ensure the proppant and thickener are mixed in a predetermined ratio. This mixture is used to thicken the liquid carbon dioxide injected into the mixing tank 602 and to add proppant. The thickener is used... The amount is 2% or less than 2% of the volume of liquid carbon dioxide, and the ratio of thickener to proppant is not less than 1:1. The specific ratio can be flexibly set according to the sand addition requirements and site conditions. The discharge port of the mixing tank 602 is connected to the sixth connecting pipe 304 through the ninth connecting pipe 605, which is used to feed the mixed proppant mixture into the heating box 305 to heat the proppant mixture to reach the set temperature, which is at least greater than the critical temperature of carbon dioxide, i.e., 31.26℃. Then, it is fed into the fracturing pipe 201 through the fifteenth connecting pipe 206 and the fourteenth connecting pipe 202 to discharge the proppant mixture into the fracture of coal seam 1.

[0099] After the proppant mixture is injected, water is injected. The second plunger pump 902 can supply pressurized water into the fracturing pipe 201 through the seventeenth connecting pipe 905, the fourth connecting pipe 302, the fifth connecting pipe 303 and the fourteenth connecting pipe 202, so that the proppant mixture in the fracturing pipe 201 and the fractures near the fracturing hole can be carried to a farther distance by the water, and the fracturing pipe 201 can be cleaned. The injection volume is 10-15 cubic meters.

[0100] S5. Gas extraction monitoring. After coal seam fracturing, the gas concentration and flow rate of multiple monitoring holes 12 arranged around the fracturing borehole 11 are monitored and collected in real time through the third flow sensor 701 and the concentration sensor 702.

[0101] Specifically, after fracturing coal seam 1, the gas concentration and extraction flow rate in multiple monitoring holes 12 arranged around the fracturing borehole 11 are independently monitored and collected in real time using a third flow sensor 701 and a concentration sensor 702. The data are compared with the extraction data of a reference borehole 13 in a similar unfractured area. The collected data are stored through a data acquisition device 8. By analyzing the extraction situation of monitoring boreholes 12 at different locations, the relationship between fracturing parameters and extraction effect is quantitatively evaluated, providing a reference for subsequent optimization of fracturing design.

[0102] The above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application. The above are merely preferred embodiments of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the protection scope of this application.

Claims

1. A downhole supercritical carbon dioxide coal rock fracturing device construction method, the downhole supercritical carbon dioxide coal rock fracturing device construction method is applied to a downhole supercritical carbon dioxide coal rock fracturing device, characterized in that, The downhole supercritical carbon dioxide fracturing coal and rock device includes: coal seam, fracturing borehole, plugging section, sealing section, fracturing section, monitoring borehole, fracturing system, temperature control system, grouting system, liquid carbon dioxide pressurization system, proppant mixing system, extraction system, and data acquisition device; The coal seam is provided with fracturing boreholes, each fracturing borehole including a plugging section, a sealing section, and a fracturing section; a number of monitoring holes are provided on the coal seam and spaced apart around the fracturing boreholes in the circumferential direction. The fracturing system is connected to the temperature control system, and the fracturing system is used to inject the fracturing fluid discharged from the temperature control system into the fracturing section of the coal seam. The grouting system is connected to the fracturing borehole of the coal seam and is used to seal the plugging section and the sealing section; The liquid carbon dioxide pressurization system is connected to the temperature control system and is used to provide fracturing fluid to the fracturing system. The proppant mixing system is located between the liquid carbon dioxide pressurization system and the temperature control system and is used to provide proppant mixture to the fracturing system. The extraction system is connected to several monitoring holes for monitoring and extracting gas from the fracturing coal seam. The data acquisition device is electrically connected to the liquid carbon dioxide pressurization system, temperature control system, fracturing system, grouting system and extraction system. The data acquisition device completes the fracturing of the coal seam and the acquisition and monitoring of data by controlling the operation of each electrically connected system. The construction method of the downhole supercritical carbon dioxide fracturing coal and rock device includes the following steps: S1. Coal seam parameter testing: Obtain coal samples from the target coal seam, test the coal seam gas pressure, gas content, coal firmness coefficient, initial venting velocity, coal failure type, and test the physical and mechanical properties of the target reservoir and the roof and floor of the coal seam. S2. Arrangement and construction of fracturing boreholes and monitoring boreholes: The fracturing boreholes penetrate the rock strata and reach a coal seam depth of at least 5m; then, with the fracturing borehole as the center of a rectangle, four monitoring boreholes are arranged along the two diagonals of the rectangle, and a reference borehole is constructed far away from the fracturing borehole, which is not affected by the fracturing area; and the original gas concentration and extraction flow rate of the eight monitoring boreholes and one reference borehole are monitored and collected in real time. S3. Fracturing borehole grouting and sealing construction: Open the high-pressure nitrogen cylinder and inject nitrogen into the capsule sealing device through the injection hose, so that it expands and squeezes the inner wall of the fracturing borehole. The plugging grouting pipe fills the plugging section with plugging grout. After it solidifies, open the second high-pressure nitrogen pressurization device to inject the sealing grout from the sealing grouting tank into the sealing section through the sealing grouting pipe. When the sealing grout flows into the grout recovery tank, the sealing is completed. S4. Fracturing Operation: Activate the first high-pressure nitrogen booster to maintain the liquid carbon dioxide pressure in the tank at 2.0~2.8MPa; the first plunger pump boosts the liquid carbon dioxide input from the storage tank, transmits it to the heating box to heat it to the set temperature, and then transports the liquid carbon dioxide to the supercritical state into the fracturing pipeline. As the supercritical carbon dioxide is continuously injected into the target coal seam, the fluid pressure in the fracturing pipeline slowly rises. After reaching the fracturing pressure and causing the coal seam to form fractures, pumping continues. The liquid pressure is maintained at high pressure or shows several peak values ​​until the pumping volume reaches the design requirement and then stops. After the target coal seam fractures are formed, proppant injection begins. Low-temperature liquid carbon dioxide, pressurized by the first plunger pump, is fed into the mixing tank. Thickener is injected into the mixing tank and stirred to form a proppant mixture. After being heated to the set temperature in the heating box, it is fed into the fracturing pipe for construction. After the proppant mixture is injected, water is injected. The second plunger pump can supply pressurized water into the fracturing pipe through the seventeenth, fourth, fifth and fourteenth connecting pipes. The liquid water can carry the proppant mixture in the fracturing pipe and the fractures near the fracturing hole to a farther distance, and can also clean the fracturing pipe. S5. Gas extraction monitoring: After coal seam fracturing, the gas concentration and flow rate of multiple monitoring holes arranged around the fracturing borehole are monitored and collected in real time through the third flow sensor and concentration sensor. The liquid carbon dioxide pressurization system includes: a first high-pressure nitrogen pressurization device, a first connecting pipe, a storage tank, a first branch, a first pressure reducing valve, a first pressure gauge, a second pressure gauge, a first plunger pump, a second connecting pipe, and a first pressure sensor; The first high-pressure nitrogen booster is connected to the storage tank via a first connecting pipe for pressurizing the material in the storage tank. The first branch is set on the first connecting pipe, and a shut-off valve, a first pressure reducing valve, and a second pressure gauge are sequentially installed on the first connecting pipe. The first branch is also equipped with a shut-off valve and a first pressure gauge. The storage tank is connected to the first plunger pump via a second connecting pipe, and a first pressure sensor and a shut-off valve are sequentially installed on the second connecting pipe. The first pressure sensor is electrically connected to the data acquisition unit. The temperature control system includes: a third connecting pipe, a fourth connecting pipe, a fifth connecting pipe, a sixth connecting pipe, a heating box, a first temperature sensor, a second pressure sensor, and a third pressure sensor; The output end of the first plunger pump is connected to the heating box through a third connecting pipe, a fourth connecting pipe, a fifth connecting pipe and a sixth connecting pipe. A first temperature sensor, a second pressure sensor and a shut-off valve are sequentially installed on the third connecting pipe. A shut-off valve is installed on the fifth connecting pipe. A shut-off valve and a third pressure sensor are sequentially installed on the sixth connecting pipe. The third pressure sensor is electrically connected to the data acquisition unit. The proppant mixing system includes: a seventh connecting pipe, a mixing tank, a proppant injection pump, an eighth connecting pipe, a ninth connecting pipe, and a first flow sensor; One end of the seventh connecting pipe is connected to the fourth connecting pipe, and the other end of the seventh connecting pipe is connected to the feed inlet of the mixing tank. The proppant injection pump is connected to the feed inlet of the mixing tank through the eighth connecting pipe. Both the seventh and eighth connecting pipes are equipped with shut-off valves. The discharge port of the mixing tank is connected to the sixth connecting pipe through the ninth connecting pipe. The ninth connecting pipe is equipped with a first flow sensor, which is electrically connected to the data acquisition unit. The grouting system includes: a second high-pressure nitrogen booster, a tenth connecting pipe, a sealing grouting tank, a sealing grouting pipe, an eleventh connecting pipe, a plugging grouting pipe, a twelfth connecting pipe, a plugging grouting tank, a sealing return grouting pipe, a thirteenth connecting pipe, a slurry recovery tank, a capsule sealing device, a high-pressure nitrogen cylinder, an injection hose, a third pressure gauge, and a second pressure reducing valve. The second high-pressure nitrogen booster is connected to the sealing grouting tank via the tenth connecting pipe. The first end of the sealing grouting pipe is located within the sealing section. The second end of the sealing grouting pipe is connected to the sealing grouting tank via the eleventh connecting pipe, and a shut-off valve is installed on the eleventh connecting pipe. The first end of the plugging grouting pipe is located within the plugging section. The second end of the plugging grouting pipe is connected to the plugging grouting tank via the twelfth connecting pipe, and a shut-off valve is installed on the twelfth connecting pipe. The first end of the sealing return grouting pipe is located at the top of the sealing section. The second end of the sealing return grouting pipe is connected to the slurry recovery tank via the thirteenth connecting pipe, and a shut-off valve is installed on the thirteenth connecting pipe. The capsule sealing device is located between the sealing section and the fracturing section of the coal seam. The high-pressure nitrogen cylinder is connected to the capsule sealing device via an injection hose, and a third pressure gauge, a second pressure reducing valve, and a shut-off valve are sequentially installed on the injection hose. The fracturing system includes: a fracturing pipe, a fourteenth connecting pipe, a second flow sensor, a second temperature sensor, a fourth pressure sensor, a fifteenth connecting pipe, and a third temperature sensor; The first end of the fracturing pipe is located within the fracturing section opened in the coal seam. The second end of the fracturing pipe is connected to the fifth connecting pipe via the fourteenth connecting pipe. The fourteenth connecting pipe is sequentially equipped with a shut-off valve, a second flow sensor, a second temperature sensor, and a fourth pressure sensor. The fifteenth connecting pipe is located between the output end of the heating box and the fourteenth connecting pipe. The fifteenth connecting pipe is sequentially equipped with a third temperature sensor and a shut-off valve. The second temperature sensor, the second flow sensor, the third temperature sensor, and the second pressure sensor are all electrically connected to the data acquisition unit.

2. The downhole supercritical carbon dioxide fracturing of coal rock device construction method according to claim 1, characterized in that, The storage tank includes a tank body, a rubber sleeve, and a sealing flange; The rubber sleeve is disposed inside the tank, and the rubber sleeve has a space for storing liquid carbon dioxide. Sealing flanges are disposed on both sides of the tank and are respectively connected to the first connecting pipe and the second connecting pipe.

3. The downhole supercritical carbon dioxide fracturing of coal rock device construction method according to claim 2, characterized in that, The extraction system includes: a third flow sensor and a concentration sensor; Each of the monitoring holes is equipped with a third flow sensor and a concentration sensor. The third flow sensor is used to monitor the gas flow rate in the monitoring hole, and the concentration sensor is used to monitor the gas concentration in the monitoring hole. The third flow sensor and the concentration sensor are electrically connected to the data acquisition unit.

4. The downhole supercritical carbon dioxide fracturing of coal rock device construction method according to claim 3, characterized in that, It also includes a hydraulic booster system, which comprises: a water supply tank, a second plunger pump, a sixteenth connecting pipe, a liquid level sensor, a seventeenth connecting pipe, and a fifth pressure sensor; The water supply tank is connected to the second plunger pump via a sixteenth connecting pipe. A liquid level sensor and a shut-off valve are sequentially installed on the sixteenth connecting pipe. The output end of the second plunger pump is connected to the fourth connecting pipe via a seventeenth connecting pipe. A fifth pressure sensor and a shut-off valve are installed on the seventeenth connecting pipe. The liquid level sensor and the fifth pressure sensor are electrically connected to the data acquisition unit.

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